CONTRASTING EFFECTS OF 2 TUMOR PROMOTERS, PHORBOL-MYRISTATE ACETATE AND OKADAIC ACID, ON T-CELL RESPONSES AND ACTIVATION OF P42 MAP-KINASE ERK-2

The induction of T-cell growth by the T-cell antigen receptor (TcR) is dependent on a co-ordinated process of phosphorylation and dephosphor ylation of intracellular proteins. An intermediary in this signalling pathway is the serine kinase, p42 mitogen-activated protein kinase (p4 2MAPK), also known as microtubule-associated protein-2 kinase (MAP-2K) . MAP-kinase is activated upon the acquisition of tyrosine as well as threonine phosphate groups and removal of either by specific tyrosine or serine/threonine phosphatases abrogates kinase activity. Okadaic ac id (OA), a tumour promoter and potent inhibitor of type 1 and 2A serin e/threonine protein phosphatases (PPI and PP2A), induced MAP-kinase ac tivity in Jurkat T cells in a dose-dependent fashion with optimal effe ct at 1 mum. Compared to rapid activation (peak < 10 min) of MAP-kinas e by another tumour promoter, the phorbol ester, PMA, the effect of OA was delayed (> 30 min) and more sustained. In spite of activating a g rowth-promoting kinase, OA differed from PMA by its lack of mitogenic activity and failure to induce CD25 [interleukin-2Ralpha (IL-2Ralpha)] expression in normal human T cells. This implies that PPI and PP2A al so act downstream of MAP-kinase to facilitate later cell cycle events. PMA induced a 42,000 MW tyrosine phosphoprotein which co-electrophore sed and co-chromatographed with ERK-2, a p42 MAP-kinase. Although OA i nduced an identical Mono-Q peak, there was less avid tyrosine phosphor ylation of p42. OA also differed from PMA to the extent by which it in duced mobility shift of the tyrosine protein kinase, p56lck, which has been implicated in p42MAPK activation in T cells. Taken together, the se results indicate that OA and PMA exert both overlapping as well as divergent effects on lymphocyte growth pathways.