Fuelling the fire : Immunometabolism of monocytes and macrophages in obesity & diabetes

Summary

The prevalence of obesity has tripled since 1975 and is projected to exponentially increase in the coming years. Obesity increases the risk of diabetes mellitus and various other complications, which may be partly driven by aberrations in innate immune cell functioning. Intracellular metabolism can be an important driver of functional properties in innate immune cells. By driving immune cell dysfunction, it may also contribute to disease progression during obesity and diabetes mellitus. For the development of diabetes mellitus and the progression of its long-term complications, dysfunctional innate immune cells form a common denominator. In this thesis, we aimed to explore the role of immunometabolism in monocytes and macrophages in the development of diabetes mellitus and associated complications. Furthermore, we investigated whether immunometabolism could be targeted in these cells to avert disease development or progression.

Increased aerobic glycolysis is a metabolic hallmark of immune cell activation. To understand the close interplay between glycolytic metabolism and functional output in humans, we define the contribution of aerobic glycolysis to the production of cytokines in chapter 2. Our results demonstrate that although different PAMPs lead to robust upregulation of cytokine and lactate production, the strength of the association between these two parameters can differ. Additionally, the production of specific cytokines is differentially associated with the upregulation of aerobic glycolysis, with stronger associations for IL-10 and IL-1RA. Inter-individual variations in lactate production revealed groups with different glycolytic flexibility, linked to the magnitude of cytokine responses. In patients with T1DM, associations between lactate and cytokine production were generally similar, although slightly attenuated, compared with healthy subjects. Overall, intra-individual differences in immune cell responses could be driven by differences in aerobic glycolysis, although specific TLRs and cytokines differentially rely on the use of aerobic glycolysis.

Many diabetes-related complications, including increased susceptibility to infections and increased cardiovascular risk, suggest an inadequate functioning of innate immune cells. Several studies have indeed demonstrated innate immune dysfunction in diabetes mellitus. However, a link to metabolic rewiring of immune cells in the diabetic microenvironment driving these functional alterations was lacking. In chapter 3, we examined diabetes-dependent alterations in monocyte function and metabolism. We revealed that a high glycemic burden, reflected by high HbA1c levels, was coupled to reduced cytokine secretion in stimulated monocytes from patients with T1DM. Interestingly, decreased cytokine secretion was associated with increased relative glycolytic rates. High HbA1c levels were subsequently linked to a pro-inflammatory transcriptional signature in circulating monocytes. This observation suggests the existence of immune tolerance in monocytes from patients with a high glycemic burden, where chronic inflammatory activation of circulating monocytes may lead to immune dysfunction upon acute activation. Together, these findings could partly explain the increased risk of infections and cardiovascular disease in patients with diabetes mellitus.

Besides driving diabetes-related complications, aberrations in innate immune responses can also contribute to the development of diabetes mellitus itself. Obesity-driven inflammation of adipose tissue can promote peripheral and systemic resistance to insulin, advancing the development of T2DM. In chapter 4, we set out to elucidate the role of UCP2 in adipose tissue macrophages in the context of adipose tissue inflammation. We confirmed the importance of UCP2 in regulating both the inflammatory response and metabolism in LPS-activated macrophages. Deletion of UCP2 resulted in a generally attenuated pro-inflammatory response to LPS, whereas glycolytic and oxidative metabolism were upregulated. However, the metabolic differences were normalized in a lipid-rich environment, potentially representative for the adipose tissue. These findings suggest that UCP2 is not a crucial component in controlling macrophage metabolism in a lipid-rich environment. Hence, in the context of obesity, deletion of UCP2 did not affect the development of adipose tissue inflammation or insulin resistance.

Metabolic activation of ATMs in obese adipose tissue leads to a unique rewiring of lipid metabolism, allowing macrophages to cope with a lipid-enriched environment. Eventually, macrophages residing in obese adipose tissue develop into foam cells characterized by excessive lipid droplet formation. To determine whether lipid accumulation itself contributes to ATM dysfunction and the development of adipose tissue inflammation or insulin resistance, we studied the function of lipid droplet-related protein HILPDA in macrophages in chapter 5. We found HILPDA expression to be strongly upregulated in obese adipose tissue, where it colocalized with crown-like structures. Fatty acids and triglycerides induced HILPDA expression, and specific myeloid deletion of HILPDA led to the abolishment of lipid droplet accumulation in macrophages after lipid loading. From a mechanistic perspective, our data revealed that HILPDA is a direct inhibitor of ATGL, and lack of lipid accumulation after HILPDA deletion was caused by enhanced ATGL-mediated lipolysis. The decreased accumulation of lipid droplets after HILPDA deletion was confirmed in adipose tissue macrophages isolated from obese adipose tissue. However, decreased lipid droplet accumulation did not lead to altered secretion of cytokines from ATMs, nor did it affect the development of adipose tissue inflammation and insulin resistance. Based on these data, we show that excessive lipid droplet accumulation in ATMs is not the sole driver of adipose tissue inflammation.

Besides the lipid-rich environment of the obese adipose tissue, the accumulation of triglycerides in lipid droplets also characterizes the metabolic response of macrophages toward classical inflammatory stimuli. In chapter 6, we studied triglyceride accumulation in pro-inflammatory macrophages and the involvement of HILPDA and ATGL in this process. The expression of HILPDA in response to inflammatory TLR ligands corresponded with the visual accumulation of lipid droplets, especially after treatment with LPS. Decreased expression of ATGL was found to be an important contributor to lipid droplet accumulation after the inflammatory activation of macrophages. Our data revealed that HILPDA is directly involved in enhancing the proteasomal degradation of ATGL. Specific deletion of HILPDA in macrophages emphasized their inflammatory phenotype in response to LPS, characterized by increased production of PGE2 and IL-6 both ex vivo and in vitro. Together, our findings suggest that ATGL-mediated lipolysis is partly responsible for the production of PGE2, which can, in turn, enhance the production of IL-6 and regulate the inflammatory response in macrophages.