Characterization and functional studies of adiponectin and leptin receptor in grass carp

Abstract

Adiponectin (AdipoQ) and leptin are adipokines secreted from white adipose tissue and with important functions in body metabolism and energy homeostasis. Their biological actions are mediated by their respective receptors, namely AdipoQ receptor (AdipoR) and leptin receptor (ObR), functionally coupled to overlapping and yet distinct post-receptor signaling mechanisms. To date, the studies for AdipoQ/ AdipoR and leptin/ObR systems are largely restricted to mammalian models and not much is known in lower vertebrates, especially in fish species. Using grass carp as a model for bony fish, the structural identities of grass carp AdipoQ, two forms of AdipoR, namely AdipoR1a and AdipoR1b, as well as two isoforms of ObR, including the full-length ObR-L and a truncated form ObR-S with loss of most of the intracellular domain, were established by 5’/3’RACE. These newly cloned cDNAs were confirmed to be single-copy genes by genomic Southern and their tissue expression profiles were elucidated by RT-PCR. Interestingly, notable signals of AdipoQ and their receptors (AdipoR1a & 1b) could be detected in the carp pituitary suggesting that the AdipoQ/AdipoR system may have autocrine/paracrine function at the pituitary level. To study the pituitary actions of AdipoQ, recombinant carp AdipoQ was produced in Escherichia. coli and its bioactivity in regulating food intake and feeding behaviors was confirmed in goldfish. In carp pituitary cells culture, carp AdipoQ was found to induce basal secretion, protein synthesis and transcript expression of growth hormone, luteinizing hormone, prolactin and somatolactin with parallel rises of follicle-stimulating hormone and pro-opiomelanocortin but not thyroid-stimulating hormone mRNA levels. Using a pharmacological approach, these stimulatory actions of AdipoQ were shown to be mediated via differential activation of AMPK/PPAR, MAPK, PI3K/Akt, PLC/PKC, and cAMP/PKA and Ca2 +/CaMK-II pathways.

In parallel studies, functional expression of carp AdipoR (AdipoR1a &1b) and ObR-L in mammalian cell lines also confirmed that the two receptors could bind with their respective ligands and are functionally coupled to AMPK/PPAR and JAK/STAT cascades, respectively. Since AdipoR and ObR expression could be mapped to the carp liver, which is a major target for pancreatic hormones, AdipoR and ObR regulation by insulin and glucagon at the hepatic level were also examined. In carp hepatocyte culture, AdipoR1a, AdipoR1b, ObR-L and ObR-S mRNA levels could be elevated by insulin treatment. Except for the involvement of only the JAK2/STAT5 pathway for AdipoR1b, the stimulatory effects of insulin on AdipoR1a, ObR-L and ObR-S gene expression were mediated via MAPK, PI3K/Akt and JAK2/ STATs pathways. In contrast, both basal as well as insulin-induced transcript expression of all the four receptors examined could be inhibited by glucagon treatment via activation of the cAMP/PKA cascade. In the carp pituitary, leptin (leptin-A and leptin-B) and its receptors, namely ObR-L and ObR-S, could be located in different pituitary cell types. In carp pituitary cells, AdipoQ was found to increase leptin-A, leptin-B, ObR-L and ObR-S mRNA levels via differential activation of AMPK/PPAR, MAPK, PI3K/Akt, cAMP /PKA and PLC/PKC pathways. Leptin treatment, interestingly, could also exert a reciprocal stimulation on AdipoQ, AdipoR1a and AdipoR1b gene expression at the pituitary cell level via MAPK, PI3K/Akt and JAK2/STAT pathways. These findings, as a whole, provide new information in a fish model on (i) AdipoQ regulation of pituitary hormone expression, (ii) modulation of AdipoR and ObR expression at the hepatic level by pancreatic hormones, and (iii) functional interaction of AdipoQ/AdipoR and leptin/ObR systems at the pituitary level.