Signal transduction for leptin-induced somatolactin gene expression in grass carp pituitary cells

Abstract

Leptin is a potent anorexic factor in mammals and known to be widely expressed at the tissue level. Although its function in feeding control is well conserved during vertebrate evolution, its protein sequence is highly variable from fish to mammals. Unlike leptin, somatolactin (SL) is a pituitary hormone unique to fish species. It is a member of the GH/PRL gene family and plays a functional role in background adaptation, lipid metabolism, acid-base balance, reproduction and immune responses. Although leptin regulation of pituitary hormones has been reported, little is known regarding its pituitary actions on SL expression. Recently, two SL isoforms, namely SL alpha and SL beta, have been cloned in grass carp and their expression have been confirmed to be under the control of central (e.g., PACAP), peripheral (e.g., IGF-I) and pituitary signals (e.g., SL itself). To shed light on the functional interactions between leptin and pituitary SL in fish model, the structural identity of leptin expressed in grass carp, namely leptin A and leptin B, have been established by molecular cloning. The two forms of leptin share a low level of a.a. sequence homology (only 23.9%) but their 3D protein structures with four helices arranged in the typical “up-up-down-down” topography are highly comparable to that of mammalian leptin. In grass carp, leptin A and B are single-copy genes, ubiquitously expressed in various tissues, and have high levels of transcript expression in the liver with leptin A as the dominant form. Recombinant proteins of grass carp leptin A and B were produced and found to inhibit both basal and NPY-induced food consumption and feeding behavior in goldfish. In primary culture of grass carp pituitary cells, leptin A and B treatment could markedly increase SL alpha mRNA levels but with a minor stimulatory effect on SL beta transcript expression. Using a pharmacological approach, SL alpha mRNA expression induced by the two leptin isoforms were shown to be mediated by JAK2/STAT5, MAPK and PI3k/Akt cascades. Similar cascades, except for the PI3K/Akt pathway, were also involved in the signal transduction for SL beta gene expression induced by leptin A and B, respectively. These findings, as a whole, provide evidence that leptin can exert direct effects at the pituitary cell level to up-regulate SL alpha and SL beta gene expression in carp species via overlapping and yet distinct post-receptor signaling mechanisms. (Project supported by GRF Grant, Research Grant Council, Hong Kong.)