Synergistic activation of tyrosine phosphorylation by o-vanadate plus calcium ionophore A23187 or aromatic 1,2-diols.

We have shown previously that treatment of WB rat liver epithelial cells with the Ca2+ ionophore A23187 provokes a rapid increase in protein-tyrosine phosphorylation that faithfully reproduces the Ca(2+)-dependent response seen with angiotensin II. In the presence of the tyrosine phosphatase inhibitor o-vanadate (2.0-200 microM), the tyrosine phosphorylation response to A23187 was increased > 10-fold in magnitude. This synergistic effect of A23187 and vanadate is clearly distinct from the combined effect of angiotensin II and vanadate, which was merely additive. Chelation of either extracellular or intracellular Ca2+ abolished the synergistic response to ionophore and vanadate, indicating its Ca2+ dependence. That divergent pathways were involved in the angiotensin II and the A23187/vanadate responses was shown definitely by studies of GN4 cells, a transformed line derived from WB cells by carcinogen treatment. GN4 cells are 2-3-fold more responsive than WB cells to angiotensin II-dependent tyrosine kinase activation, yet they completely lacked the synergistic tyrosine phosphorylation response to A23187/vanadate. To test the role of arachidonic acid metabolites in the A23187/vanadate response, cells were pretreated with either indomethacin or nordihydroguaiaretic acid (NDGA). Neither compound was inhibitory, but surprisingly, NDGA plus vanadate closely mimicked the A23187/vanadate response in WB cells and, like A23187/vanadate, was ineffective in GN4 cells. NDGA contains catechol nuclei (i.e., aromatic 1,2-diols) and therein resembles the flavonoid anti-oxidant quercetin, another compound found to increase tyrosine phosphorylation synergistically with vanadate.(ABSTRACT TRUNCATED AT 250 WORDS)