Inhibition of autoregulated TGFβ signaling simultaneously enhances proliferation and differentiation of kidney epithelium and promotes repair following renal ischemia

Abstract

We studied autocrine transforming growth factor (TGF)β signaling in kidney epithelium. Cultured proximal tubule cells showed regulated signaling that was high during log-phase growth, low during contact-inhibited differentiation, and rapidly increased during regeneration of wounded epithelium. Autoreg- ulation of signaling correlated with TGFβ receptor and Smad7 levels, but not with active TGFβ, which was barely measurable in the growth medium. Confluent differentiated cells with low receptor and high Smad7 levels exhibited blunted responses to saturating concentrations of exogenously provided active TGFβ, suggesting that TGFβ signaling ho- meostasis was achieved by cell density-dependent modulation of signaling intermediates. Antagonism of Alk5 kinase, the TGFβ type I receptor, dramatically accelerated the induction of differentiation in sparse, proliferating cultures and permitted better retention of differentiated features in regenerating cells ofwounded, confluentcultures. Alk5 antagonism accelerated the differentiation of cells in proximal tubule primary cultures while simultaneously increasing their proliferation. Consequently, Alk5-inhibited primary cultures formed confluent, differentiated mono- layers faster than untreated cultures. Furthermore, treatment with an Alk5 antagonist promoted kidney repair reflected by increased tubule differentiation and decreased tubulo-interstitial pathology during the recovery phase following ischemic injury in vivo. Our results show that autocrine TGF/ signaling in proliferating proximal tubule cells exceeds the levels that are necessary for physiological regeneration. To that end, TGF/ signaling is redundant and maladaptive during tubule repair by epithelial regeneration. Copyright © American Society for Investigative Pathology.