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Bolander FF Jr, E Ginsburg, and BK Vonderhaar

In a previous study, infection with the mouse mammary tumor virus (MMTV) was shown to increase the sensitivity of the mammary epithelium toward prolactin (PRL); furthermore, this effect could be mimicked by the binding of the MMTV envelope protein (gp52) to its cell receptor. The present work has investigated the possibility that gp52-induced changes in the PRL receptor (PRLR) were responsible for this phenomenon. In vitro, gp52 doubled the PRLR concentration in the plasmalemma of mammary epithelium without affecting the affinity. The origins of these PRLRs were twofold: first, gp52 stimulated PRLR mRNA nearly fivefold, suggesting that some of the receptors were newly synthesized. Second, there was a redistribution of PRLRs within the mammary cell: PRLRs were shifted from an internal pool to the plasma membrane. This relocation was very rapid, occurring within 30 min. There did not appear to be any contribution from alterations in PRLR degradation, since the half-life of PRLR was not affected by gp52. In summary, the MMTV increases the PRL sensitivity of mouse mammary epithelium by elevating PRLRs through both enhanced synthesis and recruitment from microsomes.

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J M Fleming, E Ginsburg, C W McAndrew, C D Heger, L Cheston, J Rodriguez-Canales, B K Vonderhaar, and P Goldsmith

Prolactin is essential for normal mammary gland development and differentiation, and has been shown to promote tumor cell proliferation and chemotherapeutic resistance. Soluble isoforms of the prolactin receptor (PrlR) have been reported to regulate prolactin bioavailability by functioning as ‘prolactin-binding proteins’. Included in this category is Δ7/11, a product of alternate splicing of the PrlR primary transcript. However, the direct interactions of prolactin with Δ7/11, and the resulting effect on cell behavior, have not been investigated. Herein, we demonstrate the ability of Δ7/11 to bind prolactin using a novel proximity ligation assay and traditional immunoprecipitation techniques. Biochemical analyses demonstrated that Δ7/11 was heavily glycosylated, similar to the extracellular domain of the primary PrlR, and that glycosylation regulated the cellular localization and secretion of Δ7/11. Low levels of Δ7/11 were detected in serum samples of healthy volunteers, but were undetectable in human milk samples. Expression of Δ7/11 was also detected in six of the 62 primary breast tumor biopsies analyzed; however, no correlation was found with Δ7/11 expression and tumor histotype or other patient demographics. Functional analysis demonstrated the ability of Δ7/11 to inhibit prolactin-induced cell proliferation as well as alter prolactin-induced rescue of cell cycle arrest/early senescence events in breast epithelial cells. Collectively, these data demonstrate that Δ7/11 is a novel regulatory mechanism of prolactin bioavailability and signaling.