Transforming growth factor-ß. Signal regulation in health and disease

Abstract: TGF-b superfamily members have been shown to play a pivotal role in multicellular organisms. Knowledge about signal transduction pathways is important in order to understand and cure diseases like cancer where TGF-b´s growth inhibitory signal often are disenabled via mutated or absent signalling components. In the present investigation, TGF-b superfamily signalling were examined regarding specific interaction between signalling molecules, as well as expression pattern within different tumours. In order to determine specificity between receptor-receptor and receptor-Smad, chimeric proteins were generated. Generation of chimeric TGF-b type I receptor / bone morphogenetic protein type IB receptor (TbR-I/BMPR-IB), and chimeric TGF-b type II receptor / activin type IIB (TbR-II/ActR-IIB), resulted in proteins that were able to form TGF-b-induced receptor complexes. TbR-II/ActR-IIB was able to transduce TGF-b signals whereas TbR-I/BMPR-IB remained silent. In addition, TbR-II/ActR-IIB was capable of phosphorylating Smad2. Co-expression of the chimeric receptors phosphorylated Smad1 upon ligand addition, indicating a functional TbR-I/BMPR-IB. Chimeric TbR-I and BMPR-IB with interchanged L45 regions altered Smad-dependent signalling. Chimeric BMPR-IB used TGF-b restricted Smad2 and chimeric TbR-I used BMP-restricted Smad1. The chimeric receptors also showed interchanged pathways in other functional reporter assays. Investigation of TGF-b family components expression pattern in various type of epithelial skin tumours revealed that basal cell carcinomas displayed a lower expression in comparison to actinic and seborrheic keratosis as well as squamous cell carcinomas. Highly proliferative cell lines were not growth inhibited after addition of TGF-b1, TGF-b2, BMP-7 and BMP-2 whereas less proliferative cell lines were. LDL was shown to repress the growth stimulatory effect of bFGF on rat mesangial cells in a dose-dependent manner. LDL up-regulated the secretion of TGF-b in a time- and dose-dependent manner. It was not possible to detect any autocrine signal from the secreted TGF-b by immunoblot against phosphorylated Smad2. However, both activated and non-activated conditioned media from mesangial cultures were able to phosphorylate Smad2. Conclusion: Receptor complex formation is crucial for signal transduction but requires certain specificity. ActR-IIBs was capable of activating TbR-I, but TbR-IIs was unable to functionally activate BMPR-IB. The L45 region is a critical determinant for downstream signalling. Exchange of L45 regions between TGF-b and BMP receptors altered the use of R-Smads, and to a high degree the induction of reporter constructs. TGF-b family components were down-regulated or even absent in the tumourogenic basal cell carcinoma. Moreover, highly proliferative cell lines were resistant to TGF-b ligands indicating that loss of TGF-b´s growth inhibitory potential is a common theme.Effects of LDL on mesangial cells may lead to glomerular diseases via up-regulation and induced secretion of TGF-b. Since a TGF-b-dependent signal could be detected after secretion there is a possibility for autocrine TGF-b signalling which may lead to sclerotic progression.

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