Characterization of Δ7/11, a functional prolactin-binding protein

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 Delta 7/11, a product of alternate splicing of the PrlR primary transcript. However, the direct interactions of prolactin with Delta 7/11, and the resulting effect on cell behavior, have not been investigated. Herein, we demonstrate the ability of Delta 7/11 to bind prolactin using a novel proximity ligation assay and traditional immunoprecipitation techniques. Biochemical analyses demonstrated that Delta 7/11 was heavily glycosylated, similar to the extracellular domain of the primary PrlR, and that glycosylation regulated the cellular localization and secretion of Delta 7/11. Low levels of Delta 7/11 were detected in serum samples of healthy volunteers, but were undetectable in human milk samples. Expression of Delta 7/11 was also detected in six of the 62 primary breast tumor biopsies analyzed; however, no correlation was found with Delta 7/11 expression and tumor histotype or other patient demographics. Functional analysis demonstrated the ability of Delta 7/11 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 Delta 7/11 is a novel regulatory mechanism of prolactin bioavailability and signaling.