Doxorubicin induces prostate cancer drug resistance by upregulation of ABCG4 through GSH depletion and CREB activation: Relevance of statins in chemosensitization.

Multidrug resistance mediated by ATP-binding cassette (ABC) transporters remains a major impediment to cancer chemotherapy. In the present study, we documented that doxorubicin (Dox) or cisplatin-induced prostate cancer (PCa) chemoresistance is predominantly mediated by the induction of ABCG4 in androgen-independent PCa cells. Treatment of DU-145 or PC-3 cells with Dox significantly enhanced the expression of ABCG4 that resulted in the efflux of intracellular Dox. However, incubation of cells with ABCG4 short hairpin RNA resulted in a significant accumulation of Dox and sensitized cells to Dox-induced cytotoxicity. Interestingly, simvastatin synergistically potentiated Dox-induced cytotoxicity by inhibiting ABCG4 in DU-145 and DU-145 Doxres cells. Mechanistically, ABCG4 expression was regulated redox-dependently by intracellular glutathione (GSH) levels. Treatment of cells with N-acetylcysteine or simvastatin restored Dox-induced depletion of GSH levels that in turn inhibited ABCG4 levels. In addition, a reduction in GSH levels by Dox caused a nuclear factor-κB dependent enhancement of c-Myc expression, which led to cAMP-regulatory element-binding protein (CREB) activation. Furthermore, chromatin immunoprecipitation experiments revealed that Dox-induced CREB activation transcriptionally upregulates ABCG4 expression. These results were further confirmed in an in vivo PCa xenograft mice model. Combination of simvastatin and Dox significantly regressed the tumor growth and size with no noticeable Dox-induced cardiotoxic side effects. Intriguingly, DU-145 cells with stably depleted ABCG4 levels not only significantly delayed the development of the tumor but also greatly sensitized the tumor to a low dose of Dox that resulted in complete tumor regression. Collectively, this data reinforces a novel function of ABCG4 in Dox-mediated chemoresistance, and as a potential therapeutic target in drug-induced PCa chemoresistance.

Metformin regulates mitochondrial biogenesis and senescence through AMPK mediated H3K79 methylation: Relevance in age-associated vascular dysfunction.

Endothelial senescence in conjunction with mitochondrial dysfunction orchestrates age-associated cardiovascular disorders. In this study we investigated the causal link between these two processes and studied the molecular mechanisms by which metformin acts to coordinate the delay of endothelial senescence via enhancing mitochondrial biogenesis/function. AMPK activators metformin and AICAR delayed endothelial senescence via SIRT1-mediated upregulation of DOT1L, leading to increased trimethylation of H3K79 (H3K79me3). Treatment of cells with either siAMPK or siSIRT1 repressed DOT1L-mediated enhancement of H3K79me3. Moreover, the increase in SIRT3 expression and mitochondrial biogenesis/function by AMPK activators was H3K79me-dependent as H3K79N mutant or siDOT1L abrogated these effects. This was confirmed by the enrichment of H3K79me3 in the SIRT3 promoter with AMPK activation. Intriguingly, enhanced PGC-1α expression by SIRT3 via AMPK activation was responsible for increased hTERT expression and delayed endothelial senescence. In contrast, SIRT3 knockdown caused increased oxidative stress and premature senescence, possibly by depleting hTERT expression. Furthermore, a chronic low dose administration of metformin significantly attenuated vascular aging and inhibited age-associated atherosclerotic plaque formation in ApoE-/- mice. Overall, the results of this study show a novel regulation of mitochondrial biogenesis/function, and cellular senescence by H3K79me acting through SIRT3, thus providing a molecular basis for metformin-mediated age-delaying effects.

A novel metadherinΔ7 splice variant enhances triple negative breast cancer aggressiveness by modulating mitochondrial function via NFĸB-SIRT3 axis.

Metadherin (MTDH) expression inversely correlates with prognosis of several cancers including mammary carcinomas. In this work, we identified a novel splice variant of MTDH with exon7 skipping (MTDHΔ7) and its levels were found significantly high in triple negative breast cancer (TNBC) cells and in patients diagnosed with TNBC. Selective overexpression of MTDHΔ7 in MDA-MB-231 and BT-549 cells enhanced proliferation, invasion, and epithelial-to-mesenchymal (EMT) transition markers in comparison to its wildtype counterpart. In contrast, knockdown of MTDHΔ7 induced antiproliferative/antiinvasive effects. Mechanistically, MTDH-NFĸB-p65 complex activated SIRT3 transcription by binding to its promoter that in turn enhanced MnSOD levels and promoted EMT in TNBC cells. Intriguingly, mitochondrial OCR through Complex-I and -IV, and glycolytic rate (ECAR) were significantly high in MDA-MB-231 cells stably expressing MTDHΔ7. While depletion of SIRT3 inhibited MTDH-Wt/Δ7-induced OCR and ECAR, knockdown of MnSOD inhibited only ECAR. In addition, MTDH-Wt/Δ7-mediated pro-proliferative/-invasive effects were greatly obviated with either siSIRT3 or siMnSOD in these cells. The functional relevance of MTDHΔ7 was further proved under in vivo conditions in an orthotopic mouse model of breast cancer. Mice bearing labeled MDA-MB-231 cells stably expressing MTDHΔ7 showed significantly more tumor growth and metastatic ability to various organs in comparison to MTDH-Wt bearing mice. Taken together, MTDHΔ7 promotes TNBC aggressiveness through enhanced mitochondrial biogenesis/function, which perhaps serves as a biomarker.

sp3 -Rich Glycyrrhetinic Acid Analogues Using Late-Stage Functionalization as Potential Breast Tumor Regressing Agents.

Late-stage functionalization (LSF) aids drug discovery efforts by introducing functional groups onto C-H bonds on pre-existing skeletons. We adopted the LSF strategy to synthesize analogues of the abundantly available triterpenoid, glycyrrhetinic acid (GA), by introducing aryl groups in the A-ring, expanding the A-ring and selectively activating one methyl group of the gem-dimethyl groups. Intriguingly, two compounds were found to preferentially accumulate in the mitochondrial compartment of MDA-MB-231 breast cancer cells, to cause depolarization of mitochondrial membrane potential and to induce antiproliferative and anti-invasive effects through enhanced mitochondrial superoxide production with parallel depletion of GSH levels. Furthermore, intraperitoneal administration of these two compounds, in comparison with GA, greatly regressed breast tumor growth and metastasis in a SCID mouse model bearing labeled MDA-MB-231 cells.