Changes in protein tyrosine phosphatase type IVA member 1 and zinc finger protein 36 C3H type-like 1 expression demonstrate altered estrogen and progestin effect in medroxyprogesterone acetate-resistant and estrogen-independent breast cancer cell models

Estrogen stimulates proliferation in hormone-responsive breast cancer cells. Progestins inhibit the estrogen-mediated growth in these cells and are used in the treatment of mammary carcinomas. We applied cDNA microarray and real-time RT-PCR methods to reveal 17β-estradiol- and medroxyprogesterone acetate (MPA)-regulated genes in MCF-7 breast cancer cells. We identified six genes, two of which were novel MPA and/or 17β-estradiol-regulated genes: protein tyrosine phosphatase type IVA, member 1 (PTP4A1) and zinc finger protein 36, C3H type-like 1 (ZFP36L1). PTP4A1 expression was upregulated by 17β-estradiol and this was opposed by MPA treatment of the cells. ZFP36L1 proved to be a direct target of MPA. Since MPA has also been shown to bind to glucocorticoid- and androgen receptors, we studied the regulation of the genes with progesterone, synthetic progestin R5020, dexamethasone and dihydrotestosterone. We also assessed the expression and hormonal regulation of PTP4A1 and ZFP36L1 mRNA in MCF-7-derived MPA-resistant and estrogen-independent sublines. The regulation of PTP4A1 expression upon 17β-estradiol and combined 17β-estradiol and MPA treatment was completely reversed in the estrogen-independent and MPA-resistant sublines, respectively. This study suggests an important role for PTP4A1 in cell growth, and shows that MPA may potentiate the effect of 17β-estradiol in the MPA-resistant breast cancer cells.