TGF-β Signaling Is Often Attenuated during Hepatotumorigenesis, but Is Retained for the Malignancy of Hepatocellular Carcinoma Cells

The role of transforming growth factor-beta (TGF-beta) signaling in hepatocarcinogenesis remains controversial. We aimed to reveal TGF-beta signaling status in human and murine tissues of hepatocellular carcinoma (HCC) and the mechanisms that mediate TGF-beta's role in regulating HCC malignancy. Here, TGF-beta pathway component expression and activation in human and murine HCC tissues were measured with quantitative RT-PCR and Western blotting assays. The role of TGF-beta receptor and Smad signaling in the growth and survival of several HCC cell lines was determined with several in vitro and in vivo approaches. We found that TGF-beta receptor II (T beta RII) expression was downregulated in two different HCC patient cohorts. Consistently, Smad3 phosphorylation was also downregulated in HCC tissues in comparison to that in adjacent normal tissues. Interestingly, many HCC cell lines were sensitive to TGF-beta and growth-inhibited by exogenous TGF-beta. However, stable knockdown of T beta RII inhibited cell growth on plastic and in soft agar, and induced apoptosis resulting in suppressed subcutaneous tumor growth and metastatic potential in vivo. Furthermore, knockdown of Smad4 also led to a significant inhibition of growth on plastic and in soft agar with concomitant increase of apoptosis, PTEN expression, and reduced nuclear accumulation of linker region-phosphorylated Smad3. Taken together, TGF-beta signaling pathway plays a dichotomous role in hepatocellular carcinogenesis. It appears to suppress HCC development, but is retained for HCC cell survival and malignancy. Furthermore, Smad4 can mediate both growth inhibitory activity induced by exogenous TGF-beta and the survival activity induced by autocrine TGF-beta revealing a delicate selection of the two opposing activities of TGF-beta during HCC evolution.