Cell Hypoxia: the Prime Cause of Cancer on Cell Level

Figure 1. Tumour hypoxia. When a small, localised tumour outgrows its vascular supply (distances >100 ?m) tumour hypoxia arises in regions with impaired oxygen delivery. Consequently, hypoxic cells switch on target genes involved in angiogenesis [vascular endothelial growth factor (VEGF)], glucose transport [glucose transporter 1 (GLUT-1)] and cell migration [urokinase-type plasminogen activator receptor (u-PAR) and plasminogen activator inhibitor 1 (PAI-1)]. Increased vascular supply to the tumour via the induction of new blood vessel formation (angiogenesis) encourages tumour growth and facilitates metastasis to distant sites. One of the functions of the mammalian hypoxic response in development and cancer is the generation of nascent vascular networks through angiogenesis. Through transcriptional regulation of the vascular endothelial growth factor A (VEGF-A) and other angiogenic factors, the Hypoxia-Inducible Factor 1 and 2 alpha (HIF-1alpha and HIF-2alpha) can increase angiogenesis in an oxygen dependent fashion, giving a survival and growth advantage to HIF wild type tumours (UCSD n.d.). Angiogenesis is an important mediator of tumor progression. As tumors expand, diffusion distances from the existing vascular supply increases resulting in hypoxia. Sustained expansion of a tumor mass requires new blood vessel formation to provide rapidly proliferating tumor cells with an adequate supply of oxygen and metabolites. The key regulator of hypoxia-induced angiogenesis is the transcription factor hypoxia inducible factor (HIF)-1 (Liao & Johnson, 2007). Deficiencies in oxygenation are widespread in solid tumors. The transcription factor HIF-1a is an important mediator of the hypoxic response of tumor cells and controls the up-regulation of a number of factors important for solid tumor expansion, including the angiogenic factor vascular endothelial growth factor (VEGF). The evidence from these experiments by Ryan et al, (2000) indicates that hypoxic response via HIF-1a is an important positive factor in solid tumor growth and that HIF-1a affects tumor expansion in ways unrelated to its regulation of VEGF expression. Cells undergo a variety of biological responses when placed in hypoxic conditions, including activation of signalling pathways that regulate proliferation, angiogenesis and death. Cancer cells have adapted these pathways, allowing tumours to survive and even grow under hypoxic conditions, and tumour hypoxia is associated with poor prognosis and resistance to radiation therapy. Many elements of the hypoxia-response pathway are therefore good candidates for therapeutic targeting (Harris, 2002). Nobel Laureate, Dr. Otto Warburg, in his article “The Prime Cause and Prevention of Cancer” (1966) wrote, “Cancer, above all other diseases, has countless secondary causes. Almost anything can cause cancer. But, even for cancer, there is only one prime cause. The prime cause of cancer is the replacement of the respiration of oxygen (oxidation of sugar) in normal body cells by fermentation of sugar… In every case, during the cancer development, the oxygen respiration always falls, fermentation appears, and the highly differentiated cells are transformed into fermenting anaerobes, which have lost all their body functions and retain only the now useless property of growth and replication.“ Dr. Otto Warburg investigated the metabolism of tumors and the respiration of cells, particularly cancer cells, and in 1931 was awarded the Nobel Prize in Physiology or Medicine for his “discovery of the nature and mode of action of the respiratory enzyme.” These conclusions are important since we have already proved the following key findings: 1. Sick patients (various chronic diseases) breathe much more than the norm. 2. Overbreathing or hyperventilation reduces CO2 content in the lungs and arterial blood. 3. Due to numerous uses of CO2 in the human body, hypocapnia (lowered CO2) leads to reduced oxygenation of all vital organs and tissues due to chest breathing, vasoconstriction, and suppressed Bohr effect. The Bohr effect explains oxygen release in capillaries or why red blood cells unload oxygen in tissues. The Bohr effect was first described in 1904 by the Danish physiologist Christian Bohr (father of famous physicist Niels Bohr). Christian Bohr stated that at lower pH (more acidic environment, e.g., in tissues), hemoglobin will bind to oxygen with less affinity. Since carbon dioxide is in direct equilibrium with the concentration of protons in the blood, increasing blood carbon dioxide content causes a decrease in pH, which leads to a decrease in affinity for oxygen by hemoglobin. Do modern scientists have a different opinion about the prime cause of cancer on cell level? It has been known for decades that malignant cells normally and constantly appear and exist in any human organism due to the billions of cell divisions and mutations. These abnormal cells, under normal conditions, are quickly detected by the immune system and destroyed. However, the work of macrophages, enzymes and other agents of the immune system is severely hampered when the conditions of hypoxia exists. That was the conclusion of various studies. For example, Dr. Rockwell from Yale University School of Medicine (USA) studied malignant changes on the cellular level and wrote, “The physiological effects of hypoxia and the associated micro environmental inadequacies increase mutation rates, select for cells deficient in normal pathways of programmed cell death, and contribute to the development of an increasingly invasive, metastatic phenotype” (Rockwell, 1997). The title of this publication is ” Oxygen delivery: implications for the biology and therapy of solid tumors”. Summarizing the results of numerous studies, a group of biological scientists from University of California (San Diego) chose the following title for their article, “The hypoxia inducible factor-1 gene is required for embryogenesis and solid tumor formation” (Ryan et al, 1998). Under normal conditions, even a group of hypoxic cells dies (or is easily destroyed). What about cells in malignant tumors? Researchers from the Gray Laboratory Cancer Research Trust (Mount Vernon Hospital, Northwood, Middlesex, UK) concluded, “Cells undergo a variety of biological responses when placed in hypoxic conditions, including activation of signalling pathways that regulate proliferation, angiogenesis and death. Cancer cells have adapted these pathways, allowing tumors to survive and even grow under hypoxic conditions…” (Chaplin et al, 1986). There is so much professional evidence about the fast growth of tumors when the condition of hypoxia is present that a large group of Californian researchers recently wrote a paper “Hypoxia – inducible factor-1 is a positive factor in solid tumor growth” (Ryan et al, 2000). Echoing their paper, a British oncologist Dr. Harris from the Weatherhill Institute of Molecular Medicine (Oxford) went further with the manuscript “Hypoxia – a key regulatory factor in tumor growth” (Harris, 2002). When the solid tumor is large enough and the disease progresses, cancer starts to invade other tissues. This process is called metastasis. Does poor oxygenation influence it? “…Therefore, tissue hypoxia has been regarded as a central factor for tumor aggressiveness and metastasis” (Kunz & Ibrahim, 2003). That was the conclusion of a group of German researchers from the University of Rostock and the University of Leipzig. Since dozens of medical and physiological studies yield the same result, what about the following title? “Tumor oxygenation predicts for the likelihood of distant metastases in human soft tissue sarcoma” (Brizel et al, 1996). This title claims that tumor oxygenation predicts chances of cancer invasion. The reader can probably guess about the effect of cancer treatment and the chances of survival for those who suffer from severe chronic hyperventilation. Indeed, “… tumor hypoxia is associated with poor prognosis and resistance to radiation therapy” (Chaplin et al, 1986). American scientists from Harvard Medical School noted “… Hypoxia may thus produce both treatment resistance and a growth advantage” (Schmaltz et al, 1998). “Low tissue oxygen concentration has been shown to be important in the response of human tumors to radiation therapy, chemotherapy and other treatment modalities. Hypoxia is also known to be a prognostic indicator, as hypoxic human tumors are more biologically aggressive and are more likely to recur locally and metastasize” (Evans & Koch, 2003). “Clinical evidence shows that tumor hypoxia is an independent prognostic indicator of poor patient outcome. Hypoxic tumors have altered physiologic processes, including increased regions of angiogenesis, increased local invasion, increased distant metastasis and altered apoptotic programs” (Denko et al, 2003). Another factor is iron metabolism. Iron is an essential element in all living organisms and is required as a cofactor for oxygen-binding proteins. Iron metabolism, oxygen homeostasis and erythropoiesis are consequently strongly interconnected. Iron needs to be tightly regulated, as iron insufficiency induces a hypoferric anemia in mammals, coupled to hypoxia in tissues, whereas excess iron is toxic, and causes generation of free radicals. Given the links between oxygen transport and iron metabolism, associations between the physiology of hypoxic response, and the control of iron availability are important. Numerous lines of investigation have proven that the HIF transcription factors function as central mediators of cellular adaptation to critically low oxygen levels in both normal and compromised tissues. Several of these target genes are involved in iron homeostasis, reflecting the molecular links between oxygen homeostasis and iron metabolism (Peyssonnaux et al, 2008). Copper is another factor in hypoxia. Cellular oxygen partial pressure is sensed by a family of prolyl-4-hydroxylase domain (PHD) enzymes that modify hypoxia-inducible factor (HIF)alpha subunits. Ceruloplasmin, the main copper transport protein in the plasma and a known HIF-1 target in vitro, was also induced in vivo in the liver of hypoxic mice. Both hypoxia and CuCl(2) increased ceruloplasmin (as well as vascular endothelial growth factor [VEGF] and glucose transporter 1 [Glut-1]) mRNA levels in hepatoma cells, which was due to transcriptional induction of the ceruloplasmin gene (CP) promoter (Martin et al, 2005). The authors of one of the studies cited above mused about the origins of all these problems, “Surprisingly little is known, however, about the natural history of such hypoxic cells” (Chaplin et al, 1986). Why do they appear? What is the source of tissue hypoxia? The answer is unknown and likely systemic but the conclusion is therefore, appearance, development and metastasis of tumors are based on cell hypoxia. References Brizel DM, Scully SP, Harrelson JM, Layfield LJ, Bean JM, Prosnitz LR, Dewhirst MW, Tumor oxygenation predicts for the likelihood of distant metastases in human soft tissue sarcoma, Cancer Research 1996, 56: p. 941-943. Chaplin DJ, Durand RE, Olive PL, Acute hypoxia in tumors: implications for modifiers of radiation effects, International Journal of Radiation, Oncology, Biology, Physics 1986 August; 12(8): p. 1279-1282. Denko NC, Fontana LA, Hudson KM, Sutphin PD, Raychaudhuri S, Altman R, Giaccia AJ, Investigating hypoxic tumor physiology through gene expression patterns, Oncogene 2003 September 1; 22(37): p. 5907-5914. Evans SM & Koch CJ, Prognostic significance of tumor oxygenation in humans, Cancer Letters 2003 May 30; 195(1): p. 1-16. Harris AL, Hypoxia: a key regulatory factor in tumor growth, National Review in Cancer 2002 January; 2(1): p. 38-47. Kunz M & Ibrahim SM, Molecular responses to hypoxia in tumor cells, Molecular Cancer 2003; 2: p. 23-31. Liao D, Johnson RS. Hypoxia: A key regulator of angiogenesis in cancer. Cancer and Metastasis Reviews. 2007 Volume 26, Number 2, 281-290, DOI: 10.1007/s10555-007-9066-y Martin F, Linden T, Katschinski DM, et al. Copper-dependent activation of hypoxia-inducible factor (HIF)-1: implications for ceruloplasmin regulation. Blood. 2005 Jun 15;105(12):4613-9. Peyssonnaux C, Nizet V, Johnson RS. Role of the hypoxia inducible factors HIF in iron metabolism. Cell Cycle. 2008 Jan 1;7(1):28-32. Rockwell S, Oxygen delivery: implications for the biology and therapy of solid tumors, Oncology Research 1997; 9(6-7): p. 383-390. Ryan H, Lo J, Johnson RS, The hypoxia inducible factor-1 gene is required for embryogenesis and solid tumor formation, EMBO Journal 1998, 17: p. 3005-3015. Ryan HE, Poloni M, McNulty W, Elson D, Gassmann M, Arbeit JM, Johnson RS, Hypoxia-inducible factor-1 is a positive factor in solid tumor growth, Cancer Res, August 1, 2000; 60(15): p. 4010 – 4015. Schmaltz C, Hardenbergh PH, Wells A, Fisher DE, Regulation of proliferation-survival decisions during tumor cell hypoxia, Molecular and Cellular Biology 1998 May, 18(5): p. 2845-2854. UCSD. (n.d.) http://biology.ucsd.edu/labs/johnson/cancer.html