EPIDERMAL GROWTH FACTOR-DEPENDENT CELL-CYCLE PROGRESSION IS ALTERED IN MAMMARY EPITHELIAL-CELLS THAT OVEREXPRESS C-MYC

Amplification and overexpression of the c-myc gene are common in prima ry human breast cancers and have been correlated with highly prolifera tive tumors. Components of the epidermal growth factor (EGF) receptor signaling pathway are also often overexpressed and/or activated in hum an breast tumors, and transgenic mouse models have demonstrated that c -myc and transforming growth factor alpha (a member of the EGF family) strongly synergize to induce mammary tumors, These bitransgenic mamma ry tumors exhibit a higher proliferation rate than do tumors arising i n single transgenics, We, therefore, chose to investigate EGF-dependen t cell cycle progression in mouse and human mammary epithelial cells w ith constitutive c-myc expression. In both species, c-myc overexpressi on decreased the doubling time of mammary epithelial cells by similar to 6 h, compared to parental lines. The faster growth rate was not due to increased sensitivity to EGF but rather to a shortening of the G(1 ) phase of the cell cycle following EGF-induced proliferation. In cell s with exogenous c-myc expression, retinoblastoma (Rb) was constitutiv ely hyperphosphorylated, regardless of whether the cells were growth-a rrested by EGF withdrawal or were traversing the cell cycle following EGF stimulation. In contrast, the parental cells exhibited a typical R b phosphorylation shift during G(1) progression in response to EGF, Th e abnormal phosphorylation status of Rb in c-myc-overexpressing cells was associated with premature activation of cdk2 kinase activity, redu ced p27 expression, and early onset of cyclin E expression. These resu lts provide one explanation for the strong tumorigenic synergism betwe en deregulated c-myc expression and EGF receptor signal transduction i n the mammary tissue of transgenic mice. In addition, they suggest a p ossible tumorigenic mechanism for c-myc deregulation in human breast c ancer.