1,25-dihydroxyvitamin D-3 and TPA activate phospholipase D in Caco-2 cells: role of PKC-alpha

1,25-Dihydroxyvitamin D-3 [1,25(OH)(2)D-3] and 12-O-tetradecanoylphorbol 13 -acetate (TPA) both activated phospholipase D (PLD) in Caco-2 cells. GF-109 203x, an inhibitor of protein kinase C (PKC) isoforms, inhibited this activ ation by both of these agonists. 1,25(OH)(2)D-3 activated PKC-alpha, but no t PKC-beta(1), -beta(II), -delta, or -zeta, whereas TPA activated PKC-alpha , -beta(1), and -delta. Chronic treatment with TPA (1 mu M, 24 h) significa ntly reduced the expression of PKC-alpha, -beta(I), and -delta and markedly reduced the ability of 1,25(OH)(2)D-3 or TPA to acutely stimulate PLD. Rem oval of Ca2+ from the medium, as well as preincubation of cells with Go-697 6, an inhibitor of Ca2+ dependent PKC isoforms, significantly reduced the s timulation of PLD by 1,25(OH)(2)D-3 or TPA. Treatment with 12-deoxyphorbol- 13-phenylacetate-20-acetate which specifically activates PKC-beta(I) and -b eta(II), however, failed to stimulate PLD. In addition, the activation of P LD by 1,25(OH)(2)D-3 or TPA was markedly reduced or accentuated in stably t ransfected cells with inhibited or amplified PKC-alpha expression, respecti vely Taken together, these observations indicate that PKC-alpha is intimate ly involved in the stimulation of PLD in Caco-2 cells by 1,25(OH)(2)D-3 or TPA.