Transforming growth factor-beta induces senescence in hepatocellular carcinoma cells and inhibits tumor growth
Senescence induction could be used as an effective treatment for hepatocellular carcinoma (HCC). However, major senescence inducers (p53 and p16Ink4a) are frequently inactivated in these cancers.We tested whether transforming growth factor-β (TGF-β) could serve as a potential senescence inducer in HCC. First, we screened for HCC cell lines with intact TGF-β signaling that leads to small mothers against decapentaplegic (Smad)-targeted gene activation. Five cell lines met this condition, and all of them displayed a strong senescence response to TGF-β1 (1-5 ng/mL) treatment. Upon treatment, c-myc was down-regulated, p21Cip1 and p15Ink4b were up-regulated, and cells were arrested at G1. The expression of p16Ink4a was not induced, and the senescence response was independent of p53 status. A short exposure of less than 1 minute was sufficient for a robust senescence response. Forced expression of p21 Cip1 and p15Ink4b recapitulated TGF-β1 effects. Senescence response was associated with reduced nicotinamide adenine dinucleotide phosphate oxidase 4 (Nox4) induction and intracellular reactive oxygen species (ROS) accumulation. The treatment of cells with the ROS scavenger N-acetyl-L-cysteine, or silencing of the NOX4 gene, rescued p21Cip1 and p15Ink4b accumulation as well as the growth arrest in response to TGF-β. Human HCC tumors raised in immunodeficient mice also displayed TGF-β1-induced senescence. More importantly, peritumoral injection of TGF-β1 (2 ng) at 4-day intervals reduced tumor growth by more than 75%. In contrast, the deletion of TGF-β receptor 2 abolished in vitro senescence response and greatly accelerated in vivo tumor growth. Conclusion: TGF-β induces p53-independent and p16Ink4a-independent, but Nox4-dependent, p21Cip1-dependent, p15Ink4b-dependent, and ROS-dependent senescence arrest in well-differentiated HCC cells. Moreover, TGF-β-induced senescence in vivo is associated with a strong antitumor response against HCC.