Brief Genetics Report Cellular Basis of Diabetic Nephropathy II. The Transforming Growth Factor- System and Diabetic Nephropathy Lesions in Type 1 Diabetes Chunmei Huang, 1 Youngki Kim, 1 Maria Luiza A. Caramori, 1 Alfred J. Fish, 2 Stephen S. Rich, 2 Michael E. Miller, 2 Gregory B. Russell, 2 and Michael Mauer 1 Transforming growth factor- (TGF-) may be critical in the development of diabetic nephropathy (DN), and genetic predisposition is an important determinant of DN risk. We evaluated mRNA expression levels of TGF- system components in cultured skin fibroblasts (SFs) from type 1 diabetic patients with fast versus slow development of DN. A total of 125 long-standing type 1 diabetic patients were ranked by renal mesangial expansion score (MES) based on renal biopsy findings and diabetes duration. Patients in the highest quintile of MES who were also microalbuminuric or proteinuric (n  16) were classified as “fast-track” for DN, while those in the lowest quintile who were also normoalbu- minuric (n  23) were classsified as “slow-track” for DN. Twenty-five normal subjects served as control sub- jects. SFs were cultured in medium with 25 mmol/l glucose for 36 h. SF mRNA expression levels for TGF- 1, TGF- type II receptor (TGF- RII), throm- bospondin-1, and latent TGF- binding protein-1 (LTBP-1) were measured by real-time RT-PCR. LTBP-1 mRNA expression was reduced in slow-track (0.99  0.38) versus fast-track patients (1.65  0.52, P  0.001) and control subjects (1.41  0.7, P  0.025). mRNA levels for TGF-1, TGF- RII, and thrombospondin-1 were similar in the three groups. Reduced LTBP-1 mRNA expression in SFs from slow-track patients may reflect genetically determined DN protection and sug- gests that LTBP-1 may be involved in the pathogenesis of DN through the regulation of TGF- activity. Diabetes 51:3577–3581, 2002 A ccumulating evidence suggests that the trans- forming growth factor- (TGF-) system plays important roles in the pathogenesis of diabetic nephropathy (DN) (1,2). This intricate system is composed of multifunctional cell-cell signaling proteins that regulate cell proliferation and metabolism of extracel- lular matrix proteins (3,4). TGF- is secreted, in most cells, in a large latent complex that has no biological activity (4,5). This complex consists of three components: a homodimer of mature TGF-, a TGF- latency–associ- ated peptide (LAP), and a latent TGF- binding protein (LTBP) (5). Cleavage of both LTBP and LAP is necessary for TGF- activation (4). Thrombospondin is important in the activation of latent TGF- (3). The binding of activated TGF- to its corresponding type II receptor, in turn recruiting the type I receptor, initiates the signaling path- way to its downstream effectors, the Smad proteins (4). Genetic predisposition or protection may be the most important DN risk determinants in type 1 diabetes. Only about one-half of patients with poor glycemic control develop DN, while some patients do so despite relatively good control (6). Moreover, familial clustering of DN has been demonstrated by several investigators (7,8). Cultured skin fibroblasts (SFs), after several passages under iden- tical in vitro conditions, probably largely reflect the genet- ically determined behavior of these cells. Thus, cellular studies in diabetic patients could help to identify genetic pathways associated with risk of, or protection from, DN (9,10). The demonstration of cell behavior variability in the TGF- system in relation to DN risk would support the concept that the TGF- pathway is involved in this genetic susceptibility. Therefore, we evaluated the mRNA expres- sion levels of several critical genes in TGF- system, including TGF-1, TGF-1 type II receptor (TGF- RII), LTBP-1, and thrombospodin-1 in cultured SFs from type 1 diabetic patients with rapid or slow development of DN lesions. SF mRNA expression levels for the TGF- type I and type III receptors, however, were too low to permit accurate measurement. The patients for this study were selected from 125 patients with 8 years of type 1 diabetes with glomerular filtration rate (GFR) 30 ml  min -1  1.73 m -2 and ade- From the 1 Department of Pediatrics, University of Minnesota Medical School, Minneapolis, Minnesota; and the 2 Department of Public Health Sciences, Wake Forest University School of Medicine, Winston-Salem, North Carolina. Address correspondence and reprint requests to Michael Mauer, MD, Pediatric Nephrology, MMC 491 UMHC, Delaware St. S.E., Minneapolis, MN 55455. E-mail: mauer002@umn.edu. Received for publication 4 April 2002 and accepted in revised form 4 September 2002. AER, albumin excretion rate; C t , threshold cycle; DMEM, Dulbecco’s modified Eagle’s medium; DN, diabetic nephropathy; ECM, extracellular matrix; GFR, glomerular filtration rate; LAP, latency-associated peptide; LTBP, latent transforming growth factor- binding protein; MES, mesangial expansion score; SF, skin fibroblast; TGF-, transforming growth factor-; TGF- RII, TGF- type II receptor. DIABETES, VOL. 51, DECEMBER 2002 3577