Signal transduction mechanisms for autocrine/paracrine regulation of somatolactin-α secretion and synthesis in carp pituitary cells by somatolactin-α and -β

Jiang Q, Wong AO. Signal transduction mechanisms for auto-crine/paracrine regulation of somatolactin-alpha secretion and synthesis in carp pituitary cells by somatolactin-alpha and -beta. Am J Physiol Endocrinol Metab 304: E176-E186, 2013. First published November 27, 2012; doi:10.1152/ajpendo.00455.2012.-Pituitary hormones can act locally via autocrine/paracrine mechanisms to modulate pituitary functions, which represents an interesting aspect of pituitary regulation other than the traditional hypothalamic input and feedback signals from the periphery. Somatolactin, a member of the growth hormone (GH)/prolactin (PL) family, is a pleiotropic hormone with diverse functions, but its pituitary actions are still unknown. Recently, two SL isoforms, SL alpha and SL beta, have been cloned in grass carp. Based on the sequences obtained, recombinant proteins of carp SL alpha and SL beta with similar bioactivity in inducing pigment aggregation in carp melanophores were produced. In carp pituitary cells, SL alpha secretion and cell content were elevated by static incubation with recombinant carp SL alpha and SL beta, respectively. These stimulatory actions occurred with a parallel rise in SL alpha mRNA level with no changes in SL beta secretion, cell content, and gene expression. In contrast, SL alpha mRNA expression could be reduced by removing endogenous SL alpha and SL beta with immunoneutralization. At the pituitary cell level, SL alpha release, cell content, and mRNA expression induced by carp SL alpha and SL beta could be blocked by inhibiting JAK2/STAT5, PI3K/Akt, MEK1/2, and p38 MAPK, respectively. Furthermore, SL alpha and SL beta induction also triggered rapid phosphorylation of STAT5, Akt, MEK1/2, ERK1/2, MKK3/6, and p38 MAPK. These results suggest that 1) SL alpha and SL beta produced locally in the carp pituitary can serve as novel autocrine/paracrine stimulators for SL alpha secretion and synthesis and 2) SL alpha production induced by local release of SL alpha and SL beta probably are mediated by the JAK2/STAT5, PI3K/Akt, and MAPK signaling pathways.