Antagonist effect of triptolide on AKT activation by truncated retinoid X receptor-alpha

Mechanisms of cancer cell death induction by triptolide: A comprehensive overview

The need for naturally occurring constituents is driven by the rise in the cancer prevalence and the unpleasant side effects associated with chemotherapeutics. Triptolide, the primary active component of "Tripterygium Wilfordii", has exploited for biological mechanisms and therapeutic potential against various tumors. Based on the recent pre-clinical investigations, triptolide is linked to the induction of death of cancerous cells by triggering cellular apoptosis via inhibiting heat shock protein expression (HSP70), and cyclin dependent kinase (CDKs) by up regulating expression of P21. MKP1, histone methyl transferases and RNA polymerases have all recently identified as potential targets of triptolide in cells. Autophagy, AKT signaling pathway and various pathways involving targeted proteins such as A-disintegrin & metalloprotease-10 (ADAM10), Polycystin-2 (PC-2), dCTP pyro-phosphatase 1 (DCTP1), peroxiredoxin-I (Prx-I), TAK1 binding protein (TAB1), kinase subunit (DNA-PKcs) and the xeroderma-pigmentosum B (XPB or ERCC3) have been exploited. Besides that, triptolide is responsible for enhancing the effectiveness of various chemotherapeutics. In addition, several triptolide moieties, including minnelide and LLDT8, have progressed in investigations on humans for the treatment of cancer. Targeted strategies, such as triptolide conjugation with ligands or triptolide loaded nano-carriers, are efficient techniques to confront toxicities associated with triptolide. We expect and anticipate that advances in near future, regarding combination therapies of triptolide, might be beneficial against cancerous cells.

The multi-faceted role of retinoid X receptor in cardiovascular diseases

Retinoid X receptors (RXRs) are members of ligand-dependent transcription factors whose effects on a diversity of cellular processes, including cellular proliferation, the immune response, and lipid and glucose metabolism. Knock out of RXRα causes a hypoplasia of the myocardium which is lethal during fetal life. In addition, the heart maintains a well-orchestrated balances in utilizing fatty acids (FAs) and other substrates to meet the high energy requirements. As the master transcriptional regulators of lipid metabolism, RXRs become particularly important for the energy needs of the heart. Accumulating evidence suggested that RXRs may exert direct beneficial effects in the heart both through heterodimerization with other nuclear receptors (NRs) and homodimerization, thus standing as suitable targets for treating in cardiovascular diseases. Although compounds that target RXRs are promising drugs, their use is limited by toxicity. A better understanding of the structural biology of RXRs in cardiovascular disease should enable the rational design of more selective nuclear receptor modulators to overcome these problems. Here, this review summarizes a brief overview of RXRs structure and versatility of RXR action in the control of cardiovascular diseases. And we also discussed the therapeutic potential of RXR ligand.

The Study of Cellular Mechanism of Triptolide in the Treatment of Cancer, Bone Loss and Cardiovascular Disease and Triptolide's Toxicity

Triptolide (TPL), the active component of Tripterygium wilfordii Hook F (Twhf) has been used to treat cancer and bone loss conditions for over two hundred years in traditional Chinese medicine (TCM). In this paper, we reviewed the specific molecular mechanisms in the treatment of cancer, bone loss and cardiovascular disease. In addition, we analyze the toxicity of TPL and collect some optimized derivatives extracted from TPL. Although positive results were obtained in most cell culture and animal studies, further studies are needed to substantiate the beneficial effects of TPL.

Retinoid X Receptor Antagonists

Retinoid X receptor (RXR) antagonists are not only useful as chemical tools for biological research, but are also candidate drugs for the treatment of various diseases, including diabetes and allergies, although no RXR antagonist has yet been approved for clinical use. In this review, we present a brief overview of RXR structure, function, and target genes, and describe currently available RXR antagonists, their structural classification, and their evaluation, focusing on the latest research.

Overview of the structure-based non-genomic effects of the nuclear receptor RXRα

The nuclear receptor RXRα (retinoid X receptor-α) is a transcription factor that regulates the expression of multiple genes. Its non-genomic function is largely related to its structure, polymeric forms and modification. Previous research revealed that some non-genomic activity of RXRα occurs via formation of heterodimers with Nur77. RXRα-Nur77 heterodimers translocate from the nucleus to the mitochondria in response to certain apoptotic stimuli and this activity correlates with cell apoptosis. More recent studies revealed a significant role for truncated RXRα (tRXRα), which interacts with the p85α subunit of the PI3K/AKT signaling pathway, leading to enhanced activation of AKT and promoting cell growth in vitro and in animals. We recently reported on a series of NSAID sulindac analogs that can bind to tRXRα through a unique binding mechanism. We also identified one analog, K-80003, which can inhibit cancer cell growth by inducing tRXRα to form a tetramer, thus disrupting p85α-tRXRα interaction. This review analyzes the non-genomic effects of RXRα in normal and tumor cells, and discusses the functional differences based on RXRα protein structure (structure source: the RCSB Protein Data Bank).

Targeting to the non-genomic activity of retinoic acid receptor-gamma by acacetin in hepatocellular carcinoma

We recently demonstrated that retinoic acid receptor-γ (RARγ) is overexpressed and acts as a tumor promoter in hepatocellular carcinoma (HCC). The oncogenic activity of RARγ is mainly attributed to its physiological interaction with p85α regulatory subunit of PI3K leading to constitutive activation of AKT. Here we report RARγ as a negative regulator of p53 signaling and thus extend the oncogenic potential of RARγ to a new role in controlling the balance between AKT and p53. A natural flavonoid acacetin is then identified to be capable of modulating RARγ-dependent AKT-p53 network. It specifically binds to RARγ and inhibits all-trans retinoic acid (atRA) stimulation of RARγ transactivation. However, the anticancer action of acacetin is independent on its modulation of RARγ-driven transcriptional activity. Acacetin induces cancer cell apoptosis through antagonizing the non-genomic effect of RARγ on AKT and p53. When bound to RARγ, acacetin prevents RARγ from its activation of AKT followed by recovery of the normal p53 signaling. Given the implication of AKT-p53 dysregulation in most HCC, targeting the non-genomic signaling of RARγ that switches AKT-p53 from a pro-survival to a pro-apoptotic program in cancer cells should be a promising strategy for developing novel anti-HCC drugs.

Phosphorylation of Human Retinoid X Receptor α at Serine 260 Impairs Its Subcellular Localization, Receptor Interaction, Nuclear Mobility, and 1α,25-Dihydroxyvitamin D3-dependent DNA Binding in Ras-transformed Keratinocytes

Human retinoid X receptor α (hRXRα) plays a critical role in DNA binding and transcriptional activity through heterodimeric association with several members of the nuclear receptor superfamily, including the human vitamin D receptor (hVDR). We previously showed that hRXRα phosphorylation at serine 260 through the Ras-Raf-MAPK ERK1/2 activation is responsible for resistance to the growth inhibitory effects of 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3), the biologically active metabolite of vitamin D3 To further investigate the mechanism of this resistance, we studied intranuclear dynamics of hVDR and hRXRα-tagged constructs in living cells together with endogenous and tagged protein in fixed cells. We find that hVDR-, hRXRα-, and hVDR-hRXRα complex accumulate in the nucleus in 1α,25(OH)2D3-treated HPK1A cells but to a lesser extent in HPK1ARas-treated cells. Also, by using fluorescence resonance energy transfer (FRET), we demonstrate increased interaction of the hVDR-hRXRα complex in 1α,25(OH)2D3-treated HPK1A but not HPK1ARas cells. In HPK1ARas cells, 1α,25(OH)2D3-induced nuclear localization and interaction of hRXRα are restored when cells are treated with the MEK1/2 inhibitor UO126 or following transfection of the non-phosphorylatable hRXRα Ala-260 mutant. Finally, we demonstrate using fluorescence loss in photobleaching and quantitative co-localization with chromatin that RXR immobilization and co-localization with chromatin are significantly increased in 1α,25(OH)2D3-treated HPK1ARas cells transfected with the non-phosphorylatable hRXRα Ala-260 mutant. This suggests that hRXRα phosphorylation significantly disrupts its nuclear localization, interaction with VDR, intra-nuclear trafficking, and binding to chromatin of the hVDR-hRXR complex.

TRC4, an improved triptolide derivative, specifically targets to truncated form of retinoid X receptor-alpha in cancer cells

The nuclear retinoid X receptor-α (RXRα) plays critical roles in cell homeostasis and in many physiological processes mainly through its transcriptional function. However, an N-terminal truncated form of RXRα, tRXRα, was frequently described in various cancer cells and tumor tissues, thus representing a new promising drug target. We recently demonstrated that triptolide (TR01) could target to the oncogenic activity of tRXRα. To improve its tumor selectivity, we developed several TR01 derivatives by introducing different amine ester groups on C-14-hydroxyl site. Interestingly, C-14 modification could differently affect the expression of tRXRα without interfering the level of its full length RXRα. Among the derivatives, TRC4 could strongly reduce tRXRα expression, while TRC5-7 increased it. The capability of inhibiting tRXRα expression was shown to be closely associated with its inactivation of AKT and induction of apoptosis in various cancer cells. Conversely, treatment of cancer cells with the tRXRα-stabilizing compounds TRC5-7 resulted in enhanced AKT activity and apoptosis-resistance. However, although TR01 could strongly reduce tRXRα expression and AKT activity, it also strongly inhibited the expression and transcriptional activity of RXRα in normal cells. Importantly, the tRXRα-selective TRC4 that did not significantly inhibit RXRα transcriptional function retained the most potency of the anticancer effect of TR01 and had no significant effect on the viability of normal cells. In conclusion, our results demonstrated that tRXRα-selective TRC4 will have potential clinical application in terms of drug target and side effects. Our findings will offer new strategies to develop improved triptolide analogs for cancer therapy.

Triptolide improves systolic function and myocardial energy metabolism of diabetic cardiomyopathy in streptozotocin-induced diabetic rats

BACKGROUND

Triptolide treatment leads to an improvement in Diabetic Cardiomyopathy (DCM) in streptozotocin-induced diabetic rat model. DCM is characterized by abnormal cardiac energy metabolism. We hypothesized that triptolide ameliorated cardiac metabolic abnormalities in DCM. We proposed (31)P nuclear magnetic resonance ((31)P NMR) spectrometry method for assessing cardiac energy metabolism in vivo and evaluating the effect of triptolide treatment in DCM rats.

METHODS

Six weeks triptolide treatment was conducted on streptozotocin-induced diabetic rats with dose of 100, 200 or 400 μg/kg/day respectively. Sex- and age-matched non-diabetic rats were used as control group. Cardiac chamber dimension and function were determined with echocardiography. Whole heart preparations were perfused with Krebs-Henseleit buffer and (31)P NMR spectroscopy was performed. Cardiac p38 Mitogen Activating Protein Kinase (MAPK) was measured using real time PCR and western blot analysis.

RESULTS

In diabetic rats, cardiac mass index was significantly higher, where as cardiac EF was lower than control group. (31)P NMR spectroscopy showed that ATP and pCr concentrations in diabetic groups were also remarkably lower than control group. Compared to non-treated diabetic rats, triptolide-treated diabetic groups showed remarkable lower cardiac mass index and higher EF, ATP, pCr concentrations, and P38 MAPK expressions. Best improvement was seen in group treated with Triptolide with dose 200 μg/kg/day.

CONCLUSIONS

(31)P NMR spectroscopy enables assessment of cardiac energy metabolism in whole heart preparations. It detects energy metabolic abnormalities in DCM hearts. Triptolide therapy improves cardiac function and increases cardiac energy metabolism at least partly through upregulation of MAPK signaling transduction.

Triptolide: progress on research in pharmacodynamics and toxicology

ETHNOPHARMACOLOGICAL RELEVANCE

Tripterygium wilfordii Hook. f. (Tripterygium wilfordii), also known as Huangteng and gelsemium elegan, is a traditional Chinese medicine that has been marketed in China as Tripterygium wilfordii glycoside tablets. Triptolide (TP), an active component in Tripterygium wilfordii extracts, has been used to treat various diseases, including lupus, cancer, rheumatoid arthritis and nephritic syndrome. This review summarizes recent developments in the research on the pharmacodynamics, pharmacokinetics, pharmacy and toxicology of TP, with a focus on its novel mechanism of reducing toxicity. This review provides insight for future studies on traditional Chinese medicine, a field that is both historically and currently important.

MATERIALS AND METHODS

We included studies published primarily within the last five years that were available in online academic databases (e.g., PubMed, Google Scholar, CNKI, SciFinder and Web of Science).

RESULTS

TP has a long history of use in China because it displays multiple pharmacological activities, including anti-rheumatism, anti-inflammatory, anti-tumor and neuroprotective properties. It has been widely used for the treatment of various diseases, such as rheumatoid arthritis, nephritic syndrome, lupus, Behcet׳s disease and central nervous system diseases. Recently, numerous breakthroughs have been made in our understanding of the pharmacological efficacy of TP. Although TP has been marketed as a traditional Chinese medicine, its multi-organ toxicity prevents it from being widely used in clinical practice.

CONCLUSIONS

Triptolide, a biologically active natural product extracted from the root of Tripterygium wilfordii, has shown promising pharmacological effects, particularly as an anti-tumor agent. Currently, in anti-cancer research, more effort should be devoted to investigating effective anti-tumor targets and confirming the anti-tumor spectrum and clinical indications of novel anti-tumor pro-drugs. To apply TP appropriately, with high efficacy and low toxicity, the safety and non-toxic dose range for specific target organs and diseases should be determined, the altered pathways and mechanisms of exposure need to be clarified, and an early warning system for toxicity needs to be established. With further in-depth study of the efficacy and toxicity of TP, we believe that TP will become a promising multi-use drug with improved clinical efficacy and safety in the future.

Triptolide induces growth inhibition and apoptosis of human laryngocarcinoma cells by enhancing p53 activities and suppressing E6-mediated p53 degradation

Triptolide, an active compound extracted from Chinese herb Leigongteng (Tripterygium wilfordii Hook F.), shows a broad-spectrum of anticancer activity through its cytotoxicity. However, the efficacy of triptolide on laryngocarcinoma rarely been evaluated, and the mechanism by which triptolide-induced cellular apoptosis is still not well understood. In this study, we found that triptolide significantly inhibited the laryngocarcinoma HEp-2 cells proliferation, migration and survivability. Triptolide induces HEp-2 cell cycle arrest at the G1 phase and apoptosis through intrinsic and extrinsic pathways since both caspase-8 and -9 are activated. Moreover, triptolide enhances p53 expression by increasing its stability via down-regulation of E6 and E6AP. Increased p53 transactivates down-stream target genes to initiate apoptosis. In addition, we found that short time treatment with triptolide induced DNA damage, which was consistent with the increase in p53. Furthermore, the cytotoxicity of triptolide is decreased by p53 knockdown or use of caspases inhibitor. In conclusion, our results demonstrated that triptolide inhibits cell proliferation and induces apoptosis in laryngocarcinoma cells by enhancing p53 expression and activating p53 functions through induction of DNA damage and suppression of E6 mediated p53 degradation. These studies indicate that triptolide is a potential anti-laryngocarcinoma drug.

PEP-1-CAT protects hypoxia/reoxygenation-induced cardiomyocyte apoptosis through multiple sigaling pathways

Background

Catalase (CAT) breaks down H₂O₂ into H₂O and O₂ to protects cells from oxidative damage. However, its translational potential is limited because exogenous CAT cannot enter living cells automatically. This study is aimed to investigate if PEP-1-CAT fusion protein can effectively protect cardiomyocytes from oxidative stress due to hypoxia/reoxygenation (H/R)-induced injury.

Methods

H9c2 cardomyocytes were pretreated with catalase (CAT) or PEP-1-CAT fusion protein followed by culturing in a hypoxia and re-oxygenation condition. Cell apoptosis were measured by Annexin V and PI double staining and Flow cytometry. Intracellular superoxide anion level was determined, and mitochondrial membrane potential was measured. Expression of apoptosis-related proteins including Bcl-2, Bax, Caspase-3, PARP, p38 and phospho-p38 was analyzed by western blotting.

Results

PEP-1-CAT protected H9c2 from H/R-induced morphological alteration and reduced the release of lactate dehydrogenase (LDH) and malondialdehyde content. Superoxide anion production was also decreased. In addition, PEP-1-CAT inhibited H9c2 apoptosis and blocked the expression of apoptosis stimulator Bax while increased the expression of Bcl-2, leading to an increased mitochondrial membrane potential. Mechanistically, PEP-1-CAT inhibited p38 MAPK while activating PI3K/Akt and Erk1/2 signaling pathways, resulting in blockade of Bcl2/Bax/mitochondrial apoptotic pathway.

Conclusion

Our study has revealed a novel mechanism by which PEP-1-CAT protects cardiomyocyte from H/R-induced injury. PEP-1-CAT blocks Bcl2/Bax/mitochondrial apoptotic pathway by inhibiting p38 MAPK while activating PI3K/Akt and Erk1/2 signaling pathways.

Regulation of proteolytic cleavage of retinoid X receptor-α by GSK-3β

We recently reported that an N-terminally truncated retinoid X receptor-α (tRXRα) produced in cancer cells acts to promote cancer cell growth and survival through AKT activation. However, how RXRα is cleaved and how the cleavage is regulated in cancer cells remain undefined. In this study, we demonstrated that calpain II could cleave RXRα protein in vitro, generating two truncated RXRα products. The cleavage sites in RXRα were mapped by Edman N-terminal sequencing to Gly(90)↓Ser(91) and Lys(118)↓Val(119). Transfection of the resulting cleavage product RXRα/90, but not RXRα/118, resulted in activation of AKT in cancer cells, similar to the effect of tRXRα. In support of the role of calpain II in cancer cells, transfection of calpain II expression vector or its activation by ionomycin enhanced the production of tRXRα, whereas treatment of cells with calpain inhibitors reduced the levels of tRXRα. Co-immunoprecipitation assays also showed an interaction between calpain II and RXRα. In studying the regulation of tRXRα production, we observed that treatment of cells with lithium chloride or knockdown of glycogen synthase kinase-3β (GSK-3β) significantly increased the production of tRXRα. Conversely, overexpression of GSK-3β reduced tRXRα expression. Furthermore, we found that the inhibitory effect of GSK-3β on tRXRα production was due to its suppression of calpain II expression. Taken together, our data demonstrate that GSK-3β plays an important role in regulating tRXRα production by calpain II in cancer cells, providing new insights into the development of new strategies and agents for the prevention and treatment of tRXRα-related cancers.

Synthesis and SAR study of modulators inhibiting tRXRα-dependent AKT activation

RXRα represents an intriguing and unique target for pharmacologic interventions. We recently showed that Sulindac and a designed analog could bind to RXRα and modulate its biological activity, including inhibition of the interaction of an N-terminally truncated RXRα (tRXRα) with the p85α regulatory subunit of phosphatidylinositol-3-OH kinase (PI3K). Here we report the synthesis, testing and SAR of a series of novel analogs of Sulindac as potential modulators for inhibiting tRXRα-dependent AKT activation. A new compound 30 was identified to have improved biological activity.