Latent transforming growth factor binding protein 4 regulates transforming growth factor beta receptor stability

Hum Mol Genet. 2015 Jul 15;24(14):4024-36. doi: 10.1093/hmg/ddv139. Epub 2015 Apr 16.

Abstract

Mutations in the gene for the latent transforming growth factor beta binding protein 4 (LTBP4) cause autosomal recessive cutis laxa type 1C. To understand the molecular disease mechanisms of this disease, we investigated the impact of LTBP4 loss on transforming growth factor beta (TGFβ) signaling. Despite elevated extracellular TGFβ activity, downstream signaling molecules of the TGFβ pathway, including pSMAD2 and pERK, were down-regulated in LTBP4 mutant human dermal fibroblasts. In addition, TGFβ receptors 1 and 2 (TGFBR1 and TGFBR2) were reduced at the protein but not at the ribonucleic acid level. Treatment with exogenous TGFβ1 led to an initially rapid increase in SMAD2 phosphorylation followed by a sustained depression of phosphorylation and receptor abundance. In mutant cells TGFBR1 was co-localized with lysosomes. Treatment with a TGFBR1 kinase inhibitor, endocytosis inhibitors or a lysosome inhibitor, normalized the levels of TGFBR1 and TGFBR2. Co-immunoprecipitation demonstrated a molecular interaction between LTBP4 and TGFBR2. Knockdown of LTBP4 reduced TGFβ receptor abundance and signaling in normal cells and supplementation of recombinant LTBP4 enhanced these measures in mutant cells. In a mouse model of Ltbp4 deficiency, reduced TGFβ signaling and receptor levels were normalized upon TGFBR1 kinase inhibitor treatment. Our results show that LTBP4 interacts with TGFBR2 and stabilizes TGFβ receptors by preventing their endocytosis and lysosomal degradation in a ligand-dependent and receptor kinase activity-dependent manner. These findings identify LTBP4 as a key molecule required for the stability of the TGFβ receptor complex, and a new mechanism by which the extracellular matrix regulates cytokine receptor signaling.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Case-Control Studies
  • Cells, Cultured
  • Cutis Laxa / genetics*
  • Disease Models, Animal
  • Down-Regulation
  • Endocytosis / genetics
  • Female
  • Fibroblasts / cytology
  • Fibroblasts / metabolism
  • Humans
  • Immunoprecipitation
  • Latent TGF-beta Binding Proteins / genetics
  • Latent TGF-beta Binding Proteins / metabolism*
  • Male
  • Mice
  • Mice, Knockout
  • Mutation
  • Phosphorylation
  • Protein-Serine-Threonine Kinases / genetics
  • Protein-Serine-Threonine Kinases / metabolism*
  • Receptor, Transforming Growth Factor-beta Type I
  • Receptor, Transforming Growth Factor-beta Type II
  • Receptors, Transforming Growth Factor beta / genetics
  • Receptors, Transforming Growth Factor beta / metabolism*
  • Signal Transduction
  • Smad2 Protein / genetics
  • Smad2 Protein / metabolism

Substances

  • LTBP4 protein, human
  • Latent TGF-beta Binding Proteins
  • Receptors, Transforming Growth Factor beta
  • SMAD2 protein, human
  • Smad2 Protein
  • Protein-Serine-Threonine Kinases
  • Receptor, Transforming Growth Factor-beta Type I
  • Receptor, Transforming Growth Factor-beta Type II
  • TGFBR1 protein, human
  • Tgfbr1 protein, mouse

Supplementary concepts

  • Cutis Laxa, Autosomal Recessive, Type I