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Prolactin Promotes the Secretion of Active Cathepsin D at the Basal Side of Rat Mammary Acini

Dipartimento di Scienze Mediche (R.C., M.D., C.I.), Università del Piemonte Orientale "A. Avogadro," 21800 Novara, Italy; UR909 Nutrition et Régulation Lipidique des Fonctions Cérébrales (S.D.) and UR1196 Génomique et Physiologie de la Lactation (E.B., M.O.-B.), Institut National de la Recherche Agronomique, F-78352 Jouy-en-Josas, France

Address all correspondence and requests for reprints to: Michèle Ollivier-Bousquet, UR1196 Génomique et Physiologie de la Lactation, Institut National de la Recherche Agronomique, F-78352 Jouy-en-Josas cedex, France. E-mail: michele.ollivier{at}jouy.inra.fr.

Cathepsin D (CD), a lysosomal aspartic protease present in mammary tissue and milk in various molecular forms, is also found in the incubation medium of mammary acini in molecular forms that are proteolytically active on prolactin at a physiological pH. Because prolactin controls the vesicular traffic in mammary cells, we studied, in vivo and in vitro, its effects on the polarized transport and secretion of various forms of CD in the rat mammary gland. CD accumulated in vesicles not involved in endocytosis in the basal region of cells. Prolactin increased this accumulation and the release of endosomal active single-chain CD at the basal side of acini. The CD-mediated proteolysis of prolactin, leading to the antiangiogenic 16-kDa form, at a physiological pH, was observed only in conditioned medium but not milk. These data support the novel concept that an active molecular form of CD, secreted at the basal side of the mammary epithelium, participates in processing blood-borne prolactin outside the cell, this polarized secretion being controlled by prolactin itself.