TP53 and Familial prostate cancer: However, in p53-expressing DU145 prostate cancer cells, they increase the expression of the proapoptotic protein Bax, through phosphorylation at Ser-15 (i.e., activation) of p53, the major regulator of the intrinsic apoptotic pathway; activation of this p53-Bax apoptotic signaling is triggered by p38 MAPK phosphorylation.