Estradiol induction of cAMP in breast cancer cells is mediated by foetal calf serum (FCS) and sex hormone-binding globulin (SHBG)

Plasma sex hormone-binding globulin (SHBG or SEP), the specific carrier for estradiol and androgens, after binding to its membrane receptor (SHBG-R), causes a significant increase of cAMP in the presence of estradiol, in both breast (MCF-7) and prostate (LNCaP) cancer cells maintained in serum-free medium. On the other hand, it has been proposed that estrogens, in addition to the well-known nuclear receptor pathway, exert their biological effect inducing cAMP, as a consequence of a direct membrane action, in breast canc er and uterine cells. The aim of the present study was to clarify this cont roversial issue by verifying if the cAMP increase in MCF-7 cells was a dire ct effect of estradiol, or if it was mediated by FCS proteins, such as bovi ne sex hormone-binding globulin; and to reevaluate the effect of human SHBG on cAMP induction in the presence of FCS. MCF-7 cells were maintained in D CC-FCS (treated with DCC to remove steroids), in SHBG-FREE/DCC-FCS (treated with DCC and with a specific affinity chromatography to remove bovine sex hormone-binding globulin), or in serum-free medium (SFM). It was observed t hat estradiol determined a significant time-dependent increase of cAMP only in MCF-7 cells maintained in 10% DCC-FCS. When cells were maintained in 10 % SHBG-FREE/DCC-FCS, estradiol had no detectable effect. However, its abili ty to increase cAMP was observed again after the addition of human SHBG, in doses ranging from 5 to 50 nM. Moreover, in the presence of 10% SHBG-FREE/ DCC-FCS, SHBG, even in the absence of estradiol, caused a significant incre ase of cAMP. In conclusion, the data reported in the present study suggest that the ability of estradiol to induce cAMP in MCF-7 cells is not due to a direct membrane effect of the hormone, but rather it is mediated by FCS. S HBG is one of the serum factors mediating estradiol action. Lastly, it was proven that SHBG triggers the cAMP pathway in MCF-7 cells in a physiologic culture condition and at physiologic concentrations. (C) 1999 Elsevier Scie nce Ltd. All rights reserved.