PfIRR Interacts with HrIGF-I and Activates the MAP-kinase and PI3-kinase Signaling Pathways to Regulate Glycogen Metabolism in Pinctada fucata.

Mao-xian He,Yu Shi

doi:10.1038/srep22063

Abstract

The insulin-induced mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) pathways are major intracellular signaling modules and conserved among eukaryotes that are known to regulate diverse cellular processes. However, they have not been investigated in the mollusk species Pinctada fucata. Here, we demonstrate that insulin-related peptide receptor of P. fucata (pfIRR) interacts with human recombinant insulin-like growth factor I (hrIGF-I), and stimulates the MAPK and PI3K signaling pathways in P. fucata oocytes. We also show that inhibition of pfIRR by the inhibitor PQ401 significantly attenuates the basal and hrIGF-I-induced phosphorylation of MAPK and PI3K/Akt at amino acid residues threonine 308 and serine 473. Furthermore, our experiments show that there is cross-talk between the MAPK and PI3K/Akt pathways, in which MAPK kinase positively regulates the PI3K pathway, and PI3K positively regulates the MAPK cascade. Intramuscular injection of hrIGF-I stimulates the PI3K and MAPK pathways to increase the expression of pfirr, protein phosphatase 1, glucokinase, and the phosphorylation of glycogen synthase, decreases the mRNA expression of glycogen synthase kinase-3 beta, decreases glucose levels in hemocytes, and increases glycogen levels in digestive glands. These results suggest that the MAPK and PI3K pathways in P. fucata transmit the hrIGF-I signal to regulate glycogen metabolism.

Highlights

Numerous insulin signaling pathways in invertebrates and vertebrates have been found to be relatively well conserved, including the ligand, its receptors, and the final effectors[14,15,16]
Insulin-like peptides (ILPs) have been found in Lymnaea stagnalis[19,20,21], Aplysia californica[22], Anodonta cygnea[23], Lottia gigantea[24], and Crassostrea gigas[25]; IRRs, which share the common characteristic of a typical tyrosine kinase (TK) domain, have been identified only in a few mollusks[6,26,27], and they are highly conserved in vertebrates
It is not known whether the insulin/IGF-I responsive machinery in P. fucata is correlated with carbohydrate/glycogen metabolism via these signaling pathways

Summary

Introduction

Numerous insulin signaling pathways in invertebrates and vertebrates have been found to be relatively well conserved, including the ligand, its receptors, and the final effectors[14,15,16]. In C. gigas, CgRas, CgPten, and CgP70S6K were found based on their sequence conservation[31] This evidence suggests that the conservation of the insulin signaling system may extend throughout bivalve species. The MAPK and PI3K signaling pathways have not been studied in P. fucata It is unknown whether pfIRR can regulate these signaling pathways, and whether these two pathways interact with each other. It is not known whether the insulin/IGF-I responsive machinery in P. fucata is correlated with carbohydrate/glycogen metabolism via these signaling pathways

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Results

Conclusion