Possible mechanism of immunosuppressive effect of scoparone (6,7-dimethoxycoumarin)

The possible mechanism of the immunosuppressive effect of scoparone (6,7-dimethoxycoumarin) was investigated. Human peripheral blood mononuclear cells (10(6) cells/ml) were stimulated with 0.25% phytohemagglutinin (PHA) and the proliferative response was determined from the uptake of tritiated thymidine. Scoparone (10(-6) to 3 x 10(-4) M) reduced the proliferative response in a dose-dependent manner. The proliferative response of mononuclear cells to mixed lymphocyte reaction was also reduced by scoparone (10(-5) to 10(-4) M). Interleukin-1, interleukin-2 production and interleukin-2 receptor expression were all reduced in the presence of scoparone. Scoparone (10 and 30 microM) significantly reduced the suppression elicited by the diabetogenic drug, alloxan (10 mM). The suppressive activity of scoparone was significantly reduced by quinacrine (a phospholipase A2 inhibitor), indomethacin (a cyclooxygenase inhibitor) and nordihydroguaiaretic acid (a lipoxygenase inhibitor). The levels of prostaglandin E2, prostaglandin F2 alpha, leukotriene B4 and 2,3-dinor-thromboxane B2 in culture medium of PHA-stimulated mononuclear cells, measured with an enzyme immunoassay, were elevated by scoparone treatment. We compared the effect of scoparone on the mononuclear cell response to genistein, a specific inhibitor of protein tyrosine kinase and demonstrated the non-additivity and cross-desensitization of the two compounds. Our results suggest that the immunosuppressive effect of scoparone may be exerted in part through inhibition of protein tyrosine kinase and release of arachidonic acid metabolites.