TOPOGRAPHICAL ASSOCIATION BETWEEN THE CYCLIN-DEPENDENT KINASES INHIBITOR P21, P53 ACCUMULATION, AND CELLULAR PROLIFERATION IN HUMAN ATHEROSCLEROTIC TISSUE

The cell cycle is controlled by cyclin-dependent protein kinases (CDKs ). The activity of these enzymes is directed by inhibitors of CDKs. Th e 21-kD protein product (P21) of the WAF1/CIP1 gene, which can be tran sactivated by the protein product of the tumor suppressor gene p53, ac ts as an inhibitor of cyclin-dependent kinases. To assess whether both P21 and p53 may play a role in the control of cellular proliferation in atherosclerotic lesions, the topographical association between p53, P21, and the proliferation marker MIB1/Ki-67, was analyzed by immunoh istochemistry in human carotid atheromatous plaques of 26 patients. p5 3 immunoreactivity (IR) was present in 26 of 26 cases in the nuclei of virtually all cell types (macrophages [MPs], smooth muscle cells [SMC s], endothelial cells [ECs]) in areas with chronic inflammation in 71. 08+/-8.28% of the nuclei. p53 staining in the control tissue from huma n coronary arteries was present in 0.3+/-0.45% of the cells (P<.002). P21-IR was present in 24 of 26 specimens in 64.38+/-10.13% of the cell s (controls: 3.8+/-1.85%, P<.002) and localized to nuclei of MPs (CD68 positive) and SMCs (alpha-actin positive), as well as ECs of microves sels present in 21 specimens (21 of 21) and luminal ECs present in 18 specimens (16 of 18). As shown by double labeling, P21-IR colocalized with p53-IR in most MPs (24 of 24), intimal SMCs (22 of 24), ECs of mi crovessels (19 of 21), and luminal ECs (10 of 16). Interestingly, few p53-positive cells did not show simultaneous P21-IR, and, conversely, not all P21-positive cells demonstrated p53-IR. MIB1/Ki-67-positive ce lls were identified in 21 of 26 tissue specimens in 3.53+/-1.79% of th e nuclei (controls: 0%, P<.002) and localized principally to MPs borde ring the atheromatous lipid core (21 of 26) and to a few scattered SMC s (16 of 26), ECs of microvessels (13 of 21), and luminal ECs (2 of 18 ). Most importantly, none of the cells coexpressing P21 and p53 were p ositive for MIB1/Ki-67-IR, indicating the absence of proliferating act ivity. In summary, this study demonstrates that P21-IR is present in t he atherosclerotic plaque and colocalizes with p53 in most MPs, SMCs, and ECs. The lack of proliferation markers in cells coexpressing p53 a nd P21 suggests that transcriptional activation of the WAF1/CIP1 gene by p53 may be involved in the control of cellular proliferation in adv anced human atherosclerotic plaques.