Genetic ablation of caveolin-1 in mammary epithelial cells increases milk production and hyper-activates STAT5a signaling

Caveolin-1 (Cav-1) is the main structural protein of caveolae, plasma membrane invaginations that have been implicated in vesicular transport, cholesterol homeostasis, and the regulation of signal transduction. Previous in vivo studies have established a correlation between Cav-1 expression levels and milk production. In the normal mouse mammary gland, Cav-1 levels were shown to be downregulated during late pregnancy and lactation, via a Ras-p42/44-MAPK- dependent mechanism. Conversely, mammary glands from Cav-1 null^-/- mice exhibit premature lactation, with augmented development of the lobulo-alveolar compartment and hyper-activation of the Jak-2/STAT5a signaling cascade. However, it remains unknown whether these phenotypes are cell-autonomous, i.e., intrinsic to the alveolar mammary epithelial cells, or whether stromal or adipocyte-secreted factors contribute. To directly address this issue, we have isolated primary mammary epithelial cells from wild-type (WT) and Cav-1 null^-/- mammary glands. We cultured them either in a 2D model (monolayers of mammary epithelial cells) or in a 3D system on exogenous basement membrane (Matrigel; to reconstitute the minimal lactating unit, i.e., the mammary acinus). We show here that Cav-1 deficient mammary epithelial cells display the ability to spontaneously generate milk droplets, and to secrete them into the acinar lumen. Interestingly, such milk production occurs in the absence of lactogenic stimulation. Our results show that monolayers of Cav-1 null mammary epithelial cells are enriched in milk droplets, as judged by both (1) phase contrast microscopy and (2) immunofluorescence analysis with an antiserum directed against mouse milk proteins. Consistently, Cav-1 deficient mammary acini display increased milk production and secretion, as evaluated by Western blot analysis and electron microscopic examination. Mechanistically, we show that loss of Cav-1 in mammary epithelial cells induces the baseline constitutive hyper-activation of STAT5a signaling, which normally controls the temporal progression of lactogenesis in the mammary gland. The possible implications of our findings for understanding mammary tumorigenesis are also discussed.