Hypoxia-inducible factors (HIFs) are transcriptional activator complexes that perform a central role in the expression of oxygen-regulated genes. These genes are involved in the proliferation and apoptosis of cells, angiogenesis, erythropoiesis, energy metabolism, vasomotor function, and so on [15,16]. Thus, HIFs are essential for normal growth and development and also participate in the pathological processes, including tumor progression and tissue regeneration [17]. Heterodimeric transcription factors (HIFs) complex are composed of α-subunits (HIF-1α, HIF-2α, and HIF-3α) and the β-subunit (HIF-1β)/aryl hydrocarbon receptor nuclear translocator (ARNT). HIF-1β/ARNT is expressed stably in cells, whereas HIF-αs are degraded under the condition of normal oxygen bioavailability and accumulate rapidly in a hypoxic environment. HIF-1α, HIF-2α, and HIF-3α bind to HIF-1β to form HIF-1, HIF-2, and HIF-3, respectively. Thus, the stability of the HIF-1α subunit seems to determine HIF-1 formation. Similarly, the formation of HIF-2 is mainly determined by the abundance of the HIF-2α subunit.

In mammalian cells, three HIF-α subunit isoforms (HIF-1α, HIF-2α, and HIF-3α) are encoded by three HIF-α genes: HIF1A, HIF2A, and HIF3A, respectively. When oxygen concentration drops to <5%, HIF-1α is stably expressed, enters the nucleus, dimerizes with HIF-1β, and binds to HIF-response elements (HRE) of targeted gene promoters [16]. When oxygen is abundant in cells (>5%), the Prolyl-4-hydroxylases (PHDs) bind to HIF-1α and hydroxylate the proline residues, which leads to the recruitment of the Von Hippel-Landau (VHL) tumor suppressor E3 ligase complex. Eventually, the proteasomal is poly-ubiquitylated and degraded [18]. In addition, factor inhibiting HIF (FIH) also restricts the binding of HIF-αs to transcriptional co-activators CBP/p300 through hydroxylating (N-terminal) asparaginyl residues when oxygen is abundant [19]. HIF-2α is regulated by oxygen in a similar manner to HIF-1α. In addition to intracellular oxygen tension, several growth factors can also regulate HIF-α subunits in a hypoxia-independent way [20]. HIF-1 has been studied more extensively than HIF-2, and HIF-1 and HIF-2 have overlapping and unique biological functions. It is reported that HIF-1α responds to acute hypoxia mainly, whereas HIF-2α is the prime subunit that responded to chronic exposure to low oxygen at high altitudes [21]. HIF-1α is generally expressed in cells and regulates downstream genes, including VEGF, GLUT-1, AK-3, ALD-A, PGK-1, PFK-L, and LDH-A through binding to HRE to regulate many metabolic enzymes [22] (Maxwell, 1999). The role of HIF-1 in promoting angiogenesis also benefits cancer development. HIF-2 regulates erythropoiesis and vascularization and is essential for embryonic development [18]. In addition, HIF-2 is also involved in the progression and metastasis of solid tumors [23]. Another HIF-α protein, HIF-3α, can bind to ARNTs to restrain HIF-1α- or HIF-2α-mediated transcription, but its transcriptional capacity is weaker than other HIFs [16,24]. HIF-3α is relatively unknown in terms of regulating the hypoxia response, and many studies have shown that HIF-3α may play a dual part as a hypoxia-inducible transcription factor in recent years [25]. The determination of genome-wide binding of the human HIF-3 and its role requires extensive scientific research (Figure 1).

More recent studies have demonstrated the role of HIF-1 in bone growth and repair. Ref. [26] used spongy scaffolds that contained dimethyloxalylglycine (DMOG) in rat calvarial defects to imitate hypoxia to up-regulate HIF-1α, and found that angiogenesis was accelerated and bone regeneration was enhanced. Ref. [27] found that HIF-1α could facilitate osseointegration of tissue-engineered bone, dental implants, and new bone formation around implants, which was verified in a canine model. Another study has shown that expression of gingival HIF-1α protein in mice was apparently increased, and the ability of bone regeneration was enhanced at the onset of periodontitis resolution, after subcutaneous injection of 1,4-dihydrophenonthrolin-4-one-3-carboxylic acid (1, 4-DPCA/hydrogel), a hydrogel-formulated PHD inhibitor [28]. Gene ablation of phd2 in chondrocytes promotes endochondral osteogenesis through up-regulation of HIF-1α signaling, resulting in a significant growth of long bones and vertebrae [29]. In the process of bone regeneration, HIF-1α not only promotes angiogenesis but also regulates metabolic adaptations by inducing glycolysis transformation to promote cell survival [30]. Hence, HIF-1 serves an indispensable role in osteogenesis and bone restoration [26,31]. When osteoblasts and other associated cells sense reduced oxygen tension, intracellular HIF-1α is stably expressed to regulate the expression of the angiogenic and osteogenic genes [32]. Additionally, the mechanisms by which HIF-1 regulates downstream genes to promote osteogenesis and bone repair are quite complex. The role of HIF-1 in mediating downstream signaling to regulate bone mass in different animal models or cells is displayed in Table 1.