INHIBITORY EFFECTS OF ZIDOVUDINE IN ERYTHROID PROGENITOR CELLS - REVERSAL WITH A COMBINATION OF ERYTHROPOIETIN AND INTERLEUKIN-3

To investigate the mechanisms that may be involved in zidovudine (AZT)-induced hematopoietic toxicity, spleen cells isolated from phenylhydrazine-treated anemic mice or murine bone marrow erythroid progenitor cells were treated with AZT (1-10 mu M) for 24 hr. A concentration-dependent inhibition of the binding of I-125-labeled erythropoietin (Epo) was observed, suggesting downregulation of Epo receptors. To determine if this effect is due to modulation of the levels of Epo receptor mRNA and to assess the effect of AZT on the expression of protooncogenes, mRNA levels were monitored by the slot blot hybridization technique. AZT caused a concentration-dependent inhibition in the levels of the mRNA of Epo receptors and c-fos, whereas the level of c-myc mRNA was unaffected. AZT also inhibited protein kinase C (PKC) activity in a concentration- and time-dependent manner, causing 50% inhibition at 10 mu M within 3 hr. Simultaneous addition of Epo or interleukin-3 (IL-3) partially reversed the inhibitory effects of AZT on the levels of the mRNAs and on PKC activity; however, a combination of Epo and IL-3 was significantly more effective. These studies demonstrate that (i) AZT-induced down-regulation of Epo receptors and c-fos expression coupled with inhibition of Epo receptor-mediated signal transduction through PKC are significant contributory factors to AZT-induced erythroid toxicity, and (ii) these inhibitory effects can be overcome by treatment with a combination of Epo and IL-3.