p53 transcription factor and cancer:
» p53 is a tumor-suppressor protein that plays an important role in preventing cancer development by inducing cell cycle arrest or apoptosis. This protein is regulated primarily by ubiquitin ligase MDM2, which binds to p53 and targets it for degradation in the proteosome. A recent genetic analysis of 3,281 tumors from 12 cancer types has found that the most frequently mutated gene in these cancer types was p53 (Kandoth et al. Nature 2013). Reactivation of wild type p53 to restart apoptosis is an effective way to kill cancer cells. About 50% of tumors overexpress a nonfunctional mutant p53 that accumulates in high concentration in tumor cells. Mutations not only disable p53 tumor suppressive function but also exhibit cancer-promoting activity by gaining oncogenic properties. Mutant p53 is an array of mutant proteins with oncogenic properties that varies among patients. The ideal cancer drug should be functional in both wild type and mutant p53 containing tumors. Depletion of mutant p53 in tumors has been shown to induce apoptosis. In reality, tumors are a heterogeneous population of mutant cells (wild type p53 and different mutations of p53). Thus, drugs that can take care of a heterogeneous population of p53 will be an effective treatment for cancer. Kevetrin activates wild type p53, degrades oncogenic mutant p53, and induces apoptosis.
Kevetrin activates wild type p53 and induces apoptosis:
» Mechanism of action studies showed that Kevetrin strongly induced apoptosis by activation of Caspase 3 and cleavage of PARP. Kevetrin induced phosphorylation of p53 at Ser15 leading to a reduced interaction between p53 and MDM2, an ubiquitin ligase for p53 that plays a central role in p53 stability. Stabilized wild type p53 induced apoptosis by inducing the expression of PUMA. In addition, Kevetrin increased expression of p53 target genes such as p21 (Waf1), an inhibitor of cell cycle progression.
» Kevetrin also induced transcription-independent p53 mediated apoptosis. Kevetrin enhanced the phosphorylation of MDM2. Phosphorylation of MDM2 alters the E3 ligase processivity. Stable monoubiquitinated form of wild type p53, accumulates in the cytoplasm and interacts with BAK or BAX proteins in mitochondria to induce apoptosis Thus Kevetrin activates both transcription dependent and transcription independent pathways to promote apoptosis.
Kevetrin induces degradation of hyperstable oncogenic mutant p53:
» Oncogenic mutant p53 is often highly expressed and is hyperstable and drives tumor formation, invasion and metastasis. Mutant p53 cancers are dependent on their hyperstable mutant p53 protein for survival. Thus targeting mutant p53 for degradation is an excellent strategy to manage tumors that are addicted to mutant p53 for survival. The underlying mechanism of mutant p53 stabilization is the formation of a stable complex between HSP90, which is highly and ubiquitously upregulated in cancer cells, and mutant p53, which inhibits E3 ligase MDM2. Kevetrin strongly destabilizes mutant p53 at the level of protein degradation. Kevetrin downregulates the HDAC6, an essential positive regulator of HSP90. Downregulation of HDAC6 releases mutant p53 from the stabilized complex and enables E3 ligase mediated degradation. Kevetrin preferentially degrades mutant p53 rather than wild type p53. Kevetrin by virtue of depleting mutant p53 may be able to dramatically chemosensitize mutant p53 cells to chemotherapeutic drugs.
Kevetrin is non-genotoxic activator of p53.
» Most currently available chemotherapeutic are genotoxic in nature and damage DNA. Our results showed that Kevetrin, in non-genotoxic manner, induces p53 leading to apoptosis. DNA damaging drugs result in rapid phosphorylation of H2A.X at Ser 139 by PI3K-like kinases. Kevetrin did not induce phosphorylation of H2A.X protein.
» These mechanisms are supported by the demonstration of potent antitumor activity in several wild type and mutant p53 human tumor xenografts, including drug-resistant cancer, treated with Kevetrin.
