ku-55933 and Glioma

In the present study, mutL homolog 1 (MLH1) small interfering (si)RNA, KU‑55933, an ataxia‑telangiectasia mutated (ATM) inhibitor, and compound C, an adenosine monophosphate‑activated protein kinase (AMPK) inhibitor, were used to investigate the mechanisms underlying temozolomide (TMZ)‑induced autophagy and to determine the role of MLH1 and ATM in autophagy. MLH1 siRNA and KU‑55933 inhibited the phosphorylation of AMPK and ULK1 and reduced the levels of autophagy. MLH1 siRNA inhibited the phosphorylation of ATM and attenuated TMZ cytotoxicity, whereas the inhibition of ATM‑AMPK augmented TMZ cytotoxicity in inherently TMZ‑sensitive glioma cells. Therefore, TMZ induced autophagy via the ATM‑AMPK pathways and the activation of ATM‑AMPK was MLH1‑dependent. The inhibition of ATM‑AMPK enhanced TMZ cytotoxicity in inherently TMZ‑sensitive glioma cells.

Topics: Adaptor Proteins, Signal Transducing; AMP-Activated Protein Kinases; Antineoplastic Agents, Alkylating; Ataxia Telangiectasia Mutated Proteins; Autophagy; Autophagy-Related Protein-1 Homolog; Cell Line, Tumor; Dacarbazine; Drug Resistance, Neoplasm; Glioma; Humans; Intracellular Signaling Peptides and Proteins; Morpholines; MutL Protein Homolog 1; Nuclear Proteins; Phosphorylation; Protein Kinase Inhibitors; Protein Serine-Threonine Kinases; Pyrones; RNA Interference; RNA, Small Interfering; Temozolomide

Autophagy is a cytoprotective process, which occurs following temozolomide (TMZ) treatment, and contributes to glioma chemoresistance and TMZ treatment failure. However, the molecular mechanisms by which TMZ induces autophagy are largely unknown. In the current study, the ataxia‑telangiectasia mutated (ATM) inhibitor KU‑55933, adenosine monophosphate‑activated protein kinase (AMPK) inhibitor compound C, and U87MG and U251 cell lines were employed to investigate the molecular mechanisms of TMZ‑induced autophagy in glioma, and to evaluate the effects of autophagy inhibition on TMZ cytotoxicity. KU‑55933 and compound C were observed to inhibit the activation of autophagy‑initiating kinase ULK1 and result in a significant decrease of autophagy as indicated by depressed LC3B cleavage and acidic vesicular organelle formation. The activation of AMPK‑ULK1 was ATM dependent. Autophagy inhibition via the AMPK inhibitor compound C augmented TMZ cytotoxicity as observed by depressed cell viability, increased γH2AX‑marked double‑strand breaks (DSBs) and elevated numbers of apoptotic glioma cells. In conclusion, TMZ induced autophagy via ATM‑AMPK‑ULK1 pathways. TMZ chemoresistance may therefore be overwhelmed by targeting AMPK, particularly for the treatment of O6‑methylguanine DNA methyltransferase‑negative gliomas.

Topics: AMP-Activated Protein Kinases; Antineoplastic Agents, Alkylating; Ataxia Telangiectasia Mutated Proteins; Autophagy; Autophagy-Related Protein-1 Homolog; Brain Neoplasms; Cell Line, Tumor; Dacarbazine; Glioma; Humans; Intracellular Signaling Peptides and Proteins; Microtubule-Associated Proteins; Morpholines; Protein Serine-Threonine Kinases; Pyrones; Signal Transduction; Temozolomide

Ataxia telangiectasia (A-T) mutated (ATM) is critical for cell cycle checkpoints and DNA repair. Thus, specific small molecule inhibitors targeting ATM could perhaps be developed into efficient radiosensitizers. Recently, a specific inhibitor of the ATM kinase, KU-55933, was shown to radiosensitize human cancer cells. Herein, we report on an improved analogue of KU-55933 (KU-60019) with K(i) and IC(50) values half of those of KU-55933. KU-60019 is 10-fold more effective than KU-55933 at blocking radiation-induced phosphorylation of key ATM targets in human glioma cells. As expected, KU-60019 is a highly effective radiosensitizer of human glioma cells. A-T fibroblasts were not radiosensitized by KU-60019, strongly suggesting that the ATM kinase is specifically targeted. Furthermore, KU-60019 reduced basal S473 AKT phosphorylation, suggesting that the ATM kinase might regulate a protein phosphatase acting on AKT. In line with this finding, the effect of KU-60019 on AKT phosphorylation was countered by low levels of okadaic acid, a phosphatase inhibitor, and A-T cells were impaired in S473 AKT phosphorylation in response to radiation and insulin and unresponsive to KU-60019. We also show that KU-60019 inhibits glioma cell migration and invasion in vitro, suggesting that glioma growth and motility might be controlled by ATM via AKT. Inhibitors of MEK and AKT did not further radiosensitize cells treated with KU-60019, supporting the idea that KU-60019 interferes with prosurvival signaling separate from its radiosensitizing properties. Altogether, KU-60019 inhibits the DNA damage response, reduces AKT phosphorylation and prosurvival signaling, inhibits migration and invasion, and effectively radiosensitizes human glioma cells.

Topics: Ataxia Telangiectasia; Ataxia Telangiectasia Mutated Proteins; Cell Cycle Proteins; Cell Movement; Cell Survival; DNA-Binding Proteins; Extracellular Signal-Regulated MAP Kinases; Fibroblasts; Gamma Rays; Glioma; Humans; Insulin; MAP Kinase Signaling System; Morpholines; Neoplasm Invasiveness; Phosphoserine; Protein Kinase Inhibitors; Protein Serine-Threonine Kinases; Proto-Oncogene Proteins c-akt; Pyrones; Radiation-Sensitizing Agents; Thioxanthenes; Tumor Suppressor Proteins

The accurate joining of DNA double-strand breaks by homologous recombination repair (HRR) is critical to the long-term survival of the cell. The three major mitogen-activated protein (MAP) kinase (MAPK) signaling pathways, extracellular signal-regulated kinase (ERK), p38, and c-Jun-NH(2)-kinase (JNK), regulate cell growth, survival, and apoptosis. To determine the role of MAPK signaling in HRR, we used a human in vivo I-SceI-based repair system. First, we verified that this repair platform is amenable to pharmacologic manipulation and show that the ataxia telangiectasia mutated (ATM) kinase is critical for HRR. The ATM-specific inhibitor KU-55933 compromised HRR up to 90% in growth-arrested cells, whereas this effect was less pronounced in cycling cells. Then, using well-characterized MAPK small-molecule inhibitors, we show that ERK1/2 and JNK signaling are important positive regulators of HRR in growth-arrested cells. On the other hand, inhibition of the p38 MAPK pathway generated an almost 2-fold stimulation of HRR. When ERK1/2 signaling was stimulated by oncogenic RAF-1, an approximately 2-fold increase in HRR was observed. KU-55933 partly blocked radiation-induced ERK1/2 phosphorylation, suggesting that ATM regulates ERK1/2 signaling. Furthermore, inhibition of MAP/ERK kinase (MEK)/ERK signaling resulted in severely reduced levels of phosphorylated (S1981) ATM foci but not gamma-H2AX foci, and suppressed ATM phosphorylation levels >85% throughout the cell cycle. Collectively, these results show that MAPK signaling positively and negatively regulates HRR in human cells. More specifically, ATM-dependent signaling through the RAF/MEK/ERK pathway is critical for efficient HRR and for radiation-induced ATM activation, suggestive of a regulatory feedback loop between ERK and ATM.

Topics: Ataxia Telangiectasia Mutated Proteins; Cell Cycle Proteins; Cell Line, Tumor; DNA Damage; DNA Repair; DNA-Binding Proteins; Glioma; Humans; MAP Kinase Signaling System; Mitogen-Activated Protein Kinase 1; Mitogen-Activated Protein Kinase 3; Morpholines; Phosphorylation; Protein Serine-Threonine Kinases; Proto-Oncogene Proteins c-raf; Pyrones; Recombination, Genetic; Tumor Suppressor Proteins