The Production of Reactive Oxygen Species in Response to Glutathione Supplementation and Acute Exercise in Patients With Type 2 Diabetes

ROS production in response to glutathione supplementation:

Today, 387 million people worldwide suffer from T2D and this number is expected to increase to 592 million in 2035. Skeletal muscle is responsible for ~75% of the total glucose uptake, making skeletal muscle the quantitatively most important tissue when it comes to insulin resistance (1). It has been suggested that oxidative stress may represent a possible causative factor in the pathophysiology of skeletal muscle insulin resistance. The link between ROS and skeletal muscle insulin resistance has been established both in vitro and in vivo (2, 3), but few studies have actually measured ROS production in skeletal muscle of T2D patients (4-6). Mitochondria are a source of ROS, and also a major target of oxidative damage (7). The mitochondrial defense system against oxidative stress relies on endogenous antioxidants. Glutathione (GSH) is the most abundant endogenous antioxidant in the cell and thus, a crucial protector against oxidative stress and insulin resistance (8). Supporting this, patients with T2D have a decreased level of GSH and an increased level of oxidized GSH (GSSG) in plasma (9) and insulin resistant subjects are reported to have an increased mitochondrial ROS production as well as a reduced GSH/GSSG ratio in skeletal muscle compared to healthy controls (3). In addition, 1 h glutathione infusion has been found to increase glucose uptake in patients with T2D by ~25% and to improve the redox environment, as reflected by an increased GSH/GSSG ratio in plasma; effects that were not seen in the healthy controls (10). The effect of prolonged oral GSH supplementation on skeletal muscle insulin sensitivity and mitochondrial ROS production in patients with T2D has, to our knowledge, never been investigated.

Research questions 1: Does oral GSH supplementation improve skeletal muscle insulin sensitivity in patients with T2D and healthy controls? And if so, can this effect be linked to a more beneficial redox state in the muscle cell? Hypothesis: Oral GSH supplementation will improve skeletal muscle insulin sensitivity in patients with T2D and this effect will be linked to a reduced mitochondrial ROS production in the skeletal muscle.

ROS production in response to acute exercise:

Acute exercise induces a marked increase in the transcription of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) (11), and therefore, PGC-1α is believed to play a key role in training-induced mitochondrial biogenesis (12). Contraction of rat skeletal muscle cells increases ROS production and PGC-1α mRNA expression, but in the presence of antioxidants, ROS production is reduced and the increase in PGC-1α mRNA is abolished (13). Also, exercise combined with allopurinol (an inhibitor of ROS production) severely attenuates the magnitude of the exercise-induced increased PGC-1α mRNA in rats, compared to exercise alone (14). These findings suggest that PGC-1α, at least in part, is regulated through a mechanism that involves ROS. Furthermore, it has been suggested that ROS regulates PGC-1α via activation of AMP-activated protein kinase (AMPK) (15). Interestingly, subjects with insulin resistance have a decreased exercise-stimulated AMPK activity, compared to lean controls (16, 17), which might explain the attenuated training-induced mitochondrial biogenesis observed in some patients with T2D (5, 17, 18), but not all (19). Whether ROS production is implicated in an abnormal training response is not known. Our current knowledge of ROS in response to acute exercise is derived from studies in animals and cells, and no study has, to our knowledge, investigated the link between ROS and mitochondrial biogenesis in patients with T2D in response to acute exercise.