Small-Molecule Hormones: Molecular Mechanisms of Action

Figure 3

Diagram of type I receptor genomic mode of action. In the circulation, the majority of the hormones form complexes with transporting proteins. The hormone enters the cell by diffusion or is actively transported by specific cell membrane proteins. In the cytoplasm, hormone-free receptors form an inactive complex with heat shock chaperone proteins. Upon hormone binding, the receptor changes its conformation, dissociates from the complex, and translocates into the nucleus. Hormone-activated receptors bind to HREs as homodimers. Recruitment of a coactivator complex possessing a histone acetyltransferase activity results in local chromatin decondensation and increases the accessibility of the promoter for transcription factor. As a result, transcription increases. SH: steroid hormone, HR: nuclear hormone receptor, HSP: heat shock protein, HRE: hormone response element, CoA: coactivator complex, HAT: histone acetyltransferase, BTF: basal transcription factors, RNA Pol II: type II RNA polymerase, and R: ribosome.