To study the role of IGF-I receptor signaling on cell cycle events we utilized MCF-7 breast cancer cells. IGF-I at physiological concentrations increased the level of p21 CIP/WAF mRNA after 4 h as well as protein after 8 h by 10-and 6-fold, respectively, in MCF-7 cells. This IGF-1 effect was reduced by 50% in MCF-7-derived cells (SX13), which exhibit a 50% reduction in IGF-1R expression, demonstrating that IGF-1 receptor activation was involved in this process. Preincubation with the ERK1/2

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... bitor U0126 significantly reduced the IGF-1 effect on the amount of p21 CIP/WAF protein in MCF-7 cells. These results were confirmed by the expression of a dominant negative construct for MEK-1 suggesting that the increase of the abundance of p21 CIP/WAF in response to IGF-1 occurs via the ERK1/2 mitogen-activated protein kinase pathway. Using an antisense strategy, we demonstrated that abolition of p21 CIP/WAF expression decreased by 2-fold the IGF-1 effect on cell proliferation in MCF-7. This latter result is explained by a delay in G 1 to S cell cycle progression due partly to a reduction in the activation of some components of cell cycle including the induction of cyclin D1 expression in response to IGF-1. MCF-7 cells transiently overexpressing p21 showed increased basal and IGF-I-induced thymidine incorporation. Taken together, these results define p21 CIP/WAF as a positive regulator in the cell proliferation induced by IGF-1 in MCF-7 cells.