Signaling pathways can be linear, but more complex patterns are common. Growth factors and many other extracellular signals cannot directly enter cells and transduce their information via membrane-bound receptors. In contrast, steroid receptors are members of the nuclear receptor superfamily and await their cognate hormones inside the cells. These two types of signaling pathways are extensively intertwined and cross-talk at many different levels. A wide range of extra-and intracellular signals, including a variety of growth factors, can activate the transcriptional activity of steroid receptors in the absence of their cognate hormones. Conversely, steroid receptors lead a double life. By coupling to signaling molecules that mediate signal transduction of extracellular factors, they can elicit very rapid nongenomic responses. The signaling pathways of steroid-independent activation of steroid receptors, on the one hand, and of nongenomic signaling by steroid receptors, on the other, display a remarkable reciprocal relationship suggesting that these two modes of signaling cross-talk may be two faces of the same coin.
DEFINITION AND OVERVIEWExtracellular signals modify intracellular processes through cognate receptors that elicit a cascade of events. However, a linear view of signal transduction falls short of describing all effects. Instead, branching, feedback, integration, and networking are characteristics of most if not all signal transduction pathways. Signaling cross-talk refers to a situation where one signal affects the output of another, seemingly distinct, signal transduction pathway.There is signaling cross-talk at all levels of signaling, from affecting availability of the signal to modifying the regulation of expression of target genes. For example, at the level of an organism, steroid hormone concentrations are influenced by signals that affect the levels of their serum binding proteins. Signals can stimulate the biosynthesis of other signals and their receptors. In breast cancer cells, estrogen induces the expression of the progesterone receptor (PR) [1], the EGF receptor [2], and several members of the EGF ligand family [3,4]. With respect to a target cell, signaling cross-talk may take place in the extracellular space or within the cell, inside or outside the nucleus.In this review, I will focus on the molecular mechanisms by which a variety of extra-and intracellular signals can modulate the activities of steroid receptors, in particular the receptors for the sex steroids, that is, the estrogen receptors (ERs) α and β, PR, and the androgen receptor (AR). These are all members of the nuclear receptor superfamily. Moreover, I will discuss how the very same nuclear receptors appear to elicit very rapid nongenomic effects by feeding into other signaling pathways. There is mounting evidence that these two nuclear receptor activities may be two faces of the same coin