Hyperglycemia-induced Tissue Damage in Diabetic Microvascular Complications (DMC)

Introduction

The term “diabetic microvascular complications” (DMC) refers to 3 specific chronic complications of diabetes: retinopathy, nephropathy, and peripheral neuropathy. These complications are consequences of microvascular and target tissue damage resulting from the unregulated metabolic state, which includes hyperglycemia, hypertension, and dyslipidemia.[1,2,3] As suggested by the common risk factors, the DMC share similar pathophysiological characteristics.[1,4,5] In fact, for patients diagnosed with 1 DMC, the American Diabetes Association (ADA) recommends evaluation for the remaining 2, as retinopathy, nephropathy, and neuropathy tend to coexist.[1]

Large clinical studies, such as the Diabetes Control and Complications Trial (DCCT), the UK Prospective Diabetes Study (UKPDS), and the Kumamoto trial, have revealed the significant role of hyperglycemia in the pathophysiology of DMC. These studies have demonstrated that tight glucose control can delay the onset and slow the progression of DMC in type 1 and type 2 diabetes.[6,7,8] In fact, patients with impaired glucose tolerance or newly diagnosed type 2 diabetes are frequently found to have evidence of DMC, suggesting that even relatively mild glycemic dysregulation contributes to the development of these microvascular complications.[9]

This article briefly describes the tissue damage associated with DMC and the clinical manifestations, including the molecular changes that develop as a result of hyperglycemia, and presents information regarding the clinical progress toward developing treatments that inhibit hyperglycemia-activated mechanisms.

 

Hyperglycemia-induced tissue damage

Hyperglycemia is a key risk factor for all 3 DMC. Certain cell types—notably mesangial cells in the kidney, the neurons in peripheral nerves and retina, and the vascular endothelial cells in retina, kidney, and peripheral nerves—are especially vulnerable to hyperglycemia because they cannot compensate for increased plasma glucose levels by decreasing glucose transport. As a result, glucose concentrations are elevated within these cells under hyperglycemic conditions, leading to subsequent damage.[10] Cellular damage due to hyperglycemia translates to tissue damage in the retina, kidneys, and peripheral nerves. This tissue damage is evident to the patient as symptoms and to the physician as impairment consistent with retinopathy, neuropathy, or nephropathy.

 

Microvascular damage

Microvascular damage contributes, in varying degrees, to the pathophysiology of all 3 DMC.[4,5,11] For example, renal microvasculature is an integral part of the glomerular filtration apparatus. The specialized function of the glomerulus depends on the blood flow to the kidney being consistently regulated and intact glomerular structure. Therefore, microvascular dysfunction, in the guise of altered capillary blood pressure, basement membrane thickening and increased mesangial matrix deposition, decreased filtration, capillary occlusion, and glomerular loss, is central to the pathogenesis of diabetic nephropathy.[12] Microvascular damage is only slightly less important in the development of diabetic retinopathy. Alterations in the retinal microvasculature, such as neovascularization and vitreous hemorrhage, can cause visual loss, but retinal neurodegeneration due to ischemia and hyperglycemia also contributes.[3,17] As such, diabetic retinopathy has features of both a microvascular and neurodegenerative complication.[17] However, some authorities believe the role of the microvasculature in diabetic peripheral neuropathy is still controversial.[13] Although microvascular disease may expose peripheral nerves to repeated ischemic attacks, the damage seen in neurons of peripheral nerves may also be the result of damage due to hyperglycemia, such as alteration in intracellular proteins leading to cellular nerve damage, and impairment of nerve conduction.[13]

 

The microvasculature consists of small arterioles, capillaries, and venules where many different physiologic processes take place, such as nutrient and metabolite exchange, fluid homeostasis, and tissue defense and repair.[11] In patients with diabetes and hyperglycemia, changes to microvascular components, particularly the vascular endothelium, lead to altered blood flow, increased vascular permeability and leakage, altered leukocyte and monocyte adhesion, changes in vascular and growth factor expression, and ultimately vascular occlusion.[4,5,14] These alterations have ramifications for tissues affected by DMC.

 

The process of microvascular narrowing and occlusion is progressive and results in impaired perfusion, ischemia, cell loss, and dysfunction in the microvasculature and affected kidney, nerve, and retinal tissues.[5] Increased vascular permeability is an early event in this process and leads to deposition of plasma proteins in and thickening of the vascular wall. Vascular wall thickening and cell adhesion to vascular walls also contribute to microvascular narrowing and occlusion as well as alterations in blood flow. Microvascular changes relevant to vascular wall thickening and cellular adhesion include increased extracellular matrix (ECM) production; basement membrane (BM) thickening; hypertrophy and hyperplasia of endothelial, smooth muscle, and mesangial cells; and increased platelet and leukocyte adhesion.[5] A summary of the microvascular structural changes associated with diabetes is provided in Table 1.

 

Table 1. Changes in microvessel structure in diabetes[11]

Anatomical Changes Described	Site
Increased basement membrane thickening	Capillaries, venules, arterioles
Increased interendothelial cell gaps	Capillaries, venules
Loss of contact between pericyte and endothelial cell	Capillaries, venules
Pericyte loss	Retina, nerve

The vascular endothelium is an active regulator of microvascular function and a target of hyperglycemic damage. Endothelium participates in control of vascular tone and permeability, coagulation and fibrinolysis, subendothelial matrix composition, and vascular inflammatory activity.[14] Endothelial cells contribute to these functions by producing a variety of molecules including collagen, nitric oxide (NO), angiotensin II (Ang II), adhesion molecules, and cytokines.[14] In general, normal endothelium promotes vasodilation, limits coagulation, and decreases inflammation. Endothelial injury thus results in vasoconstriction, alterations in membrane permeability, coagulation, matrix accumulation, and inflammation. All of these pathophysiologic states promote microvascular occlusion, cell death, and damage to kidney, nerve, and retinal tissues.

 

Retinal damage

The retinal vasculature is specialized to provide oxygen and glucose to meet the high metabolic demands of the retina.[15] Figure 1 illustrates the arrangement of cells in the retinal microvasculature.[15] The microvasculature is comprised of endothelial cells surrounded by a basement membrane and contractile pericytes, which act with the epithelium to maintain capillary integrity. [16] The endothelial cells are the main component of the blood-retinal barrier; tight junctions between endothelial cells prevent plasma protein leakage.[15] Retinal endothelial cells contain glucose transporters that promote ready uptake of glucose, consistent with the high metabolic needs of the retina. The transporter subtypes in the retina, particularly GLUT1, transport glucose at relatively low concentrations and do not downregulate transport under hyperglycemic conditions. As a result, glucose uptake is enhanced in the retina in hyperglycemic states, rendering the retinal microvasculature particularly susceptible to hyperglycemic damage. In large part, diabetic retinopathy is the result of damage to the retinal microvasculature.[15] However, it should be noted that retinal neurons are also sensitive to hyperglycemia-induced damage and that diabetic retinopathy is also a disease of macular edema and retinal neurodegeneration.[17]

 

Figure 1. Retinal capillary components

 

Diabetic retinopathy is progressive and is the most frequent cause of new cases of blindness among working-age adults.[18] It is very common among patients with diabetes. Nearly all patients with type 1 diabetes develop retinopathy within 20 years of diagnosis, and >60% of type 2 diabetes patients have retinopathy.[18] Disease progression can be monitored through clinically observable microvascular changes that are recognized as representative of disease stages. An example of observed retinal vascular changes is presented in Figure 2. Prior to changes that are evident in a clinical setting, pericytes are lost and there are changes in retinal blood flow. In the earliest observable stages, diabetic retinopathy is characterized by microvascular abnormalities such as microaneurysms, intraretinal hemorrhages, and cotton wool spots. Vascular closure signals the transition to nonproliferative diabetic retinopathy (NPDR) and leads to ischemic damage. New vessel formation, in response to ischemia, is the hallmark of advanced, proliferative diabetic retinopathy (PDR). New vessels and fibrosis develop and lead to retinal traction, retinal tears, vitreous hemorrhage, and retinal detachment. In addition, at any stage of progression, increased vascular leakiness can contribute to the fluid accumulation in the retina and macular edema.[3]

 

Figure 2. Observed retinal vascular changes

 

Hyperglycemia damages retinal microvascular cells and contributes specifically to the changes described above. Hyperglycemia-mediated endothelial damage results in loss of tight junctions and blood-retina barrier compromise, increasing vascular permeability.[15] In addition to causing endothelial cell death, hyperglycemia also induces apoptosis in pericytes, the contractile support cells that surround retinal vessels. Pericyte dropout is an early pathological event in retinopathy, and it contributes to vascular weakness, blood flow abnormalities, and aberrant vascular remodeling.[15,16]

 

Kidney damage

The glomerular filtration barrier is composed of fenestrated endothelial cells, podocytes, and basement membrane (see Figure 3).[19] Structural changes to the glomerulus that occur in diabetic nephropathy are due at least in part to effects of hyperglycemia on the microvasculature. For example, relaxation of afferent arterioles and constriction of efferent arterioles, a consequence of endothelial dysfunction, results in increased glomerular capillary pressure which can then contribute to increased albumin excretion.[12, 14] Structural damage to the glomerulus itself, including basement membrane thickening and increased mesangial volume, are also observed in diabetic nephropathy. Mesangial matrix expansion compromises glomerular capillary lumen and filtration surface area and leads to declining renal function and proteinuria.[12] Changes in glomerular membrane thickness and composition resulting from endothelial cell dysfunction also lead to decreased filtration barrier integrity.[19] Dysfunctional endothelial cells are presumed to play a role in the structural changes described above by increasing glomerular pressure, altering basement membrane composition, and influencing mesangial cell and podocyte function.[14]

 

Figure 3. Changes in glomerular structure with increased glomerular pressure

 

Structural changes in the renal glomerulus develop in parallel with clinically relevant events. The natural history of diabetic nephropathy begins with the development of microalbuminuria: low but abnormal levels (≥30 mg/day or 20 mcg/min) of urinary albumin.[20] At this stage, patients are said to have incipient nephropathy.[20] The increased urinary albumin excretion (UAE) is likely a response to elevated capillary pressure.[12] At the same time, glomerular filtration rate (GFR) increases.[12] As diabetic nephropathy progresses, UAE increases.[20] When UAE has reached ≥300 mg/day or ≥200 mcg/min, a patient is said to have overt nephropathy.[20] From this point, mesangial volume increases, resulting in decreased relative capillary filtering surface area and gradually falling GFR.[12]

 

Peripheral nerve damage

Chronic sensorimotor neuropathy is the most common type of diabetic peripheral neuropathy.[21] As many as half of all patients with sensorimotor neuropathy experience symptoms at variable times, but as many as half of all patients may be asymptomatic.[22] Symptoms may include burning pain, electrical or stabbing sensations, parasthesiae, hyperaesthesiae, or deep aching pain and are most commonly experienced in the feet and lower limbs.[22] However, sensory loss is more common than positive symptoms.[21] Clinical examination of the lower limbs of diabetes patients is therefore very important. It should not be assumed that a patient does not have neuropathy based on a lack of symptoms.[22]

Sensory, autonomic, and motor neurons of the peripheral nervous system are all susceptible to diabetic neuropathy.[21] Distal fiber loss and demyelination are characteristic of peripheral nerve damage in diabetic sensorimotor neuropathy.[13] Neurons and Schwann cells may suffer direct damage due to hyperglycemia because they are unable to decrease glucose uptake in the presence of elevated intracellular glucose.[10] However, there is also evidence that decreased blood flow due to microvascular dysfunction leads to ischemia, hypoxia, and therefore neuronal damage. This concept is supported by the observations that neuronal dysfunction correlates with the progression of vascular abnormalities and that neuronal ischemia is a characteristic of diabetic neuropathy. Further evidence can be found in studies showing that vasodilating agents improve blood flow and nerve conduction velocity (NCV) in animal models of diabetes.[21] Based on these observations, microvascular dysfunction appears to play a role in peripheral nerve damage.

 

Mechanisms of hyperglycemia-induced tissue damage

Hyperglycemia is a hypermetabolic state that leads to generation of toxic derivatives such as oxidants and glycation products. The products of glucose metabolism, in addition to causing direct damage to cells, also promote sustained activation of cell signaling pathways, such as the protein kinase C β (PKC β) pathway. Several cell signaling pathways and products of glucose metabolism have been implicated in the development of DMC. These include production of reactive oxygen species (ROS) and thus generation of oxidative stress, advanced glycation endproduct formation, increased polyol pathway activation, and increased activation of PKC β.

 

Oxidative stress

Experimental evidence from cell culture systems indicates that reactive oxygen species (ROS), generated by high glucose levels, contribute to the pathogenetic PKC activation, polyol pathway flux, and increased advanced glycation endproducts (AGE) precursor formation.[10, 23] These observations suggest that ROS may be a common mechanism for microvascular damage in diabetes. High intracellular glucose can increase mitochondrial ROS production by driving oxidative phosphorylation. As the mitochondria metabolize glucose and convert it to adenosine triphosphate energy equivalents, they also generate ROS such as superoxide and hydrogen peroxide.[10] Alternatively, free radicals are also produced during glucose auto-oxidation.[4] The increased presence of free radicals leads to increased oxidative stress. The consequences are damage to cellular proteins, leukocyte adhesion to the endothelium, inhibition of endothelial barrier function, and activation of the pathogenetic mechanisms listed above.[4]

 

Glycation

Advanced glycation endproducts are formed when glucose or other glycating agents react with long-lasting molecules, such as proteins and nucleic acids.[5] Figure 4 illustrates 3 ways that intracellular AGE precursors can contribute to tissue damage associated with DMC.[24] Intracellular metabolism of glucose in endothelial cells generates AGE precursors (glyoxal, methylglyoxal, 3-deoxyglucosone) that react with amino groups of proteins to form AGEs.[24] AGE precursors can react with and modify the function of intracellular proteins, such as transcription factors.[10] In addition, AGE precursors can diffuse out of the cell and react with extracellular proteins. Reactions with extracellular matrix proteins can disrupt interactions with other matrix proteins as well as signaling between matrix proteins and the cell.[10,24] Extracellular AGE precursors can also react with circulating blood proteins, which in turn can activate AGE receptors, causing pathological increases in inflammatory cytokine and growth factor production by macrophages and mesangial cells.[10] Impaired microvascular function is a consequence of all 3 of the AGE precursor-activated mechanisms: intracellular protein glycation, matrix protein glycation, and AGE receptor activation.[10]

 

Figure 4. Intracellular production of AGE precursors damages cells by 3 mechanisms

 

 

Polyol pathway

The polyol pathway is depicted in Figure 5.[24] A key enzyme in this pathway is aldose reductase (AR), which normally consumes the cellular cofactor NADPH to reduce and detoxify certain products of metabolic reactions.[10] Through this process, AR normally converts toxic aldehydes to inactive alcohols.[10] However, when intracellular glucose levels are high enough, AR also converts glucose to sorbitol.[24]

Several hypotheses have been proposed to explain how AR conversion of glucose to sorbitol contributes to cellular damage. It has been suggested that intracellular sorbitol accumulation could contribute to osmotic stress, but the levels of sorbitol generated are too low.[24] It has also been suggested that sorbitol is oxidized by NAD⁺, which increases the NADH:NAD⁺ ratio, inhibits glyceraldehyde-3-phosphate-dehydrogenase, increases triose phosphate concentrations, and ultimately increases methylglyoxal and diacylglycerol (DAG).[24] Methylglyoxal is an AGE precursor, and DAG activates PKC. It seems more likely, however, that this pathway is activated via poly(ADP-ribose) polymerase (PARP), which is normally activated by increased concentrations of free radicals. According to the current, most likely explanation, increased intracellular glucose drives AR activity, thereby depleting cellular NADPH stores.[24] NADPH is also required to maintain stores of the intracellular antioxidant, reduced glutathione (GSH). Therefore, elevated intracellular glucose may increase cellular damage by driving AR activity, depleting NADPH and GSH, and increasing cell susceptibility to oxidative damage.[24] Whatever pathway may actually be activated, it is clear that polyol pathway activation can lead to other pathways that contribute to DMC damage.

 

Figure 5. Hyperglycemia increases flux through the polyol pathway

 

PKC β activation

Hyperglycemia increases the activity of PKC isoforms, particularly the β and δ isoforms, as diagrammed in Figure 6.[24] Intracellular hyperglycemia may activate PKC through several mechanisms. By one mechanism, intracellular glucose is metabolized to glycolytic intermediates that contribute to de novo production of the PKC activator diacylglycerol (DAG). This is in contrast to the typical receptor-signaled generation of DAG. In a second pathway, intracellular hyperglycemia increases AGEs, which activate AGE receptors and, as a result, increase DAG and PKC activity.[24] Regardless of the mode through which it is achieved, amplified PKC activity results in cellular changes contrary to good vascular function.

As illustrated in Figure 6, PKC activation decreases eNOS production and increases endothelin-1 (ET-1) production, resulting in blood flow abnormalities; increases vascular endothelial growth factor (VEGF) levels, leading to vascular permeability and inappropriate angiogenesis; increases transforming growth factor-β (TGF-β) and plasminogen activator inhibitor-1 (PAI-1), ultimately resulting in capillary and vascular occlusion; and activates nuclear factor-κB (NF-κB), enhancing pro-inflammatory gene expression.[10,24] The end result of these alterations is impaired microvascular function, which contributes to DMC tissue dysfunction.

 

Figure 6. Hyperglycemia induces protein kinase C activation

 

Pharmacotherapies based on mechanisms overactivated by hyperglycemia

The metabolic and cell signaling processes described above are targets for oral therapeutic agents designed to treat DMC. Classes of compounds and individual agents that block these pathways are illustrated in Figure 7.

 

Figure 7. Pathogenesis-based treatments interrupt metabolic pathways to DMC

 

Oxidative stress

It is difficult to draw conclusions regarding the benefits of antioxidants^† in diabetes because the clinical data are limited and inconsistent.[25] Very high doses of conventional antioxidants, such as vitamin E, may be required for therapeutic benefit because these antioxidants have restricted ability to scavenge ROS; each antioxidant molecule can scavenge only a limited number of ROS before it reaches its capacity.[10] Catalytic antioxidants^† may be an improvement over conventional antioxidants, as they can repeatedly scavenge ROS without becoming saturated.[10] However, because catalytic antioxidants are synthetic agents, they will need to be thoroughly assessed in double-blind, randomized controlled clinical trials before they can be applied to the treatment of DMC.[25,26] Despite speculation that conventional antioxidants may provide only limited benefits, several large clinical trials have demonstrated the efficacy of α-lipoic acid, a natural antioxidant, for DPN. A meta-analysis of the Alpha-Lipoic Acid in Diabetic Neuropathy Study (ALADIN) I, ALADIN III, the Symptomatic Diabetic Neuropathy Study (SYDNEY), and the Neurological Assessment of Thioctic Acid in Neuropathy (NATHAN) II trials revealed that 600 mg/d of α-lipoic acid, delivered intravenously for 3 weeks, significantly improved both positive symptoms and deficits of DPN (Figure 8).[27] Clinical data are not available for the effect of α-lipoic acid in diabetic retinopathy (DR), but in a pilot study, α-lipoic acid maintained UAE in patients with DN.[28]

 

Figure 8. Effects of α-lipoic acid on diabetic neuropathy

Glycation

Increased AGE formation has also been implicated in the development of DMC. Agents that prevent AGE formation would thus be expected to have therapeutic value for preventing and delaying the progression of DMC. Aminoguanidine,* also known as pimagedine, is one such agent. Among its many biochemical activities, aminoguanidine binds to AGE precursors, preventing the formation of AGEs.[29] Two clinical studies suggest aminoguanidine may provide benefit in diabetic nephropathy. In a 1999 study, aminoguanidine significantly reduced UAE compared to placebo. The effect was more pronounced in patients with less advanced nephropathy.[29] In a 2004 trial, the primary endpoint of reduced serum creatinine doubling was not met. However, aminoguanidine significantly reduced 24-hour total urinary proteinuria from baseline, and estimated GFR decreased more slowly in the aminoguanidine group.[30] A transient flu-like syndrome, anemia, and autoantibody production were reported as adverse events in both trials.[29,30] The flu-like syndrome was associated with serious subsequent adverse events (myocardial infarction, congestive heart failure, and atrial fibrillation) in one study.[29] In the 2004 study, development of crescent glomerulonephritis (GN) was attributed to high antimyeloperoxidase antineutrophilic cytokine antibodies (ANCA).[30] The GN incidence was similar to that observed with hydralazine, which is structurally similar to aminoguanidine.[29]

 

Polyol pathway

Increased polyol pathway flux is associated with increased AR activity. It is thought that excessive AR activity may contribute to hyperglycemic tissue damage by depleting intracellular antioxidant stores, thereby increasing cellular susceptibility to oxidative damage.[10,24] Enhanced AR activity has also been hypothesized to cause cellular damage and dysfunction by converting glucose to sorbitol, which is metabolized to glucotoxins that, in turn, promote oxidative stress.[4] Based on the lines of evidence implicating AR in hyperglycemic damage, a number of AR inhibitors (ARIs)^† have been developed and tested in clinical trials for efficacy in treating all 3 types of DMC.

The effects of the ARIs epalrestat, ponalrestat, tolrestat, and sorbinil on DR have all been assessed in clinical trials.[31,32,33,34] Only epalrestat has demonstrated efficacy for DR (Table 2). In the epalrestat studies, patients with NPDR benefited more than patients with more advanced, preproliferative DR.[31] Improvement in visual acuity was also reported.[31] Clinical trials on the effects of ARIs on DN have shown that epalrestat maintained and tolrestat improved UAE; tolrestat also decreased glomerular filtration rate (GFR; Table 3).[35,36] Ponalrestat had no effect on DN endpoints, and no data have been published for sorbinil.[37] A number of ARIs have been assessed in clinical trials for DPN, but most ARIs have been withdrawn due to adverse events (see Table 4).[38] Interest in this approach remains, though, as demonstrated by the recent publication of a trial for ranirestat.[39] In this study, increased nerve conduction velocity (NCV) and vibration detection threshold (VDT) indicated improved nerve function following 12 and 60 weeks of treatment, respectively.[39] Currently, no ARIs are approved for the treatment of DPN in the US, although epalrestat is marketed in Japan.[22,38]

 

Table 2. Aldose reductase inhibitors and their effect on diabetic retinopathy

Agent	Dose	Result
Epalrestat	300 mg/day	Improved electroretinogram, funduscopy, visual acuity[31]
Ponalrestat	600 mg/day	No clinically significant effect[32]
Tolrestat	200 mg/day	Little effect; significantly improved fluorescein leakage[33]
Sorbinil	250 mg/day	Little effect; lower rate of microaneurysms at interim time points[34]

 

Table 3. Aldose reductase inhibitors and their effect on diabetic nephropathy

Agent	Dose	Result
Epalrestat	150 mg/day	Maintained UAE* during 5 year study[35]
Ponalrestat	600 mg/day	No effect[37]
Tolrestat	200 mg/day	Decreased UAE and GFR† over 6 month study[36]
Sorbinil		No published clinical trial data

Table 4. Aldose reductase inhibitors and their effect on diabetic neuropathy

Drug	Results	Status
Alrestatin	Minor benefits (S)	Withdrawn (toxicity)
Sorbinil	Benefits (S, EP, M)	Withdrawn (toxicity)
Tolrestat	Minor benefits (S, EP)	Withdrawn (toxicity)
Ponalrestat	No efficacy	Withdrawn (toxicity)
Zenarestat	Minor benefits (EP, M)	Withdrawn (toxicity)
Epalrestat	Clinical benefits (S, EP)	Marketed in Japan
Fidarestat	Minor benefits (S, EP)	Under investigation

S=sensory; EP=electrophysiology; M=motor

PKC β activation

Selective inhibition of PKC β is one potential approach to interrupting the pathogenesis of DMC. General inhibition of PKC isozymes might be effective in preventing and/or treating DMC, but PKC isozymes are important in signal transduction in many tissues, and nonspecific PKC inhibitors might be associated with multiorgan dysfunction and toxicity.[40, 41] Therefore, agents that specifically inhibit PKC isozymes that play a role in DMC pathogenesis, such as PKC β, may improve DMC outcomes with more limited toxic side effects.[40] Clinical trials using the selective PKC β inhibitor ruboxistaurin* have demonstrated the potential benefits of this approach.

The Protein Kinase C β Inhibitor Diabetic Retinopathy Study (PKC-DRS) Group has published initial results of a clinical trial to assess the efficacy of ruboxistaurin in treating DR.[42] Ruboxistaurin (RBX), at a dose of 32 mg/d, significantly delayed development of moderate visual loss (MVL), defined as a decrease from baseline in ETDRS visual acuity score of 15 or more letters, although ruboxistaurin did not prevent DR progression (see Figure 9).[42] In addition, ruboxistaurin significantly reduced the occurrence of sustained MVL, defined as MVL at each of 2 consecutive visits 6 or more months apart, in patients who presented with DME at the start of the study.[42] In a study of the effects of ruboxistaurin in diabetic peripheral neuropathy, Neuropathy Total Symptom Score-6 (NTSS-6) results indicated improvement in neuropathic symptoms (see Figure 10).[43] However, nerve function, as assessed by VDT, only showed significant improvement in patients with less severe neuropathy.[43] Finally, data from a pilot phase 2 clinical trial demonstrate the potential benefit of ruboxistaurin in DN.[44] Patients (N = 123) were treated for 1 year with 32 mg/d of ruboxistaurin or placebo in addition to their current treatment, which included antihypertensives such as angiotensin-converting enzyme inhibitors (ACEIs) and angiotensin receptor blockers (ARBs).[44] The primary endpoint was lowered urinary albumin:creatinine ratio (ACR).[44] At the end of the study, ACR differed significantly from baseline for the ruboxistaurin group, but not the placebo group. In addition, GFR decreased significantly in patients in the placebo group, but not those in the ruboxistaurin group.[44] The results of these studies suggest ruboxistaurin has potential as a therapeutic agent in all 3 types of DMC.

Figure 9. Ruboxistaurin delays MVL in patients with nonproliferative diabetic retinopathy (NPDR)

Figure 10. PKC β inhibition and its effect on diabetic neuropathy (change in NTSS-6 score)

Summary

Hyperglycemia contributes significantly to DMC tissue damage. It can cause direct damage to cells, such as neurons, and can also lead to microvascular injury and subsequent ischemic damage and structural changes in the affected tissues (eg, retina, kidney, peripheral neurons). Hyperglycemia mediates damage through the generation of pathophysiologic metabolites, such as glycation products and reactive oxygen species, and via activation of cell signaling pathways. More specifically, hyperglycemia induces damage in DMC by promoting oxidative stress, AGE formation, overactivation of the polyol pathway, and stimulation of PKC β signaling. Each pathway can, by itself, generate specific tissue damage. Oxidative stress damages cellular proteins, reduces NO levels, promotes leukocyte adhesion, and inhibits endothelial barrier function. AGEs modify intracellular and extracellular protein function and activate deleterious cell signaling cascades through binding to AGE-specific receptors. Overactivation of the polyol pathway predisposes cells to oxidative stress. And PKC β overactivation leads to a series of events that cause structural changes to the microvasculature (see Figure 6).

In addition to the specific actions cited above, there is considerable interaction among the mechanisms mediating hyperglycemic damage. PKC β is activated by a number of these mechanisms, making it a key component in the development of DMC (See Figure 11). Therefore, inhibiting PKC β signaling may partially inhibit the action of other cellular mechanisms leading to vascular and tissue damage. Clinical trials with ruboxistaurin, a specific PKC β inhibitor, have demonstrated the potential benefit of this approach in all 3 DMC.

Figure 11. Metabolic pathways leading to DMC

Footnotes

*This agent is not FDA-approved for the treatment of DMC.
^†Agents in this class are not FDA-approved for the treatment of DMC.

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