Role of TGF-β and Smad7 in apoptosis of HCC cells

TGF-β plays a pivotal role in cell signaling in liver. In healthy tissue and early stages of liver diesease, TGF-β signaling controls apoptosis, cell proliferation and differentiation, thereby acting as a tumor suppressor. During tumor progression in hepatocellular carcinogenesis TGF-β acquires tumor promoting functions. For urgently needed treatment development it is essential to elucidate the point of decision, when TGF-β turns from a tumor suppressor into a tumor promoter.

In this respect, we focused on apoptosis of HCC cells with special attention on Smad7, an endogenous inhibitor of TGF-β signaling. We analyzed 9 different HCC cell lines, overexpressing Smad7, or not. Western Blot Analysis of apoptotic- and anti-apoptotic proteins, like e.g. Bcl-2 & Bcl-xL, PARP, and p-p38 was performed. In addition, we investigated cell death dependent LDH release and Caspase 3 Activity. We found that overexpression of Smad7 was able to inhibit PARP cleavage and upregulated Bcl-2 levels in some HCC cell lines. However, whether this finally leads to protection of these cell against apoptosis has still to be elucidated. In other cell lines, we observed anti-apoptotic effects of Smad7 overexpression using LDH and Caspase 3 Activity Assays. This effect might suggest that overexpression of Smad7 provides neoplastic hepatocytes with a way to escape the TGF-β-dependent cell death, thereby facilitating the progression of the cancer.

In conclusion, the differences in the outcome of TGF-β signaling in dependency of Smad7 levels might reflect the heterogenicity of HCC as well as the time dependent impact of TGF-beta in hepatocellular carcinogenesis. However, these results also show that dissecting Smad7/TGF-b signaling in HCC possibly leads to the detection of the switch of TGF-beta function during liver cancer progression.