LINEAGE COMMITMENT OF TRANSFORMED HEMATOPOIETIC PROGENITORS IS DETERMINED BY THE LEVEL OF PKC ACTIVITY

Our previous work showed that haematopoietic precursors transformed by the E26 avian leukaemia virus undergo multilineage differentiation in response to the phorbol ester phorbol 12-myristate 13-acetate (PMA). Treatment of the cells with high concentrations of PMA (100 nM) favour s myelomonocytic differentiation, while lower concentrations (20 nM) i nduce predominantly eosinophil differentiation. Here we have investiga ted the role of protein kinase C (PKC) in this process and found that 100 nM, but not 20 nM, PMA dramatically down-regulates total cellular PKC activity, indicating that high PMA concentrations result in less e fficient signalling than lower PMA concentrations. Consistent with the se findings is the observation that very low PMA concentrations (1 nM) , which presumably only moderately activate PKC, induce myeloid differ entiation. This suggests the existence of two PKC thresholds which pla y a role in lineage commitment. To test the model, alpha- and epsilon- PKC isoforms were expressed in E26-transformed progenitors. These cell s exhibited myelomonocytic differentiation even in the absence of PMA, while treatment with concentrations of PMA as high as 100 nM led to t he differentiation of predominantly eosinophils and failed to down-reg ulate the exogenous PKC. Our results suggest that different levels of PKC activity result in three different phenotypes: (i) no PKC activity maintains the progenitor phenotype; (ii) low PKC activity favours mye lomonocytic differentiation; (iii) high PKC activity favours eosinophi l differentiation.