BAY 61-3606, CDKi, and Sodium Butyrate Treatments Modulate p53 Protein Level and Its Site-Specific Phosphorylation in Human Vestibular Schwannomas In Vitro
This study is done to evaluate the effect of spleen tyrosine kinase inhibitor (BAY 61-3606), cyclin-dependent kinase inhibitor (CDKi), and sodium butyrate (Na-Bu) on the level and phosphorylation of p53 protein and its binding to murine double minute 2 (MDM2) homologue in human vestibular schwannomas (VS). Primary cultures of the tumor tissues were treated individually with optimum concentrations of these small molecules in vitro. The results indicate modulation of p53 protein status and its binding ability to MDM2 in treated samples as compared to the untreated control. The three individual treatments reduced the level of total p53 protein. These treatments also decreased Ser392 and Ser15 phosphorylated p53 in tumor samples of young patients and Ser315 phosphorylated p53 in old patients. Basal level of Thr55 phosphorylated p53 protein was present in all VS samples and it remained unchanged after treatments. The p53 protein from untreated VS samples showed reduced affinity to MDM2 binding in vitro and it increased significantly after treatments. The MDM2/p53 ratio increased approximately 3-fold in the treated VS tumor samples as compared to the control. The differential p53 protein phosphorylation status perhaps could play an important role in VS tumor cell death due to these treatments that we reported previously. 1. Introduction The human vestibular schwannomas (VS) are benign tumors of the vestibular branch of the 8th cranial nerve. Bilateral VS tumors are mostly familial in nature, also known as neurofibromatosis type 2 (NF2) and as opposed to unilateral tumors which are generally sporadic. The onset age of familial forms of these tumors is earlier than that of the sporadic forms. These tumors cause high morbidity and also cause hearing loss, nausea, vertigo, tinnitus and facial paralysis, and hydrocephalus. Mortality rates are low due to this tumor. However, a higher rate of mortality is associated with secondary malignant tumors including gliomas and meningiomas [1, 2]. The p53 protein is an important cellular component with numerous key cellular functions [3, 4]. One of its functions involves its ability to bind to DNA and regulate transcription and in turn regulate a variety of cellular processes such as cell-cycle, apoptosis, DNA repair, aging, and senescence [5–8]. It also regulates a variety of cellular signals upon DNA damage, oncogenic activation, telomere erosion, change in cellular contact and adhesion, and hypoxia [5, 7, 8]. The p53 protein undergoes posttranslational modifications (PTMs) and one such important modification is
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