Nestin suppression attenuates invasive potential of endometrial cancer cells by downregulating TGF-β signaling pathway
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Amber A. Bokhari1, Tabari M. Baker1 , Batsukh Dorjbal1, Sana Waheed1, Christopher M. Zahn2, Chad A. Hamilton1,3,4, G. Larry Maxwell3,4,5, Viqar Syed1,4,6
1Uniformed Services University, Department of Obstetrics and Gynecology, Bethesda, MD 20814, USA
2American College of Obstetricians and Gynecologists, Washington, DC 20024, USA
3Women’s Health Integrated Research Center at Inova Health System, Department of Defense Gynecologic Cancer Center of Excellence, Annandale, VA 22003, USA
4John P. Murtha Cancer Center at Water Reed National Military Medical Center, Bethesda, MD 20889, USA
5Inova Fairfax Hospital, Department of Obstetrics and Gynecology, Falls Church, VA 22042, USA
6Uniformed Services University, Department of Molecular and Cell Biology, Bethesda, MD 20814, USA
Viqar Syed, email: firstname.lastname@example.org
Keywords: cell proliferation, progesterone, angiogenesis, matrix metalloproteinases, epithelial-mesenchymal-transition
Received: April 25, 2016 Accepted: September 02, 2016 Published: September 10, 2016
Nestin, an intermediate filament protein and a stem cell marker is expressed in several tumors. Until recently, little was known about the expression levels and the role of Nestin in endometrial cancer. Compared to the immortalized endometrial epithelial cell line EM-E6/E7-TERT, endometrial cancer cell lines express high to moderate levels of Nestin. Furthermore, endometrial tumors and tumor cell lines have a cancer stem-like cell subpopulation expressing CD133. Among the cancer lines, AN3CA and KLE cells exhibited both a significantly higher number of CD133+ cells and expressed Nestin at higher levels than Ishikawa cells. Knockdown of Nestin in AN3CA and KLE increased cells in G0/G1 phase of the cell cycle, whereas overexpression in Ishikawa decreased cells in G0/G1 phase and increased cells in S-phase. Nestin knockdown cells showed increased p21, p27, and PNCA levels and decreased expression of cyclin-D1 and D3. In contrast, Nestin overexpression revealed an inverse expression pattern of cell cycle regulatory proteins. Nestin knockdown inhibited cancer cell growth and invasive potential by downregulating TGF-β signaling components, MMP-2, MMP-9, vimentin, SNAIL, SLUG, Twist, N-cadherin, and upregulating the epithelial cell marker E-cadherin whereas the opposite was observed with Nestin overexpressing Ishikawa cells. Nestin knockdown also inhibited, while overexpression promoted invadopodia formation and pFAK expression. Knockdown of Nestin significantly reduced tumor volume in vivo. Finally, progesterone inhibited Nestin expression in endometrial cancer cells. These results suggest that Nestin can be a therapeutic target for cancer treatment.
Endometrial cancer is the most prevalent gynecologic cancer in the United States. When detected in early stages, these cancers have a good prognosis. However, in advanced stages the prognosis is generally poor [1, 2]. Treatment includes surgery, but for higher risk disease the addition of chemotherapy, radiation therapy, or a combination thereof is necessary. Unfortunately, if cancer progresses, systemic metastasis leads to patient morbidity and mortality [2, 3]. With an aging and increasingly obese population in the United States, the incidence and mortality rate of endometrial cancer is increasing. This worsening public health problem provides a strong rationale for a better understanding of the molecular mechanisms of endometrial carcinogenesis as well as developing effective methods to treat the disease.
Nestin belongs to a family of intermediate filament (IF) proteins. It consists of 1621 amino acids and has a molecular weight of 177.4 kDa [4, 5]. Nestin has a short N-terminus and a long C-terminus which binds to several structural proteins. The C-terminal of Nestin serves as a bridge between microtubules and other IFs proteins including vimentin, desmin, or internexin. Nestin assists in an array of signaling cascades through mobilizing and demobilizing cytoskeletal IFs proteins which culminates in changes in cell dynamics [6, 7, 8, 9].
Nestin was first identified as a neural stem cell marker [9, 10]. Nestin expression is reported in several tumor types including lung , brain , colorectal , breast , prostate  pancreas , ovary  and endometrial cancer . Recent studies revealed the involvement of Nestin in cancer cell migration, invasion, and metastasis, thus defining it as a potential therapeutic target [11, 15, 16]. In many solid tumors the Nestin expression was associated with a worse clinical outcome [14, 19, 20]. We investigated the expression and distribution of Nestin in well-annotated uterine cancers and found that high Nestin expression was more common in endometrial cancer with aggressive features and poor outcome . All of these studies suggest that Nestin is associated with tumorigenesis; nevertheless, the functions of Nestin in endometrial cancer cells are not fully elucidated. Here we report that the knockdown of Nestin expression suppresses cell proliferation, migration, and invasion by downregulating TGF-β signaling pathway components, mesenchymal markers, and enhancing epithelial markers in human endometrial cancer cell lines. Furthermore, knockdown of Nestin inhibits invadopodia formation and MMP expression resulting in attenuated tumor invasive potential. All together, these changes bring about a reduction in tumor growth and metastasis. Furthermore, we have identified progesterone as a potentially effective agent that can downregulate Nestin expression in cancer cells.
Nestin was upregulated in human endometrial cancer cell lines and correlates with number of stem-like cells
Nestin expression was compared in four endometrial cancer cell lines and an immortalized endometrial epithelial cell line. Compared with EM-E6/E7-TERT, an immortalized endometrial epithelial cell line, all cancer cell lines expressed higher levels of Nestin. The HEC-1B and Ishikawa cells showed moderate expression of Nestin compared with the AN3CA and KLE cell lines (Figure 1A). The existence of cancer stem-like cells (CSCs) has been shown in human endometrial cancer and in endometrial cancer cell lines . Stem-like cell subpopulations, referred to as “side population” (SP) cells, have features of cancer stem cells and are capable of effluxing the DNA-binding dye Hoechst 33342. Since Nestin is a stem cell marker, we examined the correlation of Nestin expression with the SP cell percentages in EM-E6/E7-TERT, HEC-1B, Ishikawa, AN3CA and KLE cell lines. The percentages of SP cells in high Nestin expressing KLE and AN3CA cells were 10.5% and 13.1% compared to moderate Nestin expressing (HEC-1B, 4 %, Ishikawa, 5%) cells and low Nestin expressing (EM-E6/E7-TERT, 1%) cells (Figure 1B).
Figure 1: Expression of Nestin in immortalized endometrial epithelial EM-E6/E7-TERT cells and endometrial cancer lines. (A) EM-E6/E7-TERT and endometrial cancer cell lines (HEC-1B, Ishikawa, KLE and AN3CA) were evaluated by Western blot for basal expression of Nestin. β-Actin was used as a loading control. Representative Western blot analyses of 3 independent experiments with similar results are shown. The values above the bands represent relative density of the bands normalized to β-actin. (B) The percentages of CD133+ (Stem-like cell subpopulation known as side population, SP) and CD133− (Main population, MP) cells are shown in Immortalized EM-E6/E7-TERT cell line and in the HEC-1B, Ishikawa, KLE and AN3CA endometrial cancer cells. The experiment was repeated twice and a representative experiment is shown. Bars are mean ± SEM and * indicates statistically significant difference between the low Nestin expressing and high Nestin expressing cancer cell lines (P < 0.05).
Nestin knockdown abrogated while overexpression enhanced the malignant phenotype of endometrial cells
To study the role of Nestin in cancer cell lines, Nestin was knocked down in AN3CA and KLE cells, which express Nestin at higher levels, and overexpressed in Ishikawa cells which exhibit moderate levels of Nestin. The expression pattern of Nestin was confirmed by Western blotting in Nestin knockdown and overexpressing cell lines (Figure 2A).
Figure 2: Knockdown of Nestin decreases and overexpression increases cell migration, colony formation and invasion. (A) Nestin knockdown and overexpressing cells were harvested, and whole-cell extracts were loaded on the gel. The blot was probed with the Nestin antibody. β-Actin was used as a loading control. Representative Western blot analyses of 3 independent experiments with similar results are shown. The values above the bands represent relative density of the bands normalized to β-actin. (B) AN3CA, KLE, Ishikawa, Nestin knockdown AN3CA, Nestin knockdown KLE, and Nestin overexpressing Ishikawa were grown in 96-well plates. Cell viability was measured by MTS assay at 12, 24, 48 and 72 hours. Results represent the group mean values of four wells ± SEM. Experiment was repeated twice. The * indicates a statistically significant difference (P < 0.05) in the cell number of AN3CA, KLE and Ishikawa cells over 72 h compared to their respective 0 h controls. The # indicates a statistically significant difference (P < 0.05) in the cell number of Nestin knockdown AN3CA, KLE and Nestin overexpressing Ishikawa cells compared to their respective 0 h controls. (C) AN3CA, KLE, Ishikawa, Nestin knockdown AN3CA, Nestin knockdown KLE, and Nestin overexpressing Ishikawa were cultured on soft agar to examine colony-forming ability or (D) on Matrigel chambers to evaluate invasion. Number of colonies on soft agar were counted 6 weeks later and cells that migrated through the Matrigel were counted following 22 h of plating. Data is the means of three experiments with triplicate wells or plates. Bars are mean ± SEM and * indicates statistically significant difference (P < 0.05).
We performed cell proliferation, colony formation, and invasion assays to elucidate the role of Nestin on endometrial cancer cells. The effect of Nestin knockdown or overexpression on endometrial cancer cell growth was assessed via a proliferation assay. High Nestin expressing AN3CA and KLE cells (Figure 2B) proliferate faster than Nestin knockdown AN3CA and KLE at all time points studied (Figure 2B). A significant increase in proliferation of AN3CA and KLE cells was observed at 12 h after plating. However, no significant difference was seen in the proliferation rate of Nestin knockdown KLE in the first 12 h. The proliferation rate was significantly reduced in Nestin knockdown AN3CA and KLE compared to Nestin expressing AN3CA and KLE cells (Figure 2B). Comparison of growth rates between Ishikawa cells moderately expressing Nestin with Nestin overexpressing Ishikawa cells showed a time dependent increase in cell proliferation. However, Nestin overexpressing Ishikawa cells proliferate at a faster rate than moderately Nestin expressing Ishikawa cells (Figure 2B).
Anchorage-independent growth in vitro is deemed to indicate the capacity of cells to establish tumors in vivo. Nestin knockdown or overxpressing cells were plated on 0.3% soft agar and colonies consisting of more than 6 cells were counted 6 weeks after plating. Nestin knockdown AN3CA and KLE cells displayed a significant (70–75%) decrease in the number of colonies compared with Nestin expressing AN3CA and KLE cells. Overexpression of Nestin in Ishikawa cells enhanced the colony formation ability (80%) compared with Ishikawa cells moderately expressing Nestin (Figure 2C). Matrigel invasion assay was used to characterize the role of Nestin on cell invasion. As shown in Figure 2D, a significant decrease of cell invasion was observed in Nestin knockdown AN3CA and KLE cells compared with their respective Nestin expressing cells. Furthermore, Nestin overexpression promoted invasiveness of Ishikawa cells compared to moderately expressing Nestin Ishikawa cells (Figure 2D).
Nestin knockdown exhibited cell cycle arrest at the G1/S phase
To determine whether Nestin knockdown or overexpression will affect the tumor cell cycle, cells were subjected to DNA flow cytometry. The cell cycle analysis demonstrated an increased cell population in the G0/G1 phase and a decreased the cells in S phase in Nestin knockdown AN3CA and KLE cells compared with their respective high Nestin expressing cells. Overexpression of Nestin in Ishikawa cells markedly reduced the cells in the G0/G1 phase and increased the cells in S phase (Figure 3A). To investigate the mechanism by which Nestin regulates cell cycle of endometrial cancer cells, we assessed the levels of cell cycle regulatory proteins. Compared to control groups, the levels of cyclin-D1, cyclin-D3 and PCNA decreased while P21 and P27 increased in Nestin knockdown AN3CA and KLE cells. In contrast, the expression of cyclin-D1, cyclin-D3 and PCNA increased while P21 and P27 levels were reduced in Nestin overexpressing Ishikawa cells compare to moderately Nestin expressing Ishikawa cells (Figure 3B). These results clearly demonstrated that knockdown of Nestin induced cell cycle arrest at G1/S phase in AN3CA and KLE cells and Nestin overexpression positively promoted G1/S phase transition via the upregulation of cyclin D1, cyclin D3, and PCNA expression.
Figure 3: Nestin knockdown suppresses and overexpression enhances cell cycle progression. (A) The columns show the percentages of cell cycle distribution between control cells, Nestin knockdown and Nestin overexpressing cells endometrial cancer cells. (B) Western blot analysis of the levels of Cyclin D1, Cyclin D3, PCNA, P21 and P27 in lysates of Nestin knockdown and Nestin overexpressing endometrial cancer cells. Representative Western blot analyses of 3 independent experiments with similar results are shown. The values above the bands represent relative density of the bands normalized to β-actin.
Nestin knockdown suppressed and overexpression elevated expression of TGF-β signaling components in endometrial cancer cells
Cancer cell proliferation and migration are regulated by a variety of signaling pathways. The TGF-β signaling pathway is implicated in the acquisition of an aggressive phenotype in human endometrial carcinoma [22–24]. We therefore examined the impact of Nestin knockdown or overexpression on the TGF-β pathway components in endometrial cancer cells. Nestin knockdown in AN3CA and KLE cells showed significant reduction of TGF-βs, TGF-βRs, p-SMAD2/3, and SMAD-4 expression. All of the TGF-β signaling components were upregulated in Nestin overexpressing Ishikawa cells. Total SMAD2/3 was not affected by Nestin knockdown or overexpression (Figure 4A).
Figure 4: Nestin knockdown suppresses and overexpression enhances cell TGF-β signaling components and alters cell proliferation, invasion and EMT markers in endometrial cancer cells. (A) Expression of TGF-β1, TGF-β2, TGF-β3, TGF-βR1, TGF-βR2, TGF-βR3, SMAD2/3, p-SMAD2/3, and SMAD-4 are shown in Nestin knockdown and overexpressing cells. The values above the bands represent relative density of the bands normalized to β-actin. (B) AN3CA, KLE, Ishikawa, Nestin knockdown AN3CA, Nestin knockdown KLE, and Nestin overexpressing Ishikawa were cultured with and without TGF-β1 (10 ng/mL) for 72 hours in the presence or absence of TGF-βR1 (SD-208, 10 mmol/L) blocker, and the effect of treatment on Nestin expression, cell viability and invasion was assessed using Western blotting, MTS and cell invasion assays. Bars are mean ± SEM and * indicates statistically significant difference (P < 0.05). (C) EMT markers were evaluated in AN3CA, KLE, Ishikawa, Nestin knockdown AN3CA, Nestin knockdown KLE, and Nestin overexpressing Ishikawa by Western blot analysis. Representative Western blot analyses of 3 independent experiments with similar results are shown.
To determine a direct association between Nestin and TGF-β, Nestin overexpressing and knockdown cells were treated with TGF-β1 in the presence or absence of TGF-βR1 blocker and consequences on Nestin expression, cell growth and invasive potential were assessed. Control (empty vector transfected cells), and Nestin overexpressing cells treated with TGF-β1 showed a marked increase in Nestin expression, cell proliferation and invasiveness. Nestin knockdown cells showed comparatively small but significant increase in Nestin expression, cell proliferation and invasion. However, these changes were not discerned when cells were cultured in the presence of TGF-βR1 blocker (Figure 4B). These results provide evidence that Nestin is required and involved in TGF-β-regulated cell proliferation and invasion.
Nestin knockdown upregulated and overexpression suppressed expression of E-cadherin in endometrial cancer cells
TGF-β signaling induces epithelial-mesenchymal transition (EMT), a process that facilitates migration and invasion of tumors. To assess the influence of Nestin knockdown or overexpression on the EMT process, the expression of EMT marker proteins was evaluated by Western blotting. Nestin knockdown AN3CA and KLE cells exhibited downregulation of the early transcriptional mesenchymal markers (SNAIL, SLUG, and TWIST), cell adhesion molecule (N-cadherin), mesenchymal intermediate filament (vimentin) and upregulation of an epithelial marker, E-cadherin, which is the suppressor of tumor cell invasion and metastasis. Overexpression of Nestin in Ishikawa cells enhanced expression of SNAIL, SLUG, TWIST, N-cadherin, and vimentin while suppressed E-cadherin (Figure 4C).
Nestin knockdown inhibited while overexpression increased invadopodia formation in cancer cells
Invadopodia are actin-enriched membrane protrusions that are important for invasive cell motility . To test whether Nestin is involved in the formation of invadopodia, Nestin knockdown AN3CA and KLE or Nestin overexpressing Ishikawa cells were examined for the expression of cortactin, which is a specific marker for invadopodia. Co-localization of F-actin and cortactin, is often considered an indicator of invadopodia formation. Using immunocytochemistry, F-actin co-localized with cortactin, appeared as yellow dots, were seen in Nestin expressing AN3CA and KLE cells. However, this co-localization was not observed in Nestin knockdown AN3CA and KLE cells. Overexpression of Nestin in Ishikawa cells resulted in greater number of invadopodia compared with moderately Nestin expressing Ishikawa cells (Figure 5A). The results suggest that increased expression of Nestin promoted while knockdown decreased the formation of invadopodia (Figure 5A). Since matrix metalloproteinases and FAK are associated with invasion and metastasis, we analyzed their expression in Nestin knockdown and overexpressing cells. Knockdown of Nestin reduced while overexpression increased pFAK, MMP-9, MMP-2 expression as well as the activity of MMP-9 and MMP-2 in endometrial cancer cells (Figure 5B and 5C).
Figure 5: Nestin knockdown inhibits and overexpression promotes invadopodia formation. (A) AN3CA, KLE, Ishikawa, Nestin knockdown AN3CA, Nestin knockdown KLE, and Nestin overexpressing Ishikawa were seeded on coverslips and fixed with 4% paraformaldehyde. The cells were labeled to visualize F-actin (phalloidin, red) and cortactin (green). Overlapping yellow dots indicate the colocalization of Factin and cortactin. Invadopodia are denoted in the control AN3CA, KLE and Nestin overexpressing Ishikawa cells by white arrows. Scale bar, 20 μm. (B) Silencing of Nestin downregulates genes involved in invadopodia formation. The downregulation of key invadopodia-related proteins were validated using Western blotting. Representative Western blot analyses of 3 independent experiments with similar results are shown. The values above the bands represent relative density of the bands normalized to β-actin. (C) MMP-2 and MMP-9 enzyme activity in conditioned media of Nestin knockdown and overexpressing cells. The assays were performed in triplicates. Bars are mean ± SEM and * indicates statistically significant difference (P < 0.05).
Nestin knockdown inhibited tumor growth in vivo
Role of Nestin in endometrial carcinogenesis in vivo was investigated by injecting Nestin expressing and Nestin knockdown AN3CA cells subcutaneously into mice (n = 6). Two mice in the Nestin expressing group were euthanized after four weeks because of large tumor size and mice appeared moribund. In the Nestin knockdown group, all mice but one failed to develop tumors. Ten weeks after injections, mice were euthanized and tumor volume was calculated. Compared to the mice injected with Nestin expressing AN3CA cells, mice injected with Nestin knockdown AN3CA cells displayed reduced tumorigenicity. The tumor size was clearly decreased in the Nestin knockdown group compared with those of the Nestin expressing group (Figure 6A). The expression of Nestin, TGF-β signaling components and EMT markers was evaluated in Nestin knockdown and Nestin expressing tumors and reduced levels of Nestin, viementin, TGF-β signaling proteins and increased levels of E-cadherin were confirmed in Nestin knockdown tumors (Figure 6B and 6C).
Figure 6: Nestin knockdown inhibits in vivo tumorigenesis. (A) Tumor sizes and volumes of Nestin expressing and Nestin knockdown groups were measured after autopsy. (B) Downregulation of TGF-β signaling proteins in Nestin knockdown group. (C) Nestin, E-cadherin, and vimentin in Nestin knockdown and Nestin expressing groups. The values above the bands represent relative density of the bands normalized to β-actin. *indicates statistically significant difference (P < 0.05).
Progesterone attenuated Nestin expression in endometrial cancer cells
Progesterone has been studied for its anti-cancer properties. The effect of progesterone on Nestin expression in AN3CA, Nestin knockdown AN3CA, KLE, Nestin knockdown KLE, Ishikawa and Ishikawa overexpressing cells was investigated. The expression levels of progesterone receptors (PR) were analyzed in Nestin knockdown and Nestin overexpressing cells using a mouse anti-human PR antibody that identifies both isoforms: PR-A and PR-B. Both forms of receptors were expressed in all cell lines tested. No marked differences were noticed in Nestin knockdown or overexpressing cell lines compared to their respective control cell lines (Figure 7A). Progesterone inhibited expression of Nestin in AN3CA, KLE, Ishikawa and Nestin overexpressing Ishikawa cells. However, progesterone showed no significant effect on Nestin knockdown AN3CA and KLE cells (Figure 7B). These results suggest that progesterone can successfully be used for inhibiting Nestin expression in cancer cells.
Figure 7: Nestin can be inhibited with progesterone in vitro. (A) Levels of progesterone receptors were assessed in the AN3CA, KLE, Ishikawa, Nestin knockdown AN3CA, Nestin knockdown KLE, and Nestin overexpressing Ishikawa cells by Western blot analysis. (B) The cellular extracts of the AN3CA, KLE, Ishikawa, Nestin knockdown AN3CA, Nestin knockdown KLE, and Nestin overexpressing Ishikawa cells treated with progesterone (20 μM) for 72 h were evaluated by Western blot analysis for the expression of Nestin. The values above the bands represent relative density of the bands normalized to β-actin.
The increased incidence of endometrial cancer is a growing public health concern. Thus, understanding the mechanisms of carcinogenesis will aid in the development of treatment options. We previously examined the expression pattern of Nestin and established its potential prognostic and predictive value in endometrial tumors. Advanced stage high grade endometrial tumors with lymphovascular space invasion (LVSI) revealed three to five times higher Nestin levels compared to early stages cancer. High Nestin expression demonstrated a correlation with worse progression-free (PFS), cancer-specific (CSS), and overall (OS) survival .
In this study we examined the role of Nestin in endometrial cancer cells in vitro and in vivo. Cancer cells expressed Nestin at higher levels than immortalized endometrial epithelial cells. Nestin expression varied among cancer lines. Nestin is a stem cell marker and stem-like cells, also known as side-population (SP) cells, have been identified in endometrial tumors and tumor cell lines based on their ability to remove intracellular Hoechst 33342 . We compared Nestin expression with the number of SP cells in cancer cell lines. Both the levels of Nestin and the number of SP cells were higher in AN3CA and KLE cells compared to Ishikawa and HEC-1B cells. Knockdown of Nestin resulted in a decreased percentage of SP cells in AN3CA and KLE cells, whereas overexpression of Nestin increased the percentage of SP cells in Ishikawa cells (results not shown). Since stem cells have important roles in the development of cancer, metastasis, recurrence of tumors and resistant to conventional cancer therapy, reducing the number of stem-like cells by Nestin knockdown may be an effective strategy for treatment of endometrial cancer. These results are in agreement with several studies that have demonstrated decreases in the cell growth, migration, invasion and metastasis in vitro and in vivo through alteration in stem-like cells population in human oral squamous cell carcinoma, breast, lung, pancreatic, and prostate cancers [27–29].
Nestin plays a role in various stages of tumor progression. We found a positive association between Nestin overexpression, increase in the number of stem-like cells, cell growth, and invasive potential of endometrial cancer cells. In breast cancer, a similar association was reported between high Nestin and rapid formation of mammospheres in vitro and formation of solid tumors in vivo . We detected high expression of Nestin in endometrial cancer cell lines (AN3CA and KLE) and in endometrial tumors  suggesting that Nestin expression is not limited to the endometrial cancer stem-like cells.
Overexpression of Nestin in pancreatic cancer increased cell motility and cause phenotypic changes associated with the epithelial-mesenchymal transition . It is well established that Nestin knockdown decreases migration of prostate cancer cells [32, 33] and suppresses proliferation of nasopharyngeal carcinoma  and lung cancer  cells by causing cell cycle arrest at G2/M phase. In the present study we found that Nestin knockdown reduced proliferation of endometrial cancer cells by inducing cell cycle arrest in G0/G1 phase, and was accompanied by changes in expression of cell cycle regulatory proteins related to G1-S transition. Furthermore, knockdown of Nestin reduced endometrial tumor growth in vivo in a xenograft model. Tumors derived from Nestin knockdown AN3CA cells were smaller and weighed significantly less compared with those originating from control AN3CA cells.
A plethora of data indicate that TGF-β signaling has tumor suppressive effects in early-stages of cancer and a tumor-promoting effect in more advanced stages [36, 37]. We and others have previously shown that TGF-β signaling pathway is activated in endometrial cancer cells [22, 23, 38]. Our findings suggest that elevated levels of Nestin in endometrial cancer cells can stimulate cell proliferation and invasion by stimulating TGF-β signaling pathway. In contrast, silencing of Nestin inhibited TGF-β signaling and revealed potent anti-tumorigenic effects on endometrial cancer cells. These findings are in agreement with studies showing a positive correlation of Nestin and TGF-β signaling in pancreatic ductal adenocarcinoma .
Previously we have shown enhanced proliferation and invasive potential of endometrial cells in response to TGF-β1 . Here we substantiated the direct interaction between Nestin and TGF-β in Nestin overexpressing cells by demonstrating a marked increase in Nestin expression which was associated with enhanced proliferation and invasive potential in response to TGF-β1 exposure. Furthermore, interaction of Nestin and TGF-β is indicated by the fact that TGF-βR1 blocker inhibited TGF-β1 induced Nestin expression and malignant phenotypes. TGF-β1 induced moderate Nestin expression and marginally affected cell proliferation and invasion in Nestin knockdown cells. The reduced effects of TGF-β1 can be attributed to reduced TGF-β receptors in Nestin knockdown cells. These findings clearly suggest that Nestin is critical for TGF-β signaling.
TGF-β signaling pathway is known to promote cancer cell invasiveness and metastatic potential by inducing the expression of the epithelial-mesenchymal-transition (EMT) transcription factors . EMT is a process where cells change from an epithelial to a mesenchymal phenotype and is an important step in the invasion and metastasis of cancer. We evaluated the expression of epithelial and mesenchymal markers as well as EMT transcription factors in Nestin knockdown endometrial cancer cells, and found enhancement of E-cadherin and attenuation of vimentin, N-cadherin and EMT transcription factors SNAIL, SLUG and TWIST. Nestin overexpression in Ishikawa cells downregulated E-cadherin and upregulated the EMT transcription factors, SNAIL, SLUG, TWIST and mescenchymal markers. Consistent with these results, the regulation of EMT by Nestin has been shown in number of cancers including gastric adenocarcinoma  and pancreatic cancer [31, 41].
Metastasis is a multistep process comprised of migration, invasion through tumor stroma, intravasation inside the primary tumor, tumor cell dissemination, extravasation and cell growth at the secondary sites . Proteolytic protrusions (invadopodia) are the drivers of metastasis . Knockdown of Nestin in AN3CA and KLE cells inhibited pFAK and MMPs along with reduction of invadopodia. The Nestin stimulated invadopodia formation and invasion is likely due to increased activation of MMP-2, MMP-9 and pFAK in Ishikawa cells. These results are in line with other studies linking inhibition of invadopodia formation by flavonoids and other drugs to reduction of metastasis . Developing effective cancer treatment requires identification of novel molecular targets essential for tumorigenesis and metastasis. The findings presented here strongly suggest an important role of Nestin in endometrial cancer.
Clinical and epidemiologic studies delineate progestins as extremely effective endometrial cancer preventive agents. Regular use of progestins reduces endometrial cancer risk and the protective effect upsurges with increasing progestin potency. In premenopausal women, usage of progestin containing oral contraceptives significantly reduce endometrial cancer risk. The ability of progesterone to antagonize proliferation and promote atrophy of the endometrium has been known . Women with pre-invasive atypical hyperplasia or early stages of endometrial cancers without myometrial invasion showed high response rates (70–90%) to progestin treatment . The advanced stage or recurrent cancers are 20–30% responsive to progestins [47, 48]. Progestin preparations are used effectively to treat endometrial cancer. They can be used in advanced and/or recurrent endometrial cancer in combination with chemotherapy . In order to explore drugs/agents that can inhibit Nestin we employed progesterone. Our results showed marked decrease in Nestin expression in AN3CA, KLE and Nestin overexpressing cells with progesterone.
In conclusion, we showed that Nestin silencing significantly reduced the growth and invasion of endometrial cancer cells through the suppression of the TGF-β signaling pathway, which is activated in endometrial tumors and is implicated in the EMT process. Furthermore, Nestin knockdown inhibits tumor metastasis by inhibiting invadopodia formation and reducing the levels of MMP-2, MMP-9 and pFAK. Thus, Nestin may be a promising therapeutic target and progesterone can be a potential therapeutic agent for the treatment of endometrial cancer.
MATERIALS AND METHODS
Cell lines and culture conditions
The immortalized human endometrial epithelial cell line EM-E6/E7-TERT was kindly provided to us by Dr. Satoru Kyo, (Kanazawa University, School of Medicine, Kanazawa, Japan). The human endometrial cancer cell lines HEC-1B, AN3CA and KLE were obtained from the American Type Culture Collection (ATCC) and Ishikawa cell line was obtained from Sigma.
Immortalized epithelial endometrial (EM-E6/E7-TERT) cells were established and characterized by Kyo and colleagues . Primary endometrial epithelial cells were transformed by the combination of human papillomavirus-E6/E7 expression and telomerase activation by the introduction of human telomerase reverse transcriptase (hTERT). The immortalized cells contained no chromosomal abnormalities, retained responsiveness to sex-steroid hormones, exhibited glandular structure on 3-dimensional culture, and lacked transformed phenotypes on soft agar or in nude mice. Ishikawa cells were derived from a grade-1 adenocarcinoma of the human endometrial epithelium from a 39-year-old woman, and express functional steroid receptors for estrogen, progesterone, and androgen. HEC-1-B cells were derived from a moderately differentiated grade- 2 adenocarcinoma of endometrial epithelium from a 71-year-old patient. AN3CA and KLE cells were derived from a 55 and 64 years Caucasian females with a poorly differentiated grade 3 endometrioid adenocarcinoma.
These cell lines were authenticated by DNA short-tandem repeat analysis by ATCC and Sigma. All cell lines were initially expanded and cryopreserved within 1 month of receipt. Cells were typically used for 3 months, at which time a fresh vial of cryopreserved cells was used.
The EM-E6/E7-TERT cells were grown in Dulbecco’s modified Eagle medium (DMEM). Both KLE and Ishikawa cells were grown in DMEM:F12, but media was supplemented with Insulin (Invitrogen) for Ishikawa cells. HEC-1B and AN3CA cells were cultured in Eagle’s minimum essential medium (Invitrogen). All five media were supplemented with 10% (v/v) FBS (Invitrogen), 100 U/mL penicillin, and 100 ug/mL streptomycin. The cells were cultured at 37οC in a humidified atmosphere containing 5% CO2. To study the interaction of Nestin and TGF-β, Nestin knockdown or overexpressing endometrial cancer cells were treated with TGF-β1 (10 ng/mL) in the presence or absence of TGF-βR1 blocker (SD-208, 10 mmol/L; Santa Cruz Biotechnology) for 72 hours and the effects on Nestin expression, cell viability and invasion were assessed. TGF-βR1 blocker was in DMSO. The final concentrations of DMSO in the media was 0.1%. Control cells were treated with equal concentration of DMSO alone. For a set of experiments, cells were treated with progesterone (20 μM/L) for 72 hours and the effects on Nestin expression was assessed by Western blotting. The progesterone solution was prepared in absolute ethanol. The final concentration of ethanol in the media was 0.1%. Untreated control cultures were exposed to equal concentrations of ethanol alone.
Generation of Nestin knockdown cell lines
Plasmid containing shRNA against Nestin was purchased from GE Dharmacon (Lafayette, CO; Cat#RHS4531-EG10763). The plasmid contains a copy of the puromycin N-acetyltransferase gene, conferring puromycin resistance to eukaryotic cells. In addition, the plasmid contains a copy of TurboGFP, allowing for visual marking of shRNA expressing cells. Lentiviral particles were generated in HEK293T packaging cells using 2nd generation packaging and envelope plasmids (pCMV-R8.74psPAX2; VSV-G/pMD2.G). To describe briefly, HEK293T cells were seeded onto 100 mm dishes. Once the cells were 50% confluent, they were transfected with a mixture of the packaging (9 μg), envelope (0.9 μg), and shRNA (9 μg) plasmid or with the packaging and envelope plasmids using Fugene 6 according to manufacturer’s instructions. After incubation at 37°C for 18 hours, the media was changed to growth media containing 30% fetal bovine serum. Media containing viral particles was harvested 24 hours later, centrifuged at 1000 × g to remove cells, aliquoted, and stored at −80°C until use. AN3CA and KLE cells were cultured either with Nestin plasmid or with an empty vector containing viral particles media, in the presence of 8 μg/mL polybrene (Santa Cruz Biotechnology; Dallas), for 48 hours to develop control and Nestin knockdown cell lines. The cells were selected using puromycin (1 μg/mL) in the appropriate media. After 7 days in selection, cell lysates were collected and Nestin protein expression was assessed. Empty vector infected AN3CA and KLE cells are denoted as control AN3CA and KLE.
Generation of Nestin overexpressing cell line
Plasmids containing the complete open reading frame of Nestin (NM006617) were purchased from Genecopoeia (Rockville, MD). The plasmid was provided in the pEZ-M68 mammalian expression vector with Nestin mRNA expression controlled by the CMV promoter. Ishikawa cells were transfected with Nestin plasmid or with empty vector plasmid using the Fugene 6 transfection reagent according to the provided instructions (Promega; Madison, WI). Forty-eight hours after transfection, cells were selected using 1μg/mL puromycin in appropriate media (Life Technologies; Grand Island, NY). After 7 days in selection, cell lysates were collected and Nestin protein expression was assessed. Empty vector transfected Ishikawa cells are referred as control Ishikawa.
Identification of side population (stem-like cells) in endometrial cancer cell lines
Control and genetically manipulated endometrial cancer cell lines (AN3CA, KLE, and Ishikawa) were grown in their respective media supplemented with penicillin, and streptomycin. Prior to the identification of a putative stem cell population in the cell lines, each cell line was grown to 70% confluence. After trypsinization cells were collected, counted, and resuspended at 1.0 × 106 cells per milliliter (mL). To identify the side population percentage in each cell line, a Hoescht dye efflux assay was performed by treating cells (1.0 × 106) with Hoechst 33342 for 90 minutes at 37°C. In order to ensure that the side population was appropriately identified, cells were treated with the ABC transporter inhibitor verapamil for 60 minutes prior to Hoechst 33342 treatment. In addition, cells were incubated with a CD133 antibody in PBS containing 10% goat serum and 0.5% bovine serum albumin (BSA) for 15 minutes at 4°C. The cells were pelleted and rinsed twice with cold PBS and subsequently incubated with an AlexaFluor-488 secondary antibody for 30 minutes at 4°C. The cells were again pelleted and rinsed twice in cold PBS, and resuspended in cold PBS containing propidium iodide (PI) for dead cell exclusion. Flow cytometry was performed on a BD LSRII flow cytometer equipped with both a violet and an ultraviolet laser. Four hundred thousand (400,000) events were collected and live single cells were electronically gated. CD133+, CD133−, Hoechsthigh and Hoechstlow populations were identified and analyzed using FlowJo software (FlowJo, LLC; Ashland, OR).
Cell viability assay
AN3CA, Nestin knockdown AN3CA, KLE, Nestin knockdown KLE, Ishikawa, and Nestin overexpressing Ishikawa cells (1,500/well) were plated in a 96-well plate and viability was examined at 12, 24, 48 and 72 h by using the CellTiter 96 AQueous One Solution cell viability assay (Promega) according to the instructions of the manufacturer. CellTiter 96 AQueous One Solution reagent (20 mL) was added into each well of the 96-well assay plate containing the samples in 100 mL of culture medium. Absorbance was measured at 490 nm using a microtiter plate reader. Assays were performed in triplicate to generate mean values for each time point. Cell number, as measured by the rate of formazan formation in cells at the time of plating was considered to be 100%. Changes in cell viability was expressed as percentage change of Nestin knockdown or Nestin overexpressing cells over their respective control cells at the 0 time point.
Soft agar colony-formation assay
The AN3CA, Nestin knockdown AN3CA, KLE, Nestin knockdown KLE, Ishikawa, and Nestin overexpressing Ishikawa cells were cultured at 5000 cells per 100-mm plate (four plates per sample) in 0.3% agar above an underlayer of 0.6% Noble agar, both containing complete medium . Plates were incubated for up to 6 weeks and the number of colonies was scored in five fields of each plate to establish the soft agar colony formation potential for each cell line with Nestin knockdown or Nestin overexpression.
Cell invasion assay
Endometrial cancer cells AN3CA, Nestin knockdown AN3CA, KLE, Nestin knockdown KLE, Ishikawa, and Nestin overexpressing Ishikawa cells were detached by trypsin and resuspended in serum-free medium. Medium containing 10% FBS medium was applied to the lower chambers of BD BioCoat Matrigel Invasion Chambers (BD Biosciences) as a chemoattractant and then cells were seeded on the upper chambers at a density of 2.5 × 104 cells per well in 100 mL of serum-free medium. The chambers were incubated for 16 to 18 hours at 37°C. At the end of incubation, noninvading cells were removed from the upper surface of the membrane by scrubbing. The cells on the lower surface of the membrane were fixed for 2 minutes in 100% methanol and stained with 1% toluidine blue in 1% sodium borate for 2 minutes. Excess stain was removed by rinsing the inserts with water. Each membrane was removed from the insert and placed on a microscope slide. A cover slip was placed on the slide and the cells were counted in five random fields per slide .
AN3CA, AN3CA Nestin knockdown, KLE, KLE knockdown cells, Ishikawa and Ishikawa Nestin overexpressing cells were plated onto tissue culture flasks and cell-cycle status was measured after 24 hours of plating. The cells (1 × 106) were collected and fixed in ice-cold 70% ethanol, and subsequently stained with a propidium iodide (PI) staining solution for 30 minutes at room temperature (PI staining solution: 10 μg/mL propidium iodide, 0.1% Triton X-100, 100 μg/mL RNase A in PBS). Cytometry was performed on a BD LSRII flow cytometer equipped with a 488nm argon laser. Propidium iodide was detected through the emission of 617nm light through 595 long pass and 610/20 band pass filters. Single cells were electronically gated and twenty thousand (20,000) events were collected. Analysis of cell cycle, by drawing gates for G1, S, and G2/M phases, was completed using FlowJo software (FlowJo LLC, Ashland, OR).
AN3CA, Nestin knockdown AN3CA, KLE, Nestin knockdown KLE, Ishikawa, and Nestin overexpressing Ishikawa cells endometrial cancer cells seeded on coverslips were fixed in 4% paraformaldehyde for 20 minutes at room temperature and permeabilized with phosphate buffered saline (PBS) containing 0.1% Triton X-100 (PBS-T) for 10 minutes, and subsequently blocked with PBS-T containing 5% normal goat serum for 1h. Cells were stained with mouse monoclonal anti-cortactin antibody (EMD Millipore) for 1h in PBS-T containing 1% normal goat serum and a secondary antibody Alexa Fluor® 488-labeled anti-mouse antibody (Invitrogen Life Technologies). To detect F-actin, cells were stained with Phalloidin–Tetramethylrhodamine B isothiocyanate (Sigma-Aldrich) for 20 minutes and the cell nuclei were stained with 1 μg/mL DAPI for 10 minutes. Fluorescent images of cells were captured using a LSM 710 laser scanning confocal microscope (Carl Zeiss Microscopy, LLC Thornwood NY).
Western blot analysis
Endometrial cancer cells AN3CA, Nestin knockdown AN3CA, KLE, Nestin knockdown KLE, Ishikawa, and Nestin overexpressing Ishikawa cells extracts were analyzed using antibodies against Nestin, cyclin D1, cyclin D3, p27 , p21 (Cell Signaling Technology), TGFβ1, TGFβ2, TGFβ3, TGFβR1, TGFβR2, TGFβR3, pSMAD2/3, SMAD2/3, SMAD4 (Santa Cruz Biotechnology), pFAK (phospho Y397), E-cadherin, N-cadherin, SNAIL, SLUG, Twist, vimentin (Cell Signaling Technology, Inc.), progesterone receptor (PgR 1294; Dako Corporation), and β-actin (Sigma-Aldrich). Equal amounts of protein were subjected to SDS-PAGE. The enhanced chemiluminescence system was used to visualize the protein bands as recommended by the manufacturer (Pierce). Protein bands were quantified using densitometry software (Bio-Rad) and normalized by using actin as a loading control. To calculate the relative intensity of each band individual bands were divided by the corresponding loading control intensity.
In vitro MMP-2 and MMP-9 activity assay
The MMP-2 and 9 activity was evaluated in the conditioned media of Nestin knockdown and overexpressing cells using MMP-2/MMP-9 colormetric drug discovery kits (Enzo, USA) according to the manufacturer’s instruction. In short, conditioned media were plated in 96 well plates with MMP substrate solution into each well for 60 minutes at 37°C. Absorbance was measured at 412 nm. The experiment was performed in triplicates.
Tumorigenicity in nude mice
Six-week-old female nude mice (nu/nu; strain 088) were purchased from Charles River Laboratories (Wilmington, MA). The animals were maintained in the Center for Laboratory Animal Medicine at the Uniformed Services University of the Health Sciences under controlled lighting (14 hours of light, 10 hours of darkness) at a temperature of 21°C and were provided with water and mouse chow ad libitum. AN3CA and Nestin knockdown AN3CA cells (3 × 106 ) were subcutaneously injected in the right flank of mouse (n = 6). After ten weeks, the mice were euthanized and tumors were excised. Tumor size was measured using a caliper and the tumor volume was determined using the formula: width2 × length × 0.5. Portions of the tumors were fixed in 10% neutral buffered formalin for routine histopathological processing. Animal experiments were carried out according to the institutional animal care guidelines of the Uniformed Services University of the Health Sciences Animal Ethics Committee.
Data are presented as the mean of triplicate determinants with SEM. Experiments carried out in triplicate were repeated at least 3 times. Statistically significant differences were determined between control and treatment groups using two-way ANOVA followed by Tukey post hoc test. A P value of less than 0.05 was considered statistically significant.
The authors thank Dr. Satoru Kyo for providing the EM-E6/E7-TERT cell line.
CONFLICTS OF INTEREST
Funding for this project is from The John P. Murtha Cancer Center at Water Reed National Military Medical Center Award (MDA905-02-2-0005, PI: Viqar Syed) and the USAMRMC (W81XWH-11-2-013, PI: Chad A. Hamilton).
The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the Uniformed Services University of the Health Sciences, the Department of the Air Force, the Department of the Army, the Department of the Navy, or the Department of Defense.
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