7,12-DIMETHYLBENZ[A]ANTHRACENE ACTIVATES PROTEIN-TYROSINE KINASES FYNAND LCK IN THE HPB-ALL HUMAN T-CELL LINE AND INCREASES TYROSINE PHOSPHORYLATION OF PHOSPHOLIPASE C-GAMMA-1, FORMATION OF INOSITOL 1,4,5-TRISPHOSPHATE, AND MOBILIZATION OF INTRACELLULAR CALCIUM

Previous studies have shown that the immunosuppressive and carcinogeni c polycyclic aromatic hydrocarbon 7,12-dimethylbenz(a)anthracene (DMBA ) impairs Ca2+-dependent transmembrane signaling in human and murine l ymphocytes. The purpose of the present studies was to analyze potentia l mechanisms of immunosuppression by DMBA and to examine effects on Ca 2+ homeostasis and antigen-receptor signaling in human T cells. DMBA p roduced a rapid and sustained increase in Ca2+ levels in HPB-ALL cells by release of cytoplasmic Ca2+. DMBA also inhibited anti-CD3/CD4 mobi lization of Ca2+ in HPB-ALL cells, with half-maximal inhibition occurr ing at almost-equal-to 4 hr. Thus, the kinetics for initial Ca2+ mobil ization and inhibition of the anti-CD3/CD4 response differed. The rapi d rise in intracellular Ca2+ induced by DMBA alone was accompanied by a rapid but transient increase in inositol 1,4,5-trisphosphate and tyr osine phosphorylation of phospholipase C-gamma1. The pattern of tyrosi ne phosphorylation induced by DMBA in HPB-ALL cells was remarkably sim ilar to that induced by anti-CD3/CD4 activation. Thus, DMBA-induced ph osphorylation may mimic antigen-receptor activation in T cells, which may lead to alterations in antigen responsiveness. The mechanism of DM BA-induced tyrosine phosphorylation of phospholipase C-gamma1 may have been due to an increase in protein-tyrosine kinase activity, since it was found that DMBA produced a >2-fold increase in the activity of th e T-cell receptor-associated Src-family kinases Fyn and Lck. The kinet ics of activation of protein-tyrosine kinases demonstrated that Fyn ac tivity was increased within 10 min of exposure to DMBA, whereas maxima l Lck activation required 30 min. Thus, it is likely that the Fyn kina se or other protein-tyrosine kinases may be responsible for the early tyrosine phosphorylation of phospholipase C-gamma1, which results in i nositol 1,4,5 -trisphosphate release and mobilization of intracellular Ca2+.