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 Az activity was characterized in two human monoc ytic cell lines, U937 and Mono Mac 6. The enzyme showed an absolute re quirement for Ca++, an alkaline pH optimum and Michaelis-Menten kineti cs in both cell lines. Differentiation of U937 and Mono Mac 6 cells wi th 1,25 (OH)(2) vitamin D-3 (10 nM, 72 h) enhanced PLA(2) activity by 82 per cent and 56 per cent, respectively. Furthermore, kinetic experi ments revealed that enzyme activity increased within 3 h when cells we re brought from the nonproliferative phase of growth to the start of a new cycle of cell proliferation. This initial activation of PLA(2) co uld be inhibited by cycloheximide and actinomycin D, indicating the re quirement of gene transcription. Taken together, these results suggest a role of cytosolic, Ca++-dependent PLA(2) in differentiation and gro wth of monocytic cells.