Transforming Growth Factor-ॆ1 Potentiates Renal Tubular Epithelial Cell Death by a Mechanism Independent of Smad Signaling

Abstract

Tubular atrophy resulting from epithelial cell loss is one of the characteristic features in the development of chronic renal interstitial fibrosis. Although the trigger(s) and mechanism for tubular cell loss remain undefined, the hyperactive transforming growth factor (TGF)-␤1 signaling has long been suspected to play an active role. Here we demonstrate that although TGF-␤1 did not induce cell death per se, it dramatically potentiated renal tubular cell apoptosis initiated by other death cues in vitro. Pre-incubation of human kidney epithelial cells (HKC) with TGF-␤1 markedly promoted staurosporine-induced cell death in a time-and dose-dependent manner. TGF-␤1 dramatically accelerated the cleavage and activation of pro-caspase-9, but not procaspase-8, in HKC cells. This event was followed by an accelerated activation of pro-caspase-3. To elucidate the mechanism underlying TGF-␤1 promotion of tubular cell death, we investigated the signaling pathways activated by TGF-␤1. Both Smad-2 and p38 mitogenactivated protein (MAP) kinase were rapidly activated by TGF-␤1, as demonstrated by the early induction of phosphorylated Smad-2 and p38 MAP kinase, respectively. We found that overexpression of inhibitory Smad-7 completely abolished Smad-2 phosphorylation and activation induced by TGF-␤1 but did not inhibit TGF-␤1-induced apoptosis. However, suppression of p38 MAP kinase with chemical inhibitor SC68376 not only abolished p38 MAP kinase phosphorylation but also obliterated apoptosis induced by TGF-␤1. These results suggest that hyperactive TGF-␤1 signaling potentiates renal tubular epithelial cell apoptosis by a Smadindependent, p38 MAP kinase-dependent mechanism.

A Novel Mechanism by which Hepatocyte Growth Factor Blocks Tubular Epithelial to Mesenchymal Transition

“…As shown in Figure 2E, blockade of Akt activation by wortmannin and p38 MAPK activation by SC68376 did not affect TGF-␤1-induced suppression of epithelial E-cadherin and de novo expression of ␣-SMA, two hallmarks of tubular EMT. Of note, these chemical inhibitors at the concentrations used were able to specifically block Akt and p38 MAPK activation initiated by TGF-␤1 in HKC cells, as reported previously (28,30). We next found that disruption of Smad signaling by overexpressing inhibitory Smad-7 completely abolished TGF-␤1-induced tubular EMT.…”

“…After transfection with equal amounts of HA-tagged SnoN expression plasmid (29) (provided by Dr. R. Weinberg, Massachusetts Institute of Technology, Cambridge, Massachusetts) or empty vector pcDNA3 (Invitrogen), HKC cells were incubated in the absence or presence of 2 ng/ml TGF-␤1 for 2 d. Whole-cell lysates were prepared and subjected to Western blot analyses. The stable cell lines overexpressing Smad-7 (HKC pSmad7 ) and mock-transfection control (HKC pcDNA3 ) were established as described previously (30).…”

Apoptosis of human endothelial cells is accompanied by proteolytic processing of latent TGF-β binding proteins and activation of TGF-β

“…[36][37][38][39] Our results also revealed that apoptosis favouring conditions lead to the activation of endogenous TGF-b. One could therefore hypothesize that the activation of endogenous TGFb is a component of the apoptotic response, which might potentiate the apoptotic cell death.…”

VEGF, a prosurvival factor, acts in concert with TGF-β1 to induce endothelial cell apoptosis

“…Induction of endothelial cell apoptosis by TGF-␤1 involves p38 MAPK activation (33), and in a variety of cell types TGF-␤1 activates p38 MAPK through a Smadindependent mechanism (34,35). Therefore, we hypothesized that TGF-␤1 activates MAPKs in endothelial cells through the action of endogenous VEGF.…”