Activation of phospholipase A 2 by 1,25 (OH) 2 vitamin D 3 and cell growth in monocytic U937 and mono mac 6 cells

Soluble phospholipase A 2 activity was characterized in two human monocytic cell lines, U937 and Mono Mac 6. The enzyme showed an absolute requirement for Ca ++ , an alkaline pH optimum and Michaelis‐Menten kinetics in both cell lines. Differentiation of U937 and Mono Mac 6 cells with 1,25 (OH) 2 vitamin D 3 (10 n M , 72 h) enhanced PLA 2 activity by 82 per cent and 56 per cent, respectively. Furthermore, kinetic experiments revealed that enzyme activity increased within 3 h when cells were brought from the nonproliferative phase of growth to the start of a new cycle of cell proliferation. This initial activation of PLA 2 could be inhibited by cycloheximide and actinomycin D, indicating the requirement of gene transcription. Taken together, these results suggest a role of cytosolic, Ca ++ ‐dependent PLA 2 in differentiation and growth of monocytic cells.