Post-transcriptional regulation of c-fms proto-oncogene expression by dexamethasone and of CSF-1 in human breast carcinomas in vitro

The c-fms proto-oncogene encodes the receptor for a hematopoietic growth factor, CSF-1. Recently, the importance of c-fms and its ligand CSF-1 in malignancies of non-hematopoietic origin, such as breast, ovarian, endometrial, pulmonary, and trophoblastic cancers has been recognized. We have previously shown that glucocorticoids induce a large increase in c-fms mRNA and protein levels in breast carcinoma cell lines. In this report, we investigate the mechanism underlying such c-fms overexpression by dexamethasone. We show that dexamethasone treatment of two breast carcinoma cell lines (BT20-c-fms expressor, and SKBR3-co-expressor of both c-fms and CSF-1) does not increase the rate of c-fms gene transcription, suggesting a post-transcriptional mechanism of regulation of c-fms expression by dexamethasone. The effect of protein synthesis inhibition was studied to help determine whether there was a role for intermediary regulatory proteins in the regulation of c-fms expression. We find that several protein synthesis inhibitors interfere with dexamethasone induction of c-fms transcripts, suggesting the existence of regulatory proteins. These regulatory proteins do not appear to be constitutively expressed, as we show no effect of protein synthesis inhibition on c-fms transcript expression in resting BT20 cells. These findings suggest that the putative regulatory proteins are induced by dexamethasone. Furthermore, the addition of a protein synthesis inhibitor, pactamycin, to dexamethasone-treated BT20 cells results in a decrease in c-fms mRNA stability. There is no decrease of c-fms transcripts over the six hours of actinomycin-D chase in the dexamethasone-treated cells. However, when pactamycin is added to dexamethasone, the calculated c-fms mRNA half-life falls to seven hours, which represents a decrease of c-fms mRNA half-life to that measured in the resting cell, and in monocytes. Taken together, these results suggest the involvement of a dexamethasone-inducible c-fms mRNA stabilizing protein in the regulation of c-fms gene expression in breast carcinoma cell lines. Interestingly, dexamethasone has no effect on CSF-1 expression in SKBR3 cells. Protein synthesis inhibition however results in a reciprocal effect, with induction and stabilization of CSF-1 transcripts, which contrasts with the destabilization of c-fms mRNA. This reciprocal effect led us to postulate that CSF-1 may also contribute to the post-transcriptional regulation of c-fms mRNA expression in breast carcinoma cells. Therefore, the effect of addition of exogenous CSF-1 on c-fms mRNA expression in BT20 cells was studied. We demonstrate that exogenous CSF-1 is able to down-regulate c-fms mRNA expression by 3-fold in BT20 cells. This suggests that CSF-1, in addition to glucocorticoids, may play a role in the post-transcriptional regulation of c-fms expression in breast carcinoma cells. (Steroids 59:514-522, 1994).