Alterations in the expression and localization of protein kinase C isoforms during mammary gland differentiation

Protein kinase C (PKC) is involved in signaling that modulates the proliferation and differentiation of many cell types, including mammary epithelial cells. In addition, changes in PKC expression or activity have been observed ring mammary carcinogenesis. In order to examine the involvement of specific PKC isoforms during normal mammary gland development, the expression and localization of PKCs α, δ, ε and ζ were examined during puberty, pregnancy, lactation, and involution. By immunoblot analysis, expression of PKC α, δ, ε and ζ proteins was increased in mammary epithelial organoids during the transition from puberty to pregnancy. In mammary gland frozen sections, PKCs α, δ, ε and ζ were stained in the luminal epithelium and myoepithelium, in varying isoform- and developmental stage-specific locations. PKC α was found in a punctate apical localization in the luminal epithelium during pregnancy. During lactation, PKC ε was present in the nucleus, and PKC ζ was concentrated in the subapical region of the luminal epithelium. Additionally, marked staining for PKCs α, δ, ε, and ζ was observed in the myoepithelial cells at the base of ducts and alveoli. This basal ductal and alveolar staining differed in intensity in a developmentally-specific fashion. During most time points (virgin, pregnant, lactating, and early involution), myoepithelial cells of the duct were more intensely stained than those lining the alveoli for PKCs α, δ, ε, and ζ. During late involution (days 9-12), the preferential staining of ducts was lost or reversed, and the myoepithelial cells lining the regressing alveolar structures stained equally (PKCs ε and ζ) or more intensely (PKCs α and δ), coincident with the thickening of the myoepithelial cells surrounding the regressing alveoli. The increased PKC isoform staining at the base of alveoli during involution suggests that alveolar regression may be influenced by alterations in signaling in the alveolar myoepithelium.