Signaling Pathways in Renal Fibrogenesis

Northwestern University At Chicago, Evanston IL

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Abstract

The signaling mechanisms underlying kidney fibrosis continue to be of significant concern and interest. This proposal presents a strategy for extending our previous studies examining how TGF-[unreadable] activates renal cell accumulation of type I collagen, an in vitro model of renal fibrogenesis. In the previous grant period, we demonstrated that numerous signaling pathways interact with TGF-[unreadable]/Smad signaling, focusing particularly on the role of the extracellular signal-regulated kinase (ERK) MAP kinase. We determined that the Smad3 and ERK pathways interact to promote TGF-[unreadable]-stimulated collagen expression in renal mesangial and tubular epithelial cells. ERK serves a critical role in supporting Smad3-mediated collagen gene transcription, but does not appear to activate the COL1A2 promoter in the absence of Smad activation; it is thus necessary but not sufficient for collagen production. ERK phosphorylates serines and threonines in the linker region (LR) domain of Smad3. However, there is considerable controversy regarding whether LR phosphorylation enhances or inhibits Smad activity. For this renewal, we propose the hypothesis that integrin-dependent ERK activity promotes TGF-[unreadable]-stimulated renal cell collagen expression through phosphorylation of linker-region serine and threonine residues of Smad3. Our preliminary data suggest that the ERK activity and collagen expression are supported by integrin engagement and subsequent, adhesion-dependent phosphorylation of tyrosine residue 397 of focal adhesion kinase (FAK). This same series of steps leads to phosphorylation of specific phosphoacceptor sites on Smad3. Here. we propose three specific aims. First, we will define the pathway from adhesion to ERK activity by identifying specific integrins that mediate ERK activation, LR phosphorylation and collagen expression; we will determine how specific cell-matrix interactions and the action of PI3-kinase, a molecule that plays a major role in the signal transduction of focal adhesion rearrangement, affect these integrin signals. Second, we will directly address the role of specific Smad3 phosphorylations in regulating a2(I) collagen promoter activity. Third, we will characterize and test a new model of kidney fibrosis for the role of TGF-[unreadable], FAK and ERK in that fibrosis. By elucidating the signaling events underlying kidney fibrosis we hope to better understand fibrogenic mechanisms and how to interrupt them.