1,25(OH)(2)-VITAMIN-D-3 STIMULATES PHOSPHOLIPASE A(2) ACTIVITY VIA A GUANINE-NUCLEOTIDE-BINDING PROTEIN IN CHICK MYOBLASTS

The steroid hormone 1,25(OH)(2)-vitamin D-3 [1,25(OH)(2)D-3] stimulated phospholipase A(2) (PLA(2)) activity in embryonic chick myoblasts releasing [H-3]arachidonic acid from the sn-2 position of phospholipids. GTP-binding protein mediation of 1,25(OH)(2)D-3-dependent PLA(2) activity was investigated in cells prelabeled with [H-3]arachidonic acid. AIF(4)(-), a G-protein activator, mimicked 1,25(OH)(2)D-3 stimulated arachidonic acid release from myoblasts in a dose-dependent manner. Consistent with the involvement of a G-protein in the activation of PLA(2) by the hormone, guanosine 5'-0-(3-thiotriphosphate) (GTP gamma S), a stable GTP analogue which activates G-protein mediated signals, strongly enhanced arachidonic acid release in myoblasts. Guanosine 5'-0-(2-thiodiphosphate) (GDP beta S), which competitively inhibits G-protein activation by GTP and its analogues, abolished 1,25(OH)(2)D-3-dependent arachidonic acid release. Bordetella pertussis toxin pretreatment significantly suppressed the hormone action whereas cholera toxin had minor effects on 1,25(OH)(2)D-3 action. Hormone-induced activation of PLA(2) was mimicked by the Ca2+ ionophore A23187 and blocked by nifedipine, but was unaffected by neomycin, a phospholipase C inhibitor, ruling out the contribution of phosphoinositide metabolism to arachidonic acid release. These results suggest that 1,25(OH)(2)D-3-stimulation of PLA(2) activity in embryonic chick myoblasts is mediated by a pertussis toxin-sensitive GTP-binding protein coupled to influx of extracellular calcium.