COEXPRESSION OF P53 AND MDM2 IN HUMAN ATHEROSCLEROSIS - IMPLICATIONS FOR THE REGULATION OF CELLULARITY OF ATHEROSCLEROTIC LESIONS

Atherosclerosis is a fibroproliferative disease of the arterial intina . It was recently found that wild-type p53 (wt p53) accumulates in hum an atherosclerotic tissue. Wt p53 is a cell cycle regulator involved i n DNA repair, DNA synthesis, cell differentiation, and apoptosis and m ight therefore make an important contribution to the cellularity of at herosclerotic plaques. The product of the MDM2 gene is a nuclear prote in which forms a complex,vith p53, thereby inhibiting the negative reg ulatory effects of wt p53 on cell cycle progression. In order to addre ss a potential role of the interaction of p53 with MDM2 for the regula tion of cellularity in atherosclerotic tissue, 22 carotid atheromatous plaques from patients undergoing endarterectomy were studied to deter mine the presence of p53 immunoreactivity (IR), MDM2 IR, cell prolifer ation as evidenced by MIB1/Ki-67 IR and DNA fragmentation by in situ t erminal transferase-mediated dUTP 3' end labelling (TUNEL), as a marke r for apoptosis. p53 IR localized to areas,vith evidence of chronic in flammation (22/22) and was observed in virtually all cell types in 68. 79 +/- 7.51 per cent of the nuclei. p53 staining in the control tissue from human internal mammary arteries was present in 0.2 +/- 0.29 per cent of the cells (P less than or equal to 0.002). MDM2 IR was present in all cases (22/22) in macrophages and smooth muscle cells (SMCs) in 60.53 +/- 8.32 per cent of the nuclei (controls: 0.8 +/- 0.65 per cen t, P less than or equal to 0.002) and co-localized with p53 IR as show n by examination of adjacent sections and by double immunofluorescence labelling. Importantly, co-immunoprecipitation and western blot analy sis revealed that p53 and MDM2 were physically associated, indicating that MDM2-p53 complex formation takes place in vivo in human atheroscl erotic tissue. Positive TUNEL staining and MIB1/Ki-67 IR present in 3. 01 +/- 1.27 per cent of the nuclei (controls: 0 per cent, P less than or equal to 0.002) localized to the same plaque compartments as p53 LR and MDM2 IR. Thus, the fate of cells with p53 accumulation may depend on the interaction and the stoichiometry of the p53 and MDM2 proteins . Cells were indeed found with strong p53 accumulation and nuclear mor phology typical for apoptosis and there were a few MIB1/Ki-67-positive cells with co-expression of MDM2, indicating a possible role for MDM2 in reversing the negative regulatory effects of p53 for cell cycle pr ogression. The nuclear co-localization of p53 IR with MDM2 IR and the co-immunoprecipitation assay indicate the presence of p53-MDM2 complex formation in vivo in human atherosclerotic tissue. The destiny of ind ividual p53 and MDM2co-expressing cells either to undergo p53-dependen t apoptosis or to re-enter the cycle of cell proliferation may depend on the relative ratios of the two proteins. p53 and MDM2 may therefore play an important role in regulating cellularity and inflammatory act ivity in human atherosclerotic plaques. (C) 1998 John Wiley & Sons, Lt d.