Stimulatory and inhibitory effects of 1,25-dihydroxyvitamin D 3 on thymocyte mitogenesis induced by phorbol ester and calcium ionophore

Abstract

Mouse medullary thymocytes have specific receptors for 1,25-dihydroxyvitamin D 3 (1,25(OH) 2D 3). The mitogenic stimulation of these cells by phytohemagglutinin in the presence or absence of the phorbol ester TPA is inhibited by 1,25(OH) 2D 3. The calcium ionophore A23187 did not reverse the inhibition by 1,25(OH) 2D 3 of phytohemagglutinin. Stimulation of thymocytes with either TPA or A23187 alone did not result in proliferation. Co-stimulation of the thymocytes with TPA and A23187 induces cell proliferation. 1,25(OH) 2D 3 markedly enhanced the TPA and A23187-induced cell proliferation even when added 4 h after the initiation of the culture. In contrast, DNA synthesis by thymocytes incubated for 4 h in the presence of TPA and A23187 and then cultured in medium containing 1,25(OH) 2D 3 but in the absence of both TPA and A23187, was inhibited by 1,25(OH) 2D 3. The extent of inhibition was comparable to the inhibition of lectin-induced stimulation by the hormone. Using monoclonal antibodies to neutralize IL-2 and block IL-2 receptors we showed that 1,25(OH) 2D 3 enhanced the IL-2-independent component of the A23187- and TPA-induced mitogenesis. In conclusion: (1) The nature and presence of the mitogenic signal determines whether 1,25(OH) 2D 3 enhances or inhibits thymocyte stimulation. (2) Both stimulatory and inhibitory actions of 1,25(OH) 2D 3 seem to take place at points distal to the initial increase in intracellular calcium or activation of protein kinase C.