Thomas G. Hofmann

Regulation of p53 activity by its interaction with homeodomain-interacting protein kinase-2

Transcriptional activity of p53, a central regulatory switch in a network controlling cell proliferation and apoptosis, is modulated by protein stability and post-translational modifications including phosphorylation and acetylation. Here we demonstrate that the human serine/threonine kinase homeodomain-interacting protein kinase-2 (HIPK2) colocalizes and interacts with p53 and CREB-binding protein (CBP) within promyelocytic leukaemia (PML) nuclear bodies. HIPK2 is activated by ultraviolet (UV) radiation and selectively phosphorylates p53 at Ser 46, thus facilitating the CBP-mediated acetylation of p53 at Lys 382, and promoting p53-dependent gene expression. Accordingly, the kinase function of HIPK2 mediates the increased expression of p53 target genes, which results in growth arrest and the enhancement of UV-induced apoptosis. Interference with HIPK2 expression by antisense oligonucleotides impairs UV-induced apoptosis. Our results imply that HIPK2 is a novel regulator of p53 effector functions involved in cell growth, proliferation and apoptosis.

The miRNA-212/132 family regulates both cardiac hypertrophy and cardiomyocyte autophagy

Pathological growth of cardiomyocytes (hypertrophy) is a major determinant for the development of heart failure, one of the leading medical causes of mortality worldwide. Here we show that the microRNA (miRNA)-212/132 family regulates cardiac hypertrophy and autophagy in cardiomyocytes. Hypertrophic stimuli upregulate cardiomyocyte expression of miR-212 and miR-132, which are both necessary and sufficient to drive the hypertrophic growth of cardiomyocytes. MiR-212/132 null mice are protected from pressure-overload-induced heart failure, whereas cardiomyocyte-specific overexpression of the miR-212/132 family leads to pathological cardiac hypertrophy, heart failure and death in mice. Both miR-212 and miR-132 directly target the anti-hypertrophic and pro-autophagic FoxO3 transcription factor and overexpression of these miRNAs leads to hyperactivation of pro-hypertrophic calcineurin/NFAT signalling and an impaired autophagic response upon starvation. Pharmacological inhibition of miR-132 by antagomir injection rescues cardiac hypertrophy and heart failure in mice, offering a possible therapeutic approach for cardiac failure.

Co-stimulatory effect of nitric oxide on endothelial NF-κB implies a physiological self-amplifying mechanism

Here we investigated the effects of the second messenger molecule NO at various concentrations on the activation of transcription factor NF‐κB, IκB‐α kinase (IKK‐α), Jun N‐terminal kinase (JNK) and apoptosis in murine endothelial cells. Low concentrations of NO alone failed to activate NF‐κB, IKK‐α and JNK. When NF‐κB was prestimulated by TNF‐α or phorbol 12‐myristate 13‐acetate, the addition of NO at low concentrations enhanced the activation of NF‐κB. This provides a mechanism for a self‐amplifying signal in the inflammatory response, since the inducible NO synthase in endothelial cells is regulated by NF‐κB. The co‐stimulatory effect of NO on NF‐κB activation was also evident from IKK‐α kinase assays and reporter gene experiments in endothelial cells. High doses of NO impaired the TNF‐α‐induced DNA‐binding activity of NF‐κB. Accordingly, these high amounts of NO also repressed the TNF‐α‐induced transactivation by NF‐κB as efficient as dexamethasone. The doses of NO required for the inhibition of NF‐κB are not cytotoxic for the endothelial cells, enabling the establishment of an autoregulatory loop for NF‐κB signaling.

Various glucocorticoids differ in their ability to induce gene expression, apoptosis and to repress NF-κB-dependent transcription

Glucocorticoids (GCs) influence a great variety of cellular functions by at least three important modes of action: the activation (or repression) of genes controlled by binding sites for the glucocorticoid receptor (GR), the induction of apoptosis in lymphocytes and the recently discovered cross‐talk to other transcription factors such as NF‐κB. In this study we systematically compared various natural and synthetic steroid hormones frequently used as therapeutic agents on their ability to mediate these three modes of action. Betamethasone, triamcinolone, dexamethasone and clobetasol turned out to be the best inducers of gene expression and apoptosis. All GCs including the antagonistic compound RU486 efficiently reduced NF‐κB‐mediated transactivation to comparable extents, suggesting that ligand‐induced nuclear localization of the GR is sufficient for transrepression. Glucocorticoid treatment of cells did not result in elevated IκB‐α expression, but impaired the tumor necrosis factor (TNF)‐α‐induced degradation of IκB‐α without affecting DNA binding of NF‐κB. The structural requirements for the various functions of glucocorticoids are discussed.

Adult Thymus Contains FoxN1− Epithelial Stem Cells that Are Bipotent for Medullary and Cortical Thymic Epithelial Lineages

Summary Within the thymus, two major thymic epithelial cell (TEC) subsets—cortical and medullary TECs—provide unique structural and functional niches for T cell development and establishment of central tolerance. Both lineages are believed to originate from a common progenitor cell, yet the cellular and molecular identity of these bipotent TEC progenitors/stem cells remains ill defined. Here we identify rare stromal cells in the murine adult thymus, which under low-attachment conditions formed spheres (termed “thymospheres”). These thymosphere-forming cells (TSFCs) displayed the stemness features of being slow cycling, self-renewing, and bipotent. TSFCs could be significantly enriched based on their distinct surface antigen phenotype. The FoxN1 transcription factor was dispensable for TSFCs maintenance in situ and for commitment to the medullary and cortical TEC lineages. In summary, this study presents the characterization of the adult thymic epithelial stem cells and demonstrates the dispensability of FoxN1 function for their stemness.

Homeodomain-Interacting Protein Kinase 2, a Novel Autoimmune Regulator Interaction Partner, Modulates Promiscuous Gene Expression in Medullary Thymic Epithelial Cells

Promiscuous expression of a plethora of tissue-restricted Ags (TRAs) by medullary thymic epithelial cells (mTECs) plays an essential role in T cell tolerance. Although the cellular mechanisms by which promiscuous gene expression (pGE) imposes T cell tolerance have been well characterized, the underlying molecular mechanisms remain poorly understood. The autoimmune regulator (AIRE) is to date the only validated molecule known to regulate pGE. AIRE is part of higher-order multiprotein complexes, which promote transcription, elongation, and splicing of a wide range of target genes. How AIRE and its partners mediate these various effects at the molecular level is still largely unclear. Using a yeast two-hybrid screen, we searched for novel AIRE-interacting proteins and identified the homeodomain-interacting protein kinase 2 (HIPK2) as a novel partner. HIPK2 partially colocalized with AIRE in nuclear bodies upon cotransfection and in human mTECs in situ. Moreover, HIPK2 phosphorylated AIRE in vitro and suppressed the coactivator activity of AIRE in a kinase-dependent manner. To evaluate the role of Hipk2 in modulating the function of AIRE in vivo, we compared whole-genome gene signatures of purified mTEC subsets from TEC-specific Hipk2 knockout mice with control mice and identified a small set of differentially expressed genes. Unexpectedly, most differentially expressed genes were confined to the CD80lo mTEC subset and preferentially included AIRE-independent TRAs. Thus, although it modulates gene expression in mTECs and in addition affects the size of the medullary compartment, TEC-specific HIPK2 deletion only mildly affects AIRE-directed pGE in vivo.

ShaPINg Cell Fate Upon DNA Damage: Role of Pin1 Isomerase in DNA Damage-Induced Cell Death and Repair

The peptidyl–prolyl cis/trans isomerase Pin1 acts as a molecular timer in proline-directed Ser/Thr kinase signaling and shapes cellular responses based on recognition of phosphorylation marks and implementing conformational changes in its substrates. Accordingly, Pin1 has been linked to numerous phosphorylation-controlled signaling pathways and cellular processes such as cell cycle progression, proliferation, and differentiation. In addition, Pin1 plays a pivotal role in DNA damage-triggered cell fate decisions. Whereas moderate DNA damage is balanced by DNA repair, cells confronted with massive genotoxic stress are eliminated by the induction of programed cell death or cellular senescence. In this review, we summarize and discuss the current knowledge on how Pin1 specifies cell fate through regulating key players of the apoptotic and the repair branch of the DNA-damage response.

A Thymic Epithelial Stem Cell Pool Persists throughout Ontogeny and Is Modulated by TGF-β

Summary Adult tissue-specific stem cells (SCs) mediate tissue homeostasis and regeneration and can give rise to all lineages in the corresponding tissue, similar to the early progenitors that generate organs in the first place. However, the developmental origins of adult SCs are largely unknown. We recently identified thymosphere-forming stem cells (TSFCs) in the adult mouse thymus, which display genuine stemness features and can generate the two major thymic epithelial cell lineages. Here, we show that embryonic TSFCs possess stemness features but differ from adult TSFCs in surface marker profile. Our findings support the model of a continuous thymic SC lineage that is maintained throughout ontogeny. TGF-β signaling differentially affects embryonic versus adult thymosphere formation, suggesting that thymic epithelial SC potency depends on both developmental stage and environmental signals. Collectively, our findings suggest that embryonic TSFCs contribute to an adult SC pool and that TSFC plasticity is controlled by TGF-β signaling.

WWOX guards genome stability by activating ATM

Common fragile sites (CFSs) tend to break upon replication stress and have been suggested to be “hot spots” for genomic instability. Recent evidence, however, implies that in the wake of DNA damage, WW domain-containing oxidoreductase (WWOX, the gene product of the FRA16D fragile site), associates with ataxia telangiectasia-mutated (ATM) and regulates its activation to maintain genomic integrity.

Report on the 6th International Conference of HPV, Polyomavirus, and UV Radiation in Skin Cancer.

The 6th International Conference of HPV, Polyomavirus, and UV Radiation in Skin Cancer was held from 18 to 20 October 2012 in Berlin, Germany. In total, 150 scientists and physicians from Europe, the United States, China, Australia, and New Zealand (including 32 invited experts and speakers) participated in this congress, which was run under the auspices of the European Skin Cancer Foundation. The necessity for this specific meeting is reflected by the strong increase in the knowledge and scientific interest in the role of oncogenic viruses and nonmelanoma skin cancer. A particular focus of this skin cancer conference was laid on UV radiation (DNA damage), cutaneous human papillomavirus (HPV) types, and polyomaviruses. Novel trends in basic research, translational research, epidemiology, and therapy were reported by international experts and 15 selected abstracts. In addition, guided poster walks were conducted to stimulate scientific discussions and to exchange ideas.