K 562 , and Its Inhibition by Alkyl-lysophospholipid from Human Patients and in Human Leukemic Cell Lines HL 60 Phosphorylation System in Various Types of Leukemic Cells-dependent Protein 2 + Phospholipid-sensitive Ca Updated

Phospholipid-sensitive Ca2+-dependent protein kinase (PL-CaPK), its endogenous substrate proteins, and regulation of the enzyme system by an antitumor agent alkyl-lysophospholipid were investigated in various types of leukemic cells (chronic myelocytic, acute myelocytic, and acute monocytic)from patients and in two cultured human leukemic cell lines (HL60 and K562). Exceedingly high levels of PL-Ca-PK, largely localized in the paniculate fraction, were found in all types and lines of leukemic cells; much lower levels of cyclic adenosine 3':5'-monophosphate-dependent protein kinase and cyclic guanosine 3':5'-monophosphate-dependent protein kinase were also found. Although numerous and similar endogenous substrates for PL-Ca-PK were found in all cell types and lines examined, substrates specific for certain leukemic cells appeared to be present. Substrate proteins for calmodulin-sensitive Ca2+-dependent protein kinase, cyclic adenosine 3':5'-monophosphate-dependent protein kinase, and cyclic guanosine 3':5'-monophosphate-dependent protein ki nase, in comparison, were much fewer or undetected. The PLCa-PK activity and the phosphatidylserine-Ca2+-stimulated phosphorylation of endogenous proteins from leukemic cells were inhibited by alkyl-lysophospholipid, which acted as a competitive inhibitor of the phospholipid cofactor of the enzyme. The findings suggested that the PL-Ca-PK system is a pre dominant protein phosphorylation system in leukemic cells and that this enzyme system may represent a site of cytotoxic action of alkyl-lysophospholipid.