IGF-Binding Protein-3 Modulates TGF-/BMP-Signaling in Glomerular Podocytes Imke Peters,* Irini Tossidou,* Johannes Achenbach,* Robert Woroniecki, † Michael Mengel, ‡ Joon-Keun Park,* Melanie Paschy,* Kirsten de Groot,* Hermann Haller,* and Mario Schiffer* *Division of Nephrology, Department of Medicine, and ‡ Department of Pathology, Hannover Medical School, Hannover, Germany; and † Division of Pediatric Nephrology, Children’s Hospital at Montefiore, Albert Einstein College of Medicine, Bronx, New York Podocyte apoptosis initiates progressive glomerulosclerosis in TGF-1 transgenic and CD2AP-knockout (CD2AP/) mice. It was previously shown that in both mouse models, activation of the TGF- pathway is the key event during development of podocyte apoptosis. Furthermore, CD2AP is an important modifier of TGF-–induced survival signaling via activation of the phosphoinositol 3-kinase/AKT signaling pathway. This article presents IGF-binding protein-3 (IGFBP-3) as a new modulator of apoptosis and survival signaling in glomerular podocytes. High expression of IGFBP-3 protein in the urine of diseased CD2AP/ mice was discovered, and IGFBP-3 expression in glomerular podocytes and parietal cells was detected. IGFBP-3 can induce changes in podocyte actin cytoskeleton, leads to apoptosis in cultured murine podocytes, and can enhance TGF-1–induced apoptosis in vitro. For studying this process on a molecular level, proapoptotic p38 mitogen-activated protein kinase pathways and antiapoptotic phosphoinositol 3-kinase/AKT pathways were examined in cultured murine podocytes. It was found that IGFBP-3 increments the level of TGF-1–induced phosphorylated p38 mitogen-activated protein kinase and decreases the phosphorylation of antiapoptotic AKT. This effect is specific for the co-stimulation of IGFBP-3 with TGF-1 because a combination of IGFBP-3 with bone morphogenic protein-7 (BMP-7), another member of the TGF- superfamily, results in apoptosis opposing signaling effects with a strong increase of phosphorylated AKT and subsequent functional effects. These results demonstrate that the IGF/IGFBP axis plays an important role in the development of podocyte apoptosis by modulation of TGF- and BMP-7–induced pro- and antiapoptotic signals. J Am Soc Nephrol 17: 1644 –1656, 2006. doi: 10.1681/ASN.2005111209 L oss of glomerular podocytes is the key mechanism of disease initiation and disease progression in focal seg- mental glomerulosclerosis (FSGS). There is a large body of evidence that a loss of glomerular podocytes of 40% results in progressive glomerular failure (1,2). TGF- has been shown to be an important mediator of progressive fibrosis, cell prolif- eration, and cell death in glomerular disease (for detailed re- view, see Bottinger et al. [3]). We demonstrated previously that TGF- is involved in survival and apoptotic responses in podo- cytes (4,5). Bone morphogenic protein-7 (BMP-7) is another member of the TGF- superfamily, so far involved primarily in kidney development and described as an active blocker of TGF-–induced profibrotic effects (6,7). TGF- and BMP sig- naling require an orchestrated pattern of downstream events (for detailed reviews, see Schiffer et al. [8] and Chen et al. [9]). The signaling is initiated after ligand binding by heteromeric complex formation of two transmembrane serine/threonine kinase receptors, known as the type I and type II receptors. This leads to receptor transphosphorylation and receptor activation. The activated receptors phosphorylate the receptor associated (R-)Smads, Smad2/3 (in the case of TGF-) or Smad1/5/8 (in the case of BMP), which form complexes with the common pathway (C-)Smad (Smad4). Only this active complex can translocate to the nucleus, associate with specific DNA-binding partners, and initiate transcriptional responses. Among several target genes, Smad6 and Smad7, the so-called inhibitory (I-)Smads, are transcriptionally induced and act as inhibitory feedback loop, leading to direct interference with the TGF-/ BMP receptors and/or the formation of inactive Smad4 com- plexes that cannot translocate to the nucleus. Smad6 preferably inhibits BMP signaling, and Smad7 preferably inhibits TGF- signaling (10,11). In epithelial cells, we and others could show that Smad7 has a dual function by inhibition of the NF-B survival signaling cascade and TGF-–independent induction of apoptosis (5,12). In renal cells, various biologic effects have been described for TGF-. The nature of the response depends on the cellular state of differentiation and the presence of other growth factors (13,14). Next to Smad-mediated signaling, non– Smad-mediated responses are initiated by TGF-/BMP bind- ing. This includes activation of the major survival pathway Received November 23, 2005. Accepted April 4, 2006. Published online ahead of print. Publication date available at www.jasn.org. I.P. and I.T. contributed equally to this work. Address correspondence to: Dr. Mario Schiffer, Division of Nephrology, Department of Medicine, Hannover Medical School, Carl-Neuberg-Strasse 1, Hannover, 30625 Germany. Phone: +49-5115-324708; Fax: +49-5115-52366; E-mail: schiffer.mario@mh- hannover.de Copyright © 2006 by the American Society of Nephrology ISSN: 1046-6673/1706-1644