Atypical PKC-ζ and PKC-ι mediate opposing effects on MCF-7 Na+/K+ ATPase activity

We demonstrated previously that in serum-starved MCF-7 breast cancer cell line, Ang II increased Na+/K(+)ATPase activity and activated the protein kinase C xi(PKC-xi) (Muscellaetal., 2002J Enclocrinol 173:315-323; 2003 J Cell Physiol 197:61 -68.). The aim of the present study was to investigate the modulation of the activity of the Na+/K(+)ATPase by PKC-xi in MCF-7 cells. Here, using serum-starved MCF-7cells, we have demonstrated that the effect of Ang II on the Na+/K(+)ATPase activity was inhibited by a synthetic myristoylated peptide with sequences based on the endogenous PKC-xi pseudosubstrate region (xi-PS) and by high doses of GF109203X, inhibitor of PKCs. When MCF-7 cells, grown in 10% fetal bovine serum (FBS), were stimulated with Ang II adose-and time-dependent inhibition of the Na+/K(+)ATPase activity was obtained. Under this growth condition we found that mRNAs for AT1, AT2, and for Na+/K(+)ATPase alpha(1) and alpha(3) subunits were unchanged; besides both the activity of the Na+/K(+)ATPase and the level of PKC-xi also were unaffected by the serum. The atypical PKC-xi level (present in very low abundance in serum-starved MCF-7) was increased and Ang II provoked its translocation from the cytosol to plasma membrane. PKC-xi was localized to the membrane, and upon Ang II treatment its cellular localization did not change. The Ang II-mediated decrease of the Na+/K(+)ATPase activity was inhibited by high doses of GF109203X but not by xi-PS, thus indicating that such effect was not due to PKC-xi activity. The treatment of cells with PKC-xi antisense oligodeoxynucleotides inhibited the effects of Ang II on the Na+/K(+)ATPase activity. Additionally, the effect of Ang II on Na+/K(+)ATPase activity was also blocked by the phosphatidylinositol 3-kinase (Pl3K) inhibitors, wortmannin and LY294002, and by the actin depolymerizing agents, cytochalasin D. In conclusion, in MCF-7 cells Ang II modulates the Na+/ K(+)ATPase activity by both atypical PKC-xi/-i. The effects of Ang II are opposite depending upon the presence of the serum-sensitive PKC-i, with the inhibitory effect possibly due to the redistribution of sodium pump from plasma membrane to the inactive intracellular pool.