Chapter II The activation of trans-acting factors in response to hypo- and hyper-osmotic stress in mammalian cells

Abstract

This chapter discusses the two trans-acting factors in the context of the effect of osmotic stress on gene expression—tonicity-responsive enhancer element binding protein (TonEBP) and heat shock transcription factor (HSF) . The osmotic stress-induced HSF activation is unique in various ways: (1) it is induced within minutes after osmotic stress, (2) it is inducible by both hypo- and hyper-osmolarity, and (3) it is uncoupled from transcriptional activation. Except for TonEBP, which has yet to be fully identified, HSF is the only known DNA binding protein that is responsive to osmotic stress. Unlike TonEBP, which responds only to hyper-osmotic stress, HSF activation is sensitive to both hypo and hyper-osmotic stress. In view of its sensitivity to both temperature and osmolarity changes, HSF can be considered not only as a molecular thermostat, but also as a molecular osmometer. HSF activation (trimerization and DNA binding) occurs rapidly within minutes under both heat stress and osmotic stress. In contrast, TonEBP activation—as measured by Tone binding activity—occurs more slowly, usually 6–10 h after hyperosmotic stress.