Diabetic kidney disease: natural history and pathophysiology





Paola FIORETTO,MD, PhD
Department of Medicine
University of Padova, Italy

Diabetic kidney disease:
natural history
and pathophysiology

 

by P. Fioretto, Italy

Diabetic nephropathy is the most common cause of end-stage renal disease worldwide. Blood glucose and blood pressure control reduce the risk of developing this complication; however, once diabetic nephropathy is established, it is only possible to slow its progression. In the last few decades, there has been a significant reduction in all chronic diabetic complications except for diabetic nephropathy. Thus, despite guidelines recommending screening and nephroprotective care, the risk of diabetic nephropathy is not decreasing among patients with type 2 diabetes. The renal lesions underlying renal dysfunction differ between type 1 and type 2 diabetes, although the clinical manifestations of diabetic nephropathy—proteinuria, decreased glomerular filtration rate, and increased blood pressure—are similar. Indeed, in type 1 diabetes, despite the presence of tubular, interstitial, and arteriolar lesions, the most important structural changes involve the glomerulus. This contrasts with type 2 diabetes, in which a substantial proportion of patients have normal glomerular structure with or without tubulointerstitial and arteriolar abnormalities, despite the presence of microalbuminuria or proteinuria. The clinical manifestationsofdiabetic nephropathy arestrongly related to structural changes, especially the degree of mesangial expansion in both type 1 and type 2 diabetes. Changes in the structure and number of podocytes may also be involved in the progression of diabetic nephropathy, especially in type 2 diabetes.

Medicographia. 2016;38:49-55 (see French abstract on page 55)

Diabetes mellitus is a worldwide, growing public health problem associated with high risks of severe microvascular and macrovascular complications. the number of patients with diabetes mellitus, mostly type 2 diabetes, is expected to increase globally from an estimated 382 million (8.3% of the adult population) in 2013 to 592 million (10.0%) by 2035.1 Diabetic nephropathy is a devastating complication of diabetes and is a major cause of illness and death. the excess mortality of diabetes occurs mainly in proteinuric diabetic patients and is a result not only of end-stage renal disease (ESRD), but also of cardiovascular disease, with the latter being a particularly common cause in type 2 diabetic patients.2 Diabetic nephropathy is the single most common cause of ESRD in Europe, Japan, and the United States, with diabetic patients accounting for 25% to 45% of all patients enrolled in ESRD programs.3,4 the rate of increase of ESRD in diabetes has been much faster than that in other diseases, and this increase is largely due to type 2 diabetes. In France in 2010, 40% of patients who started renal replacement therapy had diabetes, and of these 94% had type 2 diabetes. the total cost to the national health system was estimated to be €4 billion.5 Diabetic nephropathy is diagnosed as increased urinary albumin excretion (UaE) or decreased estimated glomerular filtration rate (GFR).2 In the US NHaNES (National Health and Nutrition Examination Survey) study, the prevalence of diabetic nephropathy (reduced GFR, increased UaE, or both) was 43%.6 In France, 29%-47% of people with type 2 diabetes have diabetic nephropathy.5 In the USa in the last two decades, there has been a significant reduction in all diabetic chronic complications—as a consequence of an improvement in diabetes management—with the exception of diabetic nephropathy.7 thus, risk of diabetic nephropathy is not decreasing among patients with type 2 diabetes, despite guidelines recommending screening and nephroprotective care.

Microalbuminuria precedes overt proteinuria and was once considered the first clinical manifestation of diabetic nephropathy8-10; however, the natural history of the disease may have changed.11-15 In the early 1980s, the risk of progression to overt proteinuria (albumin excretion rate (aER) >200 μg/ min) over a decade in microalbuminuric type 1 diabetic patients was estimated to be about 80%.8-10 However, more recently, prospective studies have demonstrated that the percentage of type 1 diabetic patients with microalbuminuria progressing to overt proteinuria over 10 years is only around 30%11-15 due to initial overestimation of risk in earlier studies, improved prognosis due to advancements in treatment, or both. In fact, the concept that microalbuminuria spontaneously regresses to normoalbuminuria in a substantial proportion of patients is now well established.11-15 In type 2 diabetes, the progression rate from microalbuminuria to proteinuria is similar (around 30% over 10 years).16 In the Steno 2 study, over a 7.8 year follow-up of 151 type 2 diabetic patients with microalbuminuria, 31% progressed to proteinuria, 31% regressed to normoalbuminuria, and 38% remained microalbuminuric.17 It is interesting that the rate of decline of GFR was much lower in patients who regressed to normoalbuminuria (2.3 ml/ min/year) than in patients who progressed to proteinuria (5.4 ml/min/year), suggesting that regression to normoalbuminuria is associated with preservation of renal function.17 Renal outcome was associated with baseline aER levels, retinopathy status, metabolic and blood pressure control, and use of renin-angiotensin system (RaS) blockade.

Key modifiable risk factors for diabetic nephropathy include hyperglycemia, hypertension, dyslipidemia, anemia, albuminuria, and lifestyle factors, such as obesity and smoking.2 Early identification of key risk factors and prompt therapeutic intervention can potentially prevent or slow the decline of renal function in patients with type 2 diabetes mellitus. large, longterm clinical trials have demonstrated that improved blood glucose control18-20 and blood pressure control21-23 slow the development and/or the progression of diabetic nephropathy. Indeed, as a consequence of better metabolic and blood pressure control (the latter especially with RaS blockers), the natural history of diabetic nephropathy has changed in the last few decades. thus, it may now be possible to delay or halt the progression towards ESRD in patients with overt diabetic nephropathy. this contrasts with the concept that by the time patients have overt nephropathy, decline of renal function is unavoidable and inexorable. Glycemic control is the main risk factor for diabetic nephropathy, and strict glycemic control is the most potent therapeutic approach for preventing the development of nephropathy. When the clinical manifestations of diabetic nephropathy are present, blood pressure control is crucial, especially that obtained using RaS blockers. Nevertheless, the reduction in progression to ESRD achieved with RaS blockade in type 2 diabetes patients is not satisfactory.21-23 Recent data from the aDVaNCE (action in Diabetes and Vascular disease: Preterax and DiamicroN MR Controlled Evaluation) study support the importance of glycemic control in preventing the transition from microalbuminuria to proteinuria and from proteinuria to ESRD.24 the long-term follow-up of this trial demonstrated a sustained reduction over 10 years in the risk of dialysis and transplantation.24 It thus appears that intensive glycemic control is effective not only at preventing the development, but also at slowing the progression of diabetic nephropathy to ESRD. We have demonstrated that prolonged normoglycemia, obtained with pancreas transplantation alone, leads to the reversal and cure of established diabetic nephropathy lesions in patients with type 1 diabetes.25 thus, glycemic control is crucial to the development, progression, and regression of diabetic nephropathy.

Pathophysiology

Although numerous factors contribute to diabetic nephropathy, it is well established that exposure to hyperglycemia is necessary for the development of this disorder. Studies in type 1 and type 2 diabetes have found that improved glycemic control can reduce the risk of diabetic nephropathy. Moreover, the development of the earliest diabetic renal lesions can be slowed or prevented by strict glycemic control, as was demonstrated in a randomized trial in type 1 diabetic kidney transplant recipients.26 In addition, intensive insulin treatment has been shown to decrease the progression rates of glomerular lesions in microalbuminuric type 1 diabetic patients.27 Regression of established diabetic glomerular lesions was demonstrated in the native kidneys of type 1 diabetic patients with prolonged normalization of glycemic levels after successful pancreas transplantation.25 these studies strongly suggest that hyperglycemia is necessary not only for diabetic nephropathy lesions to develop, but also for the maintenance of established lesions. Removal of hyperglycemia allows expression of reparative mechanisms, which ultimately result in the healing of the original diabetic glomerular injury.

Hemodynamic mechanisms may be also involved in the pathogenesis of diabetic nephropathy.2 Interestingly, patients with other causes of hyperfiltration, such as uninephrectomy, do not develop diabetic lesions. Therefore, glomerular hyperfiltration alone cannot explain the genesis of the early lesions of diabetic nephropathy. Clinical observations suggest that hemodynamic factors may be more important in modulating the rate of progression of already well-established diabetic lesions. It is worth noting that the presence of reduced GFR in normoalbuminuric patients with type 1 diabetes has been associated with the presence of more severe glomerular lesions, and these patients may be at increased risk of progression to overt diabetic nephropathy.28 Systemic blood pressure levels are implicated in progression and may be implicated in the genesis of diabetic nephropathy. Intensive blood pressure control has been associated with decreased rates of progression from normoalbuminuria to microalbuminuria and from microalbuminuria to proteinuria in both normotensive and hypertensive type 2 diabetic patients.2

Genetic predisposition to diabetic nephropathy has been strongly suggested by multiple cross-sectional studies in type 1 and type 2 diabetic siblings concordant for diabetes.29,30 Importantly, diabetic sibling pairs known to be concordant for diabetic nephropathy risk are highly concordant for diabetic glomerulopathy lesions.31 this risk, in part independent of glycemia, seems to be substantial. Searches for genetic loci related to susceptibility to diabetic nephropathy are ongoing with genomic scanning and candidate gene approaches, although neither approach has yet yielded definitive results.

Renal lesions of diabetic nephropathy appear to be mainly related to extracellular matrix accumulation, which occurs in the glomerular basement membrane (GBM) and in the tubular basement membrane and is the principal cause of mesangial expansion and a contributor to interstitial expansion late in the disease.2,32,33 Several regulatory mechanisms have been proposed to explain the linkage between high glucose levels and extracellular accumulation. these include: increased levels of transforming growth factor β; activation of protein kinase C; stimulation of extracellular matrix production through the cyclic adenosine monophosphate pathway; increased levels of advanced glycation end products; and increased activity of aldose reductase, leading to the accumulation of sorbitol. there is also growing evidence that oxidative stress increases in diabetes and is related to diabetic nephropathy. there is an association between oxidative stress, altered nitric oxide production and action, and endothelial dysfunction.

Pathology

The renal lesions underlying renal dysfunction differ between type 1 and type 2 diabetes, although the clinical manifestations of diabetic nephropathy—proteinuria, decreased GFR, and increased blood pressure—are similar. Indeed, in type 1 diabetes, although tubular, interstitial, and arteriolar lesions are present, the most important structural changes involve the glomerulus. In contrast, a substantial proportion of type 2 diabetic patients have normal glomerular structure with or without tubulointerstitial and arteriolar abnormalities, despite the presence of microalbuminuria or proteinuria.2,32,33 the clinical manifestations of diabetic nephropathy are strongly related to structural changes, especially the degree of mesangial expansion in both type 1 and type 2 diabetes. In the last few years, changes in the structure and number of podocytes have also been linked to the progression of diabetic nephropathy.2,32,33

Type 1 diabetes
Renal morphologic lesions of diabetic nephropathy in type 1 diabetic patients occur in the glomeruli, arterioles, interstitium, and tubules.2,32-34

In type 1 diabetes, glomerulopathy is the most important lesion, characterized by thickening of the GBM and mesangial expansion, although the tubules, interstitium, and arterioles also undergo substantial changes (Figure 1, page XX).34

GBM thickening, the first measurable change, has been documented as early as 1.5 to 2.5 years after the onset of type 1 diabetes. Mesangial expansion develops later; an increase in the matrix component of the mesangium can be detected as early as 5-7 years after the onset of diabetes. thereafter, these structural changes do not develop at the same rate in individual patients. Nevertheless, when renal insufficiency occurs, marked mesangial expansion and increased GBM width are present in all type 1 diabetic patients.34 Diffuse and generalized mesangial expansion, commonly termed diffuse diabetic glomerulosclerosis, is associated with nodular lesions consisting of areas of marked mesangial expansion forming large, round fibrillar mesangial zones with palisading of mesangial nuclei around the periphery of the nodule and extreme compression of the associated glomerular capillaries (Kim melstiel-Wilson nodules). Both mesangial expansion and GBM thickening are consequences of extracellular matrix accumulation, with increased deposition of collagen (types IV and VI), laminin, and fibronectin. additional structural abnormalities include glomerular enlargement, tubular basement membrane thickening, tubular atrophy, interstitial expansion, and afferent and efferent arteriolar hyalinosis.2,32-34 Bowman’s capsule thickening is also regularly present.

Figure 1
Figure 1. Glomerulus from a type 1 diabetic patient with diffuse
and nodular mesangial expansion, glomerular basement membrane
thickening, and afferent and efferent arteriolar hyalinosis
(Periodic Acid Schiff’s [PAS] stain).

Reproduced from reference 32: Fioretto et al. Semin Nephrol. 2007;27:195-207.
© 2007, Elsevier Inc.

Type 2 diabetes
In type 2 diabetes the situation is more complex. We studied a large cohort of type 2 diabetic patients with microalbuminuria and proteinuria and described heterogeneity in renal structure among these patients; in fact, only a subset had diabetic glomerulopathy, while the remaining had mild or absent glomerulopathy with or without tubulointerstitial and arteriolar changes.33,35 In proteinuric patients, less than 10% had nondiabetic renal diseases. thus, we proposed a classification system based on 3 major groups33,35:

Category C I: normal or near normal renal structure
These patients (representing 35% of cases of microalbuminuria and 15% of proteinuria) had biopsies that were normal or showed very mild glomerular, tubular, interstitial, and vascular changes.

Category C II: typical diabetic nephropathology
These patients (representing 30% of cases of microalbuminuria and 50% of proteinuria) had established diabetic lesions, with an approximately equal degree of glomerular, tubulointerstitial, and arteriolar changes. this picture is typical of that seen in type 1 diabetic patients with obvious diabetic nephropathy changes using light microscopy.

Category C III: atypical patterns of renal injury
These patients (representing 35% of cases of microalbuminuria and 35% of proteinuria) had relatively mild diabetic glomerular changes considering the disproportionately severe renal structural changes, including: (i) tubular atrophy, tubular basement membrane thickening and reduplication, and interstitial fibrosis (tubulointerstitial lesions); (ii) advanced glomerular arteriolar hyalinosis commonly associated with atherosclerosis of larger vessels; and (iii) global glomerular sclerosis. In the C III group, these patterns were present in all possible combinations (Figure 2).

It thus seems that hyperglycemia may cause different patterns of renal injury in type 2 diabetic patients compared with type 1 diabetic patients. Tubulointerstitial and vascular changes could also be related to aging, atherosclerosis, and systemic hypertension. However, hypertension was present in almost all patients in all three structural categories, and “per se” cannot account for the different lesions observed in category C III. Furthermore, mean age was similar in the patients in category C II and C III (60 years), despite different patterns of renal injury in the two groups, and our observations in a large number of age-matched normal controls indicate that aging by itself is not sufficient to explain most of the renal structural changes observed in C III patients. Thus, heterogeneity in renal structure might reflect the heterogeneous nature of type 2 diabetes.

Figure 2
Figure 2. Renal biopsies from microalbuminuric
type 2 diabetic patients (Periodic
Acid Schiff’s [PAS] stain). Panel A (left)
shows normal glomerular, tubular, interstitial,
and vascular structures. This would be
classified category C I (normal or near normal
renal structure), according to our system.
Panel B (right) shows mild mesangial expansion
relative to the severity of interstitial
fibrosis and tubular atrophy. This would be
classified as category C III (atypical pattern
of renal injury).

Reproduced from reference 35: Fioretto et al. Diabetologia.
1996;39:1569-1576. © 1996, Springer-Verlag.

Structural-functional relationships
The relationships between structural abnormalities and kidney function are better defined using electron microscopy morphometric analysis. the critical lesion in type 1 diabetes is mesangial expansion, morphometrically termed mesangial fractional volume [Vv(mes/glom)]; this is the structural parameter that best correlates with all functional parameters in type 1 diabetes.34 Indeed a highly significant inverse correlation exists between Vv(mes/glom) and GFR34; when mesangium expands it restricts and distorts glomerular capillaries and diminishes capillary filtration surface, which is strongly directly related to Vv(mes/glom) and inversely to GFR.34 Vv(mes/glom) is also related to aER and blood pressure levels.34 In contrast, GBM thickening is not related to GFR or to the presence of hypertension, but only to aER, suggesting that this lesion is involved in the pathogenesis of albuminuria rather than the loss of kidney function. Interstitial expansion and percentage of global sclerosis are also related to proteinuria, hypertension, and declining GFR.34 In the early stages of diabetic nephropathy, however, our data in a small number of patients with type 1 diabetes studied with sequential renal biopsies indicate that progression from normoalbuminuria to microalbuminuria and from microalbuminuria to early overt nephropathy is related only to progressive mesangial expansion36; by contrast, there was no progression in interstitial fibrosis or GBM thickening.36

Mesangial expansion and interstitial expansion are independent determinants of renal dysfunction in type 1 diabetes and are probably the consequence of different pathogenetic mechanisms. Indeed, mesangial expansion is mainly due to extracellular matrix accumulation; this differs from early interstitial expansion, which is due to an increase of the cellular component. Increased interstitial collagen occurs only in patients with advanced diabetic nephropathy. Thus, in type 1 diabetes, clinical nephropathy is always associated with advanced glomerular injury.

In type 2 diabetes, our findings indicate that mesangial expansion is also a crucial structural change leading to loss of renal function.34,37 Although these structural-functional relationships were significant, they were imprecise and less strong than in type 1 diabetes. Moreover, in confirmation of our observations from light microscopy studies,35 glomerular lesions were less advanced in type 2 than type 1 diabetic patients, and a substantial number of these type 2 patients had normal glomerular structure despite abnormal aER. These data are in agreement with those from a study of Pima Indians, where global glomerular sclerosis, Vv(interstitium), and GBM width were similar in patients with long-term type 2 diabetes with normoalbuminuria and those with microalbuminuria; only Vv(mes/glom) increased from early diabetes to microalbuminuria. In these patients, ultrastructural glomerular parameters were significantly abnormal only in patients with clinical nephropathy.38

Hayashi et al39 reported renal structural-functional relationships in type 2 diabetic patients similar to those observed in type 1 diabetes. In contrast, Osterby et al40 described great variability in glomerular injury in Danish type 2 diabetic patients with proteinuria; they also observed that type 2 diabetic patients tended to have less marked glomerular changes than type 1 diabetic patients with comparable renal function.

Heterogeneity in renal structure is related to the risk of progressive GFR loss.37 Over 4 years of follow-up, decline in GFR in type 2 diabetic patients with microalbuminuria and proteinuria was significantly correlated with the severity of mesangial expansion and GBM thickening.37

Role of podocytes in diabetic nephropathy
Podocytes are injured in numerous experimental and human glomerular diseases, including diabetes. White et al41 observed similar numbers of podocytes in normal subjects and in type 1 diabetic patients with abnormal aER, although there was a trend towards fewer podocytes per glomerulus in diabetic patients. Moreover, this study found no significant correlation between podocyte number and aER. these findings contrast with those of a previous study42 reporting that podocyte number decreased in type 1 diabetic patients with normal aER compared with normal controls, suggesting that diabetes per se may affect podocytes. Biorn et al43 described an increase in foot process width (FPW) in the peripheral GBM of type 1 diabetic patients with abnormal aER compared with normoalbuminuric patients, without significant differences between microalbuminuric and proteinuric patients. low podocyte number has also been described in type 2 diabetic Pima Indians with proteinuria.38

It has been suggested that in these patients podocyte loss and increased foot process width play a role in the progression to overt nephropathy. this concept was reinforced by findings from a longitudinal study, in which Pima Indians with type 2 diabetes and microalbuminuria were studied over 4 years44; the number of podocytes per glomerulus at baseline was the strongest predictor of changes in albuminuria, in that patients with the lowest number of podocytes had the highest risk of fast progression to overt proteinuria.44 We studied podocyte structure in a large cohort of Caucasian type 2 diabetic patients, with aER ranging from normal to overt proteinuria.45 the density of podocytes per glomerulus, Nv(epi/glom), was significantly lower in all diabetic patients compared with controls, and was lower in microalbuminuric and proteinuric patients than in normoalbuminuric ones. The absolute number of podocytes per glomerulus (Epi N/glom) was lower in microalbuminuric and proteinuric patients compared with controls; however, there were no significant differences between the diabetic groups. In addition, microalbuminuric and proteinuric patients had decreased length density of filtration slits over the peripheral GBM (FSlv/glom) and increased FPW compared with normoalbuminuric patients. aER was inversely related to Nv(epi/glom) and FSlv/glom and directly related to FPW, while there was no correlation with Epi N/glom. GFR was weakly related only to FSlv/glom. These results suggest that in Caucasian type 2 diabetic patients changes in podocyte structure and density occur from the early stages of diabetic nephropathy and might contribute to increasing albuminuria in these patients. Structural changes in podocytes could in part explain abnormal albuminuria in patients without diabetic glomerulopathy. these findings also suggest that in type 2 diabetic patients, the density of podocytes may be functionally more relevant than their absolute number.45

Podocytes have limited capacity to replicate and when they are lost they cannot be replaced by new cells; thus, it has been hypothesized that the loss of podocytes, along with the increase in glomerular volume caused by diabetes, necessarily requires the residual cells to cover a larger area of GBM. This could cause foot process widening and reduce the ability of the podocytes to remain attached to GBM, forming areas of bare GBM with consequent proteinuria. These areas are potential starting points for glomerular sclerosis.

Further studies on podocyte ultrastructure and on the expression of slit diaphragm proteins are necessary to better define the role of these cells in the pathogenesis of diabetic nephropathy.

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Keywords: diabetes; mesangial expansion; nephropathy; pathophysiology; podocyte; proteinuria