Lower Heat Shock Factor Activation and Binding and Faster Rate of HSP-70A Messenger RNA Turnover in Heat Sensitive Human Leukemias

Normal bone marrow progenitors and some leukemic cells develop only a limited amount of thermotolerance. Further, once developed, thermotolerance decays at a faster rate than that normally observed in cells of nonhemopoietic origin. Thermotolerance induction and maintenance correlates with reduced levels of expression of various Mr 70,000 heat shock proteins (HSP-70) mRNAs after heat shock. We have now compared the accumulation of HSP-70 proteins in heat-shocked human leukemic cells KG-1, HL-60, and K562 to that in Htl080, a colon carcinoma cell line. We have found reduced accumulation of HSP-70 proteins in all leukemic cells. The rate of decay of HSP-70A mRNA, measured following heat shock by using actinomycin D treatment to inhibit further RNA synthesis, was more rapid in KG-1 and HL-60 cells compared to Htl080 cells. The half-life of HSP-70A mRNA was 2 h in KG-1 and HL-60 cells while in Htl080 cells it was >7 h. HSP-70A mRNA is known to decay with a half-life of 2 h in unheated cells; this is increased to >7 h following heat shock. We therefore postulate that leukemic cells lack the mechanism to stabilize HSP-70A mRNA after heat shock. One postulated mechanism for HSP-70 mRNA decay rate is known to be due to the nucleotide sequences at the 3'-untranslated region. We examined the 3-untranslated region in leukemic cells. No sequence variations, however, were observed at either the genomic or the complementary DNA levels between leukemic or nonleukemic tumor cells. Heat shock factor activation and binding by gel retardation assays showed that KG-1 and HL-60 cells had a reduced heat shock factor binding to the heat shock element when compared to K562 and Htl080 cells. Furthermore, HSF-1 mRNA was found to be expressed at relatively lower levels in HL-60 cells when compared to HU080 or KG-1 cells. In conclusion, reduced HSP synthesis and accumulation of leukemic cells after heat shock correlates with the reduction in heat shock factor-heat shock element binding and a faster HSP-70A mRNA decay rate that is observed in these cells.