Elevation of heat shock protein synthesis and hsp gene transcription during monocyte to macrophage differentiation of U937 cells.

During the phorbol myristate acetate (PMA)-induced differentiation of U937 cells to a macrophage-like phenotype, the levels of the heat shock proteins hsp90, hsp72 and hsp65 increased dramatically to a peak level following 24 h of treatment, and then declined. In contrast, no significant increase was observed in the level of the constitutive hsp73 protein in this process. The observed increases in hsp levels were preceded by an increase in the transcription of each of the genes encoding these hsps, including both of the two genes which encode hsp90. The mechanism of this effect and the possible role of the hsps in the function of differentiated macrophages and in the differentiation process are discussed.