Jiang Q, Ko WK, Wong AO. Insulin-like growth factor as a novel stimulator for somatolactin secretion and synthesis in carp pituitary cells via activation of MAPK cascades. Am J Physiol Endocrinol Metab 301: E1208 -E1219, 2011. First published August 23, 2011; doi:10.1152/ajpendo.00347.2011, a member of the growth hormone/prolactin family, is a pituitary hormone unique to fish models. Although SL is known to have diverse functions in fish, the mechanisms regulating its secretion and synthesis have not been fully characterized. Using grass carp pituitary cells as a model, here we examined the role of insulin-like growth factor (IGF) in SL regulation at the pituitary level. As a first step, the antisera for the two SL isoforms expressed in the carp pituitary, SL␣ and SL, were produced, and their specificity was confirmed by antiserum preabsorption and immunohistochemical staining in the carp pituitary. Western blot using these antisera revealed that grass carp SL␣ and SL could be N-linked glycosylated and their basal secretion and cell content in carp pituitary cells could be elevated by IGF-I and -II treatment. These stimulatory effects occurred with parallel rises in SL␣ and SL mRNA levels, and these SL gene expression responses were not mimicked by insulin but blocked by IGF-I receptor inactivation. In carp pituitary cells, IGF-I and -II could induce rapid phosphorylation of IGF-I receptor, MEK1/2, ERK1/2, MKK3/6, and p38 MAPK; and SL␣ and SL secretion, protein production, and mRNA expression caused by IGF-I and -II stimulation were negated by inactivating MEK1/2 and p38 MAPK. Parallel inhibition of PI3K and Akt, however, were not effective in these regards. These results, taken together, provide evidence that IGF can upregulate SL secretion and synthesis at the pituitary level via stimulation of MAPK-but not PI3K/Akt-dependent pathways.insulin-like growth factor I receptor; mitogen-activated protein kinase; signal transduction; grass carp SOMATOLACTIN (SL), a member of the growth hormone (GH)/ prolactin (PRL) family, is a pituitary hormone unique to fish models (23). Since SL gene could not be identified in the genome databases of tetrapods, including Xenopus, mouse, and human, it is commonly accepted that the gene had been lost in tetrapods during the land invasion of amphibian ancestors (15). To date, two SL isoforms, SL␣ and SL, have been reported in bony fish, including zebrafish (53) and grass carp (Ctenopharyngodon idellus) (20). Unlike GH and PRL with expression restricted to pituitary cells in the pars distalis, SL is expressed in periodic acid-Schiff (PAS)-positive cells located in the neurointermediate lobe (NIL) of the fish pituitary (34). PAS staining is commonly used in polysaccharide and glycoprotein detection in biological samples. The sensitivity of SL cells to PAS staining and the presence of N-linked glycosylation sites on SL itself [e.g., in Atlantic cod (39) and carp SL (20)] suggest that SL is a glycoprotein. This idea is supported by the finding that a glycosylated form of SL␣ sensit...