CSF-1 activates MAPK-dependent and p53-independent pathways to induce growth arrest of hormone-dependent human breast cancer cells

The CSF-1 receptor (CSF-1R) is expressed in >50% of human breast cancers. To investigate the consequence of CSF-1R expression, hormone-dependent human breast cancer cell lines, MCF-7 and T-47D, were transfected with CSF-1R. Unexpectedly, CSF-1 substantially inhibited estradiol (E2) and insulin-dependent proliferation of MCF-7 transfectants (MCF-7fms) and prevented cyclin E/cdk2 and cyclin A/cdk2 activation, consistent with a G1 arrest. In contrast, CSF-1 increased DNA synthesis in T-47D transfectants (T-47Dfms) alone and with E2 or insulin. In response to CSF-1, there was a marked and sustained upregulation of the cyclin-dependent kinase inhibitor, p21Waf1/Cip1, in MCF-7fms but not T-47Dfms. CSF-1 also markedly upregulated cyclin D1 in MCF-7fms. The coordinate increase in cyclin D1 and p21 had the effect of decreasing the specific but not absolute activity of cyclin D1/cdk4. p53 was not involved since CSF-1 induction of p21 was unaffected by dominant-negative p53 expression. ERK activation by CSF-1 was robust and sustained in MCF-7fms and to a much lesser extent in T-47Dfms. Using pharmacological and transient transfection approaches, we showed that ERK activation was necessary and sufficient for p21 induction in MCF-7fms. Moreover, activated MEK inhibited E2-stimulated cdk2 activity. Our findings indicate that the consequence of CSF-1R-mediated signals in human breast cancer cells is dependent on the genetic background of the particular tumor.