1. Insulin-Mediated Glucose Uptake

Skeletal muscle accounts for 30-40% of body mass. As the major metabolic tissue of the body, it plays an important role in the whole-body metabolism and homeostasis. Skeletal muscle tissue is responsible for over 80% of insulin-induced glucose uptake in the postprandial state. The molecular mechanisms of insulin-mediated glucose transport are intensively studied. GLUT4 (glucose transporter type 4) is responsible for glucose uptake into adipocytes and muscle tissue.

The GLUT4 vesicles are mainly found perinuclearly at a basal state translocated to the plasma membrane by insulin-regulated vesicular traffic leading to glucose transport into the cells and a simultaneous decrease in blood glucose.

Significantly, in the case of insulin resistance and type-2 diabetes mellitus, the amount of GLUT4 is decreased, and its translocation is impaired. The insulin receptor signaling involves the insulin receptor substrate- (IRS-) 1-mediated activation of PI3K (phosphatidylinositol-3-kinase), resulting in Akt2 activation, inhibition of the Akt2 substrate AS160 (Akt substrate 160, a GTPase-activating protein for Rabs), and consequently, the activation of Rab8a and Rab14 GTPases in muscle cells. The PI3K can also activate the Rho-family GTPase Rac1 involved in the remodeling of a cortical actin network by regulating the Arp2/3 complex and cofilin influencing GLUT4 translocation. The joint activation of the PI3K-mediated Akt2/AS160 and Rac1 signaling pathways is necessary for the translocation of GLUT4.

Astaxanthin improves insulin sensitivity and glucose uptake. The oxidative stress can lead to insulin resistance by activating various kinases such as JNK, which catalyze the phosphorylation of serine residues in IRS-1, inhibiting its activity and preventing its interaction with the insulin receptor.

Furthermore, oxidative stress switches the GLUT4 sorting to the degradation of GLUT4 vesicles. The dietary astaxanthin administration improves insulin sensitivity, IRS-1 activation, Akt phosphorylation, and GLUT4 translocation in skeletal muscle leading to increased insulin sensitivity and a decrease in blood glucose level.

  1. Oxidative Stress and Insulin Resistance

Insulin resistance is a definition of the insufficient response of tissues to the effect of insulin, resulting in decreased insulin-mediated glucose uptake into the skeletal muscle, increased hepatic glucose production in the liver, and impaired suppression of lipolysis in adipose tissue.

The development of insulin resistance is an intensively studied, complex, and not fully known process. It has been widely reported that mitochondrial dysfunction is linked to insulin resistance and type-2 diabetes mellitus.

However, it remains unclear whether perturbations in mitochondrial functional capacity are causes, consequences, or key contributors to insulin resistance. Mitochondria are the primary sources of reactive oxygen species. Insulin resistance is characterized by inefficient mitochondrial coupling, a low level of ATP, and the formation of an excessive amount of ROS despite the normal to high oxygen consumption. Many studies suggested that the loss in mitochondrial content and function and consequently the decreased mitochondrial oxidative capacity lead to low lipid oxidation and accumulation of lipid excess resulting in the development of insulin resistance.

The accumulation of ROS can activate various kinases such as PKCs (protein kinases C), IKK β (inhibitor κB kinase-β), JNK (c-Jun N-terminal kinase), and p38 MAPK (mitogen-activated protein kinase), which induce the phosphorylation of serine residues in IRS-1 leading to the inhibition of its activity and directing it to proteasomal degradation.

JNK1 has a crucial role in developing insulin resistance by inhibiting IRS activity via phosphorylation of Ser307 residue preventing its interaction with the insulin receptor. Furthermore, oxidative stress inhibits the retromer function in a casein kinase-2- (CK2-) dependent manner leading to the sorting of GLUT4 vesicles to lysosomes for degradation.

  1. Astaxanthin Treatment, Insulin Sensitivity, and Muscle Metabolism

Astaxanthin accumulated in skeletal muscle and was shown to reduce hyperglycemia and lessen insulin secretion and sensitivity by improving glucose metabolism and β-cell dysfunction by GLUT4 regulation. Astaxanthin administration increased the translocation of GLUT4 transporter and also insulin-dependent glucose uptake in line with increased phosphorylation of IRS-1 tyrosine and Akt and a decrease in JNK and IRS-1 Ser307 phosphorylation in L6 muscle cells. Inflammatory cytokines (e.g., TNFα, tumor necrosis factor α) and fatty acids are released from adipose tissue and are major contributors to insulin resistance. Palmitate generates ceramide, which triggers mitochondrial oxidative stress and insulin resistance, and the role of TNFα in the generation of insulin resistance was also shown. Significantly, astaxanthin treatment restored TNFα- and palmitate-induced insulin resistance and decreased ROS generation of L6 muscle cells.

Astaxanthin treatment (8 mg/day, eight weeks) was effective in patients with type-2 diabetes mellitus: reduced visceral fat mass, serum triglyceride, very-low-density lipoprotein cholesterol concentration, and decreased systolic blood pressure. Furthermore, astaxanthin significantly reduced the fructosamine and plasma glucose concentration.

In a metabolic syndrome model SHR/NDmcr-cp (cp/cp) rats where spontaneous hypertension, obesity, hyperinsulinemia, and mild hyperlipidemia evolved, the astaxanthin treatment (22 weeks) ameliorated features of metabolic syndrome: improved insulin resistance; decreased fasting blood glucose, homeostasis of insulin resistance (HOMA-IR), triglyceride, and fatty acid levels; and induced a significant reduction of arterial blood pressure and the size of fat cells in white adipose tissue.

Astaxanthin ameliorated high-fat, cholesterol, and cholate diet-induced glucose intolerance and reversed hepatic inflammation and fibrosis in C57BL/6J mice. Moreover, astaxanthin administration in a type-2 diabetic db/db (leptin receptor mutated) mice improved the intraperitoneal glucose tolerance test and protected pancreatic β-cells against glucose toxicity by reducing blood glucose concentration and hyperglycemia-induced oxidative stress.

Insulin resistance can also be detected in another animal model (high fat and high fructose diet-fed mice). The astaxanthin treatment improved their insulin sensitivity parameters: lowered insulin and glucose levels in the plasma, ameliorated insulin signaling, and enhanced Akt phosphorylation and GLUT4 translocation in skeletal muscle.

The number and function of mitochondria influence the fatty acid utilization of the skeletal muscle. The peroxisome proliferator-activated receptor-γ coactivator-1a (PGC-1α) is a key transcriptional coactivator playing a role in the biogenesis of mitochondria in the muscle. PGC-1α was significantly elevated in skeletal muscle samples following astaxanthin intake, and cytochrome C levels were also increased in mice.

Moreover, the levels of plasma fatty acids were decreased after exercise in the astaxanthin-fed mice. The fat utilization of skeletal muscle was improved during exercise in a treadmill running model by activating carnitine palmitoyltransferase I. Interestingly, PGC-1α increases GLUT4 and has multiple roles in the pathogenesis of type-2 diabetes mellitus, but the effects of astaxanthin on the PGC-1α/GLUT4 pathway have not been studied.

It has also been demonstrated that the peroxisome proliferator-activated receptor (PPAR), which has a major role in the carbohydrate metabolism, is a novel target for astaxanthin. Depending on the cell context, the antioxidant molecule can bind dose-dependently to PPARγ and act as an antagonist or an agonist.

  1. Protective Effect of Astaxanthin on Diabetes Mellitus Complications

Diabetes mellitus increases ROS production and also decreases antioxidant defense capacity. Reactive radicals are produced in several ways; one source is the activated macrophages and neutrophils. Large amounts of ROS release lead to oxidative stress of all cell components and induces chronic inflammatory responses.

It has also been suggested that carotenoids can protect the different tissues from the long-term consequences of diabetes, including nephropathy, infectious diseases, and abnormalities in the neuronal system and eye. Several reports try to examine and discuss the mechanisms behind the biological effects of carotenoids to prevent the complications of diabetes mellitus.

Astaxanthin supplementation markedly reduced the level of inflammation-related proteins COX-2 (cyclooxygenase-2), iNOS (inducible nitric oxide synthase), MCP-1 (monocyte chemoattractant protein 1), NF-β (nuclear factor-beta) in the liver, and the ROS-induced lipid peroxidation in streptozotocin-induced diabetic rats.

In human mesangial cells, astaxanthin prevented the high-glucose exposure-induced elevated ROS production in the mitochondria to have a protective effect against diabetic neuropathy. In human neutrophil cells, astaxanthin prevented the high-glucose-induced ROS/RNS production and improved the phagocytic capacity of the cells.

The redox balance in the lymphocytes was ameliorated by astaxanthin application via lowering the catalase activities, restoring the ratio between glutathione peroxidase and glutathione reductase activities, and lowering the scores of lipid oxidation in an alloxan-induced diabetic rat model. Inflammation-related neuronal apoptosis leads to learning and memory deficits. Astaxanthin decreased the activity of apoptosis-related molecules (TNF, IL-1, and IL-6) and caspases 3 and 9 in the cortex and hippocampus of diabetic rats, improving cognitive deficits. High-glucose concentration-induced superoxide, nitric oxide, and peroxynitrite generation were also reduced by astaxanthin treatment in the proximal tubular epithelial cell, which inhibited the nuclear translocation of the NF-κB p65 subunit. Oxidative stress is the primary cause of renal fibrosis during the progression of diabetic nephropathy.

In diabetic (db/db) mice, astaxanthin administration improved the development and acceleration of diabetic nephropathy; it improved experimental diabetes-induced renal oxidative stress and prevented renal fibrosis by upregulating connexin43 and activating the antioxidant Nrf2- (NF-E2-related factor 2-) ARE (antioxidant responsive element) pathway in glomerular mesangial cells.

In streptozotocin-induced diabetic rats, 12 weeks of astaxanthin treatment ameliorated morphological changes in the kidney by decreasing fibronectin’s and collagen IV’s protein expression and through the activation of Nrf2-ARE signaling.