Celastrol inhibits gastric cancer growth by induction of apoptosis and autophagy

Post-translational modification of OCT4 in breast cancer tumorigenesis

Recurrence and drug resistance of breast cancer are still the main reasons for breast cancer-associated deaths. Cancer stem cell (CSC) model has been proposed as a hypothesis for the lethality of breast cancer. Molecular mechanisms underlying CSC maintenance are still unclear. In this study, we generated mammospheres derived from breast cancer MDA-MB231 cells and MCF7 cells to enrich CSCs and performed DNA microarray analysis. We found that the expression of carboxy terminus of HSP70-interacting protein (CHIP) E3 ubiquitin ligase was significantly downregulated in breast CSCs. CHIP depletion increased mammosphere formation, whereas CHIP overexpression reversed this effect. We identified interactomes by mass spectrometry and detected CHIP directly interacted with OCT4, a stemness factor. CHIP overexpression decreased OCT4 stability through proteasomal degradation. CHIP induced OCT4 ubiquitination, whereas H260Q, a catalytic CHIP mutant, did not. Interestingly, we determined that OCT4 was ubiquitinated at lysine 284, and CHIP overexpression did not degrade K284R mutant OCT4. CHIP overexpression decreased the proliferation and side population of breast cancer cells, but these were not occurred in K284R mutant OCT4 overexpressed cells. Only 1000 cells showing CHIP depletion or OCT4 overexpression sufficiently generated breast tumors and lung metastases in xenografted mice. Ubiquitination-defective mutant of OCT4(K284R) overexpressed cells drastically generated tumor burdens in mice. Patients with breast cancer who showed low CHIP expression had poor survival probability. Taken together, we suggest that CHIP-induced OCT4 ubiquitination is important in breast CSCs. Regulation of CHIP expression and OCT4 protein stability is a considerable approach for breast cancer therapy.

Targeting the WEE1 kinase as a molecular targeted therapy for gastric cancer

Wee1 is a member of the Serine/Threonine protein kinase family and is a key regulator of cell cycle progression. It has been known that WEE1 is highly expressed and has oncogenic functions in various cancers, but it is not yet studied in gastric cancers. In this study, we investigated the oncogenic role and therapeutic potency of targeting WEE1 in gastric cancer. At first, higher expression levels of WEE1 with lower survival probability were determined in stage 4 gastric cancer patients or male patients with accompanied lymph node metastasis. To determine the function of WEE1 in gastric cancer cells, we determined that WEE1 ablation decreased the proliferation, migration, and invasion, while overexpression of WEE1 increased these effects in gastric cancer cells. We also validated the clinical application of WEE1 targeting by a small molecule, AZD1775 (MK-1775), which is a WEE1 specific inhibitor undergoing clinical trials. AZD1775 significantly inhibited cell proliferation and induced apoptosis and cell cycle arrest in gastric cancer cells, which was more effective in WEE1 high-expressing gastric cancer cells. Moreover, we performed combination treatments with AZD1775 and anti-cancer agents, 5- fluorouracil or Paclitaxel in gastric cancer cells and in gastric cancer orthotopic-transplanted mice to maximize the therapeutic effect and safety of AZD1775. The combination treatments dramatically inhibited the proliferation of gastric cancer cells and tumor burdens in stomach orthotopic-transplanted mice. Taken together, we propose that WEE1 is over-expressed and could enhance gastric cancer cell proliferation and metastasis. Therefore, we suggest that WEE1 is a potent target for gastric cancer therapy.

Investigating the mechanism of hepatocellular carcinoma progression by constructing genetic and epigenetic networks using NGS data identification and big database mining method

The mechanisms leading to the development and progression of hepatocellular carcinoma (HCC) are complicated and regulated genetically and epigenetically. The recent advancement in high-throughput sequencing has facilitated investigations into the role of genetic and epigenetic regulations in hepatocarcinogenesis. Therefore, we used systems biology and big database mining to construct genetic and epigenetic networks (GENs) using the information about mRNA, miRNA, and methylation profiles of HCC patients. Our approach involves analyzing gene regulatory networks (GRNs), protein-protein networks (PPINs), and epigenetic networks at different stages of hepatocarcinogenesis. The core GENs, influencing each stage of HCC, were extracted via principal network projection (PNP). The pathways during different stages of HCC were compared. We observed that extracellular signals were further transduced to transcription factors (TFs), resulting in the aberrant regulation of their target genes, in turn inducing mechanisms that are responsible for HCC progression, including cell proliferation, anti-apoptosis, aberrant cell cycle, cell survival, and metastasis. We also selected potential multiple drugs specific to prominent epigenetic network markers of each stage of HCC: lestaurtinib, dinaciclib, and perifosine against the NTRK2, MYC, and AKT1 markers influencing HCC progression from stage I to stage II; celecoxib, axitinib, and vinblastine against the DDIT3, PDGFB, and JUN markers influencing HCC progression from stage II to stage III; and atiprimod, celastrol, and bortezomib against STAT3, IL1B, and NFKB1 markers influencing HCC progression from stage III to stage IV.

Phytochemicals as potent modulators of autophagy for cancer therapy

The dysregulation of autophagy is involved in the pathogenesis of a broad range of diseases, and accordingly universal research efforts have focused on exploring novel compounds with autophagy-modulating properties. While a number of synthetic autophagy modulators have been identified as promising cancer therapy candidates, autophagy-modulating phytochemicals have also attracted attention as potential treatments with minimal side effects. In this review, we firstly highlight the importance of autophagy and its relevance in the pathogenesis and treatment of cancer. Subsequently, we present the data on common phytochemicals and their mechanism of action as autophagy modulators. Finally, we discuss the challenges associated with harnessing the autophagic potential of phytochemicals for cancer therapy.

A Mechanistic Overview of Triptolide and Celastrol, Natural Products from Tripterygium wilfordii Hook F

Triptolide and celastrol are predominantly active natural products isolated from the medicinal plant Tripterygium wilfordii Hook F. These compounds exhibit similar pharmacological activities, including anti-cancer, anti-inflammation, anti-obesity, and anti-diabetic activities. Triptolide and celastrol also provide neuroprotection and prevent cardiovascular and metabolic diseases. However, toxicity restricts the further development of triptolide and celastrol. In this review, we comprehensively review therapeutic targets and mechanisms of action, and translational study of triptolide and celastrol. We systemically discuss the structure-activity-relationship of triptolide, celastrol, and their derivatives. Furthermore, we propose the use of structural derivatives, targeted therapy, and combination treatment as possible solutions to reduce toxicity and increase therapeutic window of these potent natural products from T. wilfordii Hook F.

SP1 reduces autophagic flux through activating p62 in gastric cancer cells

Gastric cancer is the most common type of gastrointestinal cancer, causing mortality worldwide. However, the underlying molecular mechanism in gastric cancer progression remains unclear. The autophagic flux was determined in gastric cancer cells overexpressing or inhibiting Sp1 transcription factor (SP1) using western blotting, reverse transcription‑polymerase chain reaction and immunofluorescence staining. Luciferase and ChIP assays were performed to detect the potential underlying mechanism of SP1 in gastric cancer cells. Lastly, immunohistochemistry was also performed on SP1 and p62 expression levels in human gastric cancer specimens. It was demonstrated that SP1 diminished autophagic flux via activating p62 in gastric cancer. Moreover, SP1 deficiency increased the rate of autophagy of gastric cancer cells. Notably, it was observed that SP1 enhanced the expression levels of p62 by directly binding to the promoter of p62. Analysis of gastric cancer specimen staining established that p62 expression levels were increased in SP1‑positve gastric tissues. The present study provided evidence for a novel mechanism regulating autophagy in gastric cancer cells.

Celastrol induces apoptosis in hepatocellular carcinoma cells via targeting ER-stress/UPR

Hepatocellular carcinoma (HCC) is one of the most serious and deadly diseases worldwide with limited options for effective treatment. Biomarker-based active compound targeting therapy may shed some light on novel drugs for HCC. The endoplasmic reticulum (ER) stress and unfolded protein response (UPR) play important roles in the regulation of cell fate and have become novel signaling targets for the development of anticancer drugs. Celastrol, a triterpene from traditional Chinese medicine, has been reported to possess anti-tumor effects on various cancers. We, along with several other research groups, have recently reported that UPR was induced by celastrol in several different cancers, including hepatocellular carcinoma. However, UPR status in HCC still remains unclear. The role of ER stress and autophagy in response to celastrol also has yet to be elucidated. Our results demonstrated that celastrol could cause G2/M phase rest and inhibit proliferation in HepG2 and Bel7402. Exposure to celastrol resulted in the activation of the intrinsic apoptotic pathway, via ER stress and the UPR. In murine syngeneic model studies celastrol inhibited H22 tumor growth via the induction of ER stress and apoptosis. Our study suggests that celastrol is a potential drug for HCC therapy via targeting ER-stress/UPR.

Celastrol alleviates angiotensin II‑mediated vascular smooth muscle cell senescence via induction of autophagy

Reactive oxygen species (ROS) production has been implicated in the promotion of cellular senescence. Celastrol, a quinone methide triterpenoid isolated from the Celastraceae family, exerts antioxidant effects and enhances autophagy in various cell types. Since autophagy serves an important role in regulating ROS, it was hypothesized that the antioxidant effect of celastrol is via enhanced autophagy, thus inhibiting cell senescence. Therefore, the present study used a Senescence β‑Galactosidase Staining kit, western blot analysis and cell cycle analysis to investigate whether celastrol alleviates angiotensin (Ang) II‑induced cellular senescence by upregulating autophagy in vascular smooth muscle cells (VSMCs). The results demonstrated that celastrol reduced Ang II‑induced senescence of VSMCs. Ang II‑induced generation of ROS and the subsequent VSMC senescence were counteracted by pretreatment with celastrol, determined by a ROS assay kit. Celastrol significantly upregulated VSMC autophagy, which reduced intracellular ROS and the subsequent cellular senescence induced by Ang II. Furthermore, celastrol markedly suppressed activity of the mechanistic target of rapamycin signaling pathway in VSMCs. In conclusion, the present study demonstrated that celastrol counteracts VSMC senescence probably by reducing ROS production via activation of autophagy, which may hold promise for the prevention and treatment of aging‑associated cardiovascular disorders such as atherosclerosis.

Oleic acid induces apoptosis and autophagy in the treatment of Tongue Squamous cell carcinomas

Oleic acid (OA), a main ingredient of Brucea javanica oil (BJO), is widely known to have anticancer effects in many tumors. In this study, we investigated the anticancer effect of OA and its mechanism in tongue squamous cell carcinoma (TSCC). We found that OA effectively inhibited TSCC cell proliferation in a dose- and time-dependent manner. OA treatment in TSCC significantly induced cell cycle G0/G1 arrest, increased the proportion of apoptotic cells, decreased the expression of CyclinD1 and Bcl-2, and increased the expression of p53 and cleaved caspase-3. OA also obviously induced the formation of autolysosomes and decreased the expression of p62 and the ratio of LC3 I/LC3 II. The expression of p-Akt, p-mTOR, p-S6K, p-4E-BP1 and p-ERK1/2 was significantly decreased in TSCC cells after treatment with OA. Moreover, tumor growth was significantly inhibited after OA treatment in a xenograft mouse model. The above results indicate that OA has a potent anticancer effect in TSCC by inducing apoptosis and autophagy via blocking the Akt/mTOR pathway. Thus, OA is a potential TSCC drug that is worthy of further research and development.

Celastrol: A Spectrum of Treatment Opportunities in Chronic Diseases

The identification of new bioactive compounds derived from medicinal plants with significant therapeutic properties has attracted considerable interest in recent years. Such is the case of the Tripterygium wilfordii (TW), an herb used in Chinese medicine. Clinical trials performed so far using its root extracts have shown impressive therapeutic properties but also revealed substantial gastrointestinal side effects. The most promising bioactive compound obtained from TW is celastrol. During the last decade, an increasing number of studies were published highlighting the medicinal usefulness of celastrol in diverse clinical areas. Here we systematically review the mechanism of action and the therapeutic properties of celastrol in inflammatory diseases, namely, rheumatoid arthritis, systemic lupus erythematosus, inflammatory bowel diseases, osteoarthritis and allergy, as well as in cancer, neurodegenerative disorders and other diseases, such as diabetes, obesity, atherosclerosis, and hearing loss. We will also focus in the toxicological profile and limitations of celastrol formulation, namely, solubility, bioavailability, and dosage issues that still limit its further clinical application and usefulness.

Celastrol inhibits migration and invasion through blocking the NF-κB pathway in ovarian cancer cells

Metastatic ovarian cancer is a major clinical challenge with poor prognosis and high mortality. Celastrol is a natural compound that has exhibits antiproliferative activity; however, its effects on metastasis-related phenotypes in ovarian cancer models are unclear. In the current study, the anti-invasive activities and associated signaling pathways of celastrol were determined in ovarian cancer cells. Cell proliferation was tested by MTT assay. Cell migration was detected by wound healing and Transwell assays, while cell invasion was detected by a Matrigel-coated Transwell method. In addition, nuclear factor (NF)-κB and matrix metalloproteinase (MMP) expression was examined by western blotting, and MMP-2/-9 activities were determined by gelatin zymography. At sub-toxic concentrations (<0.5 µM), celastrol inhibited migration and invasion in a concentration-dependent manner in SKOV-3 and OVCAR-3 cells. At the molecular level, celastrol blocked the canonical NF-κB pathway by inhibiting IκBα phosphorylation, and preventing IκBα degradation and p65 accumulation. Furthermore, the expression and activity of the NF-κB target protein MMP-9, but not MMP-2, were inhibited by celastrol. Furthermore, celastrol showed no synergistic effect with MG132, an NF-κB inhibitor. In conclusion, celastrol exhibited significant anti-invasive activities in ovarian cancer cells. Such functions may be mediated via NF-κB pathway blockade. The results of this in vitro study strengthen the value of applying celastrol as a potential clinical intervention modality for delaying ovarian cancer metastasis. This, celastrol warrants further preclinical investigation.

Celastrol-induced apoptosis in human nasopharyngeal carcinoma is associated with the activation of the death receptor and the mitochondrial pathway

Nasopharyngeal carcinoma (NPC) is a cancer that arises from the epithelium of the nasopharynx. Celastrol is a triterpene from traditional Chinese medicine, which demonstrates anti-proliferative activity in several cancer cell lines. However, the effect of celastrol on human NPC and the underlying mechanisms are not yet elucidated. The present study investigated whether celastrol induced apoptosis in human NPC cells, and the underlying molecular mechanisms were explored. Celastrol decreased the viability of HONE-1 and NPC-039 cells in a dose-dependent manner, and induced G1 and G2/M phase cell cycle arrest. The level of cleaved caspases-3, -8, and -9 and poly (ADP-ribose) polymerase 1 increased in cells treated with celastrol. There was an increase in active Bcl-2-like 11 isoform S, Bcl-2-associated X, Bcl-2 antagonist/killer and truncated BH3-interacting death antagonist, and the levels of the anti-apoptotic Bcl-2 and Bcl-2-like 1 decreased. Celastrol induced an increase in Fas, Fas-associated via death domain, TNF receptor superfamily members (TNRSF) 1A and 10B, and TNFRSF1A associated via death domain, and induced a dose-dependent reduction in mitochondrial membrane potential. Celastrol inhibited activation of mitogen-activated protein kinase (MAPK) 1/3 and 14, and induced MAPK 8/9 activation. The results indicated that celastrol induced apoptosis through the death receptor and the mitochondrial pathway in human NPC cells, and is a promising candidate in the development of drugs against NPC.

Induction of apoptosis by Dae-Hwang-Mok-Dan-Tang in HCT-116 colon cancer cells through activation of caspases and inactivation of the phosphatidylinositol 3-kinase/Akt signaling

BACKGROUND

Dae-Hwang-Mok-Dan-Tang (DHMDT), a traditional Korean medicine, contains five species of medicinal plants and has been used to treat patients with digestive tract cancer for hundreds of years; however, its anticancer mechanism is poorly understood. In the present study, we investigated the proapoptotic effects of DHMDT in human colon cancer HCT-116 cells.

METHODS

Cytotoxicity was evaluated using the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide assay. Apoptosis was detected using 4,6-diamidino-2-phenyllindile staining, agarose gel electrophoresis, and flow cytometry. The protein levels were determined using Western blot analysis. Caspase activity was measured using a colorimetric assay.

RESULTS

Treatment with DHMDT resulted in a growth inhibition coupled with apoptosis induction, which was associated with the downregulation of members of IAP (inhibitor of apoptosis protein) family, including XIAP and survivin, and the activation of caspase-9 and -3 accompanied by proteolytic degradation of poly(ADP-ribose)-polymerase and phospholipase C-γ1. DHMDT treatment also showed a correlation with the translocation of proapoptotic Bax to mitochondria, the loss of mitochondrial membrane permeabilization, and the cytochrome c release from the mitochondria to the cytosol. Moreover, DHMDT increased the levels of death receptor-associated ligands and enhanced activation of caspase-8 and cleavage of its substrate, Bid. However, the pan-caspase inhibitor could reverse DHMDT-induced apoptosis. In addition, DHMDT suppressed the phosphoinositide 3-kinase (PI3K)/Akt pathway, and treatment with a potent inhibitor of PI3K further increased the apoptotic activity of DHMDT.

CONCLUSION

Our data showed that DHMDT induces HCT-116 cell apoptosis by activating intrinsic and extrinsic apoptosis pathways and by suppressing the PI3K/Akt signal pathway; however, further studies are needed to identify the active compounds.

Functional role of autophagy in gastric cancer

Autophagy is a highly regulated catabolic pathway responsible for the degradation of long-lived proteins and damaged intracellular organelles. Perturbations in autophagy are found in gastric cancer. In host gastric cells, autophagy can be induced by Helicobacter pylori (or H. pylori) infection, which is associated with the oncogenesis of gastric cancer. In gastric cancer cells, autophagy has both pro-survival and pro-death functions in determining cell fate. Besides, autophagy modulates gastric cancer metastasis by affecting a wide range of pathological events, including extracellular matrix (ECM) degradation, epithelial-to-mesenchymal transition (EMT), tumor angiogenesis, and tumor microenvironment. In addition, some of the autophagy-related proteins, such as Beclin 1, microtubule-associated protein 1 light chain 3 (MAP1-LC3), and p62/sequestosome 1 (SQSTM1) have certain prognostic values for gastric cancer. In this article, we review the recent studies regarding the functional role of autophagy in gastric cancer.

Natural Compounds As Modulators of Non-apoptotic Cell Death in Cancer Cells

Cell death is an innate capability of cells to be removed from microenvironment, if and when they are damaged by multiple stresses. Cell death is often regulated by multiple molecular pathways and mechanism, including apoptosis, autophagy, and necroptosis. The molecular network underlying these processes is often intertwined and one pathway can dynamically shift to another one acquiring certain protein components, in particular upon treatment with various drugs. The strategy to treat human cancer ultimately relies on the ability of anticancer therapeutics to induce tumor-specific cell death, while leaving normal adjacent cells undamaged. However, tumor cells often develop the resistance to the drug-induced cell death, thus representing a great challenge for the anticancer approaches. Numerous compounds originated from the natural sources and biopharmaceutical industries are applied today in clinics showing advantageous results. However, some exhibit serious toxic side effects. Thus, novel effective therapeutic approaches in treating cancers are continued to be developed. Natural compounds with anticancer activity have gained a great interest among researchers and clinicians alike since they have shown more favorable safety and efficacy then the synthetic marketed drugs. Numerous studies in vitro and in vivo have found that several natural compounds display promising anticancer potentials. This review underlines certain information regarding the role of natural compounds from plants, microorganisms and sea life forms, which are able to induce non-apoptotic cell death in tumor cells, namely autophagy and necroptosis.

Compounds From Celastraceae Targeting Cancer Pathways and Their Potential Application in Head and Neck Squamous Cell Carcinoma: A Review

Squamous cell carcinoma of the head and neck is one of the most common cancer types worldwide. It initiates on the epithelial lining of the upper aerodigestive tract, at most instances as a consequence of tobacco and alcohol consumption. Treatment options based on conventional therapies or targeted therapies under development have limited efficacy due to multiple genetic alterations typically found in this cancer type. Natural products derived from plants often possess biological activities that may be valuable in the development of new therapeutic agents for cancer treatment. Several genera from the family Celastraceae have been studied in this context. This review reports studies on chemical constituents isolated from species from the Celastraceae family targeting cancer mechanisms studied to date. These results are then correlated with molecular characteristics of head and neck squamous cell carcinoma in an attempt to identify constituents with potential application in the treatment of this complex disease at the molecular level.

Celastrol prevents cadmium-induced neuronal cell death by blocking reactive oxygen species-mediated mammalian target of rapamycin pathway

BACKGROUND AND PURPOSE

Increasing evidence has suggested cadmium (Cd), as an inducer of ROS, is a potential pathogenic factor in human neurodegenerative diseases. Thus, it is important to find effective interventions for Cd-induced oxidative stress in the CNS. Here, we have studied the effects of celastrol, a plant-derived triterpene, on ROS production and cell death in neuronal cells, induced by Cd.

EXPERIMENTAL APPROACH

PC12, SH-SY5Y cells and primary murine neurons were used to study celastrol neuroprotection against Cd-poisoning. Cd-induced changes in cell viability, apoptosis, ROS and AMP-activated protein kinase (AMPK)/mammalian target of rapamycin (mTOR) pathway in the cells were analysed by Trypan blue exclusion, DAPI and TUNEL staining, ROS imaging, immunofluorescence staining and Western blotting. Pharmacological and genetic approaches were employed to investigate the mechanisms underlying Cd neurotoxicity.

RESULTS

Celastrol attenuated Cd-induced apoptosis by suppressing Cd activation of mTOR, which was attributed to preventing Cd inactivation of AMPK. Inhibition of AMPK with compound C or expression of dominant negative AMPKα prevented celastrol from hindering Cd-induced dephosphorylation of AMPKα, activation of mTOR and apoptosis. Inhibition of mTOR with rapamycin or knockdown of mTOR potentiated prevention by celastrol, of Cd-induced phosphorylation of p70 S6 kinase 1/eukaryotic initiation factor 4E binding protein 1 and apoptosis. Celastrol attenuated Cd-induced cell death by suppressing induction of mitochondrial ROS.

CONCLUSIONS AND IMPLICATIONS

Celastrol prevented the inactivation of AMPK by mitochondrial ROS, thus attenuating Cd-induced mTOR activation and neuronal apoptosis. Celastrol may be a promising agent for prevention of Cd-induced oxidative stress and neurodegenerative diseases.

Effects of celastrol on enhancing apoptosis of ovarian cancer cells via the downregulation of microRNA‑21 and the suppression of the PI3K/Akt‑NF‑κB signaling pathway in an in vitro model of ovarian carcinoma

Celastrol has previously been used to treat rheumatoid arthritis, bruises, back pain and additional diseases. At present, efficacy studies predominantly focus on the anti‑inflammatory, antioxidative and antitumor effects of celastrol. However, the effect of celastrol on ovarian cancer cells is not fully elucidated. In the present study, the effects of celastrol were investigated in ovarian cancer cells and the mechanisms involved were explored. In OVCAR3 cells, celastrol was observed to suppress cellular proliferation, induce apoptosis and increase caspase‑9 and ‑3 activity in a dose‑ and time‑dependent manner. The expression levels of microRNA‑21 (miRNA‑21) were reduced, in addition to a reduction in the levels of phosphoinositide 3‑kinase (PI3K)/p‑Akt‑NF (NF)‑κB following treatment with celastrol. Notably, reduced expression of miRNA‑21 replicated the effect of celastrol on OVCAR3 cells and inhibited the PI3K/p‑Akt‑NF‑κB signaling pathway in an in vitro model of ovarian carcinoma. To the best of our knowledge this is the first study to indicate that celastrol may represent a potential agent for the treatment of human ovarian carcinoma, via the induction of apoptosis through the downregulation of miRNA‑21 and the PI3K/Akt‑NF‑κB signaling pathway in an in vitro model of ovarian carcinoma.

Downregulation of miR-17-92a cluster promotes autophagy induction in response to celastrol treatment in prostate cancer cells

Celastrol has potential application for the treatment of prostate cancer. However it causes autophagy as a protective response in prostate and other types of cancers, thus unveiling the underlying mechanisms may benefit its future application. In the present study, we demonstrate that the miR-17-92a cluster plays a negative role in celastrol induced-autophagy. Dissection of miR-17-92a cluster revealed the role of miR-17 seed family (miR-20a and miR-17) in autophagy inhibition in the context of prostate cancer cells. Autophagy-related gene ATG7 was validated as a target of miR-17 seed family by dual-luciferase assay and qPCR. Celastrol induced autophagy was inhibited by miR-20a or miR-17, while the inhibitory effects were rescued in the presence of pcDNA-ATG7 lacking 3' UTR, demonstrating that these two members target ATG7 to inhibit celastrol-induced autophagy. As celastrol degrades androgen receptor (AR), a key transcription factor in prostate cancer cells, we further investigated whether AR affected miR-17-92a expression in prostate cancer cells. AR binding sites were found in the promoter and two introns of miR-17-92a. In addition, higher expression levels of miR-17-92a were observed in AR positive cells compared with AR negative cells. Ectopic expression of AR could enhance the expression of miR-17-92a cluster in AR-negative prostate cancer cells while knockdown of AR decreased miR-17-92a expression in AR-positive cells, demonstrating the regulation of AR on miR-17-92a transcription. In summary, our results demonstrate that celastrol downregulates AR and its target miR-17-92a, leading to autophagy induction in prostate cancer cells.

Ablation of human telomerase reverse transcriptase (hTERT) induces cellular senescence in gastric cancer through a galectin-3 dependent mechanism

The human Telomerase Reverse Transcriptase (hTERT) gene encodes a rate-limiting catalytic subunit of telomerase that maintains genomic integrity. Suppression of hTERT expression could induce cellular senescence and is considered a potent approach for gastric cancer therapy. However, control of hTERT expression and function remains poorly understood in gastric cancer. In this study, we demonstrated that high expression levels of hTERT in malignant tissues are correlated with poor survival probability in gastric cancer patients. Knockdown of hTERT expression retarded cell proliferation and cellular senescence, which was confirmed by increased protein expression levels of p21cip1 and p27kip1, and decreased phosphorylation of Rb. In contrast, overexpression of hTERT increased cell proliferation and decreased cellular senescence. Remarkably, the down-regulation of hTERT expression was detected in lgals3-/- mouse embryo fibroblasts (MEFs). Knockdown of galectin-3 decreased the expression of hTERT in gastric cancer cells. Galectin-3 ablation-induced cellular senescence was rescued by concomitant overexpression of hTERT. hTERT ablation-induced cellular senescence and p21cip1 and p27kip1 expression was rescued by concomitant overexpression of galectin-3. The size of tumor burdens was increased in hTERT-overexpressed gastric cancer cells xenografted mice, whereas it was repressed by concomitant depletion of galectin-3. Additionally, we determined that the N-terminal domain of galectin-3 directly interacted with hTERT. The telomeric activity of hTERT was also decreased by galectin-3 ablation. Taken together, ablation of hTERT induces cellular senescence and inhibits the growth of gastric cancer cells, suggesting that it could be a potent target in gastric cancer therapy. We also propose that galectin-3 is an important regulator of hTERT expression and telomeric activity in gastric tumorigenesis.

C-terminal domain of p42 Ebp1 is essential for down regulation of p85 subunit of PI3K, inhibiting tumor growth

Potential tumor suppressor p42, ErbB3-binding protein 1 (EBP1) inhibits phosphoinositide 3-kinase (PI3K) activity reducing the p85 regulatory subunit. In this study, we demonstrated that overexpression of p42 promoted not only a reduction of wild type of p85 subunit but also oncogenic mutant forms of p85 which were identified in human cancers. Moreover, we identified the small fragment of C-terminal domain of p42 is sufficient to exhibit tumor suppressing activity of p42-WT, revealing that this small fragment (280-394) of p42 is required for the binding of both HSP70 and CHIP for a degradation of p85. Furthermore, we showed the small fragment of p42 markedly inhibited the tumor growth in mouse xenograft models of brain and breast cancer, resembling tumor suppressing activity of p42. Through identification of the smallest fragment of p42 that is responsible for its tumor suppressor activity, our findings represent a novel approach for targeted therapy of cancers that overexpress PI3K.

Gecko proteins induce the apoptosis of bladder cancer 5637 cells by inhibiting Akt and activating the intrinsic caspase cascade

Gecko proteins have long been used as anti-tumor agents in oriental medicine, without any scientific background. Although anti-tumor effects of Gecko proteins on several cancers were recently reported, their effect on bladder cancer has not been investigated. Thus, we explored the anti-tumor effect of Gecko proteins and its cellular mechanisms in human bladder cancer 5637 cells. Gecko proteins significantly reduced the viability of 5637 cells without any cytotoxic effect on normal cells. These proteins increased the Annexin-V staining and the amount of condensed chromatin, demonstrating that the Gecko proteinsinduced cell death was caused by apoptosis. Gecko proteins suppressed Akt activation, and the overexpression of constitutively active form of myristoylated Akt prevented Gecko proteins-induced death of 5637 cells. Furthermore, Gecko proteins activated caspase 9 and caspase 3/7. Taken together, our data demonstrated that Gecko proteins suppressed the Akt pathway and activated the intrinsic caspase pathway, leading to the apoptosis of bladder cancer cells. [BMB Reports 2015; 48(9): 531-536].

Protective effects of Celastrol on diethylnitrosamine-induced hepatocellular carcinoma in rats and its mechanisms

Celastrol, an active ingredient of Tripterygium Wilfordii, is a traditional Chinese medicinal herb, which has attracted interests for its potential anti-inflammatory and anti-cancer activities. The aim of this study was to evaluate the anti-tumor effect of Celastrol against diethylnitrosamine (DEN)-induced hepatocellular carcinoma (HCC) in rats and furthermore, to explore the underlying mechanism. Sprague-Dawley rats were intragastrically administered with DEN (10mg/kg) for 6 days every week and persisting 16 weeks. The number of nodules was calculated. Hematoxylin-Eosin (HE) staining was used to evaluate the hepatic pathological lesions. The levels of serum alanine aminotransferase (ALT), glutamic oxalacetic transaminase (AST), alkaline phosphatase (ALP) and alpha fetoprotein (AFP) were analyzed by Elisa kits, and the protein levels of p53, Murine double minute (MDM) 2, Bax, Bcl-2, Bcl-xl, cytochrome C, Caspase-3, Caspase-9 and Poly (ADP-ribose) polymerase (PARP) were analyzed by western blot. The results showed that Celastrol could significantly decrease the mortality, the number of tumor nodules and the index of liver in the Celastrol groups compared with DEN-treated group. Moreover, Celastrol obviously improved the hepatic pathological lesions and decreased the elevated levels of ALT, AST, ALP and AFP. Meanwhile, Celastrol suppressed the expression of the protein MDM2, activated the intrinsic mitochondrial apoptosis pathway induced by p53, inhibited anti-apoptotic Bcl-2 and Bcl-xl, induced the pro-apoptotic Bax, cytochrome C, PARP and caspases. These results suggested that Celastrol had a good therapeutic action in reversing DEN-induced HCC rats, which may be associated with the apoptosis of hepatoma cells induced by Celastrol.

Cleaved CD44 intracellular domain supports activation of stemness factors and promotes tumorigenesis of breast cancer

CD44 plays a role in the progression of tumors and is expressed in cancer stem cells (CSCs). However, the mechanisms underlying the crosstalk of CD44 with stemness genes in CSC maintenance remains unclear. In this study, we demonstrated how the cleaved intracellular domain of CD44 (CD44ICD) activates stemness factors such as Nanog, Sox2 and Oct4, and contributes to the tumorigenesis of breast cancer. We have found that the overexpression of CD44ICD increased mammosphere formation in breast cancer cells. Treatment with a γ-secretase inhibitor (GSI), which blocks the cleavage of CD44ICD, interfered with mammosphere formation. Interestingly, CD44ICD decreased the expression levels and nuclear localization of stemness factors, but overexpression of CD44ICD reversed these effects. In addition, we showed that nuclear localization of CD44ICD is important for transcriptional activation of the stemness factors. Furthermore, CD44ICD-overexpressed cells exhibited strong tumorigenecity and greater metastatic potential than did the control cells or CD44-depleted cells in vivo in mice models. Taken together, it was supposed that CD44 promotes tumorigenesis through the interaction and nuclear-translocation of its intracellular domain and stemness factors. We suggest that the prevention of cleavage and nuclear-translocation of CD44ICD is a potential target in treating breast cancer.

Knockdown of Rap1b Enhances Apoptosis and Autophagy in Gastric Cancer Cells via the PI3K/Akt/mTOR Pathway

Gastric cancer (GC) is the fourth most common malignancy and the second leading cause of cancer mortality around the world. However, the regulatory mechanisms of GC tumorigenesis and cancer cell motility are completely unknown. We investigated the role of a RAS-related protein (Rap1b) in the progression of GC. Our results showed that the expression of Rap1b is aberrantly upregulated in GC tissue samples and human GC cell lines, and the high expression of Rap1b indicated a positive correlation with poor prognosis in patients with GC. Inhibition of endogenous Rap1b dramatically reduced the cell cycle progression but strongly enhanced the apoptosis capacity of human GC cell lines MKN-28 and SGC-7901 cells compared with the control group. Western blotting assay showed that Rap1b inhibition resulted in a significant increase in the ratio of LC3-II to LC3-I, and the levels of p62 protein were decreased in both MKN-28 and SGC-7901 cells. Furthermore, PI3K/Akt/mTOR activation was found to be maintained in a low level in the normal gastric mucosal epithelial cells, while it was significantly upregulated in GC cells, which could be decreased by Rap1b inhibition. The PI3K inhibitor LY294002 was enhanced but activator insulin-like growth factor 1 (IGF-1) blocked the Rap1b silencing-induced enhancement of apoptosis and autophagy in MKN-28 and SGC-7901 cells. In conclusion, we demonstrate that Rap1b expression is aberrantly increased in GC, resulting in the inhibition of autophagy and apoptosis of GC cells by the PI3K/Akt/mTOR pathway. This might provide a new understanding and represent a novel therapeutic target for human GC.

Celastrol induces cell cycle arrest by MicroRNA-21-mTOR-mediated inhibition p27 protein degradation in gastric cancer

Objective

Celastrol has anti-cancer effects by increase of apoptosis of gastric cancer cells. However, its role in gastric cancer cell cycle is still unclear. The aim of this study was to investigate the effect and mechanism of celastrol on gastric cancer cell cycle.

Methods

The effects of celastrol on cell cycle in BGC-823 and MGC-803 cells were assayed via flow cytometry analysis. The expression of p27 and mTOR was detected by real-time PCR and western blot. The activity of mTOR and mTORC2 was measured by mTOR and mTORC2 kinase assays. miR-21 mimic was used to up-regulate miR-21 expression and mTOR expression plasmid was used to increase mTOR level in gastric cancer cells.

Results

Celastrol caused G2/M cell-cycle arrest that was accompanied by the down-regulation of miR-21 expression. In particular, miR-21 overexpression reversed cell cycle arrest effects of celastrol. Further study showed that celastrol increased levels of the p27 protein by inhibiting its degradation. miR-21 and mTOR signaling pathway was involved in the increase of p27 protein expression in BGC-823 and MGC-803 cells treated with celastrol. Significantly, miR-21 overexpression restored the decrease of mTOR activity in cells exposed celastrol.

Conclusions

The effect of celastrol on cell cycle arrest of gastric cancer cells was due to an increase of p27 protein level via inhibiting miR-21-mTOR signaling pathway.

Autophagic Cell Death by Poncirus trifoliata Rafin., a Traditional Oriental Medicine, in Human Oral Cancer HSC-4 Cells

Poncirus trifoliata Rafin. has long been used as anti-inflammatory and antiallergic agent to treat gastrointestinal disorders and pulmonary diseases such as indigestion, constipation, chest fullness, chest pain, bronchitis, and sputum in Korea. P. trifoliata extract has recently been reported to possess anticancer properties; however, its mechanisms of action remain unclear. In this study, its antiproliferative effects and possible mechanisms were investigated in HSC-4 cells. The methanol extract of P. trifoliata (MEPT) significantly decreased the proliferation of HSC-4 cells (inhibitory concentration (IC)₅₀ = 142.7 μg/mL) in a dose-dependent manner. While there were no significant changes observed upon cell cycle analysis and ANNEXIN V and 7-AAD double staining in the MEPT-treated groups, the intensity of acidic vesicular organelle (AVO) staining and microtubule-associated protein 1 light chain (LC) 3-II protein expression increased in response to MEPT treatment. Furthermore, 3-methyladenine (3-MA, autophagy inhibitor) effectively blocked the MEPT-induced cytotoxicity of HSC-4 cells and triggered the activation of p38 and extracellular signal-regulated kinases (ERK) proteins. Taken together, our results indicate that MEPT is a potent autophagy agonist in oral cancer cells with antitumor therapeutic potential that acts through the mitogen-activated protein kinase (MAPK) pathway.

Celastrol induces apoptosis of human osteosarcoma cells via the mitochondrial apoptotic pathway

Celastrol is an active compound extracted from the root bark of Triptergium wilfordii Hook F., also known as 'Thunder of God Vine'. It is a well-known Chinese medicinal herb that was found to inhibit tumor cell growth and promote apoptosis in several tumor cell lines. However, research into its effects on osteosarcoma cell apoptosis is still extremely limited. The present study was undertaken to determine the effect of celastrol on viability and apoptosis of osteosarcoma cells and furthermore, to illuminate the molecular mechanism of celastrol-induced osteosarcoma cell apoptosis. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) colorimetric assay was used to evaluate the viability of the cells following treatment with celastrol. The effect of celastrol on the apoptotic rate of the cells was evaluated by flow cytometry using Annexin V-PE/7-AAD staining assay. Fluorescence microscopy was used to detect the morphological changes in the human osteosarcoma U-2OS cell lines. The expression of Bcl-2 family proteins, caspase-3, caspase-8, caspase-9, cytochrome c and PARP was measured by western blotting. We found that celastrol significantly inhibited the growth of osteosarcoma cells in a dose-dependent manner, particularly U-2OS cells. Furthermore, we observed that celastrol upregulated the expression of the pro-apoptotic proteins Bax and cytochrome c and altered the ratio of Bax/Bcl-2, and triggered the mitochondrial apoptotic pathway, resulting in caspase-3 and -9 activation and PARP cleavage. To conclude, the results indicate that celastrol inhibits the proliferation of human osteosarcoma cancer cells by inducing apoptosis via the mitochondrial-dependent pathway.

Targeting Notch signaling by γ-secretase inhibitor I enhances the cytotoxic effect of 5-FU in gastric cancer

Current medication for gastric cancer patients has a low success rate and the patients develop rapid tolerance to these drugs. Therefore, the development of new regimens is desired. In this study, we determined that Notch-signaling-related genes were overexpressed and activated in gastric cancer patients and gastric cancer cell lines. According to recent studies, γ-secretase inhibitors (GSIs), which function as Notch signaling inhibitors, could be used as therapeutic drugs in cancer. We demonstrated that GSI I (cbz-IL-CHO) is the most effective GSI in gastric cancer cells. We also determined the cell survival signaling-related proteins that were affected by GSI I. The levels of phosphorylated AKT were significantly decreased upon GSI I treatment, and constitutively activated myristoylated AKT completely blocked GSI I-induced apoptosis and cell survival, suggesting that inhibition of AKT signaling is critical for GSI I-mediated effects in gastric cancer cells. In order to maximize the effects and safety of GSI I, a combination treatment with GSI I and 5-FU was performed. Inhibition of gastric cancer cell proliferation with the combination treatment was significantly better than that with the single treatment. All phosphorylated forms of AKT, p44/42, JNK, and p38 were drastically changed by the combination treatment. Orthotopically transplanted gastric tumor burdens in mice were reduced using the combined treatment. The outcomes of this study clearly demonstrated the therapeutic potential of GSI I in gastric cancer, as well as the greater efficacy of the combined treatment of GSI I with 5-FU. Therefore, we suggest that further clinical trials examining the potential of combined GSI I and 5-FU treatment in gastric cancer patients be undertaken.

Increase in apoptotic effect of Panax ginseng by microwave processing in human prostate cancer cells: in vitro and in vivo studies

Background

Ginseng, which is widely used in functional foods and as an herbal medicine, has been reported to reduce the proliferation of prostate cancer cells by mechanisms that are not yet fully understood.

Methods

This study was designed to investigate the changes in ginsenoside content in ginseng after treatment with a microwave-irradiation thermal process and to verify the anticancer effects of the extracts. To confirm the anticancer effect of microwave-irradiated processed ginseng (MG), it was tested in three human prostate cancer cell lines (DU145, LNCaP, and PC-3 cells). Involvements of apoptosis and autophagy were assessed using Western blotting.

Results

After microwave treatment, the content of ginsenosides Rg1, Re, Rb1, Rc, Rb2, and Rd in the extracts decreased, whereas the content of ginsenosides 20(S)-Rg3, 20(R)-Rg3, Rk1, and Rg5 increased. Antiproliferation results for the human cancer cell lines treated with ginseng extracts indicate that PC-3 cells treated with MG showed the highest activity with an half maximal inhibitory concentration of 48 μg/mL. We also showed that MG suppresses the growth of human prostate cancer cell xenografts in athymic nude mice as an in vivo model. This growth suppression by MG is associated with the inductions of cell death and autophagy.

Conclusion

Therefore, heat processing by microwave irradiation is a useful method to enhance the anticancer effect of ginseng by increasing the content of ginsenosides Rg3, Rg5, and Rk1.

So-Cheong-Ryong-Tang induces apoptosis through activation of the intrinsic and extrinsic apoptosis pathways, and inhibition of the PI3K/Akt signaling pathway in non-small-cell lung cancer A549 cells

Background

So-Cheong-Ryong-Tang (SCRT), a traditional Korean medicine containing eight species of medicinal plant, has been used to treat patients with bronchial asthma and allergic rhinitis for hundreds of years; however, its anti-cancer potential is poorly understood. The present study was designed to evaluate the apoptotic effect of SCRT against human non-small-cell lung cancer (NSCLC) A549 cells.

Methods

The effects of SCRT on cell growth and viability were evaluated by trypan blue dye exclusion and 3-(4, 5-dimethyl-thiazol-2-yl)-2, 5-diphenyl tetrazoliumbromide (MTT) assays, respectively. Apoptosis was detected using 4,6-diamidino-2-phenyllindile (DAPI) staining, agarose gel electrophoresis and flow cytometry. The protein levels were determined by Western blot analysis. Caspase activity was measured using a colorimetric assay.

Results

SCRT treatment resulted in significantly decreased A549 cell growth and viability by induction of apoptosis. SCRT induced the translocation of pro-apoptotic Bax to the mitochondria, mitochondrial membrane permeabilization, cytochrome c release from mitochondria to cytosol, and activated caspase-9 and caspase-3. SCRT also increased death receptor-associated ligands and enhanced the activation of caspase-8 and cleavage of its substrate Bid. However, the pan-caspases inhibitor significantly blocked the SCRT-induced apoptosis, suggesting that it is a caspase-dependent pathway. In addition, SCRT suppressed the phosphorylation of phosphoinositide 3-kinase (PI3K) and Akt, and treatment with a potent inhibitor of PI3K further increased the apoptotic activity of SCRT.

Conclusions

These findings suggest that SCRT may play its anti-cancer actions partly through a suppression of the PI3K/Akt signal pathway in A549 cells, and further in vivo studies on the potential of SCRT for prevention and therapy of NSCLCs are warranted.

Celastrol ameliorates experimental colitis in IL-10 deficient mice via the up-regulation of autophagy

BACKGROUND

Celastrol had been proved effective in the treatment for IBD, probably with the modulation of oxidative stress, inflammatory cytokines and intestinal homeostasis. This study was aimed to investigate whether celastrol could ameliorate the inflammation of IL-10 deficient mice, a murine model of Crohn's disease (CD) with the induction of autophagy.

MATERIAL AND METHODS

The mice included were divided into four groups, ##WT group, IL-10(-/-) group, Cel group and Control group (celastrol+3-Methyladenine). Celastrol (2 mg/kg) treatment by gavage was administered to mice daily over one week. 3-Methyladenine (autophagy inhibitors) was administered at a dose of 30 mg/kg by intraperitoneal injection. The histological evaluation of the colon, tissue myeloperoxidase (MPO), and colon inflammation of mice in the four groups was evaluated and compared. Furthermore, the PI3K/Akt/mTOR pathway and the status of autophagy in intestine affected by celastrol were also assessed.

RESULTS

The one-week administration of celastrol ameliorated established colitis in IL-10 deficient mice, associated with a reduction of marked histological inflammation, a decreased colon MPO concentration and suppression of colonic proinflammatory cytokine. Furthermore, the decreased neutrophil infiltration in proximal colon and improvement of inflammation in the Cel group was much more obvious than that in the Control group. The Western blotting analysis of the PI3K/Akt/mTOR pathway and autophagy showed that celastrol treatment up-regulated the autophagy of colon tissue by suppressing the PI3K/Akt/mTOR signaling pathway.

CONCLUSIONS

Celastrol ameliorates experimental colitis in IL-10 deficient mice via the up-regulation of autophagy by suppressing the PI3K/Akt/mTOR signaling pathway.

Elaborating the role of natural products-induced autophagy in cancer treatment: achievements and artifacts in the state of the art

Autophagy is a homeostatic process that is highly conserved across different types of mammalian cells. Autophagy is able to relieve tumor cell from nutrient and oxidative stress during the rapid expansion of cancer. Excessive and sustained autophagy may lead to cell death and tumor shrinkage. It was shown in literature that many anticancer natural compounds and extracts could initiate autophagy in tumor cells. As summarized in this review, the tumor suppressive action of natural products-induced autophagy may lead to cell senescence, provoke apoptosis-independent cell death, and complement apoptotic cell death by robust or target-specific mechanisms. In some cases, natural products-induced autophagy could protect tumor cells from apoptotic death. Technical variations in detecting autophagy affect data quality, and study focus should be made on elaborating the role of autophagy in deciding cell fate. In vivo study monitoring of autophagy in cancer treatment is expected to be the future direction. The clinical-relevant action of autophagy-inducing natural products should be highlighted in future study. As natural products are an important resource in discovery of lead compound of anticancer drug, study on the role of autophagy in tumor suppressive effect of natural products continues to be necessary and emerging.

Glucose deprivation triggers protein kinase C-dependent β-catenin proteasomal degradation

Autophagy is a conserved process that contributes to cell homeostasis. It is well known that induction mainly occurs in response to nutrient starvation, such as starvation of amino acids and insulin, and its mechanisms have been extensively characterized. However, the mechanisms behind cellular glucose deprivation-induced autophagy are as of now poorly understood. In the present study, we determined a mechanism by which glucose deprivation induced the PKC-dependent proteasomal degradation of β-catenin, leading to autophagy. Glucose deprivation was shown to cause a sub-G₁ transition and enhancement of the LC3-II protein levels, whereas β-catenin protein underwent degradation in a proteasome-dependent manner. Moreover, the inhibition of GSK3β was unable to abolish the glucose deprivation-mediated β-catenin degradation or up-regulation of LC3-II protein levels, which suggested GSK3β-independent protein degradation. Intriguingly, the inhibition of PKCα using a pharmacological inhibitor and transfection of siRNA for PKCα was observed to effectively block glucose deprivation-induced β-catenin degradation as well as the increase in LC3-II levels and the accumulation of a sub-G₁ population. Together, our results demonstrated a molecular mechanism by which glucose deprivation can induce the GSK3β-independent protein degradation of β-catenin, leading to autophagy.

Up-regulation of neogenin-1 increases cell proliferation and motility in gastric cancer

Although elevated expression of neogenin-1 has been detected in human gastric cancer tissue, its role in gastric tumorigenesis remains unclear due to the lack of neogenin-1 studies in cancer. Therefore, we demonstrated here the function and regulatory mechanism of neogenin-1 in gastric cancer. Neogenin-1 ablation decreased proliferation and migration of gastric cancer cells, whereas its over-expression reversed these effects. Xenografted analyses using gastric cancer cells displayed statistically significant inhibition of tumor growth by neogenin-1 depletion. Interestingly, galectin-3 interacted with HSF-1 directly, which facilitated nuclear-localization and binding on neogenin-1 promoter to drive its transcription and gastric cancer cell motility. The galectin-3-increased gastric cancer cell motility was down-regulated by HSF-1 depletion. Moreover, the parallel expression patterns of galectin-3 and neogenin-1, as well as those of HSF-1 and neogenin-1, were detected in the malignant tissues of gastric cancer patients. Taken together, high-expression of neogenin-1 promotes gastric cancer proliferation and motility and its expression is regulated by HSF-1 and galectin-3 interaction. In addition, we propose further studies for neogenin-1 and its associated pathways to provide them as a proper target for gastric cancer therapy.

Design, synthesis and biological evaluation of C(6)-indole celastrol derivatives as potential antitumor agents

A new class of C(6)-indole substituted celastrol derivatives were designed and synthesized. Among all these synthesized molecules, compound 4f and 4h displayed excellent in vitro antiproliferative activities against Bel7402 cancer cells.

Celastrol stimulates hypoxia-inducible factor-1 activity in tumor cells by initiating the ROS/Akt/p70S6K signaling pathway and enhancing hypoxia-inducible factor-1α protein synthesis

Celastrol, a tripterine derived from the traditional Chinese medicine plant Tripterygium wilfordii Hook F. (“Thunder of God Vine”), has been reported to have multiple effects, such as anti-inflammation, suppression of tumor angiogenesis, inhibition of tumor growth, induction of apoptosis and protection of cells against human neurodegenerative diseases. However, the mechanisms that underlie these functions are not well defined. In this study, we reported for the first time that Celastrol could induce HIF-1α protein accumulation in multiple cancer cell lines in an oxygen-independent manner and that the enhanced HIF-1α protein entered the nucleus and promoted the transcription of the HIF-1 target genes VEGF and Glut-1. Celastrol did not influence HIF-1α transcription. Instead, Celastrol induced the accumulation of the HIF-1α protein by inducing ROS and activating Akt/p70S6K signaling to promote HIF-1α translation. In addition, we found that the activation of Akt by Celastrol was transient. With increased exposure time, inhibition of Hsp90 chaperone function by Celastrol led to the subsequent depletion of the Akt protein and thus to the suppression of Akt activity. Moreover, in HepG2 cells, the accumulation of HIF-1α increased the expression of BNIP3, which induced autophagy. However, HIF-1α and BNIP3 did not influence the cytotoxicity of Celastrol because the main mechanism by which Celastrol kills cancer cells is through stimulating ROS-mediated JNK activation and inducing apoptosis. Furthermore, our data showed that the dose required for Celastrol to induce HIF-1α protein accumulation and enhance HIF-1α transcriptional activation was below its cytotoxic threshold. A cytotoxic dose of Celastrol for cancer cells did not display cytotoxicity in LO2 normal human liver cells, which indicated that the novel functions of Celastrol in regulating HIF-1 signaling and inducing autophagy might be used in new applications, such as in anti-inflammation and protection of cells against human neurodegenerative diseases. Future studies regarding these applications are required.