Effects of protein kinase C activators on phorbol ester-sensitive and - resistant EL4 thymoma cells

Phorbol ester-sensitive EL4 murine thymoma cells respond to phorbol 12-myristate 13-acetate with activation of ERK mitogen-activated proteinkinases, synthesis of interleukin-2, and death, whereas phorbol ester-resistant variants of this cell line do not exhibit these responses.Additional aspects of the resistant phenotype were examined, using anewly-established resistant cell line. Phorbol ester induced morphologicalchanges, ERK activation, calcium-dependent activation of the c-JunN-terminal kinase (JNK), interleukin-2 synthesis, and growth inhibition insensitive but not resistant cells. A series of protein kinase C activatorscaused membrane translocation of protein kinase C's (PKCs) alpha, eta, andtheta in both cell lines. While PKC eta was expressed at higher levels insensitive than in resistant cells, overexpression of PKC eta did notrestore phorbol ester-induced ERK activation to resistant cells. Insensitive cells, PKC activators had similar effects on cell viability andERK activation, but differed in their abilities to induce JNK activationand interleukin-2 synthesis. PD 098059, an inhibitor of the mitogenactivated protein (MAP)/ERK kinase kinase MEK, partially inhibited ERKactivation and completely blocked phorbol ester-induced cell death insensitive cells. Thus MEK and/or ERK activation, but not JNK activation orinterleukin-2 synthesis, appears to be required for phorbol ester-inducedtoxicity. Alterations in phorbol ester response pathways, rather thanaltered expression of PKC isoforms, appear to confer phorbol esterresistance to EL4 cells.