Pten constrains centroacinar cell expansion and malignant transformation in the pancreas

To determine the role of the phosphatidylinositol 3-kinase (PI3-K) pathway in pancreas development, we generated a pancreas-specific knockout of Pten, a negative regulator of PI3-K signaling. Knockout mice display progressive replacement of the acinar pancreas with highly proliferative ductal structures that contain abundant mucins and express Pdx1 and Hes1, two markers of pancreatic progenitor cells. Moreover, a fraction of these mice develop ductal malignancy. We provide evidence that ductal metaplasia results from the expansion of centroacinar cells rather than transdifferentiation of acinar cells. These results indicate that Pten actively maintains the balance between different cell types in the adult pancreas and that misregulation of the PI3-K pathway in centroacinar cells may contribute to the initiation of pancreatic carcinoma in vivo.     

The PTEN and INK4A/ARF tumor suppressors maintain myelolymphoid homeostasis and cooperate to constrain histiocytic sarcoma development in humans

Histiocytic sarcoma (HS) is a rare malignant proliferation of histiocytes of uncertain molecular pathogenesis. Here, genetic analysis of coincident loss of Pten and Ink4a/Arf tumor suppressors in the mouse revealed a neoplastic phenotype dominated by a premalignant expansion of biphenotypic myelolymphoid cells followed by the development of HS. Pten protein loss occurred only in the histiocytic portion of tumors, suggesting a stepwise genetic inactivation in the generation of HS. Similarly, human HS showed genetic or epigenetic inactivation of PTEN, p16(INK4A), and p14(ARF), supporting the relevance of this genetically engineered mouse model of HS. These genetic and translational observations establish a cooperative role of Pten and Ink4a/Arf in the development of HS and provide mechanistic insights into the pathogenesis of human HS.     

Antitumor activity of a small-molecule inhibitor of human silent information regulator 2 enzymes

SIRT1 and other NAD-dependent deacetylases have been implicated in control of cellular responses to stress and in tumorigenesis through deacetylation of important regulatory proteins, including p53 and the BCL6 oncoprotein. Hereby, we describe the identification of a compound we named cambinol that inhibits NAD-dependent deacetylase activity of human SIRT1 and SIRT2. Consistent with the role of SIRT1 in promoting cell survival during stress, inhibition of SIRT1 activity with cambinol during genotoxic stress leads to hyperacetylation of key stress response proteins and promotes cell cycle arrest. Treatment of BCL6-expressing Burkitt lymphoma cells with cambinol as a single agent induced apoptosis, which was accompanied by hyperacetylation of BCL6 and p53. Because acetylation inactivates BCL6 and has the opposite effect on the function of p53 and other checkpoint pathways, the antitumor activity of cambinol in Burkitt lymphoma cells may be accomplished through a combined effect of BCL6 inactivation and checkpoint activation. Cambinol was well tolerated in mice and inhibited growth of Burkitt lymphoma xenografts. Inhibitors of NAD-dependent deacetylases may constitute novel anticancer agents.     

Three-dimensional visualization of microvessel architecture of whole-mount tissue by confocal microscopy

The three-dimensional architecture of the nascent microvascular network is a critical determinant of vascular perfusion in the setting of regenerative growth, vasculopathies and cancer. Current methods for microvessel visualization are limited by insufficient penetration and instability of endothelial immunolabels, inadequate vascular perfusion by the high-viscosity polymers used for vascular casting, and destruction of tissue stroma during the processing required for scanning electron microscopy. The aim of this study was to develop whole-mount tissue processing methods for 3D in situ visualization of the microvasculature that were also compatible with supplementary labeling for other structures of interest in the tissue microenvironment. Here, we present techniques that allow imaging of the microvasculature by confocal microscopy, to depths of up to 1500 mum below the specimen surface. Our approach includes labeling luminal surfaces of endothelial cells by i.v. injection of fluorescently conjugated lectin and filling the microvasculature with carbon or fluorescent nanoparticles/Mercox, followed by optical clearing of thick tissue sections to reduce light scatter and permit 3D visualization of microvessel morphology deep into the sample. Notably, tissue stroma is preserved, allowing simultaneous labeling of other structures by immunohistochemistry or nuclear dyes. Results are presented for various murine tissues including fat, muscle, heart and brain under conditions of normal health, as well as in the setting of a glioma model growing in the subcutaneous space or orthotopically in the brain parenchyma.     

Cancer drug discovery faces the FACT

In this issue of Science Translational Medicine, Gasparian et al. use a clever cell-based screen to identify a family of DNA-binding small molecules-curaxins-that inhibit tumor cell growth and division. The curaxins' mechanism of action pinpoints a new chromatin-remodeling factor as a therapeutic target for cancer.     

ARF functions as a melanoma tumor suppressor by inducing p53-independent senescence

Inactivation of the p53 pathway represents the most common molecular defect of human cancer. But in the setting of melanoma, a highly aggressive and invariably fatal malignancy in its advanced disseminated form, mutation/deletion of p53 is relatively rare, whereas its positive regulator ARF is often lost. Here, we show that genetic deficiency in Arf but not p53 facilitates rapid development of melanoma in a genetically engineered mouse model. This difference is accounted for, at least in part, by the unanticipated observation that, unlike fibroblasts, senescence control in melanocytes is strongly regulated by Arf and not p53. Moreover, oncogenic NRAS collaborates with deficiency in Arf, but not p53, to fully transform melanocytes. Our data demonstrate that ARF and p53, although linked in a common pathway, suppress tumorigenesis through distinct, lineage-dependent mechanisms and suggest that ARF helps restrict melanoma progression by executing the oncogene-induced senescence program in benign nevi. Thus, therapeutics designed to restore wild-type p53 function may be insufficient to counter melanoma and other malignancies in which ARF holds p53-independent tumor suppressor activity.     

High-resolution genomic profiles define distinct clinico-pathogenetic subgroups of multiple myeloma patients

To identify genetic events underlying the genesis and progression of multiple myeloma (MM), we conducted a high-resolution analysis of recurrent copy number alterations (CNAs) and expression profiles in a collection of MM cell lines and outcome-annotated clinical specimens. Attesting to the molecular heterogeneity of MM, unsupervised classification using nonnegative matrix factorization (NMF) designed for array comparative genomic hybridization (aCGH) analysis uncovered distinct genomic subtypes. Additionally, we defined 87 discrete minimal common regions (MCRs) within recurrent and highly focal CNAs. Further integration with expression data generated a refined list of MM gene candidates residing within these MCRs, thereby providing a genomic framework for dissection of disease pathogenesis, improved clinical management, and initiation of targeted drug discovery for specific MM patients.