Synergistic effects of phorbol ester and INF-gamma on the induction of indoleamine 2,3-dioxygenase in THP-1 monocytic leukemia cells.

Indoleamine 2,3-dioxygenase (IDO) is a flavin-dependent enzyme which uses superoxide anion as a cosubstrate to catalyze the decyclization of the pyrrole ring of L-tryptophan to form formylkynurenine. This enzyme is induced in some tumor cells after treatment with IFN-gamma. The mechanism of induction of IDO in tumor cells by IFN-gamma was studied in THP-1 human monocytic leukemia cells. Before the addition of IFN-gamma, no IDO could be detected in these cells. Treatment of THP-1 cells with IFN-gamma produced an induction of IDO, with peak activity occurring 72 to 96 h after addition of IFN-gamma. Because phorbol esters are known to induce many enzymes in cells, most likely through the activation of protein kinase C, the effects of PMA on the induction of IDO were determined. PMA potentiated the IFN-gamma-induced elevation of IDO, but by itself, was unable to induce enzyme activity. Maximum induction of IDO in the presence of PMA and IFN-gamma was obtained by preexposure of the cells to PMA for 48 h before the addition of IFN-gamma. Maximum induction of IDO after the addition of IFN-gamma occurred 24 to 48 h after addition of the cytokine to the culture medium. However, the induction of IDO does not appear to be potentiated through the activation of protein kinase C, because the addition of the protein kinase C inhibitor H-7 had no effect on the induction of IDO when the cells were exposed to PMA and IFN-gamma. Moreover, diacylglycerol was unable to replace PMA in these studies. Studies with cAMP and cGMP analogs suggest a role for these compounds in the regulation of IDO expression.