Involvement of p38-βTrCP-Tristetraprolin-TNFα axis in radiation pneumonitis
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Pranathi Meda Krishnamurthy1,3,*, Shirish Shukla1,2,*, Paramita Ray1, Rohit Mehra2, Mukesh K. Nyati1, Theodore S. Lawrence1 and Dipankar Ray1
1Department of Radiation Oncology, University of Michigan, Ann Arbor, MI, USA
2Department of Pathology, University of Michigan, Ann Arbor, MI, USA
3Current address: RNA Therapeutics Institute, University of Massachusetts Medical School, Worcester, MA, USA
*These authors have contributions equal to this work
Dipankar Ray, email: email@example.com
Keywords: radiation pneumonitis, TNF-α, Tristetraprolin, p38 MAPK, β-TrCP1
Received: December 21, 2016 Accepted: April 14, 2017 Published: May 10, 2017
Early release of tumor necrosis factor-alpha (TNF-α) during radiotherapy of thoracic cancers plays an important role in radiation pneumonitis, whose inhibition may provide lung radioprotection. We previously reported radiation inactivates Tristetraprolin (TTP), a negative regulator of TNF-α synthesis, which correlated with increased TNF-α release. However, the molecular events involved in radiation-induced TTP inactivation remain unclear. To determine if eliminating Ttp in mice resulted in a phenotypic response to radiation, Ttp-null mice lungs were exposed to a single dose of 15 Gy, and TNF-α release and lung inflammation were analyzed at different time points post-irradiation. Ttp-/- mice with elevated (9.5±0.6 fold) basal TNF-α showed further increase (12.2±0.9 fold, p<0.02) in TNF-α release and acute lung inflammation within a week post-irradiation. Further studies using mouse lung macrophage (MH-S), human lung fibroblast (MRC-5), and exogenous human TTP overexpressing U2OS and HEK293 cells upon irradiation (a single dose of 4 Gy) promoted p38-mediated TTP phosphorylation at the serine 186 position, which primed it to be recognized by an ubiquitin ligase (E3), beta transducing repeat containing protein (β-TrCP), to promote polyubiquitination-mediated proteasomal degradation. Consequently, a serine 186 to alanine (SA) mutant of TTP was resistant to radiation-induced degradation. Similarly, either a p38 kinase inhibitor (SB203580), or siRNA-mediated β-TrCP knockdown, or overexpression of dominant negative Cullin1 mutants protected TTP from radiation-induced degradation. Consequently, SB203580 pretreatment blocked radiation-induced TNF-α release and radioprotected macrophages. Together, these data establish the involvement of the p38-βTrCP-TTP-TNFα signaling axis in radiation-induced lung inflammation and identified p38 inhibition as a possible lung radioprotection strategy.
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