The etiology of most chronic angiogenic diseases such as rheumatoid arthritis, atherosclerosis, diabetes complications, and cancer includes the presence of pockets of hypoxic cells growing behind aerobic cells and away from blood vessels. Hypoxic cells are the result of uncontrolled growth and insufficient vascularization and have undergone a shift from aerobic to anaerobic metabolism. Cells respond to hypoxia by stimulating the expression of hypoxia inducible factor (HIF), which is critical for survival under hypoxic conditions and in embryogenesis. HIF is a heterodimer consisting of the O2-regulated subunit, HIF-1R, and the constitutively expressed aryl hydrocarbon receptor nuclear translocator, HIF-1!. Under hypoxic conditions, HIF-1R is stable, accumulates, and migrates to the nucleus where it binds to HIF-1! to form the complex (HIF-1R + HIF-1!). Transcription is initiated by the binding of the complex (HIF-1R + HIF-1!) to hypoxia responsive elements (HREs). The complex [(HIF-1R + HIF-1!) + HREs] stimulates the expression of genes involved in angiogenesis, anaerobic metabolism, vascular permeability, and inflammation. Experimental and clinical evidence show that these hypoxic cells are the most aggressive and difficult angiogenic disease cells to treat and are a major reason for antiangiogenic and conventional treatment failure. Hypoxia occurs in early stages of disease development (before metastasis), activates angiogenesis, and stimulates vascular remodeling. HIF-1R has also been identified under aerobic conditions in certain types of cancer. This review summarizes the role of hypoxia in some chronic degenerative angiogenic diseases and discusses potential functional foods to target the HIF-1R pathways under hypoxic and normoxic conditions. 

Cancer is responsible for one of every four deaths in the United States. In 2005, about 570 280 Americans are expected to die of cancer from all sites, up 10 000 more from 2004 combined cancer deaths. An estimated 1.4 million new cases were diagnosed in 2004. Despite advances in medical interventions, it is also predicted that aging and the increasing size of the U.S. population will cause the total number of cancer cases to double by 2050. The financial costs of cancer, for 2003, were estimated at more than $189 billion overall with $64 billion for direct medical costs and $125 billion for lost productivity. Arthritis, which comprises over 100 different diseases such as osteoarthritis, rheumatoid arthritis (RA), and gout, affects nearly 70 million Americans with nearly two-thirds of people affected younger than 65 years. From CDC statistics, in 1995, arthritis cost U.S. medical care nearly $22 billion and loss of productivity cost the economy about $60 billion. The number is projected to increase dramatically and so will the cost. The National Institute of Diabetes and Digestive and Kidney Diseases of the NIH estimated that in 2002, 18.2 million Americans or 6.3% of the population had diabetes. The U.S. government statistics show that, in 2002, diabetes cost the country $132 billion. Indirect costs, including disability payments, time lost from work, and premature death, totaled $40 billion; direct medical costs for diabetes care, including hospitalizations, medical care, and treatment supplies, totaled $92 billion. 

A variety of proangiogenic factors, which include hypoxia, growth factors, hormones, matrix metalloproteinases, serine proteinases, aspartic proteinases, cysteine proteinases, proteasome, signal transduction enzymes, proteins, cell adhesion molecules, metals, and oncogenes, have been identified and recognized for their ability to stimulate the angiogenesis machinery that leads to disease progression, metastasis, and death. The stimulators of angiogenesis have thus become the subject of intense research efforts, to elucidate their mechanisms of action and allow rational design of effective and selective inhibitors of angiogenesis. Tumor angiogenesis has become the focus of extensive biomedical investigations, and its inhibition is emerging as a rational and potentially valuable new approach to cancer therapy because angiogenesis is required for tumor growth and metastasis (4-6). Activated endothelial cells are the primary targets of antiangiogenic compounds. There are several advantages to consider antiangiogenic therapy over conventional therapies. In healthy individuals, angiogenesis is normally restricted, with only about 0.01% of adult endothelial cells undergoing the process of angiogenesis at any given time. Therefore, the side effects of inhibitors of angiogenesis on normal tissues should be negligible (4, 7, 8). The inhibitors of angiogenesis affect tumor growth indirectly by targeting the steps involved in the process leading to the formation of new blood vessels and are mainly cytostatic (9). 

The inhibitors of angiogenesis can be classified in different categories: (i) endothelial growth factors inhibitors, (ii) endothelial cell signaling transduction inhibitors, (iii) inhibitors of the urokinase plasminogen activator system, (iv) inhibitors of matrix metalloproteinases, (v) inhibitors of cell proliferation, (vi) inhibitors of endothelial cell survival, and (vi) inhibitors of endothelial bone marrow precursor cells (10). Although acquired resistance has never been demonstrated in preclinical trials of antiangiogenesis therapy, clinical evidence has shown a gradual loss of activity by antiangiogenic drugs such as STI571 or Gleevec when these compounds were administered in monotherapy (11, 12). Potential factors involved in the acquired resistance to antiangiogenic drugs include among others: (i) antiapoptotic functions of activated endothelial cells, (ii) genetic alterations such as p53, (iii) redundancy of tumor cell-secreted growth factors, (iv) impact of tumor microenvironment such as hypoxia and alteration of the hypoxia inducible factor-1R (HIF-1R) pathway, (v) advanced stage of the disease and age of patients while preclinical data are often collected using young animals with relatively small tumors (13). Vascular endothelial growth factor (VEGF) is one of the most potent stimulators of physiological and pathological angiogenesis. Hypoxia is one of the major stimulators for VEGF. Three strategies were suggested to improve the efficacy of antiangiogenic drugs: (i) inhibition of oncogene-mediated signal transduction, (ii) use of hypoxic cell cytotoxins, and (iii) vascular targeting agents (14). The early stages of the angiogenic diseases are critical to the spread of the diseases and could be used as targets in disease prevention (15, 16).

 Several antiangiogenic factors, some endogenous to tumor cells such as endostatin, angiostatin, thrombospondin-1 (TSP-1), tissue inhibitors of metalloproteinases (TIMPs), and others from dietary sources or synthesis, have been identified, characterized, and used in cell cultures, animal models, and clinical trials (16-21). Endostatin is a 20 kDa C-terminal proteoglycan fragment of collagen XVIII produced by hemangioendothelioma (22). Endostatin inhibits endothelial cell migration in vitro and experimental tumor growth in vivo (22). The ability of endostatin to bind Zn2+ is essential for its antiangiogenic activity (23). Angiostatin is a 38 kDa internal proteolytic fragment of plasminogen (4, 24). Metalloproteinases such as matrix metalloproteinase (MMP)-3, MMP-7, MMP-9, and MMP-12 can generate angiostatin from plasminogen (25). The antiangiogenic mechanism of angiostatin remains an enigma, and it is difficult to predict the ultimate outcome of ongoing clinical trials because the mechanism of action of angiostatin is not well-known (26). TSP-1 is a 450 kDa homotrimeric extracellular matrix protein expressed by both normal and tumor cells (27). TSP-1 has been shown to inhibit tumorigenesis, angiogenesis, and prevent metastasis in several tumor models including breast, skin, and lung carcinomas, melanoma, and malignant glioma (28). Repression of TSP-1 promotes tumor growth (31, 32). TIMPS are a family of closely related 21-32 kDa proteins found in the extra cellular matrix that regulate the activity of MMPs and have substantial influence on the activation process of MMP zymogens. 

 Anemia and the formation of methemoglobin or carboxyhemoglobin can reduce O2 transport capacity and induce hypoxia (43). Anemia has been shown to induce or aggravate hypoxia and ischemic complications and is a poor prognostic factor in lymphoma, leukemia, and many types of cancer (44-46). There are two types of hypoxia: transient and chronic hypoxia. Transient hypoxia is a temporary reduction in oxygen availability. The inadequate vascular geometry relative to the volume of oxygen-consuming tumor cells creates diffusion-limited O2 delivery, which results in chronic hypoxia (47). In response to chronic hypoxic conditions, cells in the hypoxic environment shift from aerobic (TCA cycle) to anaerobic metabolism (glycolysis, also known as Warburg effect) and respond to low O2 levels by up-regulating the synthesis of HIF. The HIF family comprises the HIF-1R, HIF-1!, HIF-2R, and HIF-3R subunits (48). HIF-1R and HIF-2R, which are both regulated by cellular oxygen concentrations in a similar fashion, have been identified as key transcription factors responsible for gene expression in response to hypoxia and up-regulated in many cancers (49). The !-subunit of HIF, also known as aryl hydrocarbon receptor nuclear translocator, is a constitutive nuclear protein present in normoxic cells (50). Under normoxic conditions, the HIF-1R subunit is undetectable because it undergoes rapid ubuquitination and proteosomal degradation (51, 52). Under hypoxic conditions, HIF-1R is stabilized, accumulates, translocates to the nucleus, and dimerizes with the HIF-1! subunit to form the (HIF-1R + HIF-1!) complex. 

The complex binds to hypoxia response elements (HREs) to form the [(HIF-1R + HIF-1!) + HREs] complex. The latter complex activates the expression of numerous hypoxia response genes of which products mediate angiogenesis, cell proliferation and survival, migration, and invasion (51). Hypoxia is associated with the induction of HIF-1R activity and expression of HIF-1R genes including VEGF, angiopoietin-2 (Ang2), erythropoietin, glycolytic enzymes, increased telomerase and carbonic anhydrase-9 and -12 activities, and increased MMP and PKC activities (17, 50, 52-54). VEGF is mostly associated with angiogenesis and vascular permeability; erythropoietin is associated with red blood cell production; glycolytic enzymes are associated with glucose metabolism and energy; and carbonic anhydrase is associated with pH and adaptation to oxidative stress (47, 55). These genes participate in the normal adaptive response of the cell to hypoxia. Under hypoxic conditions, a mutation in the tumor suppressor gene p53 can lead to clonal selection of tumor cells with mutated p53, which in turn facilitates a more malignant phenotype and diminished apoptotic potential (55, 56). Hypoxia is not the only inducer of angiogenesis. Genetic alterations, such as loss of function of tumor suppressor genes such as the von Hippel-Lindau (VHL), p53, and p16INK4a or the activation of oncogenes including ras, raf, HER2/erbB2 (neu), and src, result in increased expression of HIF-1R and HIF-1 inducible genes (56, 57). HIF-1R and HIF-2R are required for normal embryogenesis because they are central to oxygen homeostasis.

 HIF-1R and HIF-2R knockout mice died early or had syndromes of multiple organ pathology that included retinopathy, cardiac hypertrophy, mitochondrial abnormalities, hypoglycemia, altered Krebs cycle, and several biochemical abnormalities (17, 49, 58, 59). However, HIF-1R is overexpressed in a large number of human tumors and its overexpression correlates with poor prognosis, treatment failure, and mortality (51, 60). HIF-1 is therefore an important target for cancer therapy. Tumors can also contain areas of more severe hypoxia (0.1% O2) or anoxia (<0.1% O2). Under these conditions, cells undergo p53-dependent apoptosis (61). The mechanisms for survival under anoxia are HIF-1R independent and, thus, differ from the hypoxic response. 3. CLINICAL SIGNIFICANCE OF HYPOXIA Clinically, the association of hypoxia to angiogenic diseases aggressiveness has been supported by the finding that hypoxic primary diseased cells are associated with a higher rate of metastasis, genetic instability, and resistant phenotypes (43). Chronic and transient hypoxia have been detected in animal tumor models, and both types occur in human cancers as well (47). Hypoxia is often associated with low glucose concentration, high lactate levels, and low extracellular pH (62). Hypoxia can induce the production of stress proteins, which protect the hypoxic cells against drugs such as methotrexate and doxorubicin (47). However, poor outcome of treatment is not necessarily indicative of hypoxia-associated intrinsic resistance (55). The following paragraphs briefly illustrate the interplay between hypoxia and some major chronic angiogenic diseases. 3.1.

 Tumor Hypoxia. HIF-1R overexpression, as a result of either intratumoral hypoxia or genetic alteration, has been demonstrated in human cancers and leads to increased transcription of genes that encode angiogenic stimulators. The products of these genes contribute to basement membrane degradation, metastasis, and patient mortality, which are the defining features of cancer cells (63). HIF-1R is stabilized during hypoxia and is a major regulator of cell cycle arrest in primary cells during hypoxia (64, 65). Low oxygen tension in tumors was associated with increased HIF-1R overexpression, metastasis, treatment failure, and/or mortality (60, 62). Hypoxia occurs in the early stages of tumor development (before metastasis), may induce angiogenesis, and is commonly observed in noninvasive tumors such as intraductal breast cancer. Hypoxia within solid tumor cells larger than 1 mm2 reduces the sensitivity of tumor cells to conventional (both radio- and chemotherapy) modalities, influences growth, and may increase malignant progression within nonirradiated tumors (13, 66). The oxygen diffusion limit prevents diffusion of sufficient concentrations of many chemotherapeutic drugs 100-150 µm from a functional vascular capillary to be toxic to hypoxic tumor cells. HIF-1R expression can also occur under aerobic conditions in some human cancer cells. Zhong et al. (67) demonstrated that human prostatic cancer lines DU 145, PC-3, PPC-1, and TSU, most notably PC-3 cells, express HIF-1R protein and HIF-1 DNA binding activity under aerobic conditions, and expression is further increased in response to hypoxia. Under normoxic conditions, growth factors such as the epidermal growth factor (EGF) and insulin-like growth factor (IGF) induce the expression of HIF-1R (68, 69).