Research Papers:
Microglial SMAD4 regulated by microRNA-146a promotes migration of microglia which support tumor progression in a glioma environment
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Abstract
Aparna Karthikeyan1, Neelima Gupta1, Carol Tang4,5,6, Karthik Mallilankaraman2, Maskomani Silambarasan1, Meng Shi3, Lei Lu3, Beng Ti Ang2,5,7,8, Eng-Ang Ling1 and S. Thameem Dheen1
1Department of Anatomy, Yong Loo Lin School of Medicine, National University of Singapore, Singapore
2Department of Physiology, Yong Loo Lin School of Medicine, National University of Singapore, Singapore
3School of Biological Sciences, Nanyang Technological University, Singapore
4Department of Research, National Neuroscience Institute, Singapore
5Duke-NUS Medical School, Singapore
6Division of Cellular and Molecular Research, National Cancer Centre, Singapore
7Department of Neurosurgery, National Neuroscience Institute, Singapore
8Singapore Institute for Clinical Sciences, A*STAR, Singapore
Correspondence to:
S. Thameem Dheen, email: [email protected]
Keywords: microglia; glioma; TGFβ; SMAD4; microRNA-146a
Received: October 05, 2017 Accepted: March 19, 2018 Published: May 18, 2018
ABSTRACT
Glioma tumors constitute a significant portion of microglial cells, which are known to support tumor progression. The present study demonstrates that transforming growth factor-β (TGFβ) signaling pathway in microglia in a glioma environment is involved in tumor progression and pathogenesis. It has been shown that the TGFβ level is elevated in higher grades of gliomas and its signaling pathway regulates tumor progression through phosphorylation of SMAD2 and SMAD3, which form a complex with SMAD4 to regulate target gene transcription. In an in vitro cell line-based model, increased protein levels of pSMAD2/3, total SMAD2/3 and SMAD4 were observed in murine BV2 microglia cultured in glioma conditioned medium (GCM), indicative of the activated TGFβ signaling pathway in microglia associated with glioma environment. Immunofluorescence labeling further revealed the expression of SMAD4 in microglial and non-microglial cells of human glioblastomas tissue in vivo. Functional analysis through shRNA-mediated stable knockdown of SMAD4 in microglia revealed the downregulation of the expression of matrix metalloproteinase 9 (MMP9), which has been shown to be involved in tumor progression and cell migration. Further, knockdown of SMAD4 in microglia decreased the migration of microglial cells towards GCM, indicating that SMAD4 promotes microglial migration in glioma environment. In addition, SMAD4 has been shown to be post-transcriptionally regulated by microRNA-146a, which was downregulated in microglia treated with GCM. Overexpression of miR-146a resulted in decreased expression of SMAD4 together with tumor supportive gene MMP9 in microglia, and subsequently suppressed microglial migration towards GCM, possibly through regulation of SMAD4. On the other hand, the cell viability assay revealed decreased viability of glioma cells when they were treated with conditioned medium derived from SMAD4 knockdown microglia or miR-146a overexpressed microglia as compared to glioma cells treated with the medium from control microglial cells. Taken together, the present study suggests that microglial SMAD4 which is epigenetically regulated by miR-146a promotes microglial migration in gliomas and glioma cell viability.
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