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] stimulate d phospholipase A(2) (PLA(2)) activity in embryonic chick myoblasts re leasing [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) acti vity was investigated in cells prelabeled with [H-3]arachidonic acid. AIF(4)(-), a G-protein activator, mimicked 1,25(OH)(2)D-3 stimulated a rachidonic acid release from myoblasts in a dose-dependent manner. Con sistent 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, stron gly 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-dependen t arachidonic acid release. Bordetella pertussis toxin pretreatment si gnificantly suppressed the hormone action whereas cholera toxin had mi nor effects on 1,25(OH)(2)D-3 action. Hormone-induced activation of PL A(2) was mimicked by the Ca2+ ionophore A23187 and blocked by nifedipi ne, but was unaffected by neomycin, a phospholipase C inhibitor, rulin g out the contribution of phosphoinositide metabolism to arachidonic a cid release. These results suggest that 1,25(OH)(2)D-3-stimulation of PLA(2) activity in embryonic chick myoblasts is mediated by a pertussi s toxin-sensitive GTP-binding protein coupled to influx of extracellul ar calcium.