Estradiol attenuates prolactin secretion and phosphoinositide hydrolysis in MMQ cells

Abstract

We previously isolated a clonal cell line, designated MMQ, which only secretes prolactin (PRL) and whose secretory process is nonresponsive to thyrotropin releasing hormone (TRH) and angiotensin II (AII). In the present study, we injected MMQ cells into rats to determine whether the tumor cells would become responsive to secretagogues when subsequently propagated in vitro. We also investigated what effects in vivo administration of 17β-estradiol would have on secretagogue-induced PRL release and on intracellular biochemical mechanisms in these cells. MMQ cells were implanted subcutaneously in the backs of female rats. One group was injected with 100 μg polyestradiol phosphate (PEP) every 5 days, a second with saline. The inoculants grew into solid tumors within 3 weeks. The day after the tumors were removed and enzymatically dispersed, the cells, now designated MMQt cells, were perifused in vitro. Basal PRL released by MMQt cells was approximately 1 ng/min/10 7 cells and perifusions with 100 nM TRH or AII for 5 min significantly increased PRL release above baseline (integrated areas: 1.8 ± 0.4 and 5.2 ± 1.3 ng/10 7 cell, respectively; P < 0.01). Two ng/ml maitotoxin (MTX), a calcium channel activator, increased PRL release (38.2 ± 6.7 ng/10 7 cells; P < 0.01). In PEP-treated perifused MMQt cells, basal in vitro PRL release was not different from that observed in the control group, but the responses to TRH, AII and MTX were greatly attenuated (TRH: 0.6 ± 0.1, AII: 1.3 ± 0.2 and MTX: 9.2 ± 2.5 ng/10 7cells). In control MMQt cells, 100 nM TRH, 100 nM AII and 1 μM neurotensin (NTS) significantly increased inositol phosphates (InsP x) generation by 53%, 106% and 56% ( P < 0.01), respectively. In addition, All and MTX increased the intracellular calcium concentration in control MMQt cells, which correlated with their ability to enhance InsP x and PRL release. In PEP-treated cells, however, All only increased IP x generation slightly by 32% (P < 0.05), and TRH or NTS did not stimulate it at all. These data demonstrate that MMQ cells, after having been transplanted into femal rats and then re-isolated from the solid tumor (and thereafter designated MMQt cells), become responsive to TRH and to AIL These data further show that exposure of MMQt cells to elevated serum estradiol concentrations in vivo blocks the cells' ability to respond in vitro to these PRL secretagogues through less responsive intracellular biochemical events.