Early expression of the fractalkine receptor CX3CR1 in pancreatic carcinogenesis.

Pancreatic cancer, namely ductal adenocarcinoma (PDAC), is a major health problem, the death rate approaching the incidence of the disease (Hidalgo, 2010). The majority of PDACs are diagnosed at an advanced stage, beyond any possibility of cure. A better understanding of the early neoplastic changes in the pancreas might help anticipating the diagnosis and counteracting the rapid progression of pancreatic cancer. Microscopic pre-cancerous lesions (Pancreatic Intraepithelial Neoplasia, PanIN) house KRAS mutations (Jones et al, 2008; Sipos et al, 2009) and accumulate further gene damage along progression to cancer (Jones et al, 2008). However, gene damage is not the only driver of pancreatic carcinogenesis, PDAC being characterised by a dense stromal reaction, referred to as desmoplasia (Farrow et al, 2008), which is an active partner in disease progression (Hwang et al, 2008; Vonlaufen et al, 2008). Stromal cells produce cytokines and growth factors that establish an active tumour-stroma cross-talk (Farrow et al, 2008; Hwang et al, 2008; Vonlaufen et al, 2008). Mediators of the cross-talk between cancer and microenvironment include chemokines, small chemotactic cytokines, and their receptors (Mantovani et al, 2008; Wang et al, 2008; Balkwill, 2012). Tumour expression of chemotactic molecules modifies the behaviour of cancer cells and tumour progression. The gain of expression of chemokine receptors enhances cancer invasion and spread by mediating cancer cell trafficking and metastasis homing (Mantovani et al, 2010). Chemokine expression in cancer may also affect immune recognition by the host, and may promote autocrine loops favouring the survival of tumour cells (Wang et al, 2008; Balkwill, 2012). The expression of the chemokine receptor CXCR4 has been associated with an enhanced progression of PDAC, and with stemness of cells with metastatic potential (Marchesi et al, 2004; Hermann et al, 2007). Surprisingly, the expression of CXCR4 and of its ligand CXCL12 begins in the pre-invasive stages of pancreatic neoplasia (Thomas et al, 2008). The relevance of chemokine receptors in PDAC spread has been recently strengthened by the finding that tumour cells also express the chemokine receptor CX3CR1 (Marchesi et al, 2008). CX3CR1 forms a high-affinity axis (CX3-CR1-CL1) with its unique ligand CX3CL1 (also referred to as Fractalkine/Neurotactin), a mucin-chemokine hybrid expressed by endothelial cells and neurons, either as a membrane-anchored adhesion molecule or as a secreted chemoattractant (Bazan et al, 1997; Pan et al, 1997). CX3CR1 expression was associated with the degree of perineural invasion (PNI), and PDAC cells expressing CX3CR1 were found to disrupt peripheral nerves once xenografted in mice (Marchesi et al, 2008). It is unknown whether the expression of the receptor occurs already in pre-invasive stages of pancreatic carcinogenesis, or alternatively, only in invasive cancer. Furthermore, limited information is available as to the expression of the ligand in PDAC (Xu et al, 2012), and as to whether tumour features and microenvironment cross-talk may affect CX3-CR1-CL1 status. This study was aimed to investigate whether pancreatic cancer expresses CX3-CR1-CL1, already gaining the expression of these chemotactic molecules in pre-invasive stages, and whether an association exists with PDAC features and outcome. To these aims, we assessed the expression of CX3-CR1-CL1 in human PanINs and invasive cancers, as well as in PanINs of a Kras-mutated mouse model of pancreatic cancer, and tested their responsiveness to cytokines and their expression in tumours propagated in vivo.