The expression of heat shock proteins (HSPs) is known to be increased via activation of heat shock factor 1 (HSF1), and excess expression of HSPs exerts feedback inhibition of HSF1. However, the molecular mechanism to modulate such relationships between HSPs and HSF1 is not clear. In the present study, we show that stable transfection of either Hsp25 or inducible Hsp70 (Hsp70i) increased expression of endogenous HSPs such as HSP25 and HSP70i through HSF1 activation. However, these phenomena were abolished when the dominant negative Hsf1 mutant was transfected to HSP25 or HSP70i overexpressed cells. Moreover, the increased HSF1 activity by either HSP25 or HSP70i was found to result from dephosphorylation of HSF1 on serine 307 that increased the stability of HSF1. Either HSP25 or HSP70i inhibited ERK1/2 phosphorylation because of increased MKP1 phosphorylation by direct interaction of these HSPs with MKP1. Treatment of HOS and NCI-H358 cells, which showed high expressions of endogenous HSF1, with small interfering RNA (siRNA) of either HSP27 (siHSP27) or HSP70i (siHSP70i) inhibited both HSP27 and HSP70i proteins; this was because of increased ERK1/2 phosphorylation and serine phosphorylation of HSF1. The results, therefore, suggested that when the HSF1 protein level was high in cancer cells, excess expression of HSP27 or HSP70i strongly facilitates the expression of HSP proteins through HSF1 activation, resulting in severe radio-or chemoresistance.The expressions of small HSPs, especially HSP25 (HSP27), 3 and the inducible HSP70 (HSP70i) have been shown to enhance the survival of mammalian cells exposed to various types of stimuli that induce stress and apoptosis (1, 2). Considering the key role of HSPs played in protection against stress-induced damage, it is of great importance to elucidate the regulatory mechanisms responsible for HSP expression.The inducible HSP expression is regulated by the heat shock transcription factors (HSFs). In response to various inducers such as elevated temperatures, oxidants, heavy metals, and bacterial and viral infections, most HSFs acquire DNA binding activity to the heat shock element (HSE), thereby mediating transcription of the heat shock protein genes, which results in accumulation of HSPs (3, 4). Although several genes encoding multiple HSF isoforms have been identified in vertebrates (5, 6), only HSF1 is shown to be essential for the transcriptional activation of HSPs in mammalian organisms (4, 7). Under normal conditions, cellular HSF1 exists in a transcriptionally repressed state (8, 9), however, it is activated by stress in a multistep process involving trimerization, acquisition of HSE binding activity, novel phosphorylation, and trans-activation of HSP genes (3, 10, 11).In recent years, a conceptual framework for stress-induced activation and feedback repression of HSF1 has emerged (3, 12, 13). However, our recent study indicated that, in RIF cells that have relatively low HSP25 and inducible HSP70 (HSP70i) expressions, HSP25 or HSP70i mutually coregulated each other ...

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