Molecular mechanisms of cross-talk between growth factors and nuclear receptor signaling

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.