Endocrinology, Vol 116, 154-163, Copyright © 1985 by Endocrine Society

ARTICLES

Effects of estradiol on insulin receptor distribution in primary cultures of R3230AC mammary adenocarcinoma of the rat

R Hilf and DH Crofton

The influence of 17 beta-estradiol on insulin receptor distribution was studied in primary cultures of R3230AC mammary tumors prepared from intact or ovariectomized Fischer rats. [125I] Insulin binding to plasma membrane (IRS) and to solubilized cells (IRt) was measured, and intracellular insulin binding (IRi) was calculated by the difference between the two; calculated IRi agreed well with measured IRi of solubilized cells that were pretreated with trypsin to remove IRs. Scatchard analysis of saturation studies on IRs and IRt displayed typical curvilinearity and were roughly parallel with comparable Ke and Kd values for high affinity sites, but tumors from ovariectomized rats displayed more sites per cell. Insulin receptors relative to time in culture showed an early decline in IRt and IRs for both types of cells, followed by a gradual return to similar IRt values; in both types of cells, however, the proportion of IRs was increased compared to their initial distribution. Cells from both types of host animals displayed an insulin dose-related down-regulation with both IRt and IRs decreased. Short exposure (24 h) to 17 beta-estradiol in vitro also reduced IRs and IRt, but to a lesser extent than insulin, whereas longer exposure to estradiol (48 h) caused a continued reduction in IRs but not IRt. Compared to cells exposed to estradiol for 24 h in vitro, cultured cells treated with progesterone demonstrated an increased IRs, a marginal effect upon exposure to higher doses of testosterone, and reduced IRt, IRs, and IRi in response to dexamethasone. Monohydroxytamoxifen, an antiestrogen, displayed an unusual pattern, inducing a reduction in IRt and IRs at the lower doses but not at the higher doses studied. Degradation of [125I]insulin was examined in cells that were insulin down-regulated in the absence or presence of various levels of estradiol; the steroid hormone appeared to reduce degradation of [125I]insulin when lower levels of insulin were employed. The presence of estradiol in the medium enhanced the reappearance of IRt during the first 10 h after removal of the insulin that caused down-regulation. In conjunction with our previous observations in vivo, we conclude that 1) estradiol in vitro can decrease (down-regulate) insulin receptors on the plasma membrane of R3230AC mammary tumors; 2) the steroid may reduce degradation of internalized [125I]insulin; and 3) the steroid may enhance insulin receptor reappearance after insulin down-regulation. These estrogen- insulin interactions could play a role in the regulation of growth and metabolism of this hormone-responsive experimental mammary carcinoma.