DPP-4 inhibitors-renoprotection in diabetic nephropathy?

There is no doubt that rates of chronic kidney disease are escalating and that this rise is the main contributor to the increasing prevalence of diabetic nephropathy. It is also clear that the increase in kidney failure continues despite tight blood glucose and blood pressure control, as well as renin-angiotensin system blockade. In response, research into novel therapies to treat diabetic nephropathy is expanding, but to date nothing has translated into clinical use. The dipeptidyl peptidase-4 (DPP-4) inhibitors are oral, weight-neutral hypoglycemic drugs used to treat patients with type 2 diabetes. DPP-4 cleaves polypeptides with a proline/alanine in the penultimate position at the aminoterminal position. Hence, the net physiologic effect is a complex interplay between the resulting substrate/product profile in a particular disease milieu (rather than a specific signaling pathway). Cleaved substrates may either be activated, inactivated, or bear no functional relevance. DPP-4 inhibitors lower blood glucose levels by raising the half-life of short-lived endogenous incretins, such as GLP-1 and glucose-dependent insulinotropic polypeptide. However, the ability of DPP-4 to cleave a host of additional membrane-bound substrates that exert nonenzymatic properties by interacting or colocalizing with other membrane proteins/receptors suggests it may be a novel and exciting therapeutic target in diabetic nephropathy. In this issue, Kanasaki et al. (1) investigate the antifibrotic effect of linagliptin in a type 1 model of diabetic nephropathy. This study demonstrates that after 4 weeks linagliptin ameliorated diabetic kidney fibrosis, an observation that occurred in association with the inhibition of endothelial-to-mesenchymal transition (EndMT) and the restoration of microRNA (miRNA) 29s. An important aspect of this study is the use of a type 1 model of diabetes with streptozotocin. An insulin-deficient model of diabetes enables the evaluation of the effects of the DPP-4 inhibitor independent of glycemic control. This is important because DPP-4 inhibitors will not have a significant hypoglycemic effect due to the lack of a significant insulin response mediated by GLP-1. As a result, the new findings provide information regarding renal benefit of linagliptin independent of glycemic control. In diabetic nephropathy, the fibrotic pathways that lead to scarring and kidney failure are mediated primarily through transforming growth factor-b (TGF-b). Antifibrotic strategies targeting TGF-b have been limited. Hence, the findings of Kanasaki et al. are highly relevant from a clinical point of view because of their potential as an antifibrotic agent in patients with diabetes. We have previously shown that the cation-independent mannose 6-phosphate receptor (CIM6PR) is central to the activation process of TGF-b1 in human kidney proximal tubular cells exposed to high glucose (2). We also have shown that linagliptin interferes with the conversion of latent to active TGF-b in human kidney proximal tubular cells and downstream fibrotic markers (3). Given that DPP-4 and CIM6PR colocalize on the cell membrane, it is highly likely that the mechanism by which linagliptin exerts its antifibrotic effect is through interrupting this protein–protein interaction and thereby reducing the activation of TGF-b. This can be confirmed with the use of proximity ligation assays, a technique used to directly observe individual endogenous protein complexes in situ (4). A schematic diagram summarizing this concept is shown in Fig. 1. Kanasaki et al. confirmed the TGF-b/miRNA 29 interaction that resulted in EndMT as a mechanism of fibrosis. However, it should be acknowledged that multiple miRNA families have been implicated in diabetic nephropathy. These include, but are not limited to, miRNA 200, 192, and 21. All are regulated by TGF-b1, but given the cell-specific interactions of miRNA and the