Intracrine regulation at the nucleus–a further mechanism of growth factor activity?

The last 10 years have seen polypeptide growth factors emerge as a group of trophic regulatory substances distinct from hormones, albeit with a number of shared characteristics. Like hormones growth factors regulate a number of cellular functions, including proliferation, by receptor-mediated mechanisms. Unlike hormones growth factors are not, as a rule, produced by specialized cells in discrete glands. Rather, they are produced diffusely from a wide variety of cell and tissue types. Of particular importance to their classification is an appreciation of the method of transport of a growth factor to its target. Endocrine substances are elaborated in specific cell types and are transported to distantly removed target cells by means of the bloodstream. At their target site they interact with specific, high-affinity receptors. Paracrine factors differ only in that the released trophic factor travels to its target cell by diffusion. Target cells bearing receptors are situated in close proximity