**3. HIF regulation in mitochondria metabolic change**

HIF is the central master regulator of adaptation to decreased oxygen availability in both physiological and pathological conditions. It is evolutionary pressure to reestablish metabolic balance to allow normal tissue and/or even tumor to survive. Physiologically, in the woundhealing area, damaged tissue leads to hypoxia and facilitates vascular growth. However, pathologically, in the solid tumor region, oxygen demand is in continuous increase due to the uncontrollable growth of the cancer cell. Hypoxia also represents the unifying feature of the microenvironment of solid tumors. The adaptive changes of tumor survival pattern referred to as "hypoxia tumor phenotype" are greatly noticed.

#### **3.1. HIF regulation of metabolic change**

HIF upregulation in tumors plays a central role in metabolic switch from aerobic metabolism to anaerobic metabolism. In turn, all the enzymes (e.g., aldolase A and C, enolase 1, hexokinase 1 and 2, pyruvate kinase M (PKM), phosphofructokinase) and glucose transporters (GLUT1, GLUT3) involved in glycolytic pathway are upregulated [16]. Moreover, conversion of pyruvate to acetyl-CoA, TCA cycle, and mitochondrial biogenesis are inhibited through downregulation of pyruvate dehydrogenase kinase (PDK) 1 and 3 [17, 18]. Even though the glycolysis produces far less energy than TCA cycle per glucose molecule, it has a significant higher throughout. In addition, the accumulated by-products could be used as sources of carbon to produce nucleotides and lipids for proliferating cells [19]. The classic view of metabolism is that of a self-correction of homeostasis responding to microenvironment. In this model, for cancer to arise, tumor hypoxia selects cells depending on anaerobic metabolism [20]. Secondary mutations are needed to give cells the ability to transform the capability to alter existing cell metabolism in a way that supports cell growth. One example is that of mouse embryonic fibroblasts that reduce oxygen consumption when switching from 20% O2 to 1% O2 , and continued low oxygen consumption when returning to 20% O2 , suggesting HIF stable modified metabolic reprogramming [21].

The direct consequence of glycolysis is the production of lactic acid by hypoxic tumor cells leading to tumor acidosis. Intracellular acidosis poses a threat to cell survival. Readjusting intracellular pH (pHi) is a critical strategy to protect against apoptosis and cell death. HIF upregulated monocarboxylate transporter 4 (MCT4) and Na<sup>+</sup> /H<sup>+</sup> exchanger (NHE1) facilitate exportation of H+ [22, 23]. Moreover, two transmembrane carbonic anhydrases (CAs) catalyze CO2 to be hydrated to HCO3 − and H+ , CA IX, and XII overexpressed in tumors also regulated by HIF. This reaction facilitates proton generation in the extracellular space, which contributes to acidification in tumor microenvironment, while preventing acidification of intercellular milieu of cancer cell [24].
