PROTEIN-KINASE-C-ETA UP-REGULATION AND SECRETION DURING POSTNATAL RATMAMMARY-GLAND DIFFERENTIATION

The mammary gland has the ability to undergo repeated cycles of tightl y regulated postnatal proliferation, differentiation, and apoptosis-me diated regression, providing a model to investigate potential regulato rs of mammary epithelial growth and differentiation. Protein kinase C eta is a candidate regulator of mammary epithelial differentiation, as increased expression of PKC eta is often observed during the terminal differentiation of many epithelial tissues. In this study, PKC eta ex pression and localization were characterized during puberty, pregnancy , lactation and involution in isolated rat mammary epithelial cells (M EC), as well as in paraffin-embedded and frozen rat mammary gland sect ions. By Western blot analysis of whole cell lysates from purified MEG , PKC eta protein expression increased during the shift from resting t o a pregnant state. This increased PKC eta protein expression during p regnancy was associated with alveolar rather than ductal development, as immunohistochemical staining for PKC eta was increased in different iating secretory alveoli, but not ducts, By immunofluorescent staining , PKC eta was stained intensely in an intracellular reticular meshwork throughout the cytosol of alveolar epithelial cells from pregnant mam mary gland. During lactation, PKC eta was abundant in apocrine bodies budding from the alveolar epithelium, in the lumen of alveoli, and was present in milk, in association with casein, while being decreased in the cytoplasm of the luminal alveolar epithelium, Staining intensity of alveoli for PKC eta decreased further during involution, Western bl otting of subcellular fractions from isolated mammary epithelial cells demonstrated that PKC eta remained associated with the membrane and p articulate fractions throughout development. The upregulation of PKC e ta in alveolar but not ductal epithelium during pregnancy suggests an association with functional secretory differentiation.