Role of MAP kinase in the enhanced cell proliferation of long term estrogen deprived human breast cancer cells

Women with estrogen receptor (ER) positive breast cancers frequently respon d initially to inhibition of estrogen action but later relapse with re-grow th of tumor. Previously, we have utilized MCF-7 human breast cancer cells d eprived of estradiol long term (LTED cells) as the model system to study th e regrowth phenomenon and have demonstrated that these cells exhibited incr eased cell proliferation rate and increased ER functionality during the ada ptive processes. In this report, we examined the hypothesis that the mitoge n-activated protein kinase (MAP kinase) signal was involved. We found that activated MAP kinase was elevated in LTED cells and that the MAP kinase spe cific inhibitor PD98059 was able to inhibit the elevated MAP kinase and [H- 3]thymidine uptake in LTED cells, suggesting mediation of DNA synthesis and proliferation by the MAP kinase pathway. Other MAP kinase upstream inhibit ors, including genestein, RG13022, and mevastatin were also able to inhibit the [H-3]thymidine uptake in LTED cells. Interestingly, the antiestrogen, ICI 182,780 was able to block the activated MAP kinase in LTED cells. Treat ment with PD98059 did not block elevated basal ERE-CAT activity while at th e same time inhibiting [H-3]thymidine uptake in LTED cells. Furthermore, tr eatment with PD98059 partially blocked the E-2-stimulated ERE-CAT activity and [H-3]thymidine uptake in both LTED and in wild type cells, indicating t hat both MAP kinase-dependent and MAP kinase-independent pathways are invol ved in the transactivation function of ER. Taken together, our data suggest that the MAP kinase pathway is, in part, involved in the adaptive process which results in enhanced DNA synthesis and cell proliferation in the absen ce of exogenous estrogen in LTED cells.