Chronic inflammation is associated with both initiation and

Chronic inflammation is associated with both initiation and progression of many neoplastic conditions [149], and ATX may be a player in the process. ATX/LPA signaling is positively correlated with the invasive and metastatic potential of several cancers including melanoma, breast cancer, ovarian cancer, thyroid cancer, renal cell cancer, lung cancer, neuroblastoma, hepatocellular carcinoma (HCC) and glioblastoma multiforme (see [44], [150] and references therein). HCC is particularly interesting because it often develops in patients with chronic disease as a complication of liver cirrhosis [151]. This is especially true for hepatitis C, which accounts for 25% of all HCC cases and is the most significant risk factor for HCC [152]. Microarray analysis of liver samples from hepatitis C and HCC patients showed an increase in ATX mRNA levels compared to normal patients [153]. This work was followed with ATX immunohistochemical analysis of HCC biopsies derived from different etiologies, and staining was heaviest in patients with histories of either hepatitis C, hepatitis B or non-alcoholic steatohepatitis (NASH) compared to HCC patients with otherwise normal livers and without identified risk factors [154]. ATX staining was most positively correlated to both the degree of liver inflammation and cirrhosis regardless of etiology [154]. In vitro, ATX expression was elevated in HCC lines Hep3B and Huh7 compared to the well-differentiated HepG2 HCC cell line, normal embryonic CL-48 liver Kartogenin and primary hepatocyte cultures [154]. ATX expression could be further increased in Hep3B and Huh7 cell lines upon treatment with TNF-α through a nuclear factor-kappa B (NF-κB)-dependent mechanism, but not for HepG2 or CL-48 cell lines [154]. Despite all the literature on the mechanisms of LPA-mediated signaling in cancer [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], the regulation of ATX expression is poorly understood, although some progress has been made. Galectin-3, a β-galactoside binding protein known to increase the invasive potential of cancer cells, increases ATX expression at the transcriptional level by modulating the expression of the transcription factor Nuclear Factor of Activated T-cell 1 (NFAT1) [155]. Silencing galectin-3 suppresses NFAT1 protein expression, which in turn reduces ATX expression. Re-expression of ATX in galectin-3-silenced cells rescues angiogenesis, tumor growth and metastasis in vivo[155]. Similar work has been done in mantle cell lymphoma, where knock-down of the transcription factor SOX11 suppresses ATX production [156]. In breast cancer specimens, there is a positive correlation between nuclear tyrosine-phosphorylated signal transducer and activator of transcription factor 3 (pStat3) and ATX. In primary cultures of breast cancer specimens from patients, inhibition of pStat3 or reduced Stat3 expression deceases both ATX levels and cell migration [157]. However, the tumor is a heterogeneous environment composed of many different cell types including fibroblasts, endothelial cells and leukocytes in addition to cancer cells. There are also a host of soluble molecules in the tumor and this heterogeneity adds another layer of complexity to ATX/LPA signaling. Popnikolov et al. showed by that ATX and LPA3 staining correlate positively with cancer aggressiveness [158]. However, this work is particular novel because the authors showed that ATX staining is predominantly stromal, whereas cancer epithelial cells stain heaviest for LPA3[158]. In support of this observation, we showed in a syngeneic, orthotopic model of breast cancer that the 4T1 cancer cells express negligible ATX compared to the surrounding mammary fat pad [159]. As the tumor grows, the surrounding mammary fat pad significantly increases the production of mRNA and protein for ATX, suggesting that ATX is a mediator in the cross-talk between cancer cells, the tumor stroma and the mammary fat pad [159]. We have preliminary evidence to show that cytokines from the growing tumor stimulate stromal tissue to increase ATX production, and this ATX fuels tumor progression in a paracrine fashion. Pharmacological inhibition with a novel ATX inhibitor, ONO-8430506, (see Section 6.3) reduces mRNA for the pro-inflammatory IL-1β and IL-6 cytokines compared to vehicle treatment [159]. The detailed mechanisms for these effects have yet to be determined, however we present a novel model of ATX signaling in cancers which endogenously express low levels of ATX in Section 7.2. Interestingly, it is estimated that obesity contributes to at least 20 percent of cancer deaths by influencing cancer onset [160], [161]. Our results demonstrate that production of ATX in adipose tissue can explain at least part of this linkage between adiposity and breast cancer.