CELLULAR SIGNALING IN VERTEBRATE PIGMENT-CELLS

Chromatophores are specialized integumental stellate cells that synthe size and store pigments. Pigment granules are translocated within chro matophores of poikilothermic vertebrates and crustaceans in response t o photic, thermal and/or neurohormonal stimuli, allowing the animal to rapidly change color for thermoregulation, adaptation to light and ba ckground, and social behavior display, Birds and mammals do not show c olor changes, but may present slow long-term responses, such as melano cyte proliferation, melanin synthesis and melanin granule translocatio n into feathers, hair and surrounding keratinocytes. Pigment transloca tion in lower vertebrates as well as pigment production in all vertebr ates are modulated by a variety of hormones and neurotransmitters acti ng on transmembrane receptors located on the cell surface. Alpha-melan ocyte-stimulating hormone (alpha-MSH), melanin-concentrating hormone ( MCH), melatonin and catecholamines are the most important pigment cell agonists in vertebrates. The major signalling pathway leading to pigm ent dispersion and melanin synthesis appears to involve stimulation of adenylate cyclase followed by an increase in the cAMP level and activ ation of cAMP-dependent protein kinases (PKAs). Another melanogenesis- related intracellular pathway involves the activation of protein kinas e C (PKC) by diacylglycerol, and the increase in cytosolic Ca2+ by ino sitol triphosphate. Growth factors such as basic fibroblast growth fac tor (bFGF), hepatocyte growth factor (HGF) and mast cell growth factor (MGF or KIT ligand), and UV radiation modulate the melanogenic and mi togenic processes in vertebrate melanocytes as well.