7,12-DIMETHYLBENZ[A]ANTHRACENE ACTIVATES PROTEIN-TYROSINE KINASES FYN AND 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 carcinogenic polycyclic aromatic hydrocarbon 7,12-dimethylbenz(a)anthracene (DMBA) impairs Ca2+-dependent transmembrane signaling in human and murine lymphocytes. The purpose of the present studies was to analyze potential mechanisms of immunosuppression by DMBA and to examine effects on Ca2+ homeostasis and antigen-receptor signaling in human T cells. DMBA produced a rapid and sustained increase in Ca2+ levels in HPB-ALL cells by release of cytoplasmic Ca2+. DMBA also inhibited anti-CD3/CD4 mobilization of Ca2+ in HPB-ALL cells, with half-maximal inhibition occurring at almost-equal-to 4 hr. Thus, the kinetics for initial Ca2+ mobilization and inhibition of the anti-CD3/CD4 response differed. The rapid rise in intracellular Ca2+ induced by DMBA alone was accompanied by a rapid but transient increase in inositol 1,4,5-trisphosphate and tyrosine phosphorylation of phospholipase C-gamma1. The pattern of tyrosine phosphorylation induced by DMBA in HPB-ALL cells was remarkably similar to that induced by anti-CD3/CD4 activation. Thus, DMBA-induced phosphorylation may mimic antigen-receptor activation in T cells, which may lead to alterations in antigen responsiveness. The mechanism of DMBA-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 the T-cell receptor-associated Src-family kinases Fyn and Lck. The kinetics of activation of protein-tyrosine kinases demonstrated that Fyn activity was increased within 10 min of exposure to DMBA, whereas maximal Lck activation required 30 min. Thus, it is likely that the Fyn kinase or other protein-tyrosine kinases may be responsible for the early tyrosine phosphorylation of phospholipase C-gamma1, which results in inositol 1,4,5 -trisphosphate release and mobilization of intracellular Ca2+.