Research Report 2010 - MDC

investigation, as is the cause of constitutive IKK/NF-κBand c-Jun activation in Hodgkin lymphoma.Functional control of IKK by transient interactionwith Hsp90 and Cdc37The IKK complex undergoes interactions with a numberof regulatory proteins, including the chaperones Cdc37and Hsp90. We had previously shown that pharmacologicalinhibition of Hsp90 abrogates constitutive IKKactivation in Hodgkin lymphoma cells, resulting instrongly enhanced apoptosis. Using an RNAi approach,we found that Cdc37 recruits Hsp90 to the IKK complexin a transitory manner, preferentially via IKKα. Bindingis conferred by N-terminal as well as C-terminalresidues of Cdc37 and results in the phosphorylation ofCdc37. Cdc37 is essential for the maturation of de novosynthesized IKKs into enzymatically competent kinases,but not for assembly of an IKK holocomplex. MatureIKKs, T-loop phosphorylated after stimulation either byreceptor-mediated signaling or upon DNA damage, furtherrequire Hsp90-Cdc37 to generate an enzymaticallyactivated state. Thus, the Hsp90-Cdc37 chaperone-cochaperonecomplex is an essential regulatory componentin IKK signaling cascades and a potential drug targetin tumor therapy.Mutual requirements of Eda-A1/Edar/NF-κB andWnt/β-catenin signaling in early ectodermalorganogenesisTo allow a functional analysis of NF-κB in early embryonicdevelopment and in disease models, we have generatedNF-κB repressor and reporter mice. The repressormice carry a dominant negative IκBα mutant (IκBα∆N)as a conditional knock-in allele, while the reporter miceexpress an NF-κB-driven β-gal transgene. Using thesemice, we could previously demonstrate novel morphogenicfunctions for NF-κB, including an early role inthe development of epidermal organs, such as hair follicles(HF). In early HF placodes, NF-κB is specifically activatedby TNF family members Eda-A1 and its receptorEdar. Furthermore we have identified an intense molecularcross-talk of Eda-A1/Edar/NF-κB with other signalingpathways required for HF development, such as Wntand Sonic Hedgehog (SHH).According to our current knowledge, Wnt/β-cateninsignaling specifies the initial hair fate decision of epidermalkeratinocytes, while NF-κB is required for hairplacode growth by activating SHH signaling andrepressing Bone Morphogenic Protein (BMP) activity. Incollaboration with the laboratory of Dr. Sarah E. Millar(U. Pennsylvania, Philadeplhia, USA) we have nowdemonstrated that Wnt/β-catenin is initially activatedindependently of Eda-A1/Edar/NF-κB, but depends onNF-κB activity for focal hair placode patterning. In contrast,initial Eda-A1/Edar/NF-κB signaling is not activatedin skin of mice expressing the secreted Wnt inhibitorDKK1, or lacking epithelial β-catenin (Figure 2). In thiscontext we have shown that Wnt/β-catenin isabsolutely essential for NF-κB activation and have providedevidence that Edar is a direct Wnt/β-catenin targetgene. However, at later time points of hair follicledevelopment, localized Wnt activity disappears fromEda-A1/Edar/NF-κB mutant skin, implying that Eda-A1/Edar/NF-κB is required for the maintenance of Wntsignaling (Figure 2).We found that NF-κB is essential forplacodal up-regulation of Wnt10a and Lef-1, and wecould identify Wnt10b as a direct target gene of NF-κB.Our data reveal a complex interplay and interdependenceof Wnt/β-catenin and Eda-A1/Edar/NF-κB signalingpathways, which may not only have importantimplications in organ development and morphogenesis,but also in cancer and inflammatory diseases.Our current and future studies will continue to focus onthe Wnt/NF-κB cross-talk in development/morpho -genesis and disease. Furthermore, in collaboration withDr. Nathaniel Heintz (Rockefeller I., New York, NY, USA)and Ines Ibañez-Tallon (MDC) we have developed amouse model (bacTRAP: translating ribosome affinitypurification (TRAP)) that allows the specific isolation ofHF keratinocytes. This mouse model will be used toidentify novel NF-κB target genes in early HF developmentand to perform detailed gene profiling studies onthe molecular controls of HF induction.Selected publicationsSchmidt-Ullrich, R, Tobin, DJ, Lenhard, D, Schneider, P, Paus, R, Scheidereit,C. (2006). NF-κB transmits Eda A1/EdaR signaling to activate Shh andcyclin D1 expression, and controls post-initiation hair placode downgrowth.Development 133, 1045-1057.Oeckinghaus, A, Wegener, E, Welteke, V, Ferch, U, Çöl Arslan, S, Ruland, J,Scheidereit, C, Krappmann, D. (2007). TRAF6 mediated Malt1 ubiquitinationtriggers IKK/NF-κB signaling upon T cell activation. EMBO J. 26,4634-4645.Hinz, M, Broemer, M, Çöl Arslan, S, Dettmer, R, Scheidereit, C. (2007).Signal-responsiveness of IκB kinases is determined by Cdc37-assistedtransient interaction with Hsp90. J. Biol. Chem. 282, 32311-32319.Zhang, Y, Tomann, P, Andl, T, Gallant, N, Huelsken, J, Jerchow, B, Birchmeier,W, Paus, R, Piccolo, S, Mikkola, ML, Morrisey, EE, Overbeek, PA, Scheidereit,C, Millar, SE, Schmidt-Ullrich, R. (2009). Reciprocal requirements forEda/Edar/NF-κB and Wnt/β-catenin signaling pathways in hair follicleinduction. Dev. Cell, 17, 49-61.Stilmann, M, Hinz, M, Cöl Arslan, S, Zimmer, A, Schreiber, V, Scheidereit, C,(2009). A nuclear poly(ADP-ribose) dependent signalosome confers DNAdamage induced IκB kinase activation. Mol Cell, in press.Cancer Research 81