Alterations in Mitochondrial Oxidative Stress and Mitophagy in Subjects with Prediabetes and Type 2 Diabetes Mellitus

Abstract

Background and aimHyperglycemia-mediated oxidative stress impedes cell-reparative process like autophagy, which has been implicated in impairment of β-cell function in type 2 diabetes mellitus (T2DM). However, the role of mitophagy (selective mitochondrial autophagy) in progression of hyperglycemia remains elusive. This study aimed to assess the impact of increasing severity of hyperglycemia on mitochondrial stress and mitophagy.Design and methodsA case–control study included healthy controls, subjects with prediabetes, newly diagnosed T2DM (NDT2DM) and advanced duration of T2DM (ADT2DM) (n = 20 each). Mitochondrial stress indices, transcriptional and translational expression of mitophagy markers (PINK1, PARKIN, MFN2, NIX, LC3-II, and LAMP-2) and transmission electron microscopic (TEM) studies were performed in peripheral blood mononuclear cells.ResultsWith mild hyperglycemia in subjects with prediabetes, to moderate to severe hyperglycemia in NDT2DM and ADT2DM, a progressive rise in mitochondrial oxidative stress was observed. Prediabetic subjects exhibited significantly increased expression of mitophagy-related markers and showed a positive association with HOMA-β, whereas, patients with NDT2DM and ADT2DM demonstrated decreased expression, with a greater decline in ADT2DM subjects. TEM studies revealed significantly reduced number of distorted mitochondria in prediabetics, as compared to the T2DM patients. In addition, receiver operating characteristic analysis showed HbA1C > 7% (53 mmol/mol) was associated with attenuated mitophagy.ConclusionIncreasing hyperglycemia is associated with progressive rise in oxidative stress and altered mitochondrial morphology. Sustenance of mitophagy at HbA1C < 7% (53 mmol/mol) strengthens the rationale of achieving HbA1C below this cutoff for good glycemic control. An “adaptive” increase in mitophagy may delay progression to T2DM by preserving the β-cell function in subjects with prediabetes.

Metformin promotes mitophagy in mononuclear cells: a potential in vitro model for unraveling metformin's mechanism of action

“…Induction of mitophagy in these mononuclear cells may be a novel surrogate marker for the improvement of mitochondrial function in other tissues/organs, including β cells. This was substantiated by our previous studies, where we demonstrated that an adaptive increase in mitophagy in subjects with prediabetes showed a positive relationship with HOMA‐β indices . Furthermore, our interventional trial observations (unpublished data) also revealed that PINK1, a key mitophagy player, significantly contributed to the rise in HOMA‐β levels in NDT2DM patients receiving metformin therapy.…”

“…Lately, the use of peripheral blood mononuclear cells (PBMCs) has emerged as a novel tool for assessing mitochondrial function in patients with T2DM, as these systemically circulating cells are also chronically exposed to glucotoxicity, and these cells may thereby reflect the pathophysiology of T2DM . There is mounting evidence revealing that the PBMCs of patients with T2DM exhibit attenuated mitophagy and altered mitochondrial morphology and function . Besides this, these cells also express human organic cation transporter 1 (hOCT1; also known as SLC22A1) and insulin receptors, and they readily respond to varying extracellular glucose and insulin concentrations .…”

Metformin upregulates mitophagy in patients with T2DM: A randomized placebo‐controlled study

“…Work from Twig and his colleagues also reported that mitophagy regulates the mitochondrial turnover in β‐cells, which is critical for maintaining the mitochondrial homeostasis, function and survival of β‐cells, and the deregulation of this process may result in the progression of T2DM . In this regard, our previous study suggested that attenuated mitophagy, accompanied with increased mitochondrial oxidative stress in T2DM patients, might contribute to the worsening of hyperglycaemia in these patients …”

“…Total cellular RNA was extracted from the freshly isolated PBMCs using TRIzol‐Chloroform method (Invitrogen), and 2 μg cDNA was synthesized by reverse‐transcription PCR using a mastercycler gradient (Eppendorf). Transcriptional expression of mitophagy‐related genes including PINK1, PARKIN, NIX, MFN2, LC3‐II and LAMP2 (primer sequences listed in Table ) was performed on StepOnePlus real‐time PCR system (Applied Biosystems) using SYBR green chemistry as described previously . The precision of the real‐time instrument had a maximum cut‐off of 11% CV.…”

“…Accumulating evidence reveals that mononuclear cells may act as potential surrogate marker for insulin‐sensitive sites, as these cells also express insulin receptors, and are easily accessible, as compared to the other insulin‐target tissues such as skeletal muscle and adipocytes, which involve an invasive extraction procedure . In addition, these systemically circulating cells readily respond to ambient glucose levels and have been used previously in a couple of studies to demonstrate the alterations in autophagy/mitophagy, and mitochondrial phenotype and function in patients with T2DM, hence providing insights into the pathogenesis of T2DM . Further, documentation of the presence of metformin transporter, that is human organic cation transporter 1 (hOCT1; also known as SLC22A1) on mononuclear cells, uncovers new vistas in elucidating the novel mechanistic insights into metformin's action in these cells .…”