Progestin-induced caveolin-1 expression mediates breast cancer cell proliferation

Progestin regulation of gene expression was assessed in the progestin- dependent murine tumor line C4HD which requires MPA, a synthetic progestin, for in vivo growth and expresses high levels of progesterone receptor ( PR). By using suppressive subtractive hybridization, caveolin-1 was identified as a gene whose expression was increased with in vivo MPA treatment. By Northern and Western blot analysis, we further confirmed that caveolin-1 mRNA and protein expression increased in MPA- treated tumors as compared with untreated tumors. When primary cultures of C4H D cells were treated in vitro with MPA, caveolin-1 levels also increased, effect that was abolished by pre-treatment with progestin antagonist RU486. In addition, MPA promoted strong caveolin-1 promoter transcriptional activation both in mouse and human breast cancer cells. We also showed that MPA regulation of caveolin-1 expression involved in activation of two signaling pathways: MAPK and PI-3K. Short- term MPA treatment of C4HD cells led to tyrosine phosphorylation of caveolin-1 protein, where Src was the kinase involved. Additionally, we showed that MPA- induced association of caveolin- 1 and PR, which was detected by communoprecipitation and by confocal microscopy. Finally, we proved that MPA- induced proliferation of C4HD cells was inhibited by suppression ofcaveol in- 1 expression with antisense oligodeoxynucleotides to caveolin1 mRNA. Furthermore, we observed that inhibition of caveolin- 1 expression abrogated PR capacity to induced luciferase activity from a progesterone response elementdriven reporter plasmid. Comprehensively, our results demonstrated for the. rst time that caveolin- 1 expression is upregulated by progestin in breast cancer. We also demonstrated that caveolin- 1 is a downstream effector of MPA that is partially responsible for the stimulation of growth ofbreast cancer cells.