1,25-Dihydroxyvitamin D3[1,25(OH)2D3] and 12- O-tetradecanoylphorbol 13-acetate (TPA) both activated phospholipase D (PLD) in Caco-2 cells. GF-109203x, an inhibitor of protein kinase C (PKC) isoforms, inhibited this activation by both of these agonists. 1,25(OH)2D3activated PKC-α, but not PKC-β1, -βII, -δ, or -ζ, whereas TPA activated PKC-α, -β1, and -δ. Chronic treatment with TPA (1 μM, 24 h) significantly reduced the expression of PKC-α, -βI, and -δ and markedly reduced the ability of 1,25(OH)2D3or TPA to acutely stimulate PLD. Removal of Ca2+ from the medium, as well as preincubation of cells with Gö-6976, an inhibitor of Ca2+-dependent PKC isoforms, significantly reduced the stimulation of PLD by 1,25(OH)2D3or TPA. Treatment with 12-deoxyphorbol-13-phenylacetate-20-acetate, which specifically activates PKC-βI and -βII, however, failed to stimulate PLD. In addition, the activation of PLD by 1,25(OH)2D3or TPA was markedly reduced or accentuated in stably transfected cells with inhibited or amplified PKC-α expression, respectively. Taken together, these observations indicate that PKC-α is intimately involved in the stimulation of PLD in Caco-2 cells by 1,25(OH)2D3or TPA.
The Role of 1,25-Dihydroxyvitamin D in the Inhibition of Bone Formation Induced by Skeletal Unloading*