Apoptosis of Rat Vascular Smooth Muscle Cells Is Regulated by p53-Dependent and -Independent Pathways

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Abstract
Apoptosis of vascular smooth muscle cells has recently been described in culture and also in remodeling of the artery after birth. However, the genes that regulate apoptosis in smooth muscle cells are mostly unknown. We studied the regulation of apoptosis in rat smooth muscle cells stably infected with retrovirus constructs containing c- myc , adenovirus E1A, bcl - 2 , and a temperature-sensitive mutant of the tumor suppressor gene p53. Apoptosis was verified by electron microscopy and quantified by time-lapse videomicroscopy. Death was induced by c- myc and E1A when cells were deprived of serum survival factors. bcl - 2 suppressed apoptosis of cells infected with c- myc and E1A and also normal smooth muscle cells. Overexpression of wild-type p53 induced apoptosis of cells infected with E1A and c- myc but not normal cells. In contrast, expression of mutant p53, which blocks wild-type p53 function, suppressed apoptosis of cells infected with E1A or c- myc but not normal cells. Both adenovirus E1A and c- myc increased the expression of endogenous p53 protein but not p53 mRNA. Although bcl - 2 suppressed apoptosis induced by E1A and c- myc , upregulation of p53 protein induced by these agents was unaffected. We conclude that apoptosis of vascular smooth muscle cells is regulated by p53-dependent and -independent pathways. Death induced by c- myc and E1A is mediated by, and dependent on, p53. However, the suppression of apoptosis by bcl - 2 is not mediated by changes in p53 expression, and the low level of apoptosis seen in normal VSMCs upon removal of survival factors is independent of p53.