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.

Key Points• In CML, early alterations of the BMP pathway are involved in the survival of hypersensitive LSCs and the expansion of myeloid progenitors.• The leukemic niche provides higher concentrations of BMP2 and BMP4 that fuel a permanent and autonomous pool of leukemic progenitors.Leukemic stem cells in chronic phase chronic myelogenous leukemia (CP-CML) are responsible for disease persistence and eventual drug resistance, most likely because they survive, expand, and are sustained through interactions with their microenvironment. Bone morphogenetic proteins 2 (BMP2) and 4 (BMP4) regulate the fate and proliferation of normal hematopoietic stem cells, as well as interactions with their niche. We show here that the intrinsic expression of members of the BMP response pathway are deregulated in CML cells with differences exhibited in mature (CD34 2 ) and immature (CD34 1 ) compartments. These changes are accompanied by altered functional responses of primitive leukemic cells to BMP2 and BMP4 and strong increases in soluble BMP2 and BMP4 in the CML bone marrow. Using primary cells and a cell line mimicking CP-CML, we found that myeloid progenitor expansion is driven by the exposure of immature cells overexpressing BMP receptor Ib to BMP2 and BMP4. In summary, we demonstrate that deregulation of intracellular BMP signaling in primary CP-CML samples corrupts and amplifies their response to exogenous BMP2 and BMP4, which are abnormally abundant within the tumor microenvironment. These results provide new insights with regard to leukemic stem cell biology and suggest possibilities for the development of novel therapeutic tools specifically targeting the CML niche. (Blood. 2013;122(23):3767-3777) IntroductionChronic myelogenous leukemia (CML) likely arises from a stem cell (SC) transformation induced by the formation of the BCR-ABL oncogene. Without treatment, the disease evolves to an inexorable fatal blast crisis. Until recently, imatinib has become the gold standard for chronic phase (CP)-CML care.1 However, some discrete Philadelphia-positive (Ph 1 ) leukemic SCs (LSCs) may be insensitive to tyrosine kinase inhibitors (TKIs) 2-4 and, therefore, sustain detectable disease for many years. None of the therapeutic agents available to date seem to eradicate the undifferentiated BCR-ABL 1 cells that serve as a reservoir for additional oncogenic events leading to disease progression, 2,4-6 and they require continued treatment. 7,8 Therefore, CML represents a unique model to study LSC biology and to elucidate some of the mechanisms of therapeutic resistance.Complete elimination of CML clones has rarely been achieved by TKIs because of the development of a variety of cell-intrinsic and cell-extrinsic protective mechanisms. Extrinsic mechanisms are supported by the bone marrow (BM) microenvironment, the so-called "leukemic niche." Bone morphogenetic proteins (BMPs) belong to the transforming growth factor-b superfamily and regulate hematopoietic SC fate both directly at various stages of SC differentiation 9-11 and indir...