Phosphorylation of the Mdm2 oncoprotein by the c-Abl tyrosine kinase regulates p53 tumor suppression and the radiosensitivity of mice

Mdm2 phosphorylation by Akt regulates the p53 response to oxidative stress to promote cell proliferation and tumorigenesis

We have shown previously that phosphorylation of Mdm2 by ATM and c-Abl regulates Mdm2-p53 signaling and alters the effects of DNA damage in mice, including bone marrow failure and tumorigenesis induced by ionizing radiation. Here, we examine the physiological effects of Mdm2 phosphorylation by Akt, another DNA damage effector kinase. Surprisingly, Akt phosphorylation of Mdm2 does not alter the p53-mediated effects of ionizing radiation in cells or mice but regulates the p53 response to oxidative stress. Akt phosphorylation of Mdm2 serine residue 183 increases nuclear Mdm2 stability, decreases p53 levels, and prevents senescence in primary cells exposed to reactive oxidative species (ROS). Using multiple mouse models of ROS-induced cancer, we show that Mdm2 phosphorylation by Akt reduces senescence to promote Kras^G12D-driven lung cancers and carcinogen-induced papilloma and hepatocellular carcinomas. Collectively, we document a unique physiologic role for Akt-Mdm2-p53 signaling in regulating cell growth and tumorigenesis in response to oxidative stress.

c-Abl kinase regulates cell proliferation and ionizing radiation-induced G2/M arrest via phosphorylation of FHL2

Nonreceptor tyrosine kinase c‐Abl participates in several cellular processes by phosphorylating transcription factors or cofactors. c‐Abl binds and phosphorylates four‐and‐a‐half‐LIM‐only protein 2 (FHL2), but the identity of the phosphorylation sites and their contribution to cell cycle regulation is unclear. In this study, we demonstrate that c‐Abl highly phosphorylates FHL2 at Y97, Y176, Y217, and Y236 through mass spectrometry and tyrosine‐to‐phenylalanine (Y → F) mutant analysis. Proliferation was inhibited in cells expressing wild‐type (WT) FHL2 but not cells expressing the phosphorylation‐defective mutant FHL2(4YF). Moreover, FHL2 contributed to cell cycle arrest at G2/M induced by ionizing radiation (IR). FHL2 WT but not FHL2(4YF) rescued FHL2 function in FHL2‐depleted cells by causing IR‐induced G2/M arrest. These results demonstrate that c‐Abl regulates cell cycle progression by phosphorylating FHL2.

Quantitative Tyrosine Phosphoproteomic Analysis of Resistance to Radiotherapy in Nasopharyngeal Carcinoma Cells

Background

Radioresistance poses a major challenge in nasopharyngeal carcinoma (NPC) treatment. Protein tyrosine phosphorylation has emerged as a key device in the control of resistance to therapy in cancer cells.

Methods

Using tandem mass tag (TMT) labeling and phospho-antibody affinity enrichment followed by high-resolution LC-MS/MS analysis, quantitative tyrosine phosphorylome analysis was performed in CNE2 (parental) and its radioresistant subline CNE2-IR.

Results

Altogether, 233 tyrosine phosphorylation sites in 179 protein groups were identified, among which 179 sites in 140 proteins were quantified. Among the quantified proteins, 38 tyrosine phosphorylation proteins are up-regulated and 18 tyrosine phosphorylation proteins are down-regulated in CNE2-IR vs CNE2. Increased tyrosine phosphorylation in multiple receptor/protein tyrosine kinases (EPHA2, EGFR, IGF1R, ABL1 and LYN) was identified in CNE2-IR vs CNE2 cells. Intensive bioinformatic analyses revealed robust activation of multiple biological processes/pathways including E-cadherin stabilization, cell-cell adhesion, and cell junction organization in radioresistant CNE2-IR cells. Specifically, we observed that the CNE2 cells incubated with EphrinA1-Fc exhibited higher EPHA2 Y772 phosphorylation and lower E-cadherin expression, as compared with PBS control. Furthermore, an ATP-competitive EPHA2 RTK inhibitor (ALW-II-41-27, ALW) reduced EPHA2 Y772 phosphorylation and increased the expression of E-cadherin in CNE2-IR cells. Colony formation analysis showed that EFNA1 (EFNA1 is the ligand of EPHA2) treatment in CNE2 significantly promoted colony formation after 6Gy irradiation; while incubation with EPHA2 inhibitor ALW-II-41-27 in CNE2-IR cells impaired colony formation after irradiation, as compared with solvent control (DMSO).

Conclusion

In conclusion, phosphoproteomic approach allowed us to link tyrosine kinases signaling with radioresistance in NPC. Further studies are necessary to delineate the molecular function of EPHA2/E-cadherin signaling in radioresistant NPC and to explore rational combination therapy and its underlying mechanism.

α-Synucleinopathy associated c-Abl activation causes p53-dependent autophagy impairment

BACKGROUND

Studies link c-Abl activation with the accumulation of pathogenic α-synuclein (αS) and neurodegeneration in Parkinson's disease (PD). Currently, c-Abl, a tyrosine kinase activated by cellular stress, is thought to promote αS pathology by either directly phosphorylating αS or by causing autophagy deficits.

METHODS

αS overexpressing transgenic (Tg) mice were used in this study. A53T Tg mice that express high levels of human mutant A53TαS under the control of prion protein promoter. Two different approaches were used in this study. Natural aging and seeding model of synucleinopathy. In seeding model, intracortical/intrastriatal (IC/IS) stereotaxic injection of toxic lysates was done using tissue lysates from end-stage symptomatic mice. In this study, nilotinib and pifithrin-α was used as a c-Abl and p53 inhibitor, respectively. Both Tg and non-transgenic (nTg) mice from each group were subjected to nilotinib (10 mg/kg) or vehicle (DMSO) treatment. Frozen brain tissues from PD and control human cases were analyzed. In vitro cells study was implied for c-Abl/p53 genetic manipulation to uncover signal transduction.

RESULTS

Herein, we show that the pathologic effects of c-Abl in PD also involve activation of p53, as c-Abl activation in a transgenic mouse model of α-synucleinopathy (TgA53T) and human PD cases are associated with the increased p53 activation. Significantly, active p53 in TgA53T neurons accumulates in the cytosol, which may lead to inhibition of autophagy. Thus, we hypothesized that c-Abl-dependent p53 activation contributes to autophagy impairment in α-synucleinopathy. In support of the hypothesis, we show that c-Abl activation is sufficient to inhibit autophagy in p53-dependent manner. Moreover, inhibition of either c-Abl, using nilotinib, or p53, using pifithrin-α, was sufficient to increase autophagic flux in neuronal cells by inducing phosphorylation of AMP-activated kinase (AMPK), ULK1 activation, and down-regulation of mTORC1 signaling. Finally, we show that pharmacological attenuation of c-Abl activity by nilotinib treatment in the TgA53T mouse model reduces activation of p53, stimulates autophagy, decreases accumulation αS pathology, and delays disease onset.

CONCLUSION

Collectively, our data show that c-Abl activation by α-synucleinopathy causes p53 dependent autophagy deficits and both c-Abl and p53 represent therapeutic target for PD.

The oncoprotein HBXIP promotes human breast cancer growth through down-regulating p53 via miR-18b/MDM2 and pAKT/MDM2 pathways

Mammalian hepatitis B X-interacting protein (HBXIP) is an 18-kDa protein that regulates a large number of transcription factors such as TF-IID, E2F1, SP1, STAT3, c-Myc, and LXR by serving as an oncogenic transcription coactivator and plays an important role in the development of breast cancer. We previously showed that HBXIP as an oncoprotein could enhance the promoter activity of MDM2 through coactivating p53, promoting the MDM2 transcription in breast cancer. In this study we investigated the molecular mechanisms underlying the modulation of MDM2/p53 interaction by HBXIP in human breast cancer MCF-7 cells in vitro and in vivo. We showed that HBXIP could up-regulate MDM2 through inducing DNA methylation of miR-18b, thus suppressing the miR-18b expression, leading to the attenuation of p53 in breast cancer cells. In addition, HBXIP could promote the phosphorylation of MDM2 by increasing the level of pAKT and bind to pMDM2, subsequently enhancing the interaction between MDM2 and p53 for the down-regulation of p53 in breast cancer cells. In MCF-7 breast cancer xenograft nude mice, we also observed that overexpression of HBXIP promoted breast cancer growth through the miR-18b/MDM2 and pAKT/MDM2 pathways. In conclusion, oncoprotein HBXIP suppresses miR-18b to elevate MDM2 and activates pAKT to phosphorylate MDM2 for enhancing the interaction between MDM2 and p53, leading to p53 degradation in promotion of breast cancer growth. Our findings shed light on a novel mechanism of p53 down-regulation during the development of breast cancer.

Interaction of Abl Tyrosine Kinases with SOCS3 Impairs Its Suppressor Function in Tumorigenesis

Suppressor of cytokine signaling 3 (SOCS3) is involved in Bcr-Abl–induced tumorigenesis. However, how SOCS3 interacts with Bcr-Abl and is regulated by Abl kinases remains largely unknown. Since c-Abl plays a critical role in tumorigenesis, we asked whether SOCS3 is regulated by c-Abl–dependent phosphorylation. Here, we found that SOCS3 interacted with all three Abl kinases (Bcr-Abl, v-Abl, and c-Abl), and SH1 domain of the Abl kinases was critically required for such interaction. Furthermore, the SH2 domain of SOCS3 was sufficient to pull down the SH1 domain but not the full length of Bcr-Abl. Importantly, SOCS3 was highly tyrosine phosphorylated by c-Abl, leading to impairment of its ability to suppress JAK8+72 activity. In addition, disrupting the tyrosine phosphorylation of SOCS3 promoted apoptosis of c-Abl–expressing cells and impeded xenograft growth of these tumor cells in nude mice. The results demonstrate that SOCS3 is highly tyrosine phosphorylated by c-Abl and that tyrosine phosphorylation of SOCS3 is required for the survival and tumorigenesis of certain cells. Our findings provide novel insights into complicated mechanisms underlying the oncogenic function of Abl kinases.

A novel small molecule inhibitor of MDM2-p53 (APG-115) enhances radiosensitivity of gastric adenocarcinoma

BACKGROUND

Gastric cancer is the leading cause of cancer related death worldwide. Radiation alone or combined with chemotherapy plays important role in locally advanced and metastatic gastric adenocarcinoma. MDM2-p53 interaction and downstream signaling affect cellular response to DNA damage which leads to cell cycle arrest and apoptosis. Therefore, restoring p53 function by inhibiting its interaction with MDM2 is a promising therapeutic strategy for cancer. APG-115 is a novel small molecule inhibitor which blocks the interaction of MDM2 and p53. In this study, we investigated that the radiosensitivity of APG-115 in gastric adenocarcinoma in vitro and in vivo.

METHODS

The role of APG-115 in six gastric cancer cells viability in vitro was determined by CCK-8 assay. The expression level of MDM2, p21, PUMA and BAX in AGS and MKN45 cell lines was measured via real-time PCR (RT-PCR). The function of treatment groups on cell cycle and cell apoptosis were detected through Flow Cytometry assay. Clonogenic assays were used to measure the radiosensitivity of APG-115 in p53 wild type gastric cancer cell lines. Western blot was conducted to detect the protein expressions of mdm2-p53 signal pathway. Xenograft models in nude mice were established to explore the radiosensitivity role of APG-115 in gastric cancer cells in vivo.

RESULTS

We found that radiosensitization by APG-115 occurred in p53 wild-type gastric cancer cells. Increasing apoptosis and cell cycle arrest was observed after administration of APG-115 and radiation. Radiosensitivity of APG-115 was mainly dependent on MDM2-p53 signal pathway. In vivo, APG-115 combined with radiation decreased xenograft tumor growth much more significantly than either single treatment. Moreover, the number of proliferating cells (Ki-67) significantly decreased in combination group compared with single treatment group.

CONCLUSIONS

In summary, we found that combination of MDM2-p53 inhibitor (APG-115) and radiotherapy can enhance antitumor effect both in vitro and in vivo. This is the first report on radiosensitivity of APG-115 which shed light on clinical trial of the combination therapy of radiation with APG-115 in gastric adenocarcinoma.