Induction of EL4 cell resistance to syngeneic macrophage-mediated lysis by protein kinase C ligands; effects of cultured TPA-treated target cell and protein phosphorylation.

Pretreatment of EL4 cells with 12-O-tetradecanoyl phorbol-13-acetate (TPA) for 30 min renders them resistant to lysis by activated macrophages (M phi). This resistance was augmented two to three-fold when TPA-treated EL4 cells were incubated for 2-6 hr prior to co-culture with M phi. Preincubation of TPA-treated cells for 24 hr could result in 100% resistance. in this paper we show that an endogenous ligand for protein kinase C, oleoyl-2-acetate glycerol (OAG), was capable of inducing tumour cell resistance to M phi kill and, similar to the effects seen with TPA, OAG did not affect the selective binding of tumour cells to activated M phi. Another important observation on the mechanism of TPA induction of tumour cell resistance was that once the target cells were programmed to die after a minimal contact with activated M phi of 4-6 hr, TPA treatment was ineffective in altering the percent lysis 20 hr later. To investigate whether any possible correlation exists between TPA-induced protein phosphorylation and acquisition of resistance, EL4 cells were labelled with 32P and treated simultaneously with TPA, and cellular proteins were resolved by two-dimensional gel electrophoresis. Eight polypeptides (MW 24,000-70,000, pI 4.8-6.1) showed consistent increased phosphorylation as a result of TPA treatment. One-minute exposure with TPA resulted in enhanced phosphorylation of only four peptides (MW 39,000, 58,000, 63,000, 70,000) while all eight polypeptides showed increased phosphorylation by 10 min.