Stimulatory and inhibitory effects of 1,25-dihydroxyvitamin D₃ on thymocyte mitogenesis induced by phorbol ester and calcium ionophore

Mouse medullary thymocytes have specific receptors for 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃). The mitogenic stimulation of these cells by phytohemagglutinin in the presence or absence of the phorbol ester TPA is inhibited by 1,25(OH)₂D₃. The calcium ionophore A23187 did not reverse the inhibition by 1,25(OH)₂D₃ 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)₂D₃ 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)₂D₃ but in the absence of both TPA and A23187, was inhibited by 1,25(OH)₂D₃. 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)₂D₃ 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)₂D₃ enhances or inhibits thymocyte stimulation. (2) Both stimulatory and inhibitory actions of 1,25(OH)₂D₃ seem to take place at points distal to the initial increase in intracellular calcium or activation of protein kinase C.