Phospholipid-sensitive Ca2+-dependent protein phosphorylation system in various types of leukemic cells from human patients and in human leukemic cell lines HL60 and K562, and its inhibition by alkyl-lysophospholipid.

Phospholipid-sensitive Ca2+-dependent protein kinase (PL-Ca-PK), 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 particulate 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 kinase, in comparison, were much fewer or undetected. The PL-Ca-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 predominant protein phosphorylation system in leukemic cells and that this enzyme system may represent a site of cytotoxic action of alkyl-lysophospholipid.