Tyrosine 705 Phosphorylation of STAT3 Is Associated with Phenotype Severity in TGFβ1 Transgenic Mice.

Eleonora Guadagnin,Jigna Narola,Carsten G Bönnemann,Yi-Wen Chen

doi:10.1155/2015/843743

Abstract

Transforming growth factor beta 1 (TGFβ1) is a key player in skeletal muscle degenerative and regenerative processes. We previously showed that conditionally overexpressing TGFβ1 in skeletal muscles caused myofiber atrophy and endomysial fibrosis in mice. However, the disease severity varied significantly among individual mice. While 40% of mice developed severe muscle pathology and lost body weight within 2 weeks of TGFβ1 transgene induction in muscles, the rest showed milder or no phenotype. This study aims at determining whether signal transducer and activator of transcription 3 (STAT3) plays a role in the phenotypic difference and whether it can be activated by TGFβ1 directly in muscle cells. Our results show that while total STAT3 was not differentially expressed between the two groups of mice, there was significantly higher pSTAT3 (Tyr705) in the muscles of the mice with severe phenotype. Immunohistochemistry showed that pSTAT3 (Tyr705) was localized in approximately 50% of the nuclei of the muscles. We further showed that TGFβ1 induced Tyr705 phosphorylation of STAT3 in C2C12 cells within 30 minutes of treatment while total STAT3 was not affected. Our findings suggest that TGFβ1 alone can induce Tyr705 phosphorylation of STAT3 in skeletal muscle cells and contribute to disease severity in transgenic TGFβ1 mice.

Highlights

TGFβ1 belongs to the TGFβ superfamily and has been shown to regulate a wide variety of biological processes, including promotion of apoptosis, inhibition of cell growth, and induction of cell differentiation, migration, and extracellular matrix (ECM) deposition [1, 2]
After the TGFβ1 transgene was induced for two weeks, approximately 40% of mice developed severe phenotypes, including early body weight loss and severe myofiber atrophy and fibrosis [5]
To determine whether signal transducer and activator of transcription 3 (STAT3) activation, which is known to be involved in muscle atrophy induced by IL-6, is involved in the variation of phenotypic presentations, we first examined the protein expression of total STAT3 as well as two phosphorylated STAT3, pSTAT3 (Tyr705), and pSTAT3 (Ser727), in muscles collected from the two groups of mice

Summary

Introduction

TGFβ1 belongs to the TGFβ superfamily and has been shown to regulate a wide variety of biological processes, including promotion of apoptosis, inhibition of cell growth, and induction of cell differentiation, migration, and extracellular matrix (ECM) deposition [1, 2]. Several studies showed that persistent expression and activation of TGFβ1 act as negative regulator of muscle repair by inducing apoptosis in myoblasts, suppressing muscle differentiation, and causing fibrosis in the muscles [3,4,5]. TGFβ1 is believed to be responsible for the ECM deposition in skeletal muscle [6,7,8,9], which leads to endomysial and perimysial fibrosis in muscular dystrophies, including Duchenne muscular dystrophy and congenital muscular dystrophies [10,11,12]. Phosphorylation of specific receptor tyrosine residue (Tyr705 or Ser727) in response to ligand stimulation determines the activities of STAT3. Tyr705 phosphorylation of latent cytoplasmic STAT3 promotes STAT3 homodimerization or heterodimerization with other STATs, which leads to nucleus translocation and DNA binding.

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