Role Of The Raserk Pathway In Furin Gene Regulation

Ras proteins are small G-proteins with GTPase activity that are located at the cytoplasmic membrane [38]. They are activated downstream of a variety of transmembrane receptors, a process that involves the exchange of GTP for GDP. Activated Ras represents an important signalling branchpoint because they activate several signalling pathways through a number of effectors that serve to regulate myriad of cell functions including growth, survival, differentiation, and angio-genesis. As important effectors are the protein kinases of the Raf family that act upstream to a two-tiered protein kinase cascade, which includes two other cytosolic protein kinases, MEK and ERK. ERK phosphorylates many substrates involved in diverse cellular functions and was shown to play an important role in determining cell fate, selecting between diverse responses such as proliferation, differentiation, senescence or survival. Using HepG2 cells transfected with the furin P1 promoter construct, it was observed that forced expression of a dominant negative mutant form of Ras (RasN17) inhibited TGF beta 1-induced furin gene transcription, suggesting the involvement of Ras in this regulation [39]. In additional studies, the use of antisense and pharmacological inhibitors of ERK has demonstrated that the ERK signalling pathway acts downstream of Ras in furin gene regulation expression [39]. In this context, dysregulation of Ras proteins by activating mutations, overexpression or upstream activation was observed in approximately 30% of all human tumors, a feature that may explain, in part, the increased levels of this convertase found in a wide array of cancer types. Since the activated Ras-MAPK pathway is coupled to growth factor receptors, including EGF or PDGF and favor cellular proliferation, differentiation and survival, convertase activation of growth factor and/or receptors which signal through Ras, would likely amplify the Ras-MAPK signalling pathway.

In light of the emerging evidence for the interactions between the Smad and MAPK pathways and the role of furin in growth and differentiation events, it was of interest to explore the possible integration of these two pathways for the regulation of this convertase. Using HepG2 cells, it was observed that inhibition of MEK by PD98059 blocked most of the enhanced Smad2 nuclear localization induced by TGF|. In contrast, activation of ERK1/2 by activated MEK1 resulted in an enhanced nuclear localization of Smad2 [39]. These findings suggest functional interactions between the Smad and the MEK/ERK pathway in furin regulation. The cross-talk between these two signalling pathway may serve a growth/differentiation integration signal for the bioavailability of a multitude of furin-activated precursors involved in tumor growth and metastasis (Figure 2).

TC7F|i

TC7F|i

Cospas Sarsat System

fur E'i promoter traD┬╗activation

Figure 2. Cooperative model for TGF|-activated fur gene expression. The addition of TGF|1 to cells results in the activation of the TGF|-specific Smad2/4 pathway as shown. In parallel, T|RII/I activation induces a rapid and sustained phosphorylation of endogenous ERK1/2 MAPK. Cross-talk with activated MEK 1/2 or downstream MAPK cascade elements enhances Smad2 nuclear translocation where it may interact with DNA-binding proteins and direct transcription of the fur gene. The increase of furin intracellular levels will impact the bioactivation of multiple growth/cell differentiation-related factors fur E'i promoter traD┬╗activation

Figure 2. Cooperative model for TGF|-activated fur gene expression. The addition of TGF|1 to cells results in the activation of the TGF|-specific Smad2/4 pathway as shown. In parallel, T|RII/I activation induces a rapid and sustained phosphorylation of endogenous ERK1/2 MAPK. Cross-talk with activated MEK 1/2 or downstream MAPK cascade elements enhances Smad2 nuclear translocation where it may interact with DNA-binding proteins and direct transcription of the fur gene. The increase of furin intracellular levels will impact the bioactivation of multiple growth/cell differentiation-related factors

In addition to members of the vast TGF^ family, these precursor proteins include, among others, several key growth factor precursors such as platelet-derived growth factor A and B chains, growth factor proreceptors such as the insulin receptor and the hepatocyte growth factor receptor (c-Met), several integrin a-subunits, and cadherin family members, many of them were found to activate RAS-MAPK.

The outcome of Smad-MAPK interactions for enhanced furin expression should therefore not be viewed solely as a result of multifaceted TGFp signalling downstream of its receptor but also as a consequence of growth factors/receptors network acting together to amplify Smad/MAPK signal.

Was this article helpful?

0 0

Post a comment