Regulation of TGF-β effects in hepatocytes by endocytosis

TGF-β is a major cytokine involved in several aspects of liver physiology (fibrosis, regeneration, apoptosis, cancer). Hepatocytes are one of the main targets of this cytokine, with apoptosis and growth inhibition seeming to be the main outcomes. Recently an important role of endocytosis in the regulation of several signaling pathways has been demonstrated, like the EGF-R/MAPK, where different routes of signal transduction triggered at miscellaneous stages of receptor endocytosis. We addressed the importance of endocytosis in the regulation of TGF-β signaling in cell lines (Hepatoma 1–6, Hep3B) and primary mouse hepatocytes. Inhibition of endocytosis via potassium depletion and use of DN-Dynamin showed a strong reduction in the rate of phosphorylation of Smad2/3. This effect was also shown using TGF-β reporter genes (eg CAGA-Luc) and by real time PCR analysis of TGF-β responsive genes. The relevance of these regulatory mechanisms in TGF-β induced apoptosis was further investigated. The effects of endocytosis inhibition on other signaling pathways known to be induced by TGF-β (eg p38, JNK) were also analyzed. The participation of endocytic regulation of TGF-β signaling was evaluated in a mathematical model proposed for this pathway. Elucidation of new regulatory steps of TGF-β signaling in hepatocytes by endocytosis will allow a further understanding of this important pathway and the development of more specific therapeutical approaches.

Key words

Endocytosis - TGF-beta - hepatocytes - signal transduction