Effect of Lipids and Lipoproteins on Hematopoietic Cell Metabolism and Commitment in Atherosclerosis

Hematopoiesis is the process that leads to multiple leukocyte lineage generation within the bone marrow. This process is maintained throughout life thanks to a nonstochastic division of hematopoietic stem cells (HSCs), where during each division, one daughter cell retains pluripotency while the other differentiates into a restricted multipotent progenitor (MPP) that converts into mature, committed circulating cell. This process is tightly regulated at the level of cellular metabolism and the shift from anaerobic glycolysis, typical of quiescent HSC, to oxidative metabolism fosters HSCs proliferation and commitment. Systemic and local factors influencing metabolism alter HSCs balance under pathological conditions, with chronic metabolic and inflammatory diseases driving HSCs commitment toward activated blood immune cell subsets. This is the case of atherosclerosis, where impaired systemic lipid metabolism affects HSCs epigenetics that reflects into increased differentiation toward activated circulating subsets. Aim of this review is to discuss the impact of lipids and lipoproteins on HSCs pathophysiology, with a focus on the molecular mechanisms influencing cellular metabolism. A better understanding of these aspects will shed light on innovative strategies to target atherosclerosis-associated inflammation.


PHYSIOLOGICAL REGULATION OF HEMATOPOIESIS
Hematopoiesis is the physiological process by which a small pool of Hematopoietic Stem Cells (HSCs), characterized by self-renewal and pluripotency, produces the variety of red blood cells and immunecompetent leukocytes circulating within our blood. Hematopoiesis is maintained lifelong thanks to the asymmetric scheme of division rate, where one daughter cell remains an HSC and the other differentiates into a restricted progenitor with limited self-renewal capacity. From these progenitor cells, downstream precursors differentiate and proliferate in order to provide different lineages in a nonstochastic hierarchy (Figure 1).
In humans, HSC are around 3000-10,000 per femur and their division rate is estimated between once every three months to once every year [12,13]. Therefore, a proportion of HSCs maintain a relatively constant phenotype of quiescence, while a small fraction differentiates towards multipotent progenitors (MPPs), which are characterized by massively increased differentiating potential and which will produce a nonstochastic proportion of downstream cellular products [14,15]. Single-cell RNA techniques recently highlighted that multiple MPPs exist which, despite all originating from a firstly committed MPP1 [16], develop an a priori hematopoietic cells from the circulation but also controls oxygen and nutrients availability to HSCs. Limiting oxidative metabolism minimizes the possibility to generate radical oxygen species (ROS) and DNA damage, this aspect becomes relevant over lifespan, as HSCs becomes less efficient to scan for and repair damages of the genomic heritage. This appears to be relevant when somatic mutations occur in specific loci such as the epigenetic regulators TET2 (ten-eleven translocation 2), DNMT3 (DNA nucleotide  (Figure 3). Only recently we have started appreciating the cross talk between nutrients supply and cellular utilization, suggesting that a deeper knowledge in this field is crucial to identify key metabolic checkpoints that could be targeted to promote HSCs immune-metabolic reprogramming during atherosclerosis and cardiovascular diseases. Main cellular immune-metabolic circuits involved in HSC commitment during atherogenesis. Cellular pathways with enhanced expression/activity are indicated with bold arrows while those down-regulation or with reduced activity are represented with thin arrows. "Gly", Glycolysis; "TCA", (Tricarboxylic Acid Cycle); "FAO", Fatty Acids Oxygenation; "ATP", Adenosine Tri-Phosphate; "GLUT-1", Glucose Transporte-1 isoform; "HIF1-alpha", Hypoxia Inducible Factor 1 alpha; "PDK", Pyruvate Dehydrogenase Kinase; "PPAR-δ", Peroxisome proliferator-activated receptor delta; "ROS", Reactive Oxygen Radical species; "ER", Endoplasmic Reticulum; "PL" Phospholipids; "Cer",Ceramids; "LAL", Lysosomial Acid Lipase; "CE", Cholesterol Esters; "LXR", Liver-X-Receptors"; "NLRP3", NOD-Like Receptor-3; "SREBP", Sterol regulatory element-binding proteins; "GM-CSF", granulocyte-macrophage colony-stimulating factor; "TET2", ten-eleven translocation 2; "CHIP", clonal haematopoiesis of indeterminate potential; "LDL-R", Low-Density Lipoproteins Receptor; "ApoE", Apolipoprotein E; "ABCG/A", ATP-Binding Cassette transporter G/A isoforms; "LPL", Lipoprotein Lipase; "FAs" Fatty Acids; "BMAT", Bone Marrow Adipose Tissue; "Angptl4", Angiopoietin Like 4 protein.  [52], lipid dietary sources that favors the activity of macrophage autophagy [53]. Accordingly, myeloid skewing of hematopoietic cells appears proportional to the quantity of white adipose tissue volume in the mid-shaft of the bones [54,55]. Still whether BMAT, whose increased volume correlates with extent of aortic atherosclerotic calcification and predicts occurrence of atherosclerotic cardiovascular events independently from risk factors [56], acts as site of energy storage to support bone marrow function and maturation towards myeloid subsets, or negatively regulate HSCs mobilization is heavily investigated.

CELL COMMITMENT IN INFLAMMATORY CONDITIONS
Additional mechanisms support hematopoietic differentiation under inflamed and atherosclerosis related conditions. Excessive glucose uptake support HSCs inflammatory skewing, and indeed glucose transporter (Glut1) deficiency in bone marrow cells limits excessive myelopoiesis and accelerated atherosclerosis in experimental models [57]. Besides to provide faster ATP replenishment, the switch to aerobic consumption of glucose coincides with the accumulation of TCA metabolites, that could restrain the hematopoietic pluripotency instead of being used as energetic substrates [58]. This effect seems to depend on the action of ATP citrate lyase (ACLY), that by converting mitochondrial citrate to Acetyl-CoA, seizes Acetyl-CoA from the TCA cycle diverting the molecule to histone acetylation and cholesterol synthesis [59]. Similar  It is worth to speculate whether these epigenetic adaptations, such as histone modifications [62], could persist even in differentiated cells, thus affecting atherosclerosis progression by forcing prolonged differentiation over time into more aggressive immune cells [63]. This "epigenetic trained phenotype" has been confirmed in vivo, since bone marrow-derived Indeed, the deficiency of Lysosomal Acid Lipase (LAL), the key enzyme which processes lipoproteins to cleave esterified cholesterol but also TGs to generate free cholesterol and FAs [73], results in cholesterol accumulation in the lysosome and abnormal number of HSCs, CMPS and GMPs, with enhanced capacity to form colonies within the niches and displaying reduced expression of apoptotic and checkpoints proteins [74].
In experimental models, LAL deficiency results into elevated circulating myeloid subsets, with elevated infiltrating capacity into inflamed tissues and improved ability to suppress lymphoid T cells proliferation [74]. These In addition, FAs act as building blocks for several macromolecules, including sphingolipids and phospholipids, which participate in HSC engagement and mobilization during atherosclerosis (Figure 3). In parallel, the identification of metabolic "checkpoints" that couple the reprogram of energetic machinery with immune cell functionality may offer innovative ways to target the inflammatory response associated to atherosclerosis. This is the case of (i) ACLY inhibition that by, reducing the acetyl-CoA pool required for histone acetylation, affects macrophage epigenetic program thus regulating TLR-driven gene expression after LPS stimulation [92,93]; (ii) LAL activity induction to boost the antiinflammatory potential of macrophages [94]; (iii) PPAR-δ antagonism that reduces macrophage IL-1β expression [95]. At the same time, the novel understanding of immune-metabolic crosstalk in HSCs could contribute to depict collateral effects of developing lipid-lowering therapies, as the recent application of Angptl4 inhibitors for the treatment of hypertriglyceridemia that, despite promising, could bear activation of LPL in macrophages thus promoting their inflammatory activation [84].
The growing interest in HSCs biology has proposed that most of the