A novel N-myristylated synthetic octapeptide inhibits protein kinase C activity and partially reverses murine fibrosarcoma cell resistance to adriamycin.

This report shows that N-acylation of the protein kinase C (PKC) substrate Arg-Lys-Arg-Thr-Leu-Arg-Arg-Leu (RKRTLRRL) provides it with a potent inhibitory activity against PKC. N-myristoyl-RKRTLRRL inhibited Ca2(+)- and phosphatidylserine (PS)-dependent histone phosphorylation catalyzed by PKC with a 50% inhibitory concentration (IC50) of 5 microM, whereas neither RKRTLRRL nor myristic acid inhibited PKC-catalyzed histone phosphorylation at concentrations as high as 50 microM. A fully active, Ca2(+)- and PS-independent catalytic fragment of PKC can be generated by limited proteolysis. N-myristoyl-RKRTLRRL inhibited histone phosphorylation catalyzed by the catalytic fragment of PKC (IC50 = 80 microM), but neither myristic acid nor the nonmyristylated peptide inhibited the activity of the catalytic fragment at concentrations up to and including 200 microM. The Km app and Vmax app for N-myristoyl-RKRTLRRL were similar to those of RKRTLRRL. Thus, N-myristylation provided the octapeptide with an inhibitory activity against PKC but had only minor effects on its Km app and Vmax app. Kinetic analysis provided evidence that the peptide inhibited PKC noncompetitively with respect to ATP. Previously, we reported that the protein kinase inhibitor H7 partially reverses Adriamycin resistance in the multidrug resistant (MDR) murine fibrosarcoma line UV-2237M-ADRR. In this report, we show that N-myristoyl-RKRTLRRL also partially reverses Adriamycin resistance in UV-2237M-ADRR cells. These results suggest that potent and selective cell permeable PKC inhibitors may be designed by N-acylating small PKC peptide substrates.