Adaptation to hypoxia is a driving force for tumor progression that leads to therapy resistance and poor clinical outcome. Hypoxic responses are mainly mediated by hypoxia-inducible transcription factor-1 (HIF-1). One critical HIF-1 target mediating tumor progression is lysyl oxidase (LOX), which catalyzes cross-linking of collagens and elastin in the extracellular matrix, thereby regulating tissue tensile strength. Paradoxically, LOX has been reported to be both upregulated and downregulated in cancer cells, especially in colorectal cancer. Thus, we hypothesized that LOX might regulate expression of HIF-1 to create a self-timing regulatory circuit. Using human colorectal carcinoma cell lines in which HIF-1 and LOX expression could be modulated, we showed that LOX induction enhanced HIF-1 expression, whereas LOX silencing reduced it. Mechanistic investigations revealed that LOX activated the PI3K (phosphoinositide 3-kinase)-Akt signaling pathway, thereby upregulating HIF-1a protein synthesis in a manner requiring LOX-mediated hydrogen peroxide production. Consistent with these results, cancer cell proliferation was stimulated by secreted and active LOX in an HIF-1a-dependent fashion. Furthermore, nude mice xenograft assays established that HIF-1 potentiated LOX action on tumor growth in vivo. Taken together, these findings provide compelling evidence that LOX and HIF-1 act in synergy to foster tumor formation, and they suggest that HIF-1/LOX mutual regulation is a pivotal mechanism in the adaptation of tumor cells to hypoxia.

BackgroundBone metastases are highly frequent complications of breast cancers. Current bone metastasis treatments using powerful anti-resorbtive agents are only palliative indicating that factors independent of bone resorption control bone metastasis progression. Autotaxin (ATX/NPP2) is a secreted protein with both oncogenic and pro-metastatic properties. Through its lysosphospholipase D (lysoPLD) activity, ATX controls the level of lysophosphatidic acid (LPA) in the blood. Platelet-derived LPA promotes the progression of osteolytic bone metastases of breast cancer cells. We asked whether ATX was involved in the bone metastasis process. We characterized the role of ATX in osteolytic bone metastasis formation by using genetically modified breast cancer cells exploited on different osteolytic bone metastasis mouse models.Methodology/Principal FindingsIntravenous injection of human breast cancer MDA-B02 cells with forced expression of ATX (MDA-B02/ATX) to inmmunodeficiency BALB/C nude mice enhanced osteolytic bone metastasis formation, as judged by increased bone loss, tumor burden, and a higher number of active osteoclasts at the metastatic site. Mouse breast cancer 4T1 cells induced the formation of osteolytic bone metastases after intracardiac injection in immunocompetent BALB/C mice. These cells expressed active ATX and silencing ATX expression inhibited the extent of osteolytic bone lesions and decreased the number of active osteoclasts at the bone metastatic site. In vitro, osteoclast differentiation was enhanced in presence of MDA-B02/ATX cell conditioned media or recombinant autotaxin that was blocked by the autotaxin inhibitor vpc8a202. In vitro, addition of LPA to active charcoal-treated serum restored the capacity of the serum to support RANK-L/MCSF-induced osteoclastogenesis.Conclusion/SignificanceExpression of autotaxin by cancer cells controls osteolytic bone metastasis formation. This work demonstrates a new role for LPA as a factor that stimulates directly cancer growth and metastasis, and osteoclast differentiation. Therefore, targeting the autotaxin/LPA track emerges as a potential new therapeutic approach to improve the outcome of patients with bone metastases.