Clinical associations and potential cellular mechanisms linking G6PD deficiency and atherosclerotic cardiovascular disease

Fig. 1. Overview of epidemiological associations between G6PD deficiency, cardiovascular disease, and immunological disease. a G6PD is the rate-limiting enzyme in the pentose phosphate pathway (PPP), producing NADPH and other biosynthetic molecules necessary for the synthesis of ribonucleic acids, fatty acids, nucleotides, and amino acids. NADPH produced by the PPP is critical for maintaining redox homeostasis and, conversely, for the production of reactive oxygen species by NADPH oxidases. b, c G6PD deficiency is associated with worsened cardiovascular disease outcomes and a higher incidence of immune-related diseases in epidemiological studies. However, the effect of G6PD-d and a dysfunctional PPP on immune cell function in the context of atherosclerosis has yet to be explored.

Fig. 2. Proposed mechanisms linking G6PD deficiency in vascular cells to increased CVD. G6PD-d reduces the production of cellular NADPH. a NADPH is a necessary cofactor for nitric oxide (NO) synthesis by endothelial nitric oxide synthase (eNOS). G6PD-d therefore decreases bioavailable NO, potentially leading to vasoconstriction, increased vascular permeability, increased leukocyte adhesion, vascular smooth muscle cell (VSMC) proliferation, and reduced LDL clearance through insufficient NO-modification of LDL receptor (LDLR) adaptor protein ARH. b NADPH is also a necessary cofactor for the generation of glutathione (GSH), which is a key antioxidant. Decreased GSH leads to increased ROS. This may lead to vascular endothelial cell (VEC) damage and death, pro-inflammatory cytokine secretion, and expression of leukocyte adhesion molecules that recruit inflammatory immune mediators of atherosclerosis. ROS accumulation may also contribute to increased oxidation of low density lipoprotein (LDL) into oxLDL, which is inflammatory and pathogenic in ASCVD. c Interleukin 10 (IL-10) production by monocytes and other immune cells may be decreased in humans with G6PD-d, though this may be due to the…

Fig. 3. Red blood cell phenotype of G6PD-d and potential related mechanisms to explain increased CVD. G6PD-d can lead to both intravascular and extravascular red blood cell (RBC) destruction. a Although minimal, acute intravascular hemolysis can occur when G6PD-d individuals consume pro-oxidant medications or potentially due to chronic pro-oxidant stressors such as hyperlipidemia. Hemolysis releases heme and iron into the intravascular space. Free heme can chelate NO, thereby reducing bioavailable NO, and can directly generate powerful reactive nitrogen species (ONOO-, peroxynitrite). Together, these can worsen VEC oxidative stress as well as the NO-depleted phenotype previously discussed (vasoconstriction, vascular permeability, leukocyte adhesion, VSMC proliferation, LDL clearance). b Extravascular destruction of oxidatively-damaged RBCs by the reticuloendothelial system accounts for a majority of RBC destruction in G6PD-d. Increased vascular permeability due to NO-depletion may increase RBC infiltration into the vascular endothelium where RBCs are further oxidized (oxRBC). Macrophage (Mφ) consumption of oxRBCs and subsequent iron overload may lead to foam cell formation as well as…

Fig. 4. ROS as important mediators of immune cell signaling. a G6PD is important for the generation of NADPH and ribose-5-phosphate (R5P). These molecules support several reductive biosynthetic pathways including fatty acid, amino acid and nucleotide biosynthesis. In turn, these processes support cellular growth and proliferation. G6PD-d therefore may hinder growth and proliferation. b ROS are generated by NADPH-dependent and NADPH-independent processes. For example, NADPH oxidases (NOX) generate NADPH-dependent ROS while mitochondrial membrane proteins (i.e. complexes I and III), peroxisomal β-oxidation, and various other enzymes generate NADPH-independent ROS. Glutathione and thioredoxin represent two important antioxidant molecules that resolve accumulated ROS and rely on a functional G6PD enzyme for regeneration. c ROS are generated upon engagement of several immune cell receptors including toll-like receptors (TLR), B cell receptors (BCR), and T cell receptors (TCR). ROS from different sources can affect protein phosphatases, kinases and transcription factors, ultimately altering several different signaling pathways. Examples of such signaling pathways, proteins or protein complexes…

Fig. 5. Proposed effects of G6PD-d on macrophage function in ASCVD. a G6PD-d monocytes accumulate ROS at baseline that paradoxically upregulate NOX. G6PD-d monocytes may be polarized to an M1-like phenotype characterized by the production of proinflammatory TNFɑ and MCP-1. Concurrently, reduction in M2 polarization may reduce anti-inflammatory TGFβ expression. This effect of G6PD-d would be proatherogenic though further studies are needed. b Plaque resident macrophages normally clear foam cells and debris from atherosclerotic plaques through efferocytosis, thereby preventing plaque growth and necrotic core formation. This process is inherently oxidative and relies on G6PD/PPP to maintain both redox homeostasis and continual efferocytosis. In G6PD-d, efferocytes may therefore be more prone to oxidative damage and death. Efferocyte death impairs clearance of apoptotic cells and debris while also increasing inflammation through the release of DAMPs. Further studies are needed to assess this effect in the setting of atherosclerosis.

Fig. 6. Proposed effect of G6PD-d on T cell function in ASCVD. G6PD-d has generally been shown to reduce cytokine production by T cells. The impact of this effect on atherosclerosis would be dependent on whether proatherogenic or atheroprotective T cell subsets, and their cytokine production, are affected more. a Th1 and Tfh cells are considered proatherogenic. Given a reduction in their cytokines would be atheroprotective, the CVD phenotype of G6PD-d would likely not be driven by changes in these cells. b Similar to Th1 and Tfh cells, CD8 + T cell expression of TNFɑ and IFNγ is reduced by G6PD inhibition and would be atheroprotective. G6PD activity also supports granzyme B production. Reduction in granzyme B-mediated destruction of VSMCs and VECs would be atheroprotective, but impaired killing of inflammatory T cells and APCs may represent a potential proatherogenic mechanism of G6PD-d. c The roles of Th2, Th17 and Th22 cells in atherosclerosis are unclear with both atheroprotective and proatherogenic functions reported. Like Th1 and CD8 + T cells, G6PD-d may limit cytokine production by these subsets but the impact of atherosclerosis depends on whether the proatherogenic or…

Fig. 7. G6PD-d and B cells in ASCVD. a B cell subsets can be atheroprotective (B1a, B1b, MZB) or proatherogenic (FoB). Atheroprotective B cells produce IgM to oxidation specific epitopes (IgMOSE). Proatherogenic FoB cells produce IgG (IgGOSE) which can form immune complexes with oxLDL that promote inflammatory macrophage responses or may sequester and clear oxLDL, depending on the IgG subclass and its receptor. In G6PD-d, plasma cell differentiation, IgMOSE and IgGOSE may all be reduced due to b impaired ROS production following BCR/TLR engagement and c impaired redox homeostasis inhibiting proper oxidative protein folding. d ROS may also inhibit BLIMP-1 expression, which is critical in promoting plasma differentiation and IgM production. e B1a and B1b cells require fatty acids (FA) to sustain self-renewal and survival. They acquire a significant proportion of FA through autophagy. However, in G6PD-d, deficient NADPH leads to reduced FA synthesis and accumulation of ROS due to insufficient GSH. ROS accumulation may then impair autophagy. Together, these factors may contribute to reduced self-renewal of important atheroprotective B cells. The increased ROS may also inhibit B1 migration to…