Beate K. Straub

Differential pattern of lipid droplet‐associated proteins and de novo perilipin expression in hepatocyte steatogenesis

Fatty change (steatosis) is the most frequent liver pathology in western countries and is caused by a broad range of disorders such as alcohol abuse and metabolic syndrome. The surface layer of lipid droplets (LDs) contains members of a protein family that share homologous sequences and domains, the so‐called PAT proteins, named after their constituents, perilipin, adipophilin, and TIP47. We characterized the LD‐associated proteins in normal and diseased liver connected with LD accumulation. Adipophilin and TIP47 are expressed in LDs of vitamin A‐storing hepatic stellate cells and additionally in LDs of steatotic hepatocytes. Perilipin, which was thought to be characteristic for LDs of adipocytes and steroidogenic cells, becomes de novo expressed in hepatocytes of human steatotic liver. Perilipin splice variant A was found in human steatotic hepatocytes by biochemical, molecular biological, and immunohistochemical methods. Its association with LDs is different from TIP47 and adipophilin, and depends on size and localization of the LDs, suggesting that the different PAT proteins play specific roles during maturation of LDs. (HEPATOLOGY 2008.)

Integrin α3 mutations with kidney, lung, and skin disease.

Integrin α(3) is a transmembrane integrin receptor subunit that mediates signals between the cells and their microenvironment. We identified three patients with homozygous mutations in the integrin α(3) gene that were associated with disrupted basement-membrane structures and compromised barrier functions in kidney, lung, and skin. The patients had a multiorgan disorder that included congenital nephrotic syndrome, interstitial lung disease, and epidermolysis bullosa. The renal and respiratory features predominated, and the lung involvement accounted for the lethal course of the disease. Although skin fragility was mild, it provided clues to the diagnosis.

The histone acetyltransferase hMOF is frequently downregulated in primary breast carcinoma and medulloblastoma and constitutes a biomarker for clinical outcome in medulloblastoma

Loss of H4 lysine 16 (H4K16) acetylation was shown to be a common feature in human cancer. However, it remained unclear which enzyme is responsible for the loss of this modification. Having recently identified the histone acetyltransferase human MOF (hMOF) to be required for bulk H4K16 acetylation, here we examined the involvement of hMOF expression and H4K16 acetylation in breast cancer and medulloblastoma. Analysis of a recent mRNA expression profiling study in breast cancer (n = 100 cases) and an array‐CGH screen in medulloblastomas (n = 102 cases), revealed downregulation in 40% and genomic loss in 11% of cases, respectively. We investigated hMOF protein expression as well as H4K16 acetylation in large series of primary breast carcinomas (n = 298) and primary medulloblastomas (n = 180) by immunohistochemistry. In contrast to nontransformed control tissues, significant fractions of both primary breast carcinomas and medulloblastomas showed markedly reduced hMOF mRNA and protein expression. In addition, hMOF protein expression tightly correlated with acetylation of H4K16 in all tested samples. For medulloblastoma, downregulation of hMOF protein expression was associated with lower survival rates identifying hMOF as an independent prognostic marker for clinical outcome in univariate as well as multivariate analyses.

Cytoprotective signaling by activated protein C requires protease activated receptor-3 in podocytes

The cytoprotective effects of activated protein C (aPC) are well established. In contrast, the receptors and signaling mechanism through which aPC conveys cytoprotection in various cell types remain incompletely defined. Thus, within the renal glomeruli, aPC preserves endothelial cells via a protease-activated receptor-1 (PAR-1) and endothelial protein C receptor-dependent mechanism. Conversely, the signaling mechanism through which aPC protects podocytes remains unknown. While exploring the latter, we identified a novel aPC/PAR-dependent cytoprotective signaling mechanism. In podocytes, aPC inhibits apoptosis through proteolytic activation of PAR-3 independent of endothelial protein C receptor. PAR-3 is not signaling competent itself as it requires aPC-induced heterodimerization with PAR-2 (human podocytes) or PAR-1 (mouse podocytes). This cytoprotective signaling mechanism depends on caveolin-1 dephosphorylation. In vivo aPC protects against lipopolysaccharide-induced podocyte injury and proteinuria. Genetic deletion of PAR-3 impairs the nephroprotective effect of aPC, demonstrating the crucial role of PAR-3 for aPC-dependent podocyte protection. This novel, aPC-mediated interaction of PARs demonstrates the plasticity and cell-specificity of cytoprotective aPC signaling. The evidence of specific, dynamic signaling complexes underlying aPC-mediated cytoprotection may allow the design of cell type specific targeted therapies.

Adipophilin/perilipin‐2 as a lipid droplet‐specific marker for metabolically active cells and diseases associated with metabolic dysregulation

Lipid droplets (LDs) are dynamic storage compartments for energy‐rich fats that are nearly ubiquitously present in eukaryotic cells, exerting tissue‐specific functions in metabolically active cell types, and are increased in conditions following cellular damage or lipid overload. The LD–cytoplasm interface is stabilized by amphiphilic proteins of the PAT/perilipin family (perilipin/perilipin‐1, adipophilin/perilipin‐2, and TIP47/perilipin‐3). We evaluated the value of adipophilin immunohistochemistry for the diagnosis of diseases associated with LD accumulation.

ErbB-2 signals through Plexin-B1 to promote breast cancer metastasis

Diagnosis of metastatic breast cancer is associated with a very poor prognosis. New therapeutic targets are urgently needed, but their development is hampered by a lack of understanding of the mechanisms leading to tumor metastasis. Exemplifying this is the fact that the approximately 30% of all breast cancers overexpressing the receptor tyrosine kinase ErbB-2 are characterized by high metastatic potential and poor prognosis, but the signaling events downstream of ErbB-2 that drive cancer cell invasion and metastasis remain incompletely understood. Here we show that overexpression of ErbB-2 in human breast cancer cell lines leads to phosphorylation and activation of the semaphorin receptor Plexin-B1. This was required for ErbB-2-dependent activation of the pro-metastatic small GTPases RhoA and RhoC and promoted invasive behavior of human breast cancer cells. In a mouse model of ErbB-2-overexpressing breast cancer, ablation of the gene encoding Plexin-B1 strongly reduced the occurrence of metastases. Moreover, in human patients with ErbB-2-overexpressing breast cancer, low levels of Plexin-B1 expression correlated with good prognosis. Our data suggest that Plexin-B1 represents a new candidate therapeutic target for treating patients with ErbB-2-positive breast cancer.

NEMO Prevents Steatohepatitis and Hepatocellular Carcinoma by Inhibiting RIPK1 Kinase Activity-Mediated Hepatocyte Apoptosis

Summary IκB kinase/nuclear factor κB (IKK/NF-κB) signaling exhibits important yet opposing functions in hepatocarcinogenesis. Mice lacking NEMO in liver parenchymal cells (LPC) spontaneously develop steatohepatitis and hepatocellular carcinoma (HCC) suggesting that NF-κB prevents liver disease and cancer. Here, we show that complete NF-κB inhibition by combined LPC-specific ablation of RelA, c-Rel, and RelB did not phenocopy NEMO deficiency, but constitutively active IKK2-mediated NF-κB activation prevented hepatocellular damage and HCC in NEMOLPC-KO mice. Knock-in expression of kinase inactive receptor-interacting protein kinase 1 (RIPK1) prevented hepatocyte apoptosis and HCC, while RIPK1 ablation induced TNFR1-associated death domain protein (TRADD)-dependent hepatocyte apoptosis and liver tumors in NEMOLPC-KO mice, revealing distinct kinase-dependent and scaffolding functions of RIPK1. Collectively, these results show that NEMO prevents hepatocarcinogenesis by inhibiting RIPK1 kinase activity-driven hepatocyte apoptosis through NF-κB-dependent and -independent functions.

Pathology and biopsy assessment of non-alcoholic fatty liver disease.

Non-alcoholic fatty liver disease (NAFLD) is one of the most prevalent liver diseases in Western industrialized countries with dramatically rising incidence. The diagnosis of NAFLD requires the existence of steatosis in the absence of significant alcohol consumption. In cases of relevant inflammation pathogenetically linked to steatosis, it is termed non-alcoholic steatohepatitis (NASH). While pure steatosis represents a relatively harmless and rapidly reversible condition without a significant tendency to progression, NASH carries a significant morbidity and progression risk. Noninvasive methods neither reliably establish the diagnosis nor define the extent of disease in NASH, making histopathology the diagnostic gold standard. Since current therapeutic options in NASH are limited, indication for biopsy is made in the clinical context, predominantly in unclear clinical constellations, prior to invasive measures, for follow-up purposes and in the context of clinical studies. Histological hallmarks of NASH are steatosis, hepatocellular ballooning (with and without Mallory-Denk bodies), necroinflammation, and progressing disease a characteristic with perisinusoidal fibrosis. For semiquantitative assessment of necroinflammation (grading) and fibrosis (staging), a score has recently been implemented. Although histology does not reliably distinguish alcoholic steatohepatitis/alcoholic fatty liver disease from NASH/NAFLD, it may give valuable hints. NASH has a tendency for more steatosis, the so-called glycogenated nuclei, and less necroinflammatory activity. Future development of biopsy diagnosis will be coupled to the development of differential systemic therapeutic approaches. Especially in the context of clinical studies, detailed histological evaluation should be considered for the detection of predictive parameters.

An RNAi screen identifies USP2 as a factor required for TNF‐α‐induced NF‐κB signaling

Tumor necrosis factor α (TNF‐α) signaling pathways play important roles during tumorigenesis and inflammation. Ubiquitin‐dependent processes are central to the regulation of TNF‐α and nuclear factor κB (NF‐κB) signaling. We performed a targeted siRNA screen for ubiquitin‐specific proteases (USPs) and identified USP2 as a modulator of TNF‐α‐induced NF‐κB signaling. We showed that USP2 is required for the phosphorylation of IκB, nuclear translocation of NF‐κB and expression of NF‐κB‐dependent target genes and IL‐8 secretion. Our study also provides evidence for isoform‐specific functions of USP2. The immunohistochemical analysis of breast carcinomas revealed that USP2 expression is frequently downregulated. Together, our results implicate USP2 as a novel positive regulator of TNF‐α‐induced NF‐κB signaling and show that its expression is altered in tumor cells.

E–N-cadherin heterodimers define novel adherens junctions connecting endoderm-derived cells

Contradicting the “cadherin switch” model, mixed E-cadherin–N-cadherin heterodimeric adherens junctions are prevalent in a variety of endodermal cells and endoderm-derived tumors.