Camptothecin acts synergistically with imatinib and overcomes imatinib resistance through Bcr-Abl independence in human K562 cells

Paris Saponin VII Induces Apoptosis and Cell Cycle Arrest in Erythroleukemia Cells by a Mitochondrial Membrane Signaling Pathway

BACKGROUND AND PURPOSE

Leukemia is considered a top-listed ailment, according to WHO, which contributes to the death of a major population of the world every year. Paris Saponin VII (PS), a saponin which was isolated from the roots of Trillium kamtschaticum, from our group, was reported to provide hemostatic, cytotoxic and antimicrobial activities. However, its molecular mechanism underlying the anti-proliferative effects remains unclear. Thus, this study hypothesized to assess that mechanism in PS treated HEL cells.

METHODS

The MTT assay was used to analyze the PS inhibited cell viability in the HEL cells. We further found that PS could induce S phase cell cycle arrest through flow cytometry as well as the western blot analysis of intrinsic and extrinsic apoptotic molecules.

RESULTS

The MTT assay showed the IC50 concentration of PS as 0.667μM. The study revealed that PS treatment inhibits cell proliferation dose-dependently. It further caused mitochondrial membrane potential changes by PS treatment. Mechanistic protein expression revealed a dose-dependent upsurge for Bid and Bim molecules, while Bcl2 and PARP expression levels were significantly (P<0.05) down-regulated in PS treated HEL cells resulting in caspase -3 release and increased the Bim levels upon 24h of incubation.

CONCLUSION

These findings indicate that PS possesses an excellent anti-leukemic activity via the regulation of the mitochondrial pathway, leading to S phase cell cycle arrest and caspase-dependent apoptosis, suggesting it as a potential alternative chemotherapeutic agent for leukemia patients.

Activation of abl family kinases in solid tumors

Although c-Abl and Arg non-receptor tyrosine kinases are well known for driving leukemia development, their role in solid tumors has not been appreciated until recently. Accumulating evidence now indicates that c-Abl and/or Arg are activated in some solid tumor cell lines via unique mechanisms that do not involve gene mutation/translocation, and c-Abl/Arg activation promotes matrix degradation, invasion, proliferation, tumorigenesis, and/or metastasis, depending on the tumor type. However, some data suggest that c-Abl also may suppress invasion, proliferation, and tumorigenesis in certain cell contexts. Thus, c-Abl/Arg may serve as molecular switches that suppress proliferation and invasion in response to some stimuli (e.g., ephrins) or when inactive/regulated, or as promote invasion and proliferation in response to other signals (e.g., activated growth factor receptors, loss of inhibitor expression), which induce sustained activation. Clearly, more data are required to determine the extent and prevalence of c-Abl/Arg activation in primary tumors and during progression, and additional animal studies are needed to substantiate in vitro findings. Furthermore, c-Abl/Arg inhibitors have been used in numerous solid tumor clinical trials; however, none of these trials were restricted to patients whose tumors expressed highly activated c-Abl/Arg (targeted trial). Targeted trials are critical for determining whether c-Abl/Arg inhibitors can be effective treatment options for patients whose tumors are driven by c-Abl/Arg.

Nuclear ErbB4 signaling through H3K9me3 that is antagonized by EGFR-activated c-Src

The ErbB family of receptor tyrosine kinases comprises four members: EGFR (epidermal growth factor receptor)/ErbB1, HER2/ErbB2, ErbB3 and ErbB4, and plays roles in signal transduction at the plasma membrane upon ligand stimulation. Stimulation with neuregulin-1 (NRG-1) cleaves ErbB4 and releases the ErbB4 intracellular domain (4ICD) that translocates into the nucleus to control gene expression. However, little is known about the regulation of 4ICD nuclear signaling through tyrosine phosphorylation. We show here that 4ICD nuclear signaling is antagonized by EGF-induced c-Src activation via EGFR. Generation of 4ICD by NRG-1 leads to increased levels of trimethylated histone H3 on lysine 9 (H3K9me3) in a manner dependent on 4ICD's nuclear accumulation and its tyrosine kinase activity. Once EGF activates c-Src downstream of EGFR concomitantly with NRG-1-induced ErbB4 activation, c-Src associates with phospho-Tyr950 and phospho-Tyr1056 on 4ICD, thereby decreasing nuclear accumulation of 4ICD and inhibiting an increase of H3K9me3 levels. Moreover, 4ICD-induced transcriptional repression of the human telomerase reverse transcriptase (hTERT) is inhibited by EGF-EGFR-Src signaling. Thus, our findings reveal c-Src-mediated inhibitory regulation of ErbB4 nuclear signaling upon EGFR activation.

Amphiregulin promotes resistance to gefitinib in nonsmall cell lung cancer cells by regulating Ku70 acetylation

Multiple molecular resistance mechanisms reduce the efficiency of receptor tyrosine kinase inhibitors such as gefitinib in non-small cell lung cancer (NSCLC). We previously demonstrated that amphiregulin (Areg) inhibits gefitinib-induced apoptosis in NSCLC cells by inactivating the proapoptotic protein BAX. In this part of the investigation, we studied the molecular mechanisms leading to BAX inactivation. We show that Areg prevents gefitinib-mediated acetylation of Ku70. This augments the BAX-Ku70 interaction and therefore prevents BAX-mediated apoptosis. Accordingly, Areg or Ku70 knock down restore BAX activation and apoptosis in gefitinib-treated H358 cells in vitro. In addition, overexpression of the histone acetyltransferase (HAT) CREB-binding protein (CBP) or treatments with histone deacetylase (HDAC) inhibitors sensitize H358 cells to gefitinib. Moreover, a treatment with vorinostat, a HDAC inhibitor strongly sensitized tumors to gefitinib in vivo. These findings suggest new prospects in combining both HDAC and epidermal growth factor receptor inhibitors for the treatment of NSCLC.

Sensitization of imatinib-resistant CML cells to TRAIL-induced apoptosis is mediated through down-regulation of Bcr-Abl as well as c-FLIP

Resistance to imatinib is commonly associated with reactivation of Bcr-Abl signalling. However, Bcr-Abl-independent signalling pathways may be activated and contributed to imatinib resistance in some CML (chronic myelogenous leukaemia) patients. We had isolated three imatinib-resistant K562/R1, R2 and R3 variants with gradual loss of Bcr-Abl from K562 cells to develop effective therapeutic strategies for imatinib-resistant CML. Interestingly, we found that these cells became highly sensitive to TRAIL (tumour necrosis factor-related apoptosis-inducing factor) in comparison with K562 cells showing high resistance to TRAIL. Treatment of K562/R3 cells with TRAIL resulted in activation of TRAIL receptor pathway by including caspase 8 activation, Bid cleavage, cytochrome c release and caspase 3 activation. These results were accompanied by down-regulation of c-FLIP {cellular FLICE [FADD (Fas-associated death domain)-like interleukin 1beta-converting enzyme]-inhibitory protein} in imatinib-resistant K562 variants compared with K562 cells. Overexpression of c-FLIP in K562/R3 cells acquired TRAIL resistance and conversely, c-FLIP-silenced K562 cells became sensitive to TRAIL. Moreover, Bcr-Abl-silenced K562 cells showed down-regulation of c-FLIP and the subsequent overcome of TRAIL resistance. Taken together, our results demonstrated for the first time that the loss of Bcr-Abl in imatinib-resistant cells led to the down-regulation of c-FLIP and subsequent increase of TRAIL sensitivity, suggesting that TRAIL could be an effective strategy for the treatment of imatinib-resistant CML with loss of Bcr-Abl.

PTPROt inactivates the oncogenic fusion protein BCR/ABL and suppresses transformation of K562 cells

Chronic myelogenous leukemia is typified by constitutive activation of the c-abl kinase as a result of its fusion to the breakpoint cluster region (BCR). Because the truncated isoform of protein-tyrosine phosphatase receptor-type O (PTPROt) is specifically expressed in hematopoietic cells, we tested the possibility that it could potentially dephosphorylate and inactivate the fusion protein bcr/abl. Ectopic expression of PTPROt in the chronic myelogenous leukemia cell line K562 indeed resulted in hypophosphorylation of bcr/abl and reduced phosphorylation of its downstream targets CrkL and Stat5, confirming that PTPROt could inactivate the function of bcr/abl. Furthermore, the expression of catalytically active PTPROt in K562 cells caused reduced proliferation, delayed transition from G0/G1 to S phase, loss of anchorage independent growth, inhibition of ex vivo tumor growth, and increased their susceptibility to apoptosis, affirming that this tyrosine phosphatase can revert the transformation potential of bcr/abl. Additionally, the catalytically inactive PTPROt acted as a trapping mutant that was also able to inhibit anchorage independence and facilitate apoptosis of K562 cells. The inhibitory action of PTPROt on bcr/abl was also confirmed in a murine myeloid cell line overexpressing bcr/abl. PTPROt expression was suppressed in K562 cells and was relieved upon treatment of the cells with 5-azacytidine, an inhibitor of DNA methyltransferase, with concomitant hypomethylation of the PTPRO CpG island. These data demonstrate that suppression of PTPROt by promoter methylation could contribute to the augmented phosphorylation and constitutive activity of its substrate bcr/abl and provide a potentially significant molecular therapeutic target for bcr/abl-positive leukemia.