br Expression of autotaxin in cancer

Expression of autotaxin in cancer Autotaxin (ATX) is a member of the family of NPPs (Nucleotides pyrophosphatases phosphodiesterases (NPP) family with a unique lysophospholipase D (lysoPLD) activity, allowing the synthesis of LPA from lysophospholipid precursors such as the lysophosphatidylcholine [43]. ATX expression is elevated in many types of cancers such as neuroblastoma [44], glioblastoma [45], hepatocarcinoma [46], B-cell lymphoma [47], melanoma [48], renal [49], thyroid [50], breast [51] and non-small cell lung cancers [52]. Mechanisms controling ATX expression are complex and not fully understood. In thyroid carcinoma cells, EGF and b-FGF up-regulate ATX expression while other cytokines such as IL-4, IL-1β and TGFβ mediate the opposite [50]. Expression of ATX in human hepatocarcinoma L002 australia are up regulated by TNF-α through down-stream activation of the NFκB [53]. Up-regulation of ATX was also associated with the activation of the WNT/β-catenin pathway in Wilms renal tumors or with the expression of v-jun in transformed fibroblasts [54]. In contrast, overexpression of N-Myc in neuroblastoma decreases ATX expression [55]. Overexpression of α6β4 integrin in breast cancer cells MDA-MB-435 was also shown to increase ATX expression involving the transcription factor NFAT1 [56]. The mitogenic function of ATX might contribute to the role of α6β4 integrin in migration and invasion of tumor cells.
Role of ATX/lysoPLD in tumorigenesis and cancer cell invasion Overexpression of ATX in RAS-mutated NIH3T3 murine fibroblasts increased tumor development and invasiveness compared to mock-transfected cells or cells transfected with a mutated inactive form of ATX (ATX/T210A) [57]. This work revealed for the first time the role of ATX in tumorigenesis through its lysoPLD activity that was further attributed to both direct and indirect actions of ATX on angiogenesis [57]. Forced expression of ATX in the mammary gland of MMTV-ATX transgenic mice induces the development of spontaneous breast tumors, metastasis formation associated with a high inflammatory process indicating that ATX has oncogenic and pro-metastatic properties [58]. Expression of the ATX transgene resulted in the secretion of cytokines involved in tumor growth and angiogenesis such as IL-8, VEGF and b-FGF [58]. Intriguingly, MMTV-LPA1-tg and MMTV-LPA3-tg animals with forced expression of LPA1 and LPA3 in the mammary gland, respectively, developed spontaneous metastatic tumors strengthening the role of LPA in tumorigenesis and cell invasion, and the pivotal function of ATX, owing to its lysoPLD activity in cancer development [58]. Using a human breast cancer model, we showed that forced expression of ATX in the ATX-null MDA-B02 cells increased their proliferation and invasion in vitro and primary tumor growth and skeletal metastasis formation in vivo [38]. However, while inhibiting endogenous expression of ATX in mouse mammary 4T1 cells affected cell invasion in vitro and reduced spontaneous metastasis dissemination to lungs and bone in vivo, we observed that down-regulating ATX in these cells had no impact on the growth of primary tumors [38]. Recently the D. Brindley׳s group showed for the first time that systemic treatment with the pharmacological blocker of ATX/lysoPLD (ONO-8430506) delayed early growth of 4T1 primary tumors that normalized to normal 12 days after cell injections [59]. In agreement with previous observations, silencing ATX expression in 4T1 cells by pharmacological blockade of ATX inhibited spontaneous lung metastasis formation [38], [59].
Functional implications of circulating ATX in cancer Clinical implications of elevated ATX expression may vary from one cancer type to another. ENPP2 is one of 64 gene signatures associated with poor survival of patients with metastatic non-small cell stage I [60] and the recent microarray analysis of Houben and colleagues on multiple data sets reveal a high expression of ATX, especially in B-cell lymphomas, renal cell carcinomas, liver and pancreatic cancers [61]. In addition, the serum level of ATX of patients with follicular lymphoma correlated with the tumor burden and a poor clinical outcome suggesting that in this particular type of cancer serum ATX may be a useful biomarker [47]. Increased serum lysoPLD activity was also reported in patients with pancreatic cancers [62]. However, radiolabeling and distribution studies showed rapid clearance within minutes of ATX from the circulation, likely through scavenger receptors of liver sinusoidal endothelial cells [63]. This may explain the accumulation of ATX in the plasma of patients with hepatic disorders [64]. However, the pathophysiological functions of circulating ATX and LPA remain elusive. In animal models, we have shown that overexpression of ATX in breast cancer cells increased the rate of spontaneous lung and micromedullar metastasis formation and that high ATX expression confers a selective advantage in the progression of osteolytic bone lesions [38]. However, ATX mRNA expression level in biopsies of human primary breast tumors does not predict metastasis recurrence or overall survival. This observation suggests first that expression of ATX at primary tumor sites may not be an appropriate biomarker for breast cancer metastasis-free survival and second that ATX expression from the tumor microenvironment may contribute to cancer growth and metastasis. Based on tissue section analyses of orthotopic 4T1 primary tumors, Benesch and colleagues observed higher ATX staining in the stroma than in the tumor cell compartment [59]. By using human breast cancer cell lines (MDA-MB-231, MDA-B02) that do not express ATX, our recent study revealed that the treatment of animals with the ATX inhibitor (BMP22) inhibited the progression of pre-established skeletal metastases. In addition, using a preventive dosing regiment we demonstrated that BPM22 markedly inhibited early steps of tumor cell bone colonization [65]. These data demonstrated that non-tumoral ATX (NT-ATX) controls early and late stages of cancer cell metastasis at least in the bone context [65]. Furthermore we showed for the first time that ATX is stored in platelet α-granules isolated from healthy donors and released upon tumor cell-induced platelet aggregation leading to the production of LPA via the degradation of platelet-derived lysophospholipid precursors [65]. Recently, binding of ATX to the integrin β3 family members was discovered [66]. Activated platelets but not resting platelets were shown to bind to ATX, indicating the necessity of integrin αIIbβ3 activation in this process [66]. A recent report also demonstrated the cooperative action between exogenous ATX and integrins in directional cell migration, where binding of ATX to integrin enabled the uptake and subsequent redistribution of ATX to the leading edge of migrating cells [67]. We showed that the protumoral activity of NT-ATX derived from platelets was partially dependent on interaction of ATX with tumoral αvβ3 integrins [65]. Altogether these studies suggest that cellular uptake, storage, and release of integrin-bound ATX may represent a general mechanism, even in cells that do not express ATX, providing localized production of LPA in close proximity to its cell surface receptors.