Defective molecular timer in the absence of nucleotides leads to inefficient caspase activation

Molecular insights on cytochrome c and nucleotide regulation of apoptosome function and its implication in cancer

Cytochrome c (Cyt c) released from mitochondria interacts with Apaf-1 to form the heptameric apoptosome, which initiates the caspase cascade to execute apoptosis. Although lysine residue at 72 (K72) of Cyt c plays an important role in the Cyt c-Apaf-1 interaction, the underlying mechanism of interaction between Cyt c and Apaf-1 is still not clearly defined. Here we identified multiple lysine residues including K72, which are also known to interact with ATP, to play a key role in Cyt c-Apaf-1 interaction. Mutation of these lysine residues abrogates the apoptosome formation causing inhibition of caspase activation. Using in-silico molecular docking, we have identified Cyt c-binding interface on Apaf-1. Although mutant Cyt c shows higher affinity for Apaf-1, the presence of Cyt c-WT restores the apoptosome activity. ATP addition modulates only mutant Cyt c binding to Apaf-1 but not WT Cyt c binding to Apaf-1. Using TCGA and cBioPortal, we identified multiple mutations in both Apaf-1 and Cyt c that are predicted to interfere with apoptosome assembly. We also demonstrate that transcript levels of various enzymes involved with dATP or ATP synthesis are increased in various cancers. Silencing of nucleotide metabolizing enzymes such as ribonucleotide reductase subunit M1 (RRM1) and ATP-producing glycolytic enzymes PKM2 attenuated ATP production and enhanced caspase activation. These findings suggest important role for lysine residues of Cyt c and nucleotides in the regulation of apoptosome-dependent apoptotic cell death as well as demonstrate how these mutations and nucleotides may have a pivotal role in human diseases such as cancer.

Lipid quantification by Raman microspectroscopy as a potential biomarker in prostate cancer

Metastatic castration-resistant prostate cancer (mCRPC) remains incurable and is one of the leading causes of cancer-related death among American men. Therefore, detection of prostate cancer (PCa) at early stages may reduce PCa-related mortality in men. We show that lipid quantification by vibrational Raman Microspectroscopy and Biomolecular Component Analysis may serve as a potential biomarker in PCa. Transcript levels of lipogenic genes including sterol regulatory element-binding protein-1 (SREBP-1) and its downstream effector fatty acid synthase (FASN), and rate-limiting enzyme acetyl CoA carboxylase (ACACA) were upregulated corresponding to both Gleason score and pathologic T stage in the PRAD TCGA cohort. Increased lipid accumulation in late-stage transgenic adenocarcinoma of mouse prostate (TRAMP) tumors compared to early-stage TRAMP and normal prostate tissues were observed. FASN along with other lipogenesis enzymes, and SREBP-1 proteins were upregulated in TRAMP tumors compared to wild-type prostatic tissues. Genetic alterations of key lipogenic genes predicted the overall patient survival using TCGA PRAD cohort. Correlation between lipid accumulation and tumor stage provides quantitative marker for PCa diagnosis. Thus, Raman spectroscopy-based lipid quantification could be a sensitive and reliable tool for PCa diagnosis and staging.

Mechanism of neem limonoids-induced cell death in cancer: Role of oxidative phosphorylation

We have previously reported that neem limonoids (neem) induce multiple cancer cell death pathways. Here we dissect the underlying mechanisms of neem-induced apoptotic cell death in cancer. We observed that neem-induced caspase activation does not require Bax/Bak channel-mediated mitochondrial outer membrane permeabilization, permeability transition pore, and mitochondrial fragmentation. Neem enhanced mitochondrial DNA and mitochondrial biomass. While oxidative phosphorylation (OXPHOS) Complex-I activity was decreased, the activities of other OXPHOS complexes including Complex-II and -IV were unaltered. Increased reactive oxygen species (ROS) levels were associated with an increase in mitochondrial biomass and apoptosis upon neem exposure. Complex-I deficiency due to the loss of Ndufa1-encoded MWFE protein inhibited neem-induced caspase activation and apoptosis, but cell death induction was enhanced. Complex II-deficiency due to the loss of succinate dehydrogenase complex subunit C (SDHC) robustly decreased caspase activation, apoptosis, and cell death. Additionally, the ablation of Complexes-I, -III, -IV, and -V together did not inhibit caspase activation. Together, we demonstrate that neem limonoids target OXPHOS system to induce cancer cell death, which does not require upregulation or activation of proapoptotic Bcl-2 family proteins.

Transformations of the macromolecular landscape at mitochondria during DNA-damage-induced apoptotic cell death

Apoptosis is a dynamic process regulated by mitochondrion critical for cellular respiration and survival. Execution of apoptosis is mediated by multiple protein signaling events at mitochondria. Initiation and progression of apoptosis require numerous apoptogenic factors that are either released from or sequestered in mitochondria, which may transform the biomolecular makeup of the organelle. In this communication, using Raman microspectroscopy, we demonstrate that transformation in biomolecular composition of mitochondrion may be used as apoptosis marker in an individual cell. For the first time, we show that significant changes occur in the concentrations of RNA, DNA, protein, and lipid constituents of mitochondria during apoptosis. The structural analysis of proteins on mitochondria demonstrated a decrease in α-helix secondary structure content, and an increase in the levels of random coils and β-sheets on mitochondria. This may represent an additional hallmark of apoptosis. Strikingly, we observed nearly identical changes in macromolecular content of mitochondria both in the presence and absence of a key proapoptotic protein, Bax (Bcl-2-associated X protein). Increased DNA level in mitochondria corresponded with higher mitochondrial DNA (mtDNA), cellular reactive oxygen species (ROS), and mitochondrial ROS production. Upregulation of polymerase-γ (POLG), mitochondrial helicase Twinkle, and mitochondrial transcription factor A (Tfam) in response to DNA damage correlated with increased mtDNA and RNA synthesis. Elevated activity of oxidative phosphorylation complexes supports functional mitochondrial respiration during apoptosis. Thus, we define previously unknown dynamic correlation of macromolecular structure of mitochondria and apoptosis progression in the presence and absence of Bax protein. These findings open up a new approach for monitoring physiological status of cells by non invasive single-cell method.

Canine parvovirus type 2a (CPV-2a)-induced apoptosis in MDCK involves both extrinsic and intrinsic pathways

The canine parvovirus type 2 (CPV-2) causes an acute disease in dogs. It has been found to induce cell cycle arrest and DNA damage leading to cellular lysis. In this paper, we evaluated the apoptotic potential of the "new CPV-2a" in MDCK cells and elucidated the mechanism of the induction of apoptosis. The exposure of MDCK cells to the virus was found to trigger apoptotic response. Apoptosis was confirmed by phosphatidylserine translocation, DNA fragmentation assays, and cell cycle analysis. Activation of caspases-3, -8, -9, and -12 and decrease in mitochondrial potential in CPV-2a-infected MDCK cells suggested that the CPV-2a-induced apoptosis is caspase dependent involving extrinsic, intrinsic, and endoplasmic reticulum pathways. Increase in p53 and Bax/Bcl2 ratio was also observed in CPV-2a-infected cells.

Nanosecond pulsed electric fields induce poly(ADP-ribose) formation and non-apoptotic cell death in HeLa S3 cells

Nanosecond pulsed electric fields (nsPEFs) have recently gained attention as effective cancer therapy owing to their potency for cell death induction. Previous studies have shown that apoptosis is a predominant mode of nsPEF-induced cell death in several cell lines, such as Jurkat cells. In this study, we analyzed molecular mechanisms for cell death induced by nsPEFs. When nsPEFs were applied to Jurkat cells, apoptosis was readily induced. Next, we used HeLa S3 cells and analyzed apoptotic events. Contrary to our expectation, nsPEF-exposed HeLa S3 cells exhibited no molecular signs of apoptosis execution. Instead, nsPEFs induced the formation of poly(ADP-ribose) (PAR), a hallmark of necrosis. PAR formation occurred concurrently with a decrease in cell viability, supporting implications of nsPEF-induced PAR formation for cell death. Necrotic PAR formation is known to be catalyzed by poly(ADP-ribose) polymerase-1 (PARP-1), and PARP-1 in apoptotic cells is inactivated by caspase-mediated proteolysis. Consistently, we observed intact and cleaved forms of PARP-1 in nsPEF-exposed and UV-irradiated cells, respectively. Taken together, nsPEFs induce two distinct modes of cell death in a cell type-specific manner, and HeLa S3 cells show PAR-associated non-apoptotic cell death in response to nsPEFs.

Bim, a proapoptotic protein, up-regulated via transcription factor E2F1-dependent mechanism, functions as a prosurvival molecule in cancer

Proapoptotic Bcl-2 homology 3-only protein Bim plays an important role in Bax/Bak-mediated cytochrome c release and apoptosis. Here, we provide evidence for a novel prosurvival function of Bim in cancer cells. Bim was constitutively overexpressed in multiple prostate and breast cancer cells as well as in primary tumor cells. Quantitative real time PCR analysis showed that Bim was transcriptionally up-regulated. We have identified eight endogenous E2F1-binding sites on the Bim promoter using in silico analysis. Luciferase assay demonstrated that Bim expression was E2F1-dependent as mutation of the E2F1-binding sites on the Bim promoter inhibited luciferase activities. In support, E2F1 silencing led to the loss of Bim expression in cancer cells. Bim primarily localized to mitochondrial and cytoskeleton-associated fractions. Bim silencing or microinjection of anti-Bim antibodies into the cell cytoplasm resulted in cell rounding, detachment, and subsequent apoptosis. We observed up-regulation of prosurvival proteins Bcl-xL and Mcl-1, which sequester Bim in cancer cells. In addition, a phosphorylated form of Bim was also elevated in cancer cells. These findings suggest that the constitutively overexpressed Bim may function as a prosurvival molecule in epithelial cancer cells, and phosphorylation and association with Bcl-xL/Mcl-1 block its proapoptotic functions.

Resveratrol depletes mitochondrial DNA and inhibition of autophagy enhances resveratrol-induced caspase activation

We recently demonstrated that resveratrol induces caspase-dependent apoptosis in multiple cancer cell types. Whether apoptosis is also regulated by other cell death mechanisms such as autophagy is not clearly defined. Here we show that inhibition of autophagy enhanced resveratrol-induced caspase activation and apoptosis. Resveratrol inhibited colony formation and cell proliferation in multiple cancer cell types. Resveratrol treatment induced accumulation of LC3-II, which is a key marker for autophagy. Pretreatment with 3-methyladenine (3-MA), an autophagy inhibitor, increased resveratrol-mediated caspase activation and cell death in breast and colon cancer cells. Inhibition of autophagy by silencing key autophagy regulators such as ATG5 and Beclin-1 enhanced resveratrol-induced caspase activation. Mechanistic analysis revealed that Beclin-1 did not interact with proapoptotic proteins Bax and Bak; however, Beclin-1 was found to interact with p53 in the cytosol and mitochondria upon resveratrol treatment. Importantly, resveratrol depleted ATPase 8 gene, and thus, reduced mitochondrial DNA (mtDNA) content, suggesting that resveratrol induces damage to mtDNA causing accumulation of dysfunctional mitochondria triggering autophagy induction. Together, our findings indicate that induction of autophagy during resveratrol-induced apoptosis is an adaptive response.

Neem oil limonoids induces p53-independent apoptosis and autophagy

Azadirachta indica, commonly known as neem, has a wide range of medicinal properties. Neem extracts and its purified products have been examined for induction of apoptosis in multiple cancer cell types; however, its underlying mechanisms remain undefined. We show that neem oil (i.e., neem), which contains majority of neem limonoids including azadirachtin, induced apoptotic and autophagic cell death. Gene silencing demonstrated that caspase cascade was initiated by the activation of caspase-9, whereas caspase-8 was also activated late during neem-induced apoptosis. Pretreatment of cancer cells with pan caspase inhibitor, z-VAD inhibited activities of both initiator caspases (e.g., caspase-8 and -9) and executioner caspase-3. Neem induced the release of cytochrome c and apoptosis-inducing factor (AIF) from mitochondria, suggesting the involvement of both caspase-dependent and AIF-mediated apoptosis. p21 deficiency caused an increase in caspase activities at lower doses of neem, whereas p53 deficiency did not modulate neem-induced caspase activation. Additionally, neem treatment resulted in the accumulation of LC3-II in cancer cells, suggesting the involvement of autophagy in neem-induced cancer cell death. Low doses of autophagy inhibitors (i.e., 3-methyladenine and LY294002) did not prevent accumulation of neem-induced LC3-II in cancer cells. Silencing of ATG5 or Beclin-1 further enhanced neem-induced cell death. Phosphoinositide 3-kinase (PI3K) or autophagy inhibitors increased neem-induced caspase-3 activation and inhibition of caspases enhanced neem-induced autophagy. Together, for the first time, we demonstrate that neem induces caspase-dependent and AIF-mediated apoptosis, and autophagy in cancer cells.

Curcumin induces Apaf-1-dependent, p21-mediated caspase activation and apoptosis

Previous studies have demonstrated that curcumin induces mitochondria-mediated apoptosis. However, understanding of the molecular mechanisms underlying curcumin-induced cell death remains limited. In this study, we demonstrate that curcumin treatment of cancer cells caused dose- and time-dependent caspase-3 activation, which is required for apoptosis as confirmed using the pan caspase inhibitor, z-VAD. Knockdown experiments and knockout cells excluded a role of caspase-8 in curcumin-induced caspase-3 activation. In contrast, Apaf-1 deficiency or silencing inhibited the activity of caspase-3, pointing to a requisite role of Apaf-1 in curcumin-induced apoptotic cell death. Curcumin treatment led to Apaf-1 upregulation both at the protein and mRNA levels. Cytochrome c release from mitochondria to the cytosol in curcumin-treated cells was associated with upregulation of proapoptotic proteins such as Bax, Bak, Bid, and Bim. Crosslinking experiments demonstrated Bax oligomerization during curcumin-induced apoptosis, suggesting that induced expression of Bax, Bid, and Bim causes Bax-channel formation on the mitochondrial membrane. The release of cytochrome c was unaltered in p53-deficient cells, whereas absence of p21 blocked cytochrome c release, caspase activation, and apoptosis. Importantly, p21-deficiency resulted in reduced expression of Apaf-1 during curcumin treatment, indicating a requirement of p21 in Apaf-1 dependent caspase activation and apoptosis. Together, our findings demonstrate that Apaf-1, Bax, and p21 as novel potential targets for curcumin or curcumin-based anticancer agents.