Intestinal phosphate absorption occurs through both a paracellular mechanism involving tight junctions and an active transcellular mechanism involving the type II sodium-dependent phosphate cotransporter NPT2b (SLC34a2). To define the contribution of NPT2b to total intestinal phosphate absorption, we generated an inducible conditional knockout mouse, Npt2b Inorganic phosphate is an essential mineral critical for cellular processes and bone mineralization. Severe disruptions in serum phosphate have pathologic consequences. 1,2 Hypophosphatemic disorders are associated with rickets, osteomalacia, and a host of secondary dysfunctions. 3 In contrast, hyperphosphatemia associated with chronic kidney disease (CKD) is linked tightly to increased risk of cardiovascular morbidity and mortality. 4 -6 Recent studies show that elevated phosphate concentrations within the high normal range in individuals with functional kidneys also are correlated with increased cardiovascular risk and mortality. 7,8 Thus, an elevated serum phosphate level is an emerging health risk. Despite the importance of maintaining a relatively narrow serum phosphate range, nearly 70% of dietary phosphate is absorbed, resulting in transient postprandial increases in serum phosphate concentrations. 9 Normalization of serum phosphate appears to be managed primarily within the renal proximal tubule by the type II sodium-dependent phosphate cotransporters NPT2a (SLC34a1) and NPT2c (SLC34a3). Genetic knockout mouse models demonstrate that 80% and 20% of total urinary phosphorus are managed by the Npt2a and Npt2c transporters, respectively. 10,11 Chronic and acute regulation of these renal transporters is modulated by changes in dietary and serum phosphate

Patients with abetalipoproteinemia, a disease caused by defects in the microsomal triglyceride transfer protein (MTP), do not produce apolipoprotein B-containing lipoproteins. It was hypothesized that small molecule inhibitors of MTP would prevent the assembly and secretion of these atherogenic lipoproteins. To test this hypothesis, two compounds identified in a high-throughput screen for MTP inhibitors were used to direct the synthesis of a highly potent MTP inhibitor. This molecule (compound 9) inhibited the production of lipoprotein particles in rodent models and normalized plasma lipoprotein levels in Watanabe-heritable hyperlipidemic (WHHL) rabbits, which are a model for human homozygous familial hypercholesterolemia. These results suggest that compound 9, or derivatives thereof, has potential applications for the therapeutic lowering of atherogenic lipoprotein levels in humans.

The incidence of cardiovascular events and mortality strongly correlates with serum phosphate in individuals with CKD. The Npt2b transporter contributes to maintaining phosphate homeostasis in the setting of normal renal function, but its role in CKD-associated hyperphosphatemia is not well understood. Here, we used adenine to induce uremia in both Npt2b-deficient and wild-type mice. Compared with wild-type uremic mice, Npt2b-deficient uremic mice had significantly lower levels of serum phosphate and attenuation of FGF23. Treating Npt2b-deficient mice with the phosphate binder sevelamer carbonate further reduced serum phosphate levels. Uremic mice exhibited high turnover renal osteodystrophy; treatment with sevelamer significantly decreased the number of osteoclasts and the rate of mineral apposition in Npt2b-deficient mice, but sevelamer did not affect bone formation and rate of mineral apposition in wild-type mice. Taken together, these data suggest that targeting Npt2b in addition to using dietary phosphorus binders may be a therapeutic approach to modulate serum phosphate in CKD.

The intrarenal renin-angiotensin system (RAS) plays a key role in the development of diabetic nephropathy. Recently, we showed that combination therapy with an AT 1 receptor blocker (ARB) and an activated vitamin D analog produced excellent synergistic effects against diabetic nephropathy, as a result of blockade of the ARB-induced compensatory renin increase. Given the diversity of vitamin D analogs, here we used a pro-drug vitamin D analog, doxercalciferol (1␣-hydroxyvitamin D 2), to further test the efficacy of the combination strategy in long-term treatment. Streptozotocin-induced diabetic DBA/2J mice were treated with vehicle, losartan, doxercalciferol (0.4 and 0.6 g/kg), or losartan and doxercalciferol combinations for 20 wk. Vehicle-treated diabetic mice developed progressive albuminuria and glomerulosclerosis. Losartan alone moderately ameliorated kidney injury, with renin being drastically upregulated. A similar therapeutic effect was seen with doxercalciferol alone, which markedly suppressed renin and angiotensinogen expression. The losartan and doxercalciferol combination most effectively prevented albuminuria, restored glomerular filtration barrier structure, and dramatically reduced glomerulosclerosis in a dose-dependent manner. These effects were accompanied by blockade of intrarenal renin upregulation and ANG II accumulation. These data demonstrate an excellent therapeutic potential for doxercalciferol in diabetic renal disease and confirm the concept that blockade of the compensatory renin increase enhances the efficacy of RAS inhibition and produces synergistic therapeutic effects in combination therapy.renin-angiotensin system; compensatory renin increase; albuminuria; glomerulosclerosis DIABETIC NEPHROPATHY (DN) is the most common renal complication of diabetes mellitus and a leading cause of end-stage renal disease, accounting for 44% of new cases in 2005 (9). It is well established that the renin-angiotensin system (RAS) is a major mediator of progressive renal injury. Since renal interstitial angiotensin (ANG) II levels are much higher than in the plasma (28), the local RAS in the kidney is believed to play the major damaging role in diabetic nephropathy. Kidney cells, including mesangial cells and podocytes, are able to synthesize all components of the RAS, including renin, the (pro)renin receptor, angiotensinogen (AGT), and ANG II receptors independently of the systemic RAS, making the kidney capable of maintaining a high level of local ANG II. Intrarenal renin and AGT levels are induced in diabetic animals (4, 48). In vitro studies showed that when exposed to high glucose levels, mesangial cells and podocytes increase renin and ANG II production (13,38,42). Intrarenal ANG II promotes the progression of renal injury via multiple pathways that increase glomerular permeability, induce oxidative stress, and promote the synthesis of profibrotic and proinflammatory factors and extracellular matrix (8,15). The consequence of the progression of diabetic renal injury is the development of protein...

Steatosis in the liver is a common feature of obesity and type 2 diabetes and the precursor to the development of nonalcoholic steatohepatitis (NASH), cirrhosis, and liver failure. It has been shown previously that inhibiting glycosphingolipid (GSL) synthesis increases insulin sensitivity and lowers glucose levels in diabetic rodent models. Here we demonstrate that inhibiting GSL synthesis in ob/ob mice not only improved glucose homeostasis but also markedly reduced the development of hepatic steatosis. The ob/ob mice were treated for 7 weeks with a specific inhibitor of glucosylceramide synthase, the initial enzyme involved in the synthesis of GSLs. Besides lowering glucose and hemoglobin A1c (HbA1c) levels, drug treatment also significantly reduced the liver/body weight ratio, decreased the accumulation of triglycerides, and improved several markers of liver pathology. Drug treatment reduced liver glucosylceramide (GL1) levels in the ob/ob mouse. Treatment also reduced the expression of several genes associated with hepatic steatosis, including those involved in lipogenesis, gluconeogenesis, and inflammation. In addition, inhibiting GSL synthesis in dietinduced obese mice both prevented the development of steatosis and partially reversed preexisting steatosis. Conclusion: These data indicate that inhibiting GSL synthesis ameliorates the liver pathology associated with obesity and diabetes, and may represent a novel strategy for treating fatty liver disease and NASH. (HEPATOLOGY 2009;50:85-93.)