Regulation of estrogen receptor levels in endometrial cancer cells

Recent experimental results from our laboratories revealed the following facts: Addition of GMP to homogenates or cytosol prepared from endometrial tissue or cultured endometrial adenocarcinoma cells during the assay for specific estrogen binders markedly increases specific binding levels. The effect is completed in about 15 min at 4 C (Fleming et al, 1983). Cyclic AMP has the opposite effect and in many cases lowers the number of binding sites to undetectable levels. ATP, a nucleotide that stimulates a particulate form of guanylate cyclase, Na2MoO4, a compound that can elevate cGMP levels (Fleming and Blumenthal, unpublished) and GTP, a metabolic precursor of cGMP, increase specific estradiol binding in the presence of plasma membranes and soluble factors. Cyclic AMP reduces the levels of estrogen binding when added to cell homogenates or to cytosol and counteracts the effects of cGMP, MoO4, ATP and GTP. ATP is required for the expression of cGMP and cAMP effects on estradiol binding. It is therefore likely that phosphorylations are involved in the generation and inactivation of estrogen binding sites. Divalent cation requirements for these effects also suggest participation of protein kinases in these processes. The reported effects of nucleotides and molybdate have been observed in specimens of histologically normal endometrium, in specimens of endometrial carcinoma, in two endometrial adenocarcinoma cell lines, HEC-1 and HEC-50 (Suzuki et al, 1980), and in two breast cancer cell lines, CG-5, a variant of MCF-7 obtained in Iacobelli's laboratory (Natoli et al, 1983), and in T47D) (Fleming et al, in press) Rapid changes in the levels of estrogen binding capacity observed in endometrial cells in culture can be associated with changes in cGMP/cAMP ratios shown, to vary during the cell cycle. Although it has not yet been demonstrated that cGMP-induced increases in specific estrogen binding can enhance responses to available estrogens, such possibility is of potential importance. Reduction of estrogen receptor levels in patients with cancers of estrogen sensitive tissues may inhibit tumor growth promoted by endogenous estrogen. Cho-Chung et al have recently reported that cholera toxin causes a reduction in estrogen receptor levels and arrests hormone dependent growth of DMBA-induced mammary carcinoma in rats (Cho-Chung et al, 1983). They postulated that the effect of cholera toxin is mediated by a cAMP effect on the estrogen receptor, an hypothesis supported by the observation that only tumors containing receptor responded to treatment. Conversely, cGMP-induced increases in specific estrogen binders may be useful in promoting a response of tumors to estr