4.1.1. HIF-1 and Mitochondrial Population

The population of mitochondria is mainly related to the type of cells and energy demand. Generally speaking, there are more mitochondria in the cells with vigorous metabolism and more energy requirements. In the same type of cells, the number of mitochondria is relatively stable, while the mitochondrial population will be changed with the fluctuation of cell function. Thus, mitochondrial number is an important cue for cellular homeostasis.

The population of mitochondria is closely related to multiple mitochondrial production and degradation processes, including mitochondrial biogenesis, autophagy, and mitochondrial dynamics [23,24]. Numerous studies have found that HIF-1 mediates mitochondrial biogenesis, mitophagy, and mitochondrial dynamics to regulate mitochondrial population.Mitochondrial biogenesis can regulate mitochondrial number by the process which existing mitochondria generate new mitochondria to maintain mitochondrial homeostasis. Previous research has suggested that HIF-1 reduced mitochondrial number by inhibiting the PPAR-γ coactivator-1 (PGC-1) family members PGC-1α and PGC-1β, which are the essential transcription factors of mitochondrial biogenesis. Slot et al. [25] demonstrated that hypoxia inhibited PPAR/PGC-1α signaling and HIF-1α-mediated mitochondrial component expression in C2C12 cells. Lu et al. [26] demonstrated that in an in vitro model of mice genioglossus myoblast, the HIF-1α protein inhibited adenosine 5′-monophosphate-activated protein kinase (AMPK) under hypoxic conditions. Downregulation of HIF-1α increased the expression of the myogenin, PGC-1β and pAMPKα1, promoted the differentiation of myoblasts, and protected mitochondrial integrity. Inhibition of the AMPK pathway inhibited mitochondrial biogenesis, decreased the level of PGC-1β, and increased apoptosis. Zhang et al. [27] found that HIF-1 inhibited cancer-myc (C-MYC) activity in renal carcinoma cells lacking von Hippel–Lindau tumor suppressors (VHL). C-MYC activity was inhibited by HIF-1, which mediated these effects. As a result of the C-MYC-dependent reduction in PGC-1β expression, mitochondrial numbers were reduced, resulting in a low energy requirement for proliferation and survival of renal carcinoma cells.Autophagy in mitochondria is the process of catabolizing cytoplasmic organelles to eliminate defective structures or to recycle them for future use. It plays key physiological cellular functions and is an important mechanism for controlling mitochondrial number. Recent studies demonstrated that Bcl-2/adenovirus E1B 19kDa-interacting protein 3 (BNIP3), a known HIF-1 target gene that had been implicated in autophagy, could reduce mitochondrial population [28]. To ensure the adaptive metabolic responses of cells in hypoxia, this mechanism required the HIF-1-dependent expression of BNIP3 and constitutive expression of Beclin-1 and Atg5, which lowered mitochondrial numbers and prevented a increase in ROS [29]. However, Fu et al. [30] verified that HIF-1α-BNIP3-mediated mitophagy functioned as a protective mechanism for acute kidney damage by inhibiting apoptosis and ROS production. In addition, Madhu et al. [31] verified hypoxia regulated mitophagy, through HIF-1α, by controlling BNIP3 translocation to mitochondria.Mitochondrial dynamics refers to the dynamic process of mitochondrial fusion and fission that is controlled by GTPases. The key regulatory proteins included OMM fusion mitochondrial fusion protein 1/2 (Mfn1/2), while IMM fusion exogenous hydrogen sulfide regulates optic atrophy 1 (OPA1)-mediated mitochondrial fission dynamin-related protein 1 (DRP1) [32]. HIF-1 regulated the above-mentioned key proteins of mitochondrial fusion and fission, participated in mitochondrial dynamics, and affected the number of mitochondria. In the process of mitochondrial fission, on the one hand, most research indicated that HIF-1α positively regulated DRP-1 to promote mitochondrial fission. According to Marsboom et al. [33], HIF-1 activation caused mitochondrial fission in human models of pulmonary arterial hypertension (PAH) by phosphorylation of DRP1 at serine 616 via cyclin B1/CDK1-dependent inhibition, whereas in normal pulmonary smooth muscle cells (PASMC), DRP1-mediated fission was caused by HIF-1 activation by cobalt chloride (CoCl2) or desferrioxamine. Therefore, HIF-1α inhibition could reduce PASMC proliferation by preventing fission through DRP1 inhibition. As observed by Wan et al. [34], hypoxia increased Drp1 transcription and expression in glioblastoma U251 cells, while stimulating mitochondrial fission. Echinomycin’s inhibition of HIF-1α prevented the hypoxia-induced expression of Drp1. In particular, the Drp1 inhibitor Mdivi-1 effectively attenuated hypoxia-induced mitochondrial fission and migration in U251 cells. Additionally, it was also reported that HIF-1α negatively regulated DRP-1 and Mfn2 to reduce mitochondrial fission. According to Pan et al. [35], the HIF/miR125a/Mfn2 pathways regulated mitochondrial fission, which influenced PANC1 cell survival, growth, metabolism, and migration, posing a potential treatment for pancreatic cancer (PC). Jiang et al. [36] verified that HIF-1α exerted a protective effect against tubular injury in a mouse model of diabetic nephropathy (DN), which was mediated via modification of mitochondrial dynamics via HO-1 overexpression. In studies of mitochondrial fusion, it was found that HIF-1α affected Mfn1/2 to change mitochondrial fusion. Chiche et al. [37] revealed that when exposed to Hypoxia over long periods of time, some cancer cells were capable of evading apoptosis by initiating mitochondrial fusion, targeting BNIP3 and BNIP3L in mitochondrial membranes, and updating the expression of Mfn1. Therefore, these cells had a selective growth advantage; this was a process dependent on the regulation of HIF-1α. Chen et al. [38] found that in both PASMC and PAH rats, HIF-1α regulated mitochondrial dynamics during hypoxia-induced pulmonary vasculature remodeling by directly lowering Drp1 expression and increasing the expression of Mfn2 (Figure 1). 4.1.3. HIF-1 and Mitochondrial Structure

In normal physiological conditions, mitochondria are oblong under an electron microscope, with complete inner and outer membranes, cristae and matrix, and a clear intermembrane space. Mitochondria are connected in the cell, presenting a three-dimensional tubular network structure. Mitochondrial morphology is the structural basis for its function.

HIF-1 regulated mitochondrial morphology, such as size, shape, and structure, which might underlie functional changes or might be secondary to functional regulation [43,44]. Chiche et al. [37] incubated tumor cells in hypoxia, such as LS174Tr cells and renal cells carcinoma 786-Ocells, which had unusually enlarged mitochondria. Induced by tetracycline (Tet) of HIF-1α or the mutation of VHL, it was constitutive for stable HIF-1α protein through a HIF-1-dependent mechanism. Chen et al. [45] treated rat mesangial cells (RMC) with bicalutamide (Bic) to up-regulate the expression of HIF-1α. In the majority of the mitochondria, cristae were lacking and the mitochondria were moderately or severely swollen. It also indicated the impairment of mitochondrial metabolic function. Chen et al. [38] discovered that HIF-1α regulated Drp1 expression directly in hypoxia-induced pulmonary vascular remodeling.