EFFECT OF HIV TYPE-1 TAT PROTEIN ON BUTYRIC ACID-INDUCED DIFFERENTIATION IN A HEMATOPOIETIC PROGENITOR-CELL LINE

The trans-activator protein (Tat) of HIV-1 plays an important role in viral pathogenesis. Since Tat has been shown to alter expression of a number of host cellular genes, we have investigated the role of Tat in modulating gene expression and differentiation in hematopoietic proge nitor cells. Tat protein was introduced in K562 cells, a human hematop oietic progenitor cell line, by either scrape-loading onto HeLa (HL)-t at cells or direct electroporation of an affinity-purified glutathione S-transferase (GST)-Tat fusion protein. Under these conditions, butyr ic acid-induced hemoglobin production in K562 cells was suppressed by 65 and 52%, respectively. However, coculturing with wild-type HeLa cel ls or electroporation with the control GST protein did not decrease he moglobin production. To confirm the presence of bioactive Tat protein within K562 cells, the cells were transiently transfected with a pHIV/ LTR-CAT prior to the introduction of Tat. A 30- to 40-fold induction i n CAT gene expression was observed in the transfected K562 cells, whic h were either cocultured with HL-tat or were electroporated with GST-T at. Simultaneous transient transfection of K562 cells with a TAR expre ssion plasmid, to compete for the availability of Tat protein, signifi cantly downregulated the HIV LTR trans-activation by Tat. In addition, overexpression of the TAR RNAs in K562 cells was able to downregulate the suppressive effect of Tat on butyric acid-induced differentiation . RT-PCR analysis of the total RNAs isolated from these cells demonstr ated that Tat protein suppressed the butyric acid-induced gamma-globin gene expression by an average of 54% without affecting the level of g lyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNAs. These data indi cate that the viral Tat protein plays a significant role in abrogating erythroid differentiation in K562 cells.