Before discussing hypoxia pathways, it is crucial to differentiate between hypoxia training and hypoxia insult[
13]. Hypoxia training involves mild, intermittent, and controlled hypoxia stimuli designed to enhance an individual’s resistance and cellular adaptation to hypoxia insult. Examples include intermittent hypoxia cycles and frequent physical exercise, which may alleviate PD symptoms by improving hypoxia pathways[
12,
14,
15]. In contrast, hypoxia insult is typically caused by pathological processes, toxins, chronic hypoxia, or a severe decrease in ambient oxygen levels, leading to cellular injury and exacerbation of PD symptoms[
1,
16]. Different treatments, animal models, and
in vitro models reflect various hypoxia conditions, resulting in paradoxical effects within the same pathway. Understanding the nuanced role of hypoxia can help clarify the complex evidence surrounding hypoxia pathways.
At the molecular level, low oxygen levels reduce the availability of electron acceptors in the mitochondrial electron transport chain, leading to increased reactive oxygen species (ROS) production and diminished ATP synthesis. These changes can directly cause neuronal damage and, in rare cases, contribute to PD symptoms[
17,
18,
19]. Hypoxia-inducible factors (HIFs) and nuclear factor erythroid 2-related factor 2 (Nrf2) are central regulators of cellular adaptation to hypoxia, and their neuroprotective functions are well established [
20,
21,
22]. Hypoxic exposure has been shown to induce α-synuclein(α-syn) overexpression and oligomer formation, leading to cell injury in HEK293 cells[
23]. Given that hypoxia can influence the pathogenesis of PD, regulating HIF-1 and Nrf2 activity may be a crucial neuroprotective strategy for promoting the survival of dopaminergic neurons.
2.1. HIF Pathway
HIFs are transcription factors that respond to low oxygen levels by regulating molecular adaptations to maintain oxygen supply and energy metabolism[
24]. HIFs are part of the PER-ARNT-SIM (PAS) subfamily within the basic helix-loop-helix (bHLH) family of heterodimeric transcription factors[
25]. Currently, three HIF members have been identified: HIF-1, HIF-2 (also known as endothelial PAS domain-containing protein 1), and HIF-3[
26,
27,
28]. Northern blot analysis reveals minimal expression of HIF-3 in brain tissue, suggesting a limited connection between HIF-3 and PD[
29].
HIF-1 and HIF-2 are both heterodimeric transcription factors composed of α- and β-subunits. HIF-1α and HIF-2α share 48% amino acid sequence similarity, with 83% identity in their basic bHLH domains and approximately 70% homology in their PER-ARNT-SIM regions[
30]. Additionally, the oxygen-dependent degradation domains of these HIF-α subunits, including the two critical proline residues that interact with prolyl hydroxylase (PHD), also exhibit a high degree of homology[
31]. This significant sequence and structural similarity suggest that HIF-1 and HIF-2 have similar regulatory mechanisms involving PHD-von Hippel-Lindau (pVHL) complex and target genes.
Both HIF-1 and HIF-2 share the same β-subunit, HIF-1β, which is stably expressed and located in the nucleus, binding to hypoxia response elements (HREs), upstream of hypoxia-inducible genes[
32,
33]. Under normoxic conditions, HIF-α subunits are continuously degraded through hydroxylation, which is catalyzed by PHD and factor inhibiting HIF (FIH). Prolyl hydroxylation promotes interaction with the pVHL E3 ubiquitin ligase complex, leading to degradation by the 26S proteasome[
34]. During moderate hypoxia, PHD activity is impaired, but FIH can still perform hydroxylation. This allows HIF-α to enter the nucleus, where it binds with HIF-1β to form the functional HIF complex, which then binds to HREs to promote the transcription of target genes involved in PHD regulation.
Under severe hypoxia, reduced oxygen levels inhibit PHD activity, preventing the degradation of HIF-α subunits (both HIF-1α and HIF-2α)[
35]. Consequently, HIF-α escapes the ubiquitin-proteasome system (UPS), binds to the coactivator p300/CBP, and is transported into the nucleus to activate transcription with HIF-1β[
36]. Typically, these genes enhance oxygen supply to tissues and facilitate metabolic adaptation to hypoxia. Despite their considerable similarities, HIF-1 and HIF-2 exhibit distinct functions and spatio-temporal regulations[
37,
38].
HIF-1α is rapidly activated during acute, severe hypoxia (1–2% O
2). A higher prevalence of HIF-1α polymorphisms has been observed in PD patients, with some single nucleotide variants associated with an increased risk of developing PD (
Figure 1)[
39]. Hypoxia has been reported to upregulate the transcription and expression of tyrosine hydroxylase, the rate-limiting enzyme for dopamine synthesis, and the dopamine transporter (DAT), both crucial for dopaminergic neuronal function, via HIF-1α regulation[
1,
40]. Conditional knockout mice lacking HIF-1α exhibit reduced levels of vascular endothelial growth factor (VEGF) and TH in midbrain-derived neural precursor cells, leading to decreased differentiation of dopaminergic neurons[
41,
42,
43]. This evidence indicates that HIF-1α plays a crucial role in the survival and functioning of dopaminergic neurons in the SNpc region.
Additionally, activation of HIF-1α is associated with suppressed mitochondrial function. HIF-1α negatively regulates mitochondrial function by repressing the transcriptional activity and inhibiting the activity of cellular myelocytomatosis viral oncogene in VHL-deficient renal cell carcinoma, which may contribute to mitochondrial dysfunction in PD[
44]. However, the function of HIF-1α appears to be diminished in PD patients. Gene expression profiling analysis reveals decreased levels of HIF-1α and its target genes (including VEGF and hexokinase) in PD patients, with an upregulation of PHD2 in the SNpc homogenate of PD patients compared to age-matched controls.
The evidence suggests that HIF-1α exhibits promising neuroprotective functions against PD. However, paradoxical findings indicate that during severe hypoxia in the brain, HIF-1α may also facilitate inflammasome formation, mitochondrial dysfunction, and cell death[
45]. For instance, in neuron-specific conditional knockout mice, deficiency of neuronal HIF-1α and HIF-2α improves neuronal survival in the early acute phase following ischemic stroke. Conversely, similar experimental designs have yielded different outcomes, with some studies showing that neuron-specific HIF-1α knockdown increases tissue damage and reduces survival rates in response to transient focal cerebral ischemia. These discrepancies might be due to variations in murine stroke models used[
46]. Additionally, HIF-1α regulates genes involved in autophagy and apoptosis, such as BNIP3 and Noxa (or PMAIP1, phorbol-12-myristate-13-acetate-induced protein 1)[
47,
48]. This adds an extra layer of complexity to its role in cellular regulation, making HIF-1α a potential double-edged sword in the context of hypoxia, oxidative stress, or ischemia. While moderate stress levels might activate the neuroprotective aspects of HIF-1α, even slight deviations could trigger detrimental effects[
49]. These paradoxes highlight the intricate and sophisticated role of HIF-1α, which must be carefully considered in PD therapy development.
In contrast, HIF-2α gradually accumulates during prolonged, moderate hypoxia (<5% O
2) and regulates similar genes[
50,
51]. Although research on the correlation between HIF-2 and PD is limited, HIF-2 likely has functions that overlap significantly with those of HIF-1. This is suggested by the comparable upregulation of HIF-1-regulated genes observed in response to short hypoxic episodes in brain tissue from HIF-1 knockout mice[
52]. In post-mortem PD brains, accumulation of HIF-2α is observed as a marker of chronic hypoxia[
53]. Additionally, data suggest that HIF-2α promotes α-syn hyperphosphorylation at the serine 129 site (pS129-αSyn) and abnormal aggregation of pS129-αSyn by upregulating alkaline ceramidase 2 (Acer2), which leads to cognitive impairment in mice. This finding is consistent with elevated levels of pS129-αSyn in the plasma of patients with obstructive sleep apnea, a condition associated with chronic intermittent hypoxia[
54]. Thus, like HIF-1α, HIF-2α also exhibits paradoxical effects, potentially being either neuroprotective or detrimental to neurons.
2.2. Nrf2/HO-1 Pathway
As a central regulator of cellular redox states, Nrf2 plays a crucial role in protecting cells against oxidative stress by activating the transcription of a wide range of antioxidant and anti-inflammatory genes. This regulation impacts metabolic pathways and initiates the production of NADPH and ATP[
55]. Nrf2 is regulated by Kelch-like ECH-associated protein 1 (KEAP1), a substrate adaptor that forms a complex with Cullin-3 (CUL3) to target specific proteins for degradation. Under normal conditions, KEAP1 and CUL3 form a ubiquitin E3 ligase complex that polyubiquitinates Nrf2, leading to its rapid degradation via the proteasome system. Whether under normoxia or hypoxia, Nrf2 degradation through the UPS depends on direct interaction with KEAP1, but this degradation is more rapid in normoxia than in hypoxia[
56], aligning with Nrf2’s role as a hypoxia regulator [
57].
Under oxidative stress or specific conditions, Nrf2 translocates from the cytoplasm to the nucleus, where it forms a heterodimer with small MAF (sMAF) proteins and binds to the antioxidant response element (ARE) to activate a range of phase II detoxification genes, thus functioning as a major regulatory transcription factor (
Figure 2). The beneficial role of Nrf2 in neurodegenerative conditions and the potential of specific Nrf2 activators as therapeutic agents are well-documented[
58]. While the antioxidant functions of Nrf2 have been a primary focus,
in vitro studies also show that Nrf2 accelerates the clearance of α-syn and mutant leucine-rich repeat kinase 2 (LRRK2) through the UPS[
59]. As a transcription factor, Nrf2 exerts its neuroprotective effects by activating multiple protective genes involved in various molecular events of PD[
60]. Among these genes, one notable candidate is heme oxygenase-1 (HO-1).
HO-1 is a potent antioxidant enzyme that degrades heme into carbon monoxide, free iron, and biliverdin[
61]. The promoter region of the HO-1 gene contains various binding sites for transcription factors such as Nrf2 and HIF-1[
62]. Significantly elevated levels of HO-1 have been observed in the plasma of early-stage PD patients and in the cerebrospinal fluid of children following severe traumatic brain injury[
63]. These observations suggest an underlying connection between HO-1 and PD.
Consistent with findings on Nrf2,
in vitro studies have shown that overexpression of HO-1 enhances the degradation of α-syn through the UPS. The α-syn mutant A30P exhibits resistance to HO-1-dependent degradation, leading to increased toxic intracellular aggregation. This finding suggests that Nrf2 may facilitate α-syn clearance through the Nrf2/HO-1 pathway[
64]. Furthermore, HO-1 helps prevent dopaminergic neuronal death by promoting the expression of neurotrophic factors and enhancing the antioxidant response in both
in vivo and
in vitro PD models[
65]. Chemicals that activate the Nrf2/HO-1 pathway have demonstrated therapeutic effects against PD models induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 1-Methyl-4-phenylpyridinium (MPP+), and other agents[
66,
67]. Nevertheless, the neuroprotective function of HO-1 appears to be closely associated with both the Nrf2 and HIF-1 pathways, highlighting the potential of targeting the Nrf2/HO-1 and HIF-1/HO-1 pathways for PD therapy.