Investigating the role of MyD88-NF-kB signalling in regulating the inflammatory response to the emerging pre-neoplastic cells

Abstract

Chronic inflammation has long been considered an enabling characteristic of the cancer microenvironment. The release of growth and survival factors, extracellular matrix-modifying enzymes and other bioactive molecules by inflammatory cells into the tumour microenvironment contributes to the acquisition of cancer hallmark capabilities. It is now known that oncogenes can activate signals that promote the formation of an inflammatory microenvironment, even in the absence of external stimuli, placing inflammation as an early phenomenon in the timeline of tumour development. The over-expression of an oncogene in pre-neoplastic cells has been shown to promote the release of cytokines and other pro-inflammatory markers and consequent recruitment of neutrophils and macrophages. The recruitment of neutrophils has been previously associated with increased proliferation of oncogene-transformed cells, however, the mechanisms by which neutrophils exercise this trophic role are yet poorly understood. Nuclear factor kappa B (NF-kB) transcription factors are crucial elements for the regulation of inflammation, immune response as well as cellular stress response. Activation of NF-kB signalling pathway leads to the expression of mitogenic and anti-apoptotic factors, thus having the potential to promote tumorigenesis. MyD88 is an adaptor protein that mediates TLR/IL-1-R activation of the NF-kB pathway. MyD88 has been shown to play both positive and negative roles in cancer development. It is unclear however, whether the MyD88-NF-kB signalling pathway plays a role during the earliest stages of preneoplastic development of tumour initiation. To better understand the role of MyD88-NF-kB in oncogene driven inflammation during preneoplastic cell development, I was involved in the development of a zebrafish (Danio rerio) tissue specific inducible model which allowed the temporal control of HRasG12V oncogene expression for generation of pre-neoplastic cells (PNCs) and the detection of the earliest signalling events that initiate the process of tumour development. Using reporter lines, generated elsewhere, I demonstrate the activation of MyD88-NF-kB signalling pathway in both PNCs and recruited neutrophils. I also evaluate the effect of its downregulation, through the expression of the murine dominant negative mutant of IkBa, on PNCs proliferation and modulation of recruited neutrophil behaviour. Alongside the study of NF-kB as an inducer of tumour initiation, I also provide evidence of heterogeneity within the neutrophil population recruited to the PNCs and the different behaviours they exhibit. This work demonstrates the potential of zebrafish for the study of tumour initiation highlighting the involvement of NF-kB signalling pathway in the establishment of an inflammatory milieu that allows the progression of this first stage of tumorigenesis.