Insulin/Insulin-like Growth Factor-I and Estrogen Cooperate to Stimulate Cyclin E-Cdk2 Activation and Cell Cycle Progression in MCF-7 Breast Cancer Cells through Differential Regulation of Cyclin E and p21WAF1/Cip1

Abstract

Estrogens and insulin/insulin-like growth factor-I (IGF-I) are potent mitogens for breast epithelial cells and, when co-administered, induce synergistic stimulation of cell proliferation. To investigate the molecular basis of this effect, a MCF-7 breast cancer cell model was established where serum deprivation and concurrent treatment with the pure estrogen antagonist, ICI 182780, inhibited growth factor and estrogen action and arrested cells in G(0)/G(1) phase. Subsequent stimulation with insulin or IGF-I alone failed to induce significant S-phase entry. However, these treatments increased cyclin D1, cyclin E, and p21 gene expression and induced the formation of active Cdk4 complexes but resulted in only minor increases in cyclin E-Cdk2 activity, likely due to recruitment of the cyclin-dependent kinase (CDK) inhibitor p21(WAF1/Cip1) into these complexes. Treatment with estradiol alone resulted in a greater increase in cyclin D1 gene expression but markedly decreased p21 expression, with a concurrent increase in Cdk4 and Cdk2 activity and subsequent synchronous entry of cells into S phase. Co-administration of insulin/IGF-I and estrogen induced synergistic stimulation of S-phase entry coincident with synergistic activation of high molecular mass (approximately 350 kDa) cyclin E-Cdk2 complexes lacking p21. To determine if the ability of estrogen to deplete p21 was central to these effects, cells stimulated with insulin and estradiol were infected with an adenovirus expressing p21. Induction of p21 to levels equivalent to those following treatment with insulin alone markedly inhibited the synergism between estradiol and insulin on S-phase entry. Thus the ability of estradiol to antagonize the insulin-induced increase in p21 gene expression, with consequent activation of cyclin E-Cdk2, is a central component of the synergistic stimulation of breast epithelial cell proliferation induced by simultaneous activation of the estrogen and insulin/IGF-I signaling pathways.