Synergism and Antagonism in the Effects of 1α,25-Dihydroxyvitamin D₃, Retinoic Acid, Dexamethasone, and a Tumor-promoting Phorbol Ester on the Functional Capability of P388D1 Cells: Phagocytosis and Transglutaminase Activity

Abstract

1α,25-Dihydroxyvitamin D₃ [1,25(OH)₂D₃] and retinoic acid (RA) induce a high-phagocytic phenotype in the macrophage-like tumor cell line P388D1. A concurrent cultivation of P388D1 cells in the presence of suboptimal concentrations of both agents led to an extent of induction of phagocytic activity that surpassed the additive effect of either of the agents alone; i.e., 1,25(OH)₂D₃ and RA synergistically induce the phagocytic capability of P388D1 cells. Dexamethasone and 4β-phorbol-12β-myristate-13α-acetate (TPA) did not induce a high-phagocytic phenotype in P388D1 cells and affected differentially the high-phagocytic phenotype induced by RA and 1,25(OH)₂D₃. Dexamethasone inhibited the phagocytic activity induced by RA (80% at 24 h), while it had small suppressive effects on that induced by 1,25(OH)₂D₃. TPA suppressed the phagocytic activity induced by RA (60% within 96 h) while potentiating the expression of the high-phagocytic phenotype induced by 1,25(OH)₂D₃ (50% increase with 96 h). The observed effects did not involve modulation of prostaglandin synthesis or intracellular cyclic adenosine 3′:5′-monophosphate. Expression of transglutaminase activity in P388D1 cells was also modulated differentially by the four agents; 1,25(OH)₂D₃ treatment had no effect on enzyme level, RA and TPA suppressed it, and dexamethasone increased it. The data suggest that: (a) 1,25(OH)₂D₃ and RA act via disparate mechanisms that can operate simultaneously; (b) the elements induced in P388D1 cells by 1,25(OH)₂D₃ and RA, and which are responsible for the phagocytic activity, differ in their sensitivity to dexamethasone and TPA; and (c) transglutaminase activity in P388D1 cells is readily manipulable, but there seems to be no straightforward correlation between the level of its activity and the phagocytic capability of the cells or their rate of proliferation.