Growth arrest of human B lymphocytes is accompanied by induction of the low molecular weight mammalian heat shock protein (Hsp28)

A large number of protein and molecular markers have been identified that delineate the early stages of human B cell activation and proliferation. In contrast, few if any molecules are transiently expressed precisely as activated B cells stop proliferating and undergo growth arrest. We demonstrate that the low molecular weight heat shock protein (hsp28) exhibits unique induction kinetics that specifically demarcates this interval. After mitogenic activation of unstimulated splenic B cells, hsp28 protein and phosphorylation transiently increase coinciding precisely with the peak of cellular proliferation and the onset of growth arrest. Although most neoplastic B cells constitutively express hsp28, three cell lines were identified that were hsp28^-. No differences in phenotype or growth kinetics were detected between hsp28⁺ and hsp28^- neoplastic B cells demonstrating that hsp28 expression is not essential for cell growth. However, when treated with phorbol ester or heat shock, these hsp28^- cell lines synthesize hsp28 followed by the onset growth arrest. The consistency with which hsp28 induction transiently delineates the interval from peak proliferation to the onset of growth arrest suggests hsp28 itself is likely to be involved in regulating this process.