Hormonal regulation of tumor suppressor proteins in breast cancer cells.

This laboratory is studying hormonal regulation of tumor suppressor proteins, p53 and retinoblastoma (pRB). Estrogen receptor and progesterone receptor positive human breast cancer cell lines, T47D and MCF-7, were utilized for determining influence of hormonal and antihormonal agents on the level of expression of p53, state of phosphorylation of pRB, and rate of cell proliferation. The expression of p53 in T47D cells grown for 4-5 days in culture medium containing charcoal-treated (stripped) fetal bovine serum declined gradually to 10% of the level seen in control (whole serum, non charcoal-treated) groups. Supplementation of culture medium containing stripped serum with 0.1-1 nM estradiol (E(2)) restored p53 to its level seen in the control within 6-24 h. Under above conditions, treatment of cells with R5020 or RU486 reduced (15-30%) the level of p53. Incubation of cells in E(2)-containing growth medium caused cell proliferation and hyperphosphorylation of pRB; the latter effect was seen maximally between 24-72 h. The E(2)-induced hyperphosphorylation of pRB and increase in the level of p53 were sensitive to the presence of ICI and 4-hydroxy tamoxifen (OHT). T47D and MCF-7 cells were also transiently transfected with a P1CAT reporter plasmid containing c-Myc responsive element and the levels of chloramphenicol acetyltransferase (CAT) activity were observed in response to various treatments. E(2) and OHT caused P1CAT induction as seen by increased CAT activity: E(2) caused an endogenous increase in the expression of an ICI-sensitive c-Myc form. These data suggest that estrogen upregulates p53 expression while progesterone downregulates this process. Further, E(2) regulates p53 level and pRB activity in a coordinated manner.