Activin/follistatin system in grass carp pituitary cells: - Regulation by local release of growth hormone and luteinizing hormone and its functional role in growth hormone synthesis and secretion.

Leo T.O Lee,Roger S K Fung,Jin Bai,Anderson O L Wong,Karen W Y Yuen

doi:10.1371/journal.pone.0179789

Abstract

Gonadotrophin regulation by activin/follistatin system is well-documented, but the corresponding effect on growth hormone (GH) has not been fully characterized and with little information available in lower vertebrates, especially in fish models. In grass carp, local interactions of GH and luteinizing hormone (LH) can induce GH release and gene expression at pituitary level via autocrine/paracrine mechanisms. To shed light on the role of activin/follistatin system in GH regulation by local actions of GH and LH, grass carp activin βA and βB were cloned, shown to be single-copy genes expressed in the pituitary, and confirmed to encode activin proteins capable of transactivating promoter with activin-responsive elements. In grass carp pituitary cells, activin A and B were effective in reducing GH secretion and GH cell content with concurrent drop in GH mRNA level whereas the opposite was true for follistatin, the activin-binding protein known to neutralize the effects of endogenous activin. Treatment with activin A and B not only could suppress basal but also inhibit GH mRNA expression induced by GH and human chorionic gonadotropin (hCG), a functional analogue of LH in fish model. Apparently, down-regulation of GH mRNA by activin was mediated by reducing GH transcript stability with concurrent inhibition on GH promoter activity via the SMAD pathway. In reciprocal experiments, GH treatment was found to up-regulate activin βA, activin βB and follistatin mRNA levels in carp pituitary cells but the opposite was noted by removing endogenous GH with GH antiserum. Interestingly, parallel treatment with hCG could also inhibit basal as well as GH-induced activin βA, activin βB and follistatin gene expression. These results, as a whole, indicate that the pituitary activin/follistatin system can serve as a regulatory target for local interactions of GH and LH and contribute to GH regulation by autocrine/paracrine mechanisms in the carp pituitary.

Highlights

Activin, a member of TGFβ superfamily, was first isolated from follicular fluid by its stimulatory action on follicle-stimulating hormone (FSH) release in pituitary cells [1, 2] It is a dimeric protein composed of two β subunits, which are known to have two major forms, namely βA and βB subunits
As a whole, provide evidence that the activin/follistatin system is involved in growth hormone (GH) regulation in grass carp and pituitary expression of activin and follistatin can be modified by functional interactions of GH and luteinizing hormone (LH) released locally in the carp pituitary. These findings suggest that the activin/follistatin system may be as a new component of the intrapituitary feedback loop for GH regulation mediated by local interactions of gonadotrophs and somatotrophs in carp species
To establish the evolutionary relationship of carp activin βA and βB with the corresponding nucleotide sequences reported in other species, phylogenetic analysis of these two newly cloned cDNAs based on neighbour-joining method was performed with MEGA 6.0

Summary

Introduction

A member of TGFβ superfamily, was first isolated from follicular fluid by its stimulatory action on follicle-stimulating hormone (FSH) release in pituitary cells [1, 2] It is a dimeric protein composed of two β subunits, which are known to have two major forms, namely βA and βB subunits. The biological effects of activin are mediated via type I (ActRI) and type II activin receptors (ActRII) [11], and to a less extent by type II BMP receptor [12]. These receptors are members of serine/threonine kinase receptor family functionally coupled to the SMAD pathway, especially with SMAD2 and SMAD3 for activin signalling [11, 13]. Aberrant signalling via activin and/or its receptors can be linked with cancer progression including epithelial-to-mesenchymal transition, cancer cell migration and invasion and neo-angiogenesis during tumorigenesis [14]

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Conclusion