Differential estrogenic regulation of small M(r) heat shock protein expression in osteoblasts

Polymerase chain reaction amplification and sequencing of heat shock protein (HSP) 27-related transcripts present in cDNAs generated from heat-shocked osteoblast RNA revealed the expression of three related open reading frames that encode proteins of 208, 197, or 175 amino acids. Their expression as recombinant proteins in Escherichia coli demonstrated that they encode 27,000, 25,000, or 22,000 M(r) proteins. Northern blot analysis of heat shock protein 27-related transcript expression revealed that, while HSP 27 expression (850-nucleotide transcript) was induced by heat shock alone, the expression of a smaller transcript was facilitated by estrogen treatment prior to heat shock. This corresponded with the co-induction of 18,000 and 22,000 M(r) proteins by estrogen-pretreated, heat-shocked osteoblasts as revealed by fluorography. The identification of these multiple small M(r) heat shock-induced proteins demonstrated that the stress response of mammalian cells is composed of multiple, related proteins whose expression is differentially regulated. The importance of estrogen-regulation of the small M(r) heat shock protein component of the stress response may be of particular significance to osteoblast physiology.