Autophagy and its link to type II diabetes mellitus

Autophagy, a double-edged sword for cell survival, is the research object on 2016 Nobel Prize in Physiology or Medicine. Autophagy is a molecular mechanism for maintaining cellular physiology and promoting survival. Defects in autophagy lead to the etiology of many diseases, including diabetes mellitus (DM), cancer, neurodegeneration, infection disease and aging. DM is a metabolic and chronic disorder and has a higher prevalence in the world as well as in Taiwan. The character of diabetes mellitus is hyperglycemia resulting from defects in insulin secretion, insulin action, or both. Type 2 diabetes mellitus (T2DM) is characterized by insulin resistance and failure of producing insulin on pancreatic beta cells. In T2DM, autophagy is not only providing nutrients to maintain cellular energy during fasting, but also removes damaged organelles, lipids and miss-folded proteins. In addition, autophagy plays an important role in pancreatic beta cell dysfunction and insulin resistance. In this review, we summarize the roles of autophagy in T2DM.


Introduction
Professor Yoshinori Ohsumi, the 2016 laureate in Physiology or Medicine, discovered the mechanisms for autophagy [1][2][3][4]. This pathway plays a crucial role in physiological cellular homeostasis and human diseases [5]. Autophagy has been known to serve as a double-edged sword for promoting survival character and/ or activating cell death ( Fig. 1) [6][7][8][9][10][11]. In addition, autophagy, a catabolic process, degrades cellular components and damaged organelles [12,13]. Recently, autophagic machinery is involved in the pathophysiology of type 2 diabetes mellitus (T2DM) disease, and it regulates normal function of pancreatic beta cells. On the other hand, enhanced autophagy acts as an important protective mechanism against to oxidative stress on insulin-target tissues such as liver, adipose tissue and skeletal muscle [14][15][16][17][18][19]. In this review, we outline the relationship among autophagy, pancreatic beta cells and T2DM. Furthermore, we highlight recent findings on the novel agents to specifically target autophagy in T2DM.

Programmed cell death (PCD)
PCD is an important physiological process during organ development, tissue homeostasis. This process is a protective mechanism against cellular stress, drug, external environment and tumor suppressive mechanism. It is generally divided into three distinct types including: (1) apoptosis; (2) autophagic cell death; and (3) necroptosis. Each type of cell death exhibits the specific morphological, molecular and biochemical characteristics [20]. We summary the characteristics of the three types as listed in Table 1.

Assays for monitoring autophagy and pharmacological regulated agents
The features of autophagy are the massive accumulation of autophagic vacuoles (autophagosomes) in the cytoplasm of cells. Hereby, we present a series of methods to monitoring autophagy in Table 2. (1) Transmission electron microscopy (TEM) is used to observe autophagosome number, volume, and content analysis; (2) The lysosomal enzymes activity, assessment of the number, size, and location of lysosomes are examined by the uptake of fluorescent dyes (monodansylcadaverine (MDC), acridine orange (AO), neutral Red, LysoSensor Blue, Lyso-Tracker Red); (3) Autophagy-related proteins such as ATGs and LC3 are detected by western blotting or fluorescent protein tagging; (4) Autophagyrelated gene expression levels are measured by western blotting or real-time PCR. Table 3 is a list of the pharmacological agents for assessing autophagy effects such as inhibition of lysosomal enzyme activities, fusion of organelles, or inter-compartmental transfer of molecules.
At the terminal stage of autophagy, the autophagosome fuses with lysosomes to form autophagolysosomes. Autophagy allows the orderly degradation and recycling of cellular components [99]. The purpose of autophagy is to ensure quality control of organelles and proteins, as well as protection of intracellular homeostasis in stress and nutrient efficiency [94][95][96][97][98][99].

Type 2 diabetes mellitus (T2DM)
Diabetes mellitus (DM), commonly referred to as diabetes, is a metabolic and chronic disease in the world [100,101]. DM patients have high blood sugar levels over a prolonged period. The character in DM is a relative or absolute lack of insulin, resulting in hyperglycemia [102]. Symptoms of hyperglycemia are frequent urination, increased thirst, and increased hunger. Acute complications of DM can include nonketotic hyperosmolar coma, diabetic ketoacidosis and death. Serious complications of DM include cardiovascular disease, stroke, chronic kidney failure, nephropathy, foot ulcers, neuropathy and damage to the eyes [103][104][105][106]. In 2014, approximately 422 million people were diagnosed with DM according to World Health Organization (WHO) report [107,108]. In Taiwan, DM is ranked as the fifth leading cause of death in 2015 on the basis of statistics by the Ministry of Health and Welfare, R.O.C. (Taiwan) [101,109].
There are three main types of diabetes mellitus: (1) Type 1 diabetes (T1D): also called insulin-dependent, juvenile or childhood-onset diabetes. T1D is characterized by deficient insulin production in the body. The pathology in T1D is described as an autoimmune disease because the pancreatic beta cells (insulinproducing tissue) are destructed in the islets of Langerhans [110]. T1D is diagnosed most in children and young adults. People with T1D require daily administration of insulin to regulate the amount of glucose in their blood [111]. Environmental factors and genetic influence play an important role in T1D [112,113].
(2) Type 2 diabetes (T2D): formerly called non-insulin-dependent (NIDDM) or adult onset diabetes. T2D is the most common type of diabetes with prevalence in Taiwan. T2D begins with insulin resistance in which cells fail to respond to and uptake of insulin in the body [114][115][116]. Insulin resistance can be enhanced by weight reduction and exercise [117]. (3) Gestational diabetes: pregnant women without a previous history of diabetes develop high blood sugar levels [118,119].
The physiological defects in T2D that is reduced insulin sensitivity, insulin resistance and combined with impaired insulin secretion (Fig. 4). T2D occurs as a result of obesity, poor diet, physical inactivity, increasing age, family history and ethnicity. The defective or mutant insulin receptor may be caused no response to insulin in body tissues. Controversially, patients with T2D in the early stage often have a normal or high bone mineral density (BMD), associated with obesity and hyperinsulinemia, as well as altered level of insulin. When cells are insensitive to insulin (or insulin resistance), the pancreatic beta cells produce more and more insulin, which leads to the higher insulin concentration in blood (hyperinsulinemia). The pancreatic beta cells desperately secrete insulin and then gradually decline. T2D at late stage is characterized by insufficient secretion of insulin from the pancreatic beta cells, coupled with impaired insulin action in target tissues such as muscle, liver and fat. Hyperglycemia results when insulin secretion is unable to compensate for insulin resistance [120][121][122][123][124]. Mechanisms in the development and pharmacological treatments of T2D are summarized in Fig. 5 and Table 4.

Autophagy and type 2 diabetes (T2D)
Autophagy has been known to regulate the function of pancreatic beta cells and insulin-target tissues (skeletal muscle, liver and adipose tissue). T2D progression through impaired pancreatic beta cells function and development of insulin resistance has been associated with autophagy [125][126][127][128]. Upon insulin resistance, pancreatic cells enhance their insulin secretion (hyperinsulinemia) to compensate for hyperglycemia on the early onset of T2D (Fig. 5). In contrast, the number of pancreatic cells is progressive diminution through apoptotic cell death on the late onset of T2D [125,[129][130][131].
Many studies suggest that enhanced autophagy acts as a protective mechanism against oxidative stress in pancreatic beta cells [128,132]. In vivo studies demonstrated that Atg7-deficient mice showed a decrease in the number of pancreatic beta cells, impairment of glucose tolerance and reduction in insulin secretion [133]. The insulin resistant mice (beta-cell-specific Atg7 knockout mice) model has been shown that autophagy plays a crucial role in the development of diabetes and in preserving the structure and function of pancreatic beta cells. Accumulation of autophagosomes in the pancreatic beta cell has been demonstrated in db/db mouse model [134][135][136]. Fujitani et al. showed that reduced insulin secretion was associated with pancreatic beta cell degeneration and impaired glucose in autophagy-deficient mice [136][137][138]. However, constitutively activated autophagy has injurious effects on pancreatic beta cells and chronic activation of autophagy causes autophagic cell death [135,[139][140][141][142][143].

Conclusion
The pancreatic beta cells control the releases of insulin and play an important role in the progression of T2D. Autophagy might function as a protective and pro-survival role on pancreatic beta cell death in T2D. Metformin has been widely used in the clinic therapy in T2D and has a protective effect on pancreatic beta cells from injury by activating autophagy through AMPK pathway [144,145]. Therefore, it is urgent to understand the relationship  Fig. 4 -Etiology of type 2 DM. Two major physiological defects associated with T2D are reduced insulin sensitivity, insulin resistance and combined with impaired insulin secretion. Obesity, poor diet, physical inactivity, increasing age, family history and ethnicity lead to a higher risk of T2D.
of autophagy and T2D. We summarize the role of autophagy and apoptosis in T2D in Fig. 6. It is expected to develop new drugs and more effective agents targeted in autophagy for the therapy of T2D.
[  [15] Kawamori R. What is the natural history of type 2 diabetes mel- Enhances insulin sensitivity in liver and peripheral tissues by activation of AMP activated protein kinase. Glycogen hydrolysis and Gluconeogenesis inhibition.