FRAX597, a small molecule inhibitor of the p21-activated kinases, inhibits tumorigenesis of neurofibromatosis type 2 (NF2)-associated Schwannomas.

The p21-activated kinases (PAKs) are immediate downstream effectors of the Rac/Cdc42 small G-proteins and implicated in promoting tumorigenesis in various types of cancer including breast and lung carcinomas. Recent studies have established a requirement for the PAKs in the pathogenesis of Neurofibromatosis type 2 (NF2), a dominantly inherited cancer disorder caused by mutations at the NF2 gene locus. Merlin, the protein product of the NF2 gene, has been shown to negatively regulate signaling through the PAKs and the tumor suppressive functions of Merlin are mediated, at least in part, through inhibition of the PAKs. Knockdown of PAK1 and PAK2 expression, through RNAi-based approaches, impairs the proliferation of NF2-null schwannoma cells in culture and inhibits their ability to form tumors in vivo. These data implicate the PAKs as potential therapeutic targets. High-throughput screening of a library of small molecules combined with a structure-activity relationship approach resulted in the identification of FRAX597, a small-molecule pyridopyrimidinone, as a potent inhibitor of the group I PAKs. Crystallographic characterization of the FRAX597/PAK1 complex identifies a phenyl ring that traverses the gatekeeper residue and positions the thiazole in the back cavity of the ATP binding site, a site rarely targeted by kinase inhibitors. FRAX597 inhibits the proliferation of NF2-deficient schwannoma cells in culture and displayed potent anti-tumor activity in vivo, impairing schwannoma development in an orthotopic model of NF2. These studies identify a novel class of orally available ATP-competitive Group I PAK inhibitors with significant potential for the treatment of NF2 and other cancers.

Pirin inhibits cellular senescence in melanocytic cells.

Cellular senescence has been widely recognized as a tumor suppressing mechanism that acts as a barrier to cancer development after oncogenic stimuli. A prominent in vivo model of the senescence barrier is represented by nevi, which are composed of melanocytes that, after an initial phase of proliferation induced by activated oncogenes (most commonly BRAF), are blocked in a state of cellular senescence. Transformation to melanoma occurs when genes involved in controlling senescence are mutated or silenced and cells reacquire the capacity to proliferate. Pirin (PIR) is a highly conserved nuclear protein that likely functions as a transcriptional regulator whose expression levels are altered in different types of tumors. We analyzed the expression pattern of PIR in adult human tissues and found that it is expressed in melanocytes and has a complex pattern of regulation in nevi and melanoma: it is rarely detected in mature nevi, but is expressed at high levels in a subset of melanomas. Loss of function and overexpression experiments in normal and transformed melanocytic cells revealed that PIR is involved in the negative control of cellular senescence and that its expression is necessary to overcome the senescence barrier. Our results suggest that PIR may have a relevant role in melanoma progression.

Pirin delocalization in melanoma progression identified by high content immuno-detection based approaches.

BACKGROUND: Pirin (PIR) is a highly conserved nuclear protein originally isolated as an interactor of NFI/CTF1 transcription/replication factor. It is a member of the functionally diverse cupin superfamily and its activity has been linked to different biological and molecular processes, such as regulation of transcription, apoptosis, stress response and enzymatic processes. Although its precise role in these functions has not yet been defined, PIR expression is known to be deregulated in several human malignancies. RESULTS: We performed immunohistochemical analysis of PIR expression in primary samples from normal human tissues and tumors and identified a dislocation of PIR to the cytoplasm in a subset of melanomas, and a positive correlation between cytoplasmic PIR levels and melanoma progression. PIR localization was subsequently analyzed in vitro in melanoma cell lines through a high content immunofluorescence based approach (ImmunoCell-Array). CONCLUSIONS: The high consistency between in vivo and in vitro results obtained by immunohistochemistry and ImmunoCell-Array provides a validation of the potential of ImmunoCell-Array technology for the rapid screening of putative biological markers, and suggests that cytoplasmic localization of PIR may represent a characteristic of melanoma progression.

The Tumor Suppressor ARID1A Controls Global Transcription via Pausing of RNA Polymerase II.

AT-rich interactive domain-containing proteins 1A and 1B (ARID1A and ARID1B) are mutually exclusive subunits of the chromatin remodeler SWI/SNF. ARID1A is the most frequently mutated chromatin regulator across all cancers, and ovarian clear cell carcinoma (OCCC) carries the highest prevalence of ARID1A mutations (∼57%). Despite evidence implicating ARID1A in tumorigenesis, the mechanism remains elusive. Here, we demonstrate that ARID1A binds active regulatory elements in OCCC. Depletion of ARID1A represses RNA polymerase II (RNAPII) transcription but results in modest changes to accessibility. Specifically, pausing of RNAPII is severely impaired after loss of ARID1A. Compromised pausing results in transcriptional dysregulation of active genes, which is compensated by upregulation of ARID1B. However, a subset of ARID1A-dependent genes is not rescued by ARID1B, including many p53 and estrogen receptor (ESR1) targets. Our results provide insight into ARID1A-mediated tumorigenesis and unveil functions of SWI/SNF in modulating RNAPII dynamics.

Molecular signature of retinoic acid treatment in acute promyelocytic leukemia.

Acute promyelocytic leukemia (APL) is a distinct subtype of acute myeloid leukemia characterized by a block of differentiation at the promyelocytic stage. APL patients respond to pharmacological concentrations of all-trans retinoic acid (RA) and disease remission correlates with terminal differentiation of leukemic blasts. The PML/RAR oncogenic transcription factor is responsible for both the pathogenesis of APL and for its sensitivity to RA. In order to identify physiological targets of RA therapy, we analysed gene expression profiles of RA-treated APL blasts and found 1056 common target genes. Comparing these results to those obtained in RA-treated U937 cell lines revealed that transcriptional response to RA is largely dependent on the expression of PML/RAR. Several genes involved in the control of differentiation and stem cell renewal are early targets of RA regulation, and may be important effectors of RA response. Modulation of chromatin modifying genes was also observed, suggesting that specific structural changes in local chromatin domains may be required to promote RA-mediated differentiation. Computational analysis of upstream genomic regions in RA target genes revealed nonrandom distribution of transcription factor binding sites, indicating that specific transcriptional regulatory complexes may be involved in determining RA response.

Notch1 is required for Kras-induced lung adenocarcinoma and controls tumor cell survival via p53.

The Notch pathway has been implicated in a number of malignancies with different roles that are cell- and tissue-type dependent. Notch1 is a putative oncogene in non-small cell lung cancer (NSCLC) and activation of the pathway represents a negative prognostic factor. To establish the role of Notch1 in lung adenocarcinoma, we directly assessed its requirement in Kras-induced tumorigenesis in vivo using an autochthonous model of lung adenocarcinoma with concomitant expression of oncogenic Kras and deletion of Notch1. We found that Notch1 function is required for tumor initiation via suppression of p53-mediated apoptosis through the regulation of p53 stability. These findings implicate Notch1 as a critical effector in Kras-driven lung adenocarcinoma and as a regulator of p53 at a posttranslational level. Moreover, our study provides new insights to explain, at a molecular level, the correlation between Notch1 activity and poor prognosis in patients with NSCLC carrying wild-type p53. This information is critical for design and implementation of new therapeutic strategies in this cohort of patients representing 50% of NSCLC cases.

WT1: a weak spot in KRAS-induced transformation.

Activating mutations in the Ras alleles are found frequently in tumors, making the proteins they encode highly attractive candidate therapeutic targets. However, Ras proteins have proven difficult to target directly. Recent approaches have therefore focused on identifying indirect targets to inhibit Ras-induced oncogenesis. For example, RNAi-based negative selection screens to identify genes that when silenced in concert with activating Ras mutations are incompatible with cellular proliferation, a concept known as synthetic lethality. In this issue of the JCI, Vicent et al. report on the identification of Wilms tumor 1 (Wt1) as a Kras synthetic-lethal gene in a mouse model of lung adenocarcinoma. Silencing of Wt1 in cells expressing an endogenous allele of activated Kras triggers senescence in vitro and has an impact on tumor progression in vivo. These findings are of significant interest given previous studies suggesting that the ability of oncogenic Kras to induce senescence versus proliferation depends on its levels of expression.