Diabetic nephropathy is the most common cause of renal failure, accounting for more than half of all cases of end-stage renal disease in the United States. Renal disease will affect between 20-40% of diabetics in their lifetime. Diabetic nephropathy involves steadily increasing proteinuria, accompanied by elevated blood pressure, with a progressive decline in GFR. There is also a greatly increased risk of cardiovascular disease.
Proteinuria is both a consequence of the glomerular damage in diabetes mellitus, and a cause of further damage, since it leads to inflammation and fibrosis in the renal tubules and a loss of functional nephrons. A specific assay for the small blood protein albumin is a more sensitive test to determine proteinuria than is a typical urine dipstick test for protein. If high values are measured, this is termed albuminuria. The table below gives values that are used to define moderately increased albuminuria (a precursor to diabetic nephropathy) and established disease. As with many physiological variables (e.g. blood pressure, hyperglycemia), cut-off values are used for definitions, however, the reality is that proteinuria is a continuous variable, and the risk of renal damage increases with the degree of proteinuria, no matter the amount.
|Albumin||Albumin-creatinine ratio (ACR)*|
|normal||20 mg/day||less than 30 mg/g|
|moderately increased albuminuria||30-300 mg/day||30-300 mg/g|
|severely increased albuminuria
||greater than 300 mg/day||greater than 300 mg/g|
|*Because of the inconvenience of 24 hour urine collection, a spot urine collection (ideally obtained first thing in the morning) can be used and the urinary albumin to creatinine ratio is determined. Finding the ratio to creatinine avoids the confounding effects of variations in urine volume.|
Diabetic nephropathy causes characteristic changes to the structure of the glomerulus known as glomerulosclerosis. In glomerulosclerosis, there are increased extracellular deposits inside the renal corpuscle, with a decreased surface area available for filtration. The glomerular basement membrane becomes thicker, yet more leaky. Historically, these changes had been attributed to effects on endothelial cells and mesangial cells, the cells that support the capillary loops. However, more recent research suggests that effects on podocytes may be the most important factor in causing diabetic glomerulosclerosis.
The most important consequence of glomerulosclerosis is that it causes a leaky filtration membrane, so there is abnormally increased filtration of protein. Key factors are the loss of podocytes and the disruption of their cytoskeleton, which results in a change in podocyte shape known as podocyte effacement. Proteinuria then causes damage to the renal tubules and further loss of nephrons.
The precise factors that are responsible for diabetic nephropathy
are still being determined. In diabetes mellitus, two changes
Hyperglycemia causes there to be abnormal glycosylation (glycation) of cellular proteins, and through a series of chemical reactions, these evolve to form what are known as advanced glycation end-products (AGEs). AGEs can induce abnormal cellular changes by signaling through a receptor (known as RAGE), which is expressed by cells in the glomerulus. This abnormal signaling may act to disrupt important paracrine signaling between podocytes and endothelial cells required for normal maintenance of the filtration membrane. It can also cause abnormalities in extracellular proteins, thus perhaps affecting the glomerular basement membrane and the connective tissue supporting the capillary loops. Hyperglycemia may also stimulate the formation of oxygen radicals that can further damage tissues.
Insulin signaling may be required for both the survival of podocytes, and for the maintenance of proper podocyte structure. In a very elegant recent experiment, investigators specifically eliminated the insulin receptor from podocytes in mice. These mice develop proteinuria and glomerulosclerosis in the absence of hyperglycemia. Examination of the glomerulus with electron microscopy revealed changes in the architecture of the podocytes and thickening of the glomerular basement membrane. Tests also revealed evidence of podocyte apoptosis. This data shows that the changes of glomerulosclerosis can occur in response to a decrease in insulin signaling that is specific to podocytes.
Naturally, treatment to limit progression of diabetic nephropathy involves good glycemic control and good blood pressure control. Clinical trials have shown that drugs that target angiotensin II are able to reduce proteinuria and slow the progression of diabetic nephropathy. These drugs include ACE inhibitors, angiotensin II receptor blockers, and the direct renin inhibitor aliskiren. These drugs appear to provide renal protection that goes beyond their beneficial effect on blood pressure. This may be due to the fact that there are intrarenal effects of angiotensin II, in addition to its effects on peripheral resistance or its stimulation of aldosterone secretion and the regulation of ECF volume. Angiotensin II preferentially causes constriction of the efferent arteriole, which increases pressure in the glomerular capillaries and would promote more filtration of protein. As well, angiotensin II affects expression of important proteins in the filtration membrane, and it also has pro-inflammatory effects. Thus, reducing angiotensin II signaling helps to preserve the filtration membrane and limit proteinuria.
The recent data showing the importance of insulin signaling in podocytes supports the idea that treatment of diabetics with insulin-sensitizing drugs (metformin and thiazolidinediones) may be a better approach to prevent diabetic nephropathy, but this has yet to be tested.
These are references for the recent study about the effects of
insulin signaling in podocytes, and an accompanying comment
paper. The links are to the Pubmed citations.