Several lines of evidence have implicated members of the developmentally important T-box gene family in cell cycle regulation and in
cancer. Importantly, the highly related T-box factors Tbx2 and Tbx3 can suppress senescence through repressing the
cyclin-dependent kinase inhibitors p19(ARF) and p21(WAF1/CIP1/SDII). Furthermore, Tbx2 is up-regulated in several
cancers, including
melanomas where it was shown to function as an anti-senescence factor, suggesting that this may be one of the mechanisms by which T-box
proteins contribute to the oncogenic process. However, very little is known about whether Tbx2 is regulated by p21-mediated stress-induced senescence signaling pathways. In this study, using the MCF-7
breast cancer cell line known to overexpress Tbx2, we show that in response to stress induced by ultraviolet irradiation the
Tbx2 protein is specifically phosphorylated by the
p38 mitogen-activated protein kinase. Using site-directed mutagenesis and in vitro
kinase assays, we have identified
serine residues 336, 623, and 675 in the
Tbx2 protein as the p38 target sites and show that these sites are phosphorylated in vivo. Importantly, we show by Western blotting, immunofluorescence, and reporter assays that this phosphorylation leads to increased
Tbx2 protein levels, predominant nuclear localization of the
protein, and an increase in the ability of Tbx2 to repress the p21(WAF1/CIP1/SDII) promoter. These results show for the first time that the ability of Tbx2 to repress the p21 gene is enhanced in response to a stress-induced senescence pathway, which leads to a better understanding of the regulation of the anti-senescence function of Tbx2.