Differential translocation of protein kinase C isozymes by phorbol esters, EGF, and ANG II in rat liver WB cells

Abstract

The protein kinase C (PKC) family represents an important group of enzymes whose activation is associated with their translocation from the cytosol to different cellular membranes. In this study, the spatial distribution of PKC-α, -δ and -ε in rat liver epithelial (WB) cells has been examined by Western blot analysis after subcellular fractionation. Cytosolic, membrane, nuclear, and cytoskeletal fractions were obtained from cells stimulated with phorbol 12-myristate 13-acetate (PMA), angiotensin II (ANG II), or epidermal growth factor (EGF). PMA caused most of the PKC-α, -δ and -ε initially present in the cytosol to be transported to the membrane and nuclear fractions. In contrast, both ANG II and EGF induced only a minor translocation of PKC-α to the membrane fraction but caused a statistically significant membrane-directed movement of PKC-δ and -ε. Translocation of PKC-δ and -ε to the nucleus induced by ANG II and EGF was transient and quantitatively smaller than that induced by PMA. PKC-δ and -ε were present in the cytoskeleton of resting cells, but although PMA, ANG II, and EGF caused some changes in their content, these were variable, suggesting that the cytoskeleton fraction was heterogeneous. PKC depletion inhibited ANG II-induced mitogenesis and the sustained activation of Raf-1 and extracellular regulated protein kinase (ERK). However, although PKC depletion inhibited EGF-induced mitogenesis, the maximum EGF-induced activation of the ERK pathway was only slightly retarded. We hypothesize that PKC-δ and -ε are involved in mitogenesis via both ERK-dependent and ERK-independent mechanisms. These results support the notion that specific PKC isozymes exert spatially defined effects by virtue of their directed translocation to distinct intracellular sites.