C71-19 The Multiple Faces Of S6k: More Than A Regulator Of Translation

The multiple faces of S6K: More than a regulator of translation Okeke, Peace; Dalal, Shloak, Qin, Wenyi, Owens, Shuzi; Jeffers, Ann; Javier Lopez, Gaddam, Shiva; Idell, Steven; Tucker, Torry A. Rationale Chronic lung injury can lead to tissue scarring and fibrosis due to the appearance and persistence of profibrotic myofibroblasts. This type of injury is often attributed to elevated and sustained activation of TGF-β. We and others have shown that TGFβ and other profibrotic mediators activate the PI3K/Akt/mTOR signaling pathway facilitating myofibroblast differentiation. Therapeutic interventions aimed at the mTORC1 signaling complex have demonstrated efficacy in addressing certain lung injuries, including lymphangioleiomyomatosis. Our previous research indicated that the selective inhibition of mTORC1 with agents such as rapamycin or its analogues paradoxically activates noncanonical signaling pathways associated with TGF-β. However, the mechanism of this effect is not understood. Methods Immunofluorescence, phase contrast microscopy, western blotting, quantitative PCR, siRNA transfection, small molecule inhibition of cell signaling pathways, and other cell biology techniques were used in the performance of these analyses. Results To elucidate the underlying mechanism of this effect, we selectively targeted components of the mTORC1 signaling pathway to identify the primary effector involved. The S6 kinase family was determined to act as a negative regulator of TGF-β-mediated signaling. Distinct mechanistic inhibition of S6K1/2 kinase activity led to increased primary Akt phosphorylation (Thr308) and enhanced activation of the mTORC2 signaling pathway, as indicated by phosphorylation of Akt at Ser473 and NDRG1 at Thr346. In targeted studies, downregulation of S6K1 expression demonstrated no effect on Akt phosphorylation or TGF-β-induced MesoMT Conversely, downregulation of S6K2 markedly inhibited both canonical and noncanonical TGF-β signaling and blocked induction of MesoMT. Parallel studies demonstrated that these effects were not confined to HPMCs as fibroblasts exhibited comparable responses. Collectively, these findings indicate a novel negative feedback loop in TGF-β receptor signaling mediated through mTORC1 pathway activation. Conclusions These studies strongly support the further study and therapeutic targeting of S6 Kinase specifically isoform two in the This work was supported by NIH NHLBI grant HL115466 and HL142853 the Texas Lung Injury Institute.