Estrogen receptor alpha allows the adipokine, leptin, to function as a growth factor for breast cancer cells.

A126

Obesity is a risk factor for postmenopausal estrogen receptor (ER) positive breast cancer. Recently, our laboratory reported that obesity-prone MMTV-TGF-α transgenic mice that develop ER positive mammary tumors (MT) had shorter MT latency, were younger at death and had higher serum leptin levels compared to controls. In additional studies MMTV-TGF-α mice were crossbred with obese leptin deficient or leptin receptor deficient mice but no MTs developed. The leptin receptor has been identified in breast tumors and in ER positive human breast cancer cell lines. Also addition of leptin to these cells increased proliferation and activated leptin signaling pathways. In contrast, the ER alpha (ERα) negative MDA-MB-231 (MDA-wt) cell line was not reported to respond to leptin. Integration of these findings led us to hypothesize that the adipokine, leptin, is a growth factor for ERα positive breast cancer. To further define the role of leptin and ERα in breast cancer progression, the ERα negative MDA-wt cell line was modified to stably express ERα (line MDA-ERα5) so that only a single gene difference could be compared. Both cell lines expressed similar amounts of the signaling (long) isoform of the leptin receptor (Ob-Rb). Importantly, leptin (100 ng/ml) reduced cell number of the parental ERα negative MDA-wt cells by 11% compared to untreated cells but increased the growth rate of the ERα positive, MDA-ERα5, cells by 21% even in the absence of estradiol. Estradiol alone (10 ng/ml) or in combination with leptin (100 ng/ml) also increased MDA-ERα5 cell growth by 41% and 57% respectively whereas the growth of the MDA-wt cells was decreased by 10% and 8% respectively. Previously, ERα positive MCF-7 cells were reported to increase aromatase P450 levels in response to leptin, however, leptin concentrations between 25 and 100 ng/ml did not increase aromatase P450 in either the parental MDA-wt or MDA-ERα5 cells. A second adipokine, adiponectin, is inversely correlated with body mass index in humans and has been proposed to act as a brake on breast cancer cell growth. Both receptors for adiponectin, AdipoR1/R2, were expressed by MDA-wt and MDA-ERα5 cells. In addition, the expression of AdipoR1 was down regulated by 40% in MDA-ERα5 cells after 24 hours at 100 ng/ml leptin but not in MDA-wt cells. In contrast, the level of AdipoR2 was down regulated in both cell lines. In an in vivo experiment, MDA-ERα5 cells grew equally well in the presence or absence of exogenous estradiol when implanted into intact female Rag1 athymic mice. We are currently evaluating leptin signaling pathways in vitro. In general, these results indicate that the effect of leptin on breast cancer cell growth is mediated by the presence or absence of the estrogen receptor. (Support: Breast Cancer Research Foundation and Hormel Foundation)