Regulation of protein kinase C-delta and -epsilon isoforms by phorbol ester treatment of LLC-PK1 renal epithelia

Background. LLC-PK1 renal epithelia are a widely used model for proximal tu bular physiology and differentiation. Protein kinase C (PKC) has been obser ved to play a role in both processes. This study examines the subcellular d istribution and down-regulation of PKC-delta and PKC-epsilon isoforms in ph orbol eater-treated LLC-PK1 epithelia. Methods. Cells were treated with 10(-7) mol/L 12-O-tetradecanoyl phorbol 13 -acetate (TPA) for up to seven days and were extracted as total cell lysate s as well as cytosolic, membrane-associated (Triton-X soluble) and a third (Triton-X insoluble) fraction. The expression and cellular localization of PKC-delta and PKC-epsilon isoforms were then detected using Western immunob lot and immunofluorescence. Results. Based on the use of an anti-PKC-delta monoclonal antibody, TPA was observed to cause a rapid decrease in total PKC-delta content, which then returned to near control levels by seven days of treatment. Immunofluoresce nce indicated that PKC-delta had a cytoskeletal localization within the cel ls, and a subtle cytoskeletal rearrangement occurred upon exposure to TPA. Western immunoblots showed that PKC-delta did not undergo the expected memb rane translocation upon activation by TPA, but simply disappeared immediate ly from the cytosolic compartment. Conventional cell fractionation procedur es such as homogenization and Triton extraction prior to Western immunoblot will, however, fail to evaluate completely PKC-delta in LLC-PK1 epithelia because of the highly stringent measures necessary to extract PKC-delta fro m the cytoskeletal compartment of these cells. Furthermore, we observed tha t a second (polyclonal) PKC-delta antibody may recognize phosphorylated for ms of PKC-delta, which went unrecognized by the other antibody. PKC-epsilon was present in the cytosol, membrane, and Triton-X-insoluble fractions of the cells. TPA treatment resulted in a partial translocation of PKC-epsilon to both the membrane and Triton-X-insoluble fractions of the cell, but tot al PKC-epsilon remained essentially unchanged. Conclusions. The present data indicate that the localization of PKC-delta a nd subsequent redistribution within the LLC-PK1 cells in response to TPA tr eatment is highly unique and distinct from that of PKC-E and PKC-ol. An imp ortant methodological finding is that one given antibody may not recognize all phosphoproteins of a given PKC isoform.