Caveolin-1 is a critical regulator of TGF-β mediated hepatocyte apoptosis

Caveolae are small plasma membrane invaginations formed by caveolin-1. Besides the clathrin dependent internalization route, TGF-β ligand/receptor complexes internalize via caveolae. The lack of caveolin-1 leads to an increase of canonical Smad signaling via Smad2 and Smad3. However, we found that the knock down of caveolin-1 in hepatocytes abrogated the TGF-β mediated activation of the AKT pathway. Because TGF-β mediates hepatocyte dedifferentiation (EMT) as well as hepatocyte apoptosis in vitro, we analyzed the consequences of a loss of AKT activation upon TGF-β treatment. Interestingly, a Caspase 3 assay revealed massive TGF-β induced apoptosis in cells lacking caveolin-1. To elucidate whether the AKT pathway is responsible for this phenomenon, we used an AKT inhibitor and stimulated cells with TGF-β. Although no basal increase of apoptosis could be observed, TGF-β potently amplified Caspase 3 activity in inhibitor treated cells compared to controls. Hence, we applied a Caspase 3 assay with siControl and siCaveolin-1 treated hepatocytes additionally transduced with LacZ as control or caAKT adenoviruses. Indeed, the constitutive active form of AKT eliminated the boost in apoptosis in siCaveolin-1 hepatocytes to the level of siControl cells. This implies that hepatocytes show a basal apoptotic rate upon TGF-β challenge which is independent of the AKT survival pathway and mainly relying on Smad3. Nevertheless, AKT activation is mandatory to prevent exorbitant apoptosis in hepatocytes. Noteworthy, the physiological level of caveolin-1 in hepatocytes is sufficient to prevent apoptosis as the overexpression of caveolin-1 did neither reduce TGF-β mediated apoptosis nor influence AKT phosphorylation. We therefore speculate that the endogenous caveolin-1 levels are adequate to allow formation of caveolae which act as signalosomes in TGF-β signaling. To conclude, we show that caveolin-1 is critical for TGF-β mediated activation of AKT and hepatocyte apoptosis.

AKT - caveolae - liver