Making Sense of Cross-Talk between Steroid Hormone Receptors and Intracellular Signaling Pathways: Who Will Have the Last Word?

In classical models of nuclear steroid hormone receptor function, ligand binds receptor, heat shock proteins dissociate, and receptor dimers enter or are withheld in the nucleus and interact with coregulatory molecules to mediate changes in gene expression. The footnotes, "receptors become phosphorylated" and "dynamic nucleo-cytoplasmic shuttling occurs" describe well-accepted, but less well-understood aspects of receptor action. Recently, the idea that several protein kinases are activated in response to steroid hormone binding to cognate cytoplasmic or membrane-associated receptors has become fashionable. However, the precise role of steroid hormone receptor phosphorylation and our understanding of which cytoplasmic kinases are activated and their functional significance remain elusive. This review provides an overview of the primary ways in which steroid hormone receptor and growth factor cross-talk occurs, using the human progesterone receptor (PR) as a model. The functional consequences of PR phosphorylation by protein kinases classically activated in response to peptide growth factors and novel extranuclear or nongenomic functions of PR as potential independent initiators of signal transduction pathways are discussed. Intracellular protein kinases are emerging as key mediators of steroid hormone receptor action. Cross-talk between steroid receptor- and growth factor-initiated signaling events may explain how gene subsets are coordinately regulated by mitogenic stimuli in hormonally responsive normal tissues, and is suspected to play a role in their cancer biology.