Metabolic syndrome results from multiple risk factors that arise from insulin resistance induced by abnormal fat deposition. Chronic inflammation owing to obesity primarily results from the recruitment of pro-inflammatory M1 macrophages into the adipose tissue stroma, as the adipocytes within become hypertrophied. During obesity-induced inflammation in adipose tissue, pro-inflammatory cytokines are produced by macrophages and recruit further pro-inflammatory immune cells into the adipose tissue to boost the immune response. Here, we provide an overview of the biology of macrophages in adipose tissue and the relationship between other immune cells, such as CD4+ T cells, natural killer cells, and innate lymphoid cells, and obesity and type 2 diabetes. Finally, we discuss the link between the human pathology and immune response and metabolism and further highlight potential therapeutic targets for the treatment of metabolic disorders.

In vertebrates, the initial step in heme biosynthesis is the production of 5-aminolevulinic acid (ALA) by ALA synthase (ALAS). ALA formation is believed to be the rate-limiting step for cellular heme production. Recently, several cohort studies have demonstrated the potential of ALA as a treatment for individuals with prediabetes and type-2 diabetes mellitus. These studies imply that a mechanism exists by which ALA or heme can control glucose metabolism. The ALAS1 gene encodes a ubiquitously expressed isozyme. Mice heterozygous null for ALAS1 (A1^+/-s) experience impaired glucose tolerance (IGT) and insulin resistance (IR) beyond 20-weeks of age (aged A1^+/-s). IGT and IR were remedied in aged A1^+/-s by the oral administration of ALA for 1 week. However, the positive effect of ALA proved to be reversible and was lost upon termination of ALA administration. In the skeletal muscle of aged A1^+/-s an attenuation of mitochondrial function is observed, coinciding with IGT and IR. Oral administration of ALA for 1-week brought about only a partial improvement in mitochondrial activity however, a 6-week period of ALA treatment was sufficient to remedy mitochondrial function. Studies on differentiated C2C12 myocytes indicate that the impairment of glucose metabolism is a cell autonomous effect and that ALA deficiency ultimately leads to heme depletion. This sequela is evidenced by a reduction of glucose uptake in C2C12 cells following the knockdown of ALAS1 or the inhibition of heme biosynthesis by succinylacetone. Our data provide in vivo proof that ALA deficiency attenuates mitochondrial function, and causes IGT and IR in an age-dependent manner. The data reveals an unexpected metabolic link between heme and glucose that is relevant to the pathogenesis of IGT/IR.

Heme serves as a prosthetic group in hemoproteins, including subunits of the mammalian mitochondrial electron transfer chain. The first enzyme in vertebrate heme biosynthesis, 5-aminolevulinic acid synthase 1 (ALAS1), is ubiquitously expressed and essential for producing 5-aminolevulinic acid (ALA). We previously showed that Alas1 heterozygous mice at 20–35 weeks (aged-A1^+/−s) manifested impaired glucose metabolism, mitochondrial malformation in skeletal muscle, and reduced exercise tolerance, potentially linked to autophagy dysfunction. In this study, we investigated autophagy in A1^+/−s and a sarcopenic phenotype in A1^+/−s at 75–95 weeks (senile-A1^+/−s). Senile-A1^+/−s exhibited significantly reduced body and gastrocnemius muscle weight, and muscle strength, indicating an accelerated sarcopenic phenotype. Decreases in total LC3 and LC3-II protein and Map1lc3a mRNA levels were observed in aged-A1^+/−s under fasting conditions and in Alas1 knockdown myocyte-differentiated C2C12 cells (A1KD-C2C12s) cultured in high- or low-glucose medium. ALA treatment largely reversed these declines. Reduced AMP-activated protein kinase (AMPK) signaling was associated with decreased autophagy in aged-A1^+/−s and A1KD-C2C12s. AMPK modulation using AICAR (activator) and dorsomorphin (inhibitor) affected LC3 protein levels in an AMPK-dependent manner. Our findings suggest that heme deficiency contributes to accelerated sarcopenia-like defects and reduced autophagy in skeletal muscle, primarily due to decreased AMPK signaling.

5-Aminolevulinic acid (ALA) is the rate-limiting intermediate in heme biosynthesis in vertebrate species; a reaction catalyzed by the mitochondrial ALA synthase 1 (ALAS1) enzyme. Previously we reported that knockdown of the ubiquitously expressed ALAS1 gene in mice disrupts normal glucose metabolism, attenuates mitochondrial function and results in a prediabetic like phenotype when animals pass 20-weeks of age (Saitoh et al., 2018). Contrary to our expectations, the cytosolic and mitochondrial heme content of ALAS1 heterozygous (A1+/-) mice were similar to WT animals. Therefore, we speculated that regulatory "free heme" may be reduced in an age dependent manner in A1+/- mice, but not total heme. Here, we examine free and total heme from the skeletal muscle and liver of WT and A1+/- mice using a modified acetone extraction method and examine the effects of aging on free heme by comparing the amounts at 8-12 weeks and 30-36 weeks of age, in addition to the mRNA abundance of ALAS1. We found an age-dependent reduction in free heme in the skeletal muscle and liver of A1+/- mice, while WT mice showed only a slight decrease in the liver. Total heme levels showed no significant difference between young and aged WT and A1+/- mice. ALAS1 mRNA levels showed an age-dependent reduction similar to that of free heme levels, indicating that ALAS1 mRNA expression levels are a major determinant for free heme levels. The free heme pools in skeletal muscle tissue were almost 2-fold larger than that of liver tissue, suggesting that the heme pool varies across different tissue types. The expression of heme oxygenase 1 (HO-1) mRNA, which is expressed proportionally to the amount of free heme, were similar to those of free heme levels. Taken together, this study demonstrates that the free heme pool differs across tissues, and that an age-dependent reduction in free heme levels is accelerated in mice heterozygous for ALAS1, which could account for the prediabetic phenotype and mitochondrial abnormality observed in these animals.