THE ROLE OF P53 IN COORDINATED REGULATION OF CYCLIN D1 AND P21 GENE-EXPRESSION BY THE ADENOVIRUS E1A AND E1B ONCOGENES

Expression of the cyclin D1 gene is induced when quiescent fibroblasts are stimulated to reenter the cell cycle by addition of growth factor s. Moderate ectopic expression of cyclin D1 in early G1 facilitates pr ogression through G1, When transiently overexpressed at the G1/S bound ary, cyclin D1 prevents S phase entry, suggesting a dual role for this protein in cellular growth control. It was shown that the retinoblast oma protein (pRB) can activate cyclin D1 gene expression; furthermore, there is evidence that expression of the cyclin D1 gene is down-regul ated by the SV40 large T and adenovirus E1A genes, both of which were shown to target pRB, We now report that in diploid human fibroblasts f unctional inactivation of pRB by adenovirus E1A is not sufficient for efficient repression Of cyclin D1 gene expression, since the E1B gene product, in addition to E1A, is required for repression of the cyclin D1 gene. Since E1B was shown to target p53, we investigated the role o f p53 for expression of the cyclin D1 gene. In a cell line with temper ature-sensitive p53, cyclin D1 is moderately expressed at the restrict ive temperature. Induction of p53 function by temperature shift leads to an increase of cyclin D1 mRNA and protein, parallel to the activati on of p21(WAF-1/CIP1) gene expression in this system, When the capabil ity of adenovirus gene products to affect expression of either gene wa s analysed, we found that infection of Ad5 drastically reduced cyclin D1 and p21(WAF-1/CIP1) gene expression in cells where p53 function is limiting. Under these conditions E1A and E1B cooperate to reduce the c yclin D1 level, while p21(WAF-1)/(CIP1) expression was found insensiti ve to E1A expression. In cells containing elevated p53 function, modul ation of gene expression by E1B was severely compromised; under these conditions, expression of E1A reduced expression of cyclin D1 without affecting p21(WAF-1/CIP1). The data suggest that E1A and E1B cooperate to inhibit expression of cyclin D1 and identify the cyclin D1 gene as a new downstream target for p53.