Altered structure and expression of RB1 gene and increased phosphorylation of pRb in human vestibular schwannomas

Preclinical validation of anti-nuclear factor-kappa B therapy to inhibit human vestibular schwannoma growth

Vestibular schwannomas (VSs), the most common tumors of the cerebellopontine angle, arise from Schwann cells lining the vestibular nerve. Pharmacotherapies against VS are almost non-existent. Although the therapeutic inhibition of inflammatory modulators has been established for other neoplasms, it has not been explored in VS. A bioinformatic network analysis of all genes reported to be differentially expressed in human VS revealed a pro-inflammatory transcription factor nuclear factor-kappa B (NF-κB) as a central molecule in VS pathobiology. Assessed at the transcriptional and translational level, canonical NF-κB complex was aberrantly activated in human VS and derived VS cultures in comparison to control nerves and Schwann cells, respectively. Cultured primary VS cells and VS-derived human cell line HEI-193 were treated with specific NF-κB siRNAs, experimental NF-κB inhibitor BAY11-7082 (BAY11) and clinically relevant NF-κB inhibitor curcumin. Healthy human control Schwann cells from the great auricular nerve were also treated with BAY11 and curcumin to assess toxicity. All three treatments significantly reduced proliferation in primary VS cultures and HEI-193 cells, with siRNA, 5 μM BAY11 and 50 μM curcumin reducing average proliferation (±standard error of mean) to 62.33% ± 10.59%, 14.3 ± 9.7%, and 23.0 ± 20.9% of control primary VS cells, respectively. These treatments also induced substantial cell death. Curcumin, unlike BAY11, also affected primary Schwann cells. This work highlights NF-κB as a key modulator in VS cell proliferation and survival and demonstrates therapeutic efficacy of directly targeting NF-κB in VS.

BAY 61-3606, CDKi, and sodium butyrate treatments alter gene expression in human vestibular schwannomas and cause cell death in vitro

Background:

Disrupted kinase and signaling pathways are found in many human cancers and they are implicated in carcinogenesis. Therefore, kinases have been important targets for the development of cancer therapeutics. Human vestibular schwannomas (VS) are the third most common intracranial tumours which occur in the vestibular branch of VIII^th cranial nerve. Sodium butyrate (Na-Bu) is a potent histone deacetylase inhibitor (HDACi) and with therapeutic efficacy. Spleen tyrosine kinase (Syk) has been implicated in many immunological consequences and is a putative target for cancer treatment.

Aims and objectives:

The present study was undertaken in order to evaluate the effect Na-Bu, 2,4-Diamino-5-oxo-pyrimidine hydrochloride (CDKi), a broad spectrum kinase inhibitor and BAY 61-3606 (Syk inhibitor) on the survival of VS tumour tissues in vitro and their possible effects on cell survival/death and levels of a few key proteins in the treated cells as compared to the untreated cells.

Materials and methods

Fresh tumour tissues were collected randomly from 16 patients with sporadic, VS tumours, minced into pieces and maintained in primary cultures. Twenty four hours later these cells were exposed to Na-Bu, BAY 61-3606 or CDKi. Forty eight hours after exposure, the tissue lysates were analysed by western blotting for expression of pRb and other proteins involved in cell survival/death.

Summary and significance of the findings:

The tissue samples used were positive for S100A protein, the maker for schwann cells confirming the VS tumour samples. The three individual treatments led to morphological change, DNA fragmentation and cell death and significantly reduced level of total and phosphorylated forms of pRb protein and drastically reduced EGF-R protein. These treatments also modulated levels of other proteins involved in cell survival/death such as PI3K, Caspase 3, TGF-β1, JNK, ASK1, Shh, NF-κB, p21^cip1/waf1. The Untreated cells had uncleaved PARP-1 protein and the treated cells had cleaved PARP-1. The results show that the observed cell death in treated cells perhaps is mediated by modulation of the levels and processing of certain key proteins. The possible development of these components as therapeutics is discussed.

miRNA-7 attenuation in Schwannoma tumors stimulates growth by upregulating three oncogenic signaling pathways

Micro RNAs (miRNA) negatively regulate protein-coding genes at the posttranscriptional level and are critical in tumorigenesis. Schwannomas develop from proliferation of dedifferentiated Schwann cells, which normally wrap nerve fibers to help support and insulate nerves. In this study, we carried out high-throughput miRNA expression profiling of human vestibular schwannomas by using an array representing 407 known miRNAs to explore the role of miRNAs in tumor growth. Twelve miRNAs were found to be significantly deregulated in tumor samples as compared with control nerve tissue, defining a schwannoma-typical signature. Among these miRNAs, we focused on miR-7, which was one of the most downregulated in these tumors and has several known oncogene targets, including mRNAs for epidermal growth factor receptor (EGFR) and p21-activated kinase 1 (Pak1). We found that overexpression of miR-7 inhibited schwannoma cell growth both in culture and in xenograft tumor models in vivo, which correlated with downregulation of these signaling pathways. Furthermore, we identified a novel direct target of miR-7, the mRNA for associated cdc42 kinase 1 (Ack1), with the expression levels of miR-7 and Ack1 being inversely correlated in human schwannoma samples. These results represent the first miRNA profiling of schwannomas and the first report of a tumor suppressor function for miR-7 in these tumors that is mediated by targeting the EGFR, Pak1, and Ack1 oncogenes. Our findings suggest miR-7 as a potential therapeutic molecule for schwannoma treatment, and they prompt clinical evaluation of drugs that can inhibit the EGFR, Pak1, and Ack1 signaling pathways to treat this tumor type.

Altered structure and deregulated expression of the tumor suppressor gene retinoblastoma (RB1) in human brain tumors

A total of 40 human brain tumor samples were analyzed for tumor-specific alterations at the RB1 gene locus. Gliomas were more prevalent in younger males and meningiomas in older females. Southern blot analysis revealed loss of heterozygosity (LOH) at the intron 1 locus of RB1 gene in 19.4% of informative cases and this is the first report showing LOH at this locus in human brain tumors. Levels of RB1 mRNA and protein, pRb, and the percentage of hyperphosphorylated form of pRb were also analyzed in these tumors. Normal human fibroblast cell line WI38 was used as control in northern and western analysis. Normal sized RB1 mRNA and protein were present in all the tumor samples. Majority of the gliomas had 2.0-fold or higher levels of RB1 mRNA and most meningiomas had less than 2.0-fold of RB1 mRNA compared to control WI38 cells. The total pRb levels were 2.0-fold or higher in all the tumor samples compared to control. More than 50% of pRb existed in hyperphosphorylated form in all gliomas except two. However, six out of 13 meningiomas had less than 50% of total pRb in the hyperphosphorylated form. These results indicate that the increased percentage of hyperphosphorylated form of pRb in gliomas could provide growth advantage to these tumors. Presence of LOH at the RB1 gene locus and the increased levels of RB1 RNA and protein and increased percentage of hyperphosphorylated form of pRb are indicative of an overall deregulation of pRb pathway in human brain tumors.

Age dependent phosphorylation and deregulation of p53 in human vestibular schwannomas

Tumor-specific alterations at the p53 gene locus in 30 human vestibular schwannomas (VS) comprising 10 confirmed NF2 cases and 20 sporadic cases were analyzed. We found loss of heterozygosity (LOH) at the first intron of the p53 gene locus in 54% of the informative cases. This is the first report showing LOH at the p53 gene locus in a significant number of human VS and both sporadic and NF2 cases show the LOH event. Increased levels of normal size p53 mRNA and p53 protein were found in all the tumors analyzed. Thus p53 appears to be deregulated in all the tumors suggesting that p53 alterations may be associated with tumor progression in VS. There was a negative significant correlation of patients' age and percentage of Ser 392 phosphorylated p53 protein. The tumor samples obtained from younger patients of 35 yr and below showed higher percentage of Ser 392 phosphorylated p53 protein compared to the tumors of older patients. The increased percentage of Ser 392 phosphorylated p53 protein indicates that it could be involved in the acceleration of tumor growth in the younger patients. Our results suggest that age dependent phosphorylation of p53 protein and deregulation of p53 gene has a role in the development of human vestibular schwannomas.