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Mechanisms of secretion and extracellular functions of hGBP-1

Final Report Summary - NGL - HGBP-1 (Mechanisms of secretion and extracellular functions of hGBP-1)

Project objectives

The human guanylate-binding protein 1 (GBP-1) is an inducible protein, whose expression is stimulated by mediators of inflammation like the inflammatory cytokines interferon gamma, interleukin-1-beta and tumour necrosis factor alpha. Accordingly, the expression of GBP-1 in human tissues occurs in a context of inflammation. GBP-1 is expressed by various cell types but a strong association with blood vessels has been observed in vivo. Therefore the biological effects of GBP-1 have been particularly studied in endothelial cells, the cells forming the blood vessels. In these cells GBP-1 inhibits cell growth and migration in response to inflammatory signals. These effects are especially relevant for diseases with a high inflammatory component, like autoimmune diseases (lupus erythematodes or rheumatoid arthritis). In addition, endothelial cells are able to secrete GBP-1. This represents a specific feature of endothelial cells and has not been observed in any other cell type expressing GBP-1. We have shown previously that secreted GBP-1 is released in the blood where its level is increased in several inflammatory diseases.

In the present project, we first investigated the mechanisms responsible for the localisation and the trafficking of GBP-1 in the cell. We showed that GBP-1 localises in vesicular structures and at the plasma membrane of the cells. This cellular distribution is well in agreement with the fact that GBP-1 is secreted since secreted proteins are transported within vesicles to the plasma membrane, where they are released from the cell. In addition, we identified the molecular mechanisms regulating the association of GBP-1 with intracellular membranes.

Consecutively, we examined the mechanisms regulating the secretion of GBP-1. Surprisingly, we found that GBP-1 is released from the cells in two different forms: the full-length protein and a shorter form. We hypothesised that the short form might correspond to a cleavage product of the full-length GBP-1. Using computed analysis we found that cleavage by caspase-1 could explain the formation of the short GBP-1 form. Caspase-1 is a protease which cuts other proteins at a specific sequence. For example, caspase-1 cleaves the precursor form of interleukin-1-beta, and this cleavage is required for the activation and the secretion of the cytokine. We showed that caspase-1 is responsible for the cleavage of GBP-1 and is necessary for the secretion of both the full-length and the short form of the protein.

Inflammatory mechanisms are also very important in cancer. We previously showed that GBP-1 is expressed in blood vessels and immune cells infiltrating colorectal carcinomas. This expression occurred in a context of a strong anti-tumour immune reaction dominated by interferon gamma. In this context, the proliferation of GBP-1-expressing vessels is reduced and the presence of GBP-1 is associated with a better survival of the patients. In the present project, we showed that in most colorectal carcinoma, tumour cells have lost the ability to express GBP-1. We then investigated if the loss of GBP-1 represents a growth advantage for tumour cells. We found that GBP-1 inhibits growth, migration and invasion in colorectal carcinoma cells as well as the formation of tumours in vivo. Similar anti-tumorigenic effects of GBP-1 could be observed in breast carcinoma cells.

During this project we characterised the cellular localisation and the trafficking of GBP-1 and we determined the mechanisms of GBP-1 secretion. In addition, we unravelled new anti-tumorigenic effects of GBP-1. The expression and the secretion of GBP-1 are induced under inflammatory conditions and can be used as marker for inflammation in different diseases. In addition, the loss of GBP-1 expression in cancer cells might take part to the mechanisms of tumour cells evasion from the immune system.

Contact

Dr Nathalie Britzen-Laurent and Prof. Dr Michael Stuerzl, Erlangen University Medical Center, Erlangen, Germany.