Islet β cell failure in type 2 diabetes (original) (raw)
Mitochondrial dysfunction with production of ROS. Increased metabolism of glucose and FFAs through mitochondrial oxidation will result in an increased mitochondrial membrane potential and superoxide production (71, 79, 80). Increased superoxide production causes increased exposure of the cell to ROS and activation of uncoupling protein 2 (UCP2) (71, 81). In addition, chronically elevated levels of FFAs and glucose induce the UCP2 gene in β cells (82, S57). Increased UCP2 helps to safely dissipate the elevated mitochondrial membrane potential and promotes fuel detoxification because oxidation of these fuels becomes increasingly coupled to heat rather than ATP production. However, this occurs at the expense of ATP synthesis efficiency and consequently insulin secretion (71, 81). In other words, uncoupling of oxidative phosphorylation, resulting in impaired insulin-secretory capacity but reduced ROS production, is the price the β cell pays for its survival in the presence of fuel surfeit. Islets that attempt to compensate for insulin resistance with abnormally high rates of glucose oxidation (e.g., the DBA2 mouse; ref. 59) or have an inherited or acquired deficiency in mitochondrial function (e.g., IUGR offspring; ref. 72) will be at risk of functional failure and oxidative damage. While it is important to realize that the production of ROS can have important regulatory functions on intermediary metabolism by altering the cellular redox state (83), the β cell has limited defense against excess ROS production because the expression levels of ROS-detoxifying enzymes in the β cell are particularly low in comparison with those in other cells (23). Overall, these considerations are compatible with the view that ROS may contribute to both early and late phases of β cell failure (84) (Figure 3). A note of caution with respect to the role of ROS in β cell failure is found in a study showing that ROS production is actually reduced by elevated glucose in purified rat β cells (85). However, it is difficult to extend this in vitro situation to chronic fuel surfeit exposure in vivo and to situations in which β cells are already stressed or have preexisting defects in mitochondrial function. Thus, we showed that elevated FFAs markedly enhance apoptosis and ROS production in β cells stressed by incubation in the absence of serum and are much less toxic in healthy cells cultured in the presence of serum (86).
Mechanisms of β cell failure in T2D. Islet β cell compensation for insulin resistance is sustained provided β cells are robust, resulting in long-term maintenance of NGT. Compensation processes, however, fail if there are genetic or acquired factors that result in susceptible β cells. The defect(s) create weak link(s) in the compensation process that promote β cell dysfunction by mechanisms with initiator roles that result in IGT and early T2D. Hyperglycemia, once established, promotes a further series of mechanisms, under the umbrella of glucotoxicity, that cause severe β cell failure and overt and late T2D. AMPK/Mal-CoA, AMPK/malonyl-CoA signaling network.
Impaired anaplerosis and cataplerosis. The anaplerosis/cataplerosis pathways provide metabolic coupling factors important for insulin secretion, and the activity of these pathways is increased in healthy compensating islets. Evidence indicates that disruption of this pathway causes loss of GSIS (Figure 3). For example, inhibition of PC (the first step in anaplerosis from glucose-derived pyruvate) by phenylacetic acid inhibited GSIS in both ZL and ZF islets (12). Furthermore, exposure of MIN-6 cells to palmitate caused reduction in PC activity and was associated with impaired GSIS (87). In a similar study, exposure of another insulinoma cell line, INS 832/13, to elevated FFAs (oleate and palmitate) caused impairment in pyruvate cycling as measured by 13C NMR (88). In the latter study, provision of a membrane-permeant ester of malate (used to reestablish anaplerosis) to FFA-treated INS 832/13 cells or lipid-laden ZDF islets caused at least partial recovery of GSIS (88). At variance from the cell line studies, exposure of rat islets in vitro to elevated oleate caused increased pyruvate cycling and enhanced basal insulin release and GSIS (89). It may be that saturated FFAs (such as palmitate) are inhibitory, whereas monounsaturated FFAs like oleate activate the anaplerosis/cataplerosis pathways. Although the role for defects in β cell anaplerosis/cataplerosis in human T2D is unknown, there is defective expression of PC in 2 diabetic rodent models, Goto-Kakizaki and ZDF rats (90, 91). Furthermore, improved GSIS following insulin treatment in the Goto-Kakizaki rat was paralleled by recovery of β cell PC expression (91).
Dysregulation of TG/FFA cycling and lipolysis. The islet β cell TG/FFA cycle provides a lipid signaling pathway by which the glucose oxidation K +ATP channel pathway of nutrient-secretion coupling can be amplified (35). In compensating islets this not only allows for greater insulin secretion without a need for very high rates of glucose oxidation, but also provides a mechanism by which ATP produced by glucose oxidation can be consumed via a “futile” cycle that transforms excessive energy from fuels (FFAs and glucose) into heat. The ATP consumption occurs in the acyl-CoA synthase step of the cycle, in which FFA is activated to LC-CoA (92). Thus, TG/FFA cycling could potentially protect the β cell from fuel surfeit and excessive increases in mitochondrial membrane potential and ROS production. In support for a role of dysfunctional TG/FFA cycling in β cell failure are studies in which enzymes of the cycle are disrupted. Pharmacological inhibition of the lipolysis arm is known to inhibit GSIS (93, S58), whereas overexpression of hormone-sensitive lipase (HSL), which prevents accumulation of TGs, also impairs secretion (94). We showed impaired GSIS in fasted male HSL-knockout mice, and this was reversed by provision of exogenous FFAs (95). One study showed that a higher capacity of islets to accumulate TGs is associated with reduced islet FFA-induced cytotoxicity (96). Further, MIN-6 cells that express a high level of stearoyl-CoA desaturase 1 (SCD1) (an enzyme that desaturates FAs and favors TG synthesis) are resistant to lipid-induced toxicity compared with low expressers (97). TG/FFA cycling, particularly in response to monounsaturated FFAs, may also divert FFA metabolism away from toxic lipids such as ceramide (98, S28, S59). To summarize, while TG/FFA cycling in the β cell has only recently been appreciated, it has potentially important roles in β cell compensation processes and in the protection of islets from damage during compensation. Disruption of this cycle could contribute to β cell failure in T2D (Figure 3) such that this new aspect of islet biology warrants further investigation.
Altered AMPK/malonyl-CoA signaling and FAO. It is important to note that both ZF rat islets (which have high rates of TG/FFA cycling; ref. 35) and FFA-resistant MIN-6 cells (which have high expression of SCD1; ref. 97) maintain normal or elevated FAO and do not develop lipoapoptosis. Oxidative clearance of FFAs is likely to a be a major factor in preventing steatosis in the islet, as it is in other tissues such as liver (99). The AMPK/malonyl-CoA signaling network has a major role in regulating FAO (50, S60). AMPK senses cellular energy status and is activated by an increase in the AMP/ATP ratio brought on by fasting or exercise. AMPK activates cellular energy production (e.g., glucose oxidation and FAO) and reduces energy consumption (e.g., FA synthesis and esterification) (50, S60). Malonyl-CoA, on the other hand, promotes nutrient storage (4, 29, 50). Importantly, AMPK phosphorylates acetyl-CoA carboxylase and malonyl-CoA decarboxylase, the enzymes that regulate malonyl-CoA synthesis and degradation, respectively, with the effect of lowering malonyl-CoA (50, S60). In addition to conditions of food deprivation and exercise, AMPK can be activated by the adipokines, including adiponectin (100, S61) and leptin (101), and pharmacological agents such as metformin and the thiazolidinediones (101, 102). Chronically reduced activity of β cell AMPK and increased malonyl-CoA levels, due to consistent overnutrition and reduced physical activity, together with the hypoadiponectinemia and leptin resistance of the metabolic syndrome, will result in downregulation of β cell FAO pathways and therefore place β cells at risk of lipoapoptosis (50). Altered AMPK/malonyl-CoA signaling underpins the mechanism of glucolipotoxicity (50), as discussed below.
Lipotoxicity and/or lipoadaptation. There is much written about lipotoxicity and the β cell; however, in our experience elevated lipids appear to be relatively benign to β cells, provided that they are not dramatically elevated in vitro and that glucose is not simultaneously elevated. In fact, the emerging evidence indicates that elevated FFAs and hyperlipidemia are major signals that permit β cell adaptation to insulin resistance, as exemplified by the obese ZF rat (35). In our view, β cell lipotoxicity studies in vitro and in vivo have investigated either the adaptive process of the β cell to FFAs or glucolipotoxicity as described in the next section. In other words, β cell studies made with reasonable concentrations of FFAs (both in vivo and in vitro), in the absence of hyperglycemia, have investigated what may be termed “lipoadaptation” rather than “lipotoxicity.” It is certainly possible that islets may be more susceptible to lipotoxic damage at normal glucose levels if the islets have preexisting defects in FA detoxification processes, such as disorders in FAO. Acquired defects in islet FAO, due for example to reduced adiponectin levels associated with central adiposity (S62), could also lower the hyperglycemic threshold at which lipotoxicity occurs.
Glucolipotoxicity. According to the glucolipotoxicity hypothesis, toxic actions of FFAs on tissues will become apparent in the context of hyperglycemia (4, 30). This derives from the fact that elevated glucose, via the AMPK/malonyl-CoA signaling network, curtails fat oxidation and consequently the detoxification of fat, while at the same time promoting partitioning of FFAs into complex lipids, some of which are cytotoxic (4, 30). In support of this hypothesis, we demonstrated a marked synergistic effect of high glucose and saturated FFAs in inducing apoptosis in both rat INS 832/13 and human islet β cells (103). Clearly, cell death was markedly increased in the presence of elevated glucose and FFAs, thereby confirming the occurrence of islet cell glucolipotoxicity. In order to show that AMPK/malonyl-CoA signaling was involved in the mechanism, we studied glucolipotoxicity in INS 832/13 β cells cultured with and without the pharmacological agents AICAR and metformin (activators of AMPK that promote FAO), triacsin C (inhibitor of the activation of FFAs to LC-CoA such that FAO and esterification processes are inhibited), and etomoxir (inhibitor of carnitine palmitoyltransferase–1 activity such that FAO alone is inhibited) (103). AICAR, metformin, and triacsin C all reduced glucolipotoxicity, whereas etomoxir increased it (103). This hypothesis is also consistent with the finding that hyperglycemic ZDF rats develop steatosis, whereas normoglycemic ZF rats are resistant to steatosis. It is also consistent with another study in ZDF rats that showed that antecedent hyperglycemia, not hyperlipidemia, led to increased islet TG content and reduced insulin gene expression (46). Also supportive is a study of subjects with elevated FFAs and/or IGT (104). Deterioration in acute insulin response over time was greatest in the subjects that had both elevated FFAs and IGT (104).
While it seems likely that glucolipotoxicity is involved in progressive β cell damage once hyperglycemia is established (Figure 3), does it have a role in early β cell damage? How high do glucose concentrations need to be for this mechanism to be activated? Considering that the key determinant in this mechanism is the status of the AMPK/malonyl-CoA network and its effect on partitioning of lipids, it could be envisaged that overnutrition with inactivity, together with hypoadiponectinemia and elevated FFAs related to visceral obesity, would place an individual at high risk of glucolipotoxicity (50). In such circumstances only minor degrees of postprandial hyperglycemia may be needed to tip the balance toward β cell damage. The emerging evidence indicates that exaggerated postprandial glucose excursions in IGT subjects is a predictive factor of diabetes and cardiovascular disease (105). Thus, the Study to Prevent NIDDM (STOP-NIDDM) trial has shown that acarbose treatment of IGT subjects to reduce elevated postprandial glucose levels decreased the risk of progression to diabetes by 36% (106).
In our in vitro studies of glucolipotoxicity, saturated FAs were cytotoxic, whereas monounsaturated FAs (oleate) were protective (103). This is certainly not unique to our work on β cells (98, S59) or to this cell type (107, S63). In considering why oleate is protective, it is of interest that monounsaturated FAs are more readily partitioned into TGs (107) and may therefore more effectively promote TG/FFA cycling. Furthermore, unlike saturated FAs, monounsaturated FAs do not cause accumulation of ceramides (98) or deplete mitochondrial cardiolipin levels (107), processes that have both been linked to cytotoxicity. β Cell lipid overload could potentially cause β cell apoptosis by suppressing the antiapoptotic factor Bcl-2 (108, 109).
Islet β cell exhaustion and ER stress. The compensating islet β cell places a high demand on the ER for the synthesis of proinsulin. The β cell exhaustion theory views an imbalance in the relationship between insulin secretion and production that results in depletion of releasable insulin to be a significant component of β cell failure in T2D (6, 54, S64, S65) (Figure 3). This is supported by the findings that resting of the β cell by somatostatin (110) or diazoxide (111) results in recovery of function. Additive to this is a defect in proinsulin synthesis, with ER stress as the potential mechanism (112, S64, S66). The unfolded protein response in the ER is protective in the early stages but can initiate cell death by apoptosis if severe (112, S64, S66). In support of a role of ER stress in T2D, it has been reported that the Akita mouse has a folding mutation in proinsulin that activates the ER stress response, resulting in diabetes with loss of β cell mass (113). In addition, chronic exposure of INS1 cells to palmitate causes lipid deposition in the ER and an ER stress response (114). It is certainly possible that excessive demands on ER — particularly if β cell mass is suboptimal or if there are inherited defects in the proinsulin gene, as in the Akita mouse — could cause β cell damage by this mechanism.