SM22α-induced activation of p16INK4a/retinoblastoma pathway promotes cellular senescence caused by a subclinical dose of γ-radiation and doxorubicin in HepG2 cells

Smooth muscle protein 22-alpha (SM22α) is known as a transformation- and shape change-sensitive actin cross-linking protein found in smooth muscle tissue and fibroblasts; however, its functional role remains uncertain. We reported previously that SM22α overexpression confers resistance against anti-cancer drugs or radiation via induction of metallothionein (MT) isozymes in HepG2 cells. In this study, we demonstrate that SM22α overexpression leads cells to a growth arrest state and promotes cellular senescence caused by treatment with a subclinical dose of γ-radiation (0.05 and 0.1 Gy) or doxorubicin (0.01 and 0.05 μg/ml), compared to control cells. Senescence growth arrest is known to be controlled by p53 phosphorylation/p21WAF1/Cip1 induction or p16INK4a/retinoblastoma protein (pRB) activation. SM22α overexpression in HepG2 cells elevated p16INK4a followed by pRB activation, but did not activate the p53/p21WAF1/Cip1 pathway. Moreover, MT-1G, which is induced by SM22α overexpression, was involved in the activation of the p16INK4a/pRB pathway, which led to a growth arrest state and promoted cellular senescence caused by damaging agents. Our findings provide the first demonstration that SM22α modulates cellular senescence caused by damaging agents via regulation of the p16INK4a/pRB pathway in HepG2 cells and that these effects of SM22α are partially mediated by MT-1G.

Research highlights
► SM22α overexpression in HepG2 cells leads cells to a growth arrest state, and the treatment of a subclinical dose of γ-radiation or doxorubicin promotes cellular senescence.
► SM22α overexpression elevates p16INK4a followed by pRB activation, but there are no effects on p53/p21WAF1/Cip1 pathway.
► SM22α-induced MT-1G activates p16INK4a/pRB pathway, which promotes cellular senescence by damaging agents.