TGF beta signalling and its role in tumour pathogenesis.

Transforming growth factor beta (TGF-beta) is a multifunctional cytokine involved in the regulation of cell proliferation, differentiation and survival/or apoptosis of many cells. Knock-out experiments in mice for the three isoforms of TGF-beta have demonstrated their importance in regulating inflammation and tissue repair. TGF-beta is implicated in the pathogenesis of human diseases, including tissue fibrosis and carcinogenesis. TGF-beta receptors act through multiple intracellular pathways. Upon binding of TGF-beta with its receptor, receptor-regulated Smad2/3 proteins become phosphorylated and associate with Smad4. Such complex translocates to the nucleus, binds to DNA and regulates transcription of specific genes. Negative regulation of TGF-beta/Smad signalling may occur through the inhibitory Smad6/7. Furthermore, TGF-beta-activated kinase-1 (TAK1) is a component of TGF-beta signalling and activates stress-activated kinases: p38 through MKK6 or MKK3 and c-Jun N-terminal kinases (JNKs) via MKK4. In the brain TGF-beta, normally expressed at the very low level, increases dramatically after injury. Increased mRNA levels of the three TGF-beta isoforms correlate with the degree of malignancy of human gliomas. TGF-betas are secreted as latent precursors requiring activation into the mature form. TGF-beta may contribute to tumour pathogenesis by direct support of tumour growth and influence on local microenvironment, resulting in immunosuppression, induction of angiogenesis, and modification of the extracellular matrix. TGF-beta1,2 may stimulate production of vascular endothelial growth factor (VEGF) as well as plasminogen activator inhibitor (PAI-I), that are involved in vascular remodelling occurring during angiogenesis. Blocking of TGF-beta action inhibits tumour viability, migration, metastases in mammary cancer, melanoma and prostate cancer model. Reduction of TGF-beta production and activity may be a promising target of therapeutic strategies to control tumour growth.