Altered translation of GATA1 in Diamond-Blackfan anemia

Ribosomal protein haploinsufficiency occurs in diverse human diseases including Diamond-Blackfan anemia (DBA)[superscript 1, 2], congenital asplenia[superscript 3] and T cell leukemia[superscript 4]. Yet, how mutations in genes encoding ubiquitously expressed proteins such as these result in cell-type– and tissue-specific defects remains unknown[superscript 5]. Here, we identify mutations in GATA1, encoding the critical hematopoietic transcription factor GATA-binding protein-1, that reduce levels of full-length GATA1 protein and cause DBA in rare instances. We show that ribosomal protein haploinsufficiency, the more common cause of DBA, can lead to decreased GATA1 mRNA translation, possibly resulting from a higher threshold for initiation of translation of this mRNA in comparison with other mRNAs. In primary hematopoietic cells from patients with mutations in RPS19, encoding ribosomal protein S19, the amplitude of a transcriptional signature of GATA1 target genes was globally and specifically reduced, indicating that the activity, but not the mRNA level, of GATA1 is decreased in patients with DBA associated with mutations affecting ribosomal proteins. Moreover, the defective hematopoiesis observed in patients with DBA associated with ribosomal protein haploinsufficiency could be partially overcome by increasing GATA1 protein levels. Our results provide a paradigm by which selective defects in translation due to mutations affecting ubiquitous ribosomal proteins can result in human disease.National Institutes of Health (U.S.) (Grant P01 HL32262)National Institutes of Health (U.S.) (Grant U54 HG003067-09

Active Pin1 is a key target of all-trans retinoic acid in acute promyelocytic leukemia and breast cancer

A common key regulator of oncogenic signaling pathways in multiple tumor types is the unique isomerase Pin1. However, available Pin1 inhibitors lack the required specificity and potency. Using mechanism-based screening, here we find that all-trans retinoic acid (ATRA)--a therapy for acute promyelocytic leukemia (APL) that is considered the first example of targeted therapy in cancer, but its drug target remains elusive--inhibits and degrades active Pin1 selectively in cancer cells by directly binding to the substrate phosphate- and proline-binding pockets in the Pin1 active site. ATRA-induced Pin1 ablation degrades the fusion oncogene PML-RARα and treats APL in cell and animal models and human patients. ATRA-induced Pin1 ablation also inhibits triple negative breast cancer cell growth in human cells and in animal models by acting on many Pin1 substrate oncogenes and tumor suppressors. Thus, ATRA simultaneously blocks multiple Pin1-regulated cancer-driving pathways, an attractive property for treating aggressive and drug-resistant tumors

Systematic analysis of BRAFV600E melanomas reveals a role for JNK/c-Jun pathway in adaptive resistance to drug-induced apoptosis

Drugs that inhibit RAF/MEK signaling, such as vemurafenib, elicit profound but often temporary anti-tumor responses in patients with BRAFV600E melanoma. Adaptive responses to RAF/MEK inhibition occur on a timescale of hours to days, involve homeostatic responses that reactivate MAP kinase signaling and compensatory mitogenic pathways, and attenuate the anti-tumor effects of RAF/MEK inhibitors. We profile adaptive responses across a panel of melanoma cell lines using multiplex biochemical measurement, single-cell assays, and statistical modeling and show that adaptation involves at least six signaling cascades that act to reduce drug potency (IC50) and maximal effect (i.e., Emax ≪ 1). Among these cascades, we identify a role for JNK/c-Jun signaling in vemurafenib adaptation and show that RAF and JNK inhibitors synergize in cell killing. This arises because JNK inhibition prevents a subset of cells in a cycling population from becoming quiescent upon vemurafenib treatment, thereby reducing drug Emax. Our findings demonstrate the breadth and diversity of adaptive responses to RAF/MEK inhibition and a means to identify which steps in a signaling cascade are most predictive of phenotypic response

A High-Throughput, Cell-Based Screening Method for siRNA and Small Molecule Inhibitors of mTORC1 Signaling Using the In Cell Western Technique

The mTORC1 pathway is a central regulator of cell growth, and defective mTORC1 regulation plays a causative role in a variety of human diseases, including cancer, tumor syndromes such as the tuberous sclerosis complex (TSC) and lymphangioleiomyomatosis (LAM), and metabolic diseases such as diabetes and obesity. Given the importance of mTORC1 signaling in these diseases, there has been significant interest in developing screening methods suitable for identifying inhibitors of mTORC1 activation. To this end, we have developed a high-throughput, cell-based assay for the detection of rpS6-phosphorylation as a measure of mTORC1 signaling. This assay takes advantage of the “In Cell Western” (ICW) technique using the Aerius infrared imaging system (LI-COR® Biosciences). The ICW procedure involves fixation and immunostaining of cells in a manner similar to standard immunofluorescence methods but takes advantage of secondary antibodies conjugated to infrared-excitable fluorophores for quantitative detection by the Aerius® scanner. In addition, the cells are stained with an infrared-excitable succinimidyl ester dye, which covalently modifies free amine groups in fixed cells and provides a quantitative measure of cell number. We present validation data and pilot screens in a 384-well format demonstrating that this assay provides a statistically robust method for both small molecule and siRNA screening approaches designed to identify inhibitors of mTORC1 signaling

Ribosome loading onto the mRNA cap is driven by conformational coupling between eIF4G and eIF4E

The eukaryotic initiation factor 4G (eIF4G) is the core of a multicomponent switch controlling gene expression at the level of translation initiation. It interacts with the small ribosomal subunit interacting protein, eIF3, and the eIF4E/cap-mRNA complex in order to load the ribosome onto mRNA during cap-dependent translation. We describe the solution structure of the complex between yeast eIF4E/cap and eIF4G (393-490). Binding triggers a coupled folding transition of eIF4G (393-490) and the eIF4E N terminus resulting in a molecular bracelet whereby eIF4G (393-490) forms a right-handed helical ring that wraps around the N terminus of eIF4E. Cofolding allosterically enhances association of eIF4E with the cap and is required for maintenance of optimal growth and polysome distributions in vivo. Our data explain how mRNA, eIF4E, and eIF4G exists as a stable mRNP that may facilitate multiple rounds of ribosomal loading during translation initiation, a key determinant in the overall rate of protein synthesis

Small-molecule inhibition of the interaction between the translation initiation factors eIF4E and eIF4G.

International audienceAssembly of the eIF4E/eIF4G complex has a central role in the regulation of gene expression at the level of translation initiation. This complex is regulated by the 4E-BPs, which compete with eIF4G for binding to eIF4E and which have tumor-suppressor activity. To pharmacologically mimic 4E-BP function we developed a high-throughput screening assay for identifying small-molecule inhibitors of the eIF4E/eIF4G interaction. The most potent compound identified, 4EGI-1, binds eIF4E, disrupts eIF4E/eIF4G association, and inhibits cap-dependent translation but not initiation factor-independent translation. While 4EGI-1 displaces eIF4G from eIF4E, it effectively enhances 4E-BP1 association both in vitro and in cells. 4EGI-1 inhibits cellular expression of oncogenic proteins encoded by weak mRNAs, exhibits activity against multiple cancer cell lines, and appears to have a preferential effect on transformed versus nontransformed cells. The identification of this compound provides a new tool for studying translational control and establishes a possible new strategy for cancer therapy