The major heat-shock protein hsp 68 is not induced by stress in mouse erythroleukemia cell lines

Abstract

Most mammalian cells respond to brief incubation at elevated temperatures by enhanced or new synthesis of a set of heat-shock proteins (hsp). In mouse cells, as determined by SDS – one-dimensional gel electrophoresis, the most prominent hsps have molecular masses of approximately 89 000, 70 000, and 68 000 Da. When the heat-shock response of the mouse erythroleukemia cell line DIB, or two other DBA/2 cell lines (707C1 and 745C2), was examined by [³⁵S]methionine labelling, following heat shocks of 10 min at 42 or 44 °C, or 1 h at 45 °C, no protein band corresponding to hsp 68 was observed. However, the synthesis of both hsp 89 and hsp 70 was enhanced. Northern blot analysis of cytoplasmic RNA extracted from control and stressed cells indicated that hsp 68 mRNA was absent, even after stresses of up to 1 h at 45 °C. Differentiation induced by dimethyl sulphoxide (DMSO) (monitored by the induction of globin synthesis) had no effect on hsp 68 expression in D1B cells; also, hsp 68 could not be induced at various stages of differentiation (0–72 h). Southern blot analysis showed that all three hsp-68 genes were present and not rearranged, and apparently did not carry any deletion in their 5′ ends. To determine whether methylation could be involved in maintaining the genes in their silent state, we treated cells with 10 μM 5-azacytidine for 48 h. No hsp 68 expression was observed following such treatment in either undifferentiated or DMSO-induced differentiated D1B cells. Furthermore, Southern blot analysis of MspI/HpaII-digested genomic D1B DNA did not display any differences in methylation patterns around the promoter region of the probed gene compared with control cells, indicating that methylation is not involved in hsp-68 repression. When chimeric plasmids carrying the bacterial chloramphenicol acetyl transferase gene under regulation of the mouse hsp-68 or Drosophila hsp-70 promoters were transfected into D1B cells, minimal (2-fold) or no induction was observed, in contrast with the 60-fold induction seen in a control myeloma cell line. These results suggest a trans-acting mechanism of hsp-68 repression in erythroleukemia cells.