Programmed cell death (apoptosis) is the most common mechanism of cell death in eukaryotes. The ability of cancer cells to evade and inhibit apoptosis has become a hallmark feature of cancer. This is accomplished through a family of proteins known as the inhibitor of apoptosis proteins (IAPs). X-Linked inhibitor of apoptosis protein (XIAP) is one of the best characterized IAPs. XIAP suppresses apoptosis by forming complexes with cysteine-aspartic proteases (caspase), through one of its baculovirus IAP repeat (BIR) domains. Its activity is endogenously antagonized by a second mitochondria derived activator of caspase (Smac). The anti-apoptotic behaviour of XIAP and the critical role it plays in the apoptotic program makes the Smac-XIAP interaction an important drug target. To this end, our laboratory is interested in synthesizing biologically related Smac mimetics which can induce apoptosis in a MDA-MB-231 cell line. Efforts have focused on (1) understanding BIR domain binding sites which allow for this interaction, and (2) the design and synthesis of molecules which are much more effective at inducing apoptosis compared to other well known analogues. Through the synthesis and evaluation of various divalent Smac mimetics we have been able to support the hypothesis that the likely binding site on XIAP is the BIR3 domain. As well, through the synthesis of a library of novel compounds, as described in the thesis, we have been able to assess the nature of the linker which joins the two tetrapeptide units. In our effort to understand which domains Smac binds with, various divalent analogues were synthesized containing MeAVPI-linker-IPVMeA (forward-reverse) and MeAVPI-linker-MeAVPI (forward-forward) sequence, which incorporated linkers with varying degrees of flexibility. We hypothesized that the forward-forward divalent mimetics would have decreased activity compared to the peptides synthesized in a forward-reverse fashion. Lastly, information gathered from structure activity relationship (SAR) studies have shown that substituting the lysine (P2) and isoleucine residues (P4) in the AVPI protein can create more potent inducers of apoptosis than its native AVPI sequence. As one of the most potent Smac mimetic that has been previously made known contains an alkyne bridge at P2 and a large hydrophobic moiety at P4, we hypothesized that similar Smac mimetics containing a propargyl glycine residue at P2 and a bulky hydrophobic moiety at P4 will be much more potent in inducing apoptosis.

Poly Brominated Diphenyl Ethers (PBDEs) are known flame retardants with highly persistent and lipophilic in nature. The continued usage of PBDE in various products amplifies the human burden of PBDEs. It is therefore, important to study the potential toxicological and/or biological effects of PBDE exposure in human. In this study we investigated the mode of action of PBDE induced toxicity in human liver by exposing human hepatocarcinoma cells in a time (24-72h) and dose (0-100μM) dependent manner. The highest test dose caused an inhibition in cell viability up to 50% after 72h, whereas lower doses (<50μM) showed slight increase in cell viability. Likewise, higher doses caused significant accumulation of intracellular ROS over time. Further, increase in caspase-3 enzyme levels and DNA fragmentation showed that, lower brominated PBDEs induce liver toxicity through accumulation of toxic metabolites and reactive oxygen species over time leading to caspase-mediated apoptotic cell death.

Lethal toxin (LeTx) of Bacillus anthracis kills murine macrophages in a caspase-1 and Nod-like-receptor-protein 1b (Nlrp1b)-dependent manner. Nlrp1b detects intoxication, and self-associates to form a macromolecular complex called the inflammasome, which activates the pro-caspase-1 zymogen. I heterologously reconstituted the Nlrp1b inflammasome in human fibroblasts to characterize the role of the FIIND domain of Nlrp1b in pro-caspase-1 activation. Amino-terminal truncation analysis of Nlrp1b revealed that Nlrp1b1100-1233, containing the CARD domain and amino-terminal 42 amino acids within the FIIND domain was the minimal region that self-associated and activated pro-caspase-1. Residues 1100EIKLQIK1106 within the FIIND domain were critical for self-association and pro-caspase-1 activation potential of Nlrp1b1100-1233, but not for binding to pro-caspase-1. Furthermore, residues 1100EIKLQIK1106 were critical for cell death and pro-caspase-1 activation potential of full-length Nlrp1b upon intoxication. These data suggest that after Nlrp1b senses intoxication, the FIIND domain promotes self-association of Nlrp1b, which activates pro-caspase-1 zymogen due to induced pro-caspase-1 proximity.

Lethal toxin (LeTx) of Bacillus anthracis kills murine macrophages in a caspase-1 and Nod-like-receptor-protein 1b (Nlrp1b)-dependent manner. Nlrp1b detects intoxication, and self-associates to form a macromolecular complex called the inflammasome, which activates the pro-caspase-1 zymogen. I heterologously reconstituted the Nlrp1b inflammasome in human fibroblasts to characterize the role of the FIIND domain of Nlrp1b in pro-caspase-1 activation. Amino-terminal truncation analysis of Nlrp1b revealed that Nlrp1b1100-1233, containing the CARD domain and amino-terminal 42 amino acids within the FIIND domain was the minimal region that self-associated and activated pro-caspase-1. Residues 1100EIKLQIK1106 within the FIIND domain were critical for self-association and pro-caspase-1 activation potential of Nlrp1b1100-1233, but not for binding to pro-caspase-1. Furthermore, residues 1100EIKLQIK1106 were critical for cell death and pro-caspase-1 activation potential of full-length Nlrp1b upon intoxication. These data suggest that after Nlrp1b senses intoxication, the FIIND domain promotes self-association of Nlrp1b, which activates pro-caspase-1 zymogen due to induced pro-caspase-1 proximity.

Human pathogens survive anti-pathogen host immune assault by either circumventing or evading the host immune response. Bartonella henselae, an intracellular pathogen previously shown to disrupt intrinsic apoptotic messengers to enhance its survival, exploits multiple facets of the cellular apoptotic mechanisms. Cellular pathways affected by apoptotic processes were assessed using real-time reverse-transcriptase-polymerase-chain-reaction (rRT-PCR) to measure the effect of B. henselae on cell regulator gene expression (TRADD, FADD, caspase-8 and caspase-3), caspase activity, DNA cell cycle analysis, cell regulator protein expression and overall cell viability and morphology. The presence of B. henselae suppresses overall gene expression for TRADD and FADD and it dramatically suppresses ceramide-induced TRADD and FADD gene expression. The presence of B. henselae has a noticeable effect on ceramide-induced caspase-8 and caspase-3 gene expression. Only caspase-3 enzymatic activity was ceramide-induced and likewise supressed by the presence of B. henselae, whereas caspase-6 and caspase-8 were unaffected and equivalent to controls. The presence of B. henselae inhibits ceramide-induced DNA fragmentation, maintains overall cell morphology and enhances host cell viability. Lastly, B. henselae inhibits the time-dependant ceramide-induction of TRADD protein and suppresses ubiquitous FADD protein expression. We demonstrated that B. henselae inhibits apoptotic induction in a systematic manner following exogenous apoptotic induction. B. henselae protection of microvascular endothelial cells from apoptosis induction begins at the modulation of cell surface receptor-dependent signaling. B. henselae minimizes, but does not completely abrogate, the cytotoxic effect of the apoptogenic shingolipid ceramide on human microvascular endothelial cells (CDC.EU.HMEC-1). Broadening our understanding of the sequence of cell regulator suppression events by intracellular pathogens will provide insight into disease manifestation. Further, understanding how infected cells initiate and conclude apoptosis will open new avenues into the study of disease treatment.

C-Phycocyanin (C-PC) from blue-green algae has been reported to have various pharmacological characteristics, including anti-inflammatory and anti-cancer activities. In this study, the beta-subunit of C-PC (ref to as C-PC/beta) was expressed and purified from bacteria E. coli BL-21. The recombinant C-PC/beta has been demonstrated to have anticancer properties. Under the treatment of 5 microM of the recombinant C-PC/beta, four different cancer cell lines accrued a high proliferation inhibition and apoptotic induction. The C-PC/beta interacts with membrane-associated beta-tubulin and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) has been found. Under the treatment of the C-PC/beta, depolymerization of microtubulin and actin-filament was observed. The cells underwent apoptosis with increase of Caspase-3 and Caspase-8 activities. Cell cycle was arrested at G0/G1 phase under the treatment of C-PC/beta. In addition, the nuclear level of GAPDH decreased significantly. Inhibition of cancer cell proliferation and induction of apoptosis may potentate C-PC/beta as a promising cancer prevention or therapy agent.

PIDD has previously been described as a death domain (DD)-containing protein that is inducible upon p53 activation and plays a role in programmed cell death. It has previously been shown that PIDD interacts with RAIDD (RIP-associated ICE/CED3 homologous protein with a death domain) in a cytoplasmic complex known as the PIDDosome, which results in the activation of capsase-2 and ultimately in cell death in response to DNA damage. Despite earlier studies on PIDD, however, the physiological role of PIDD has not been elucidated. Thus, we have generated PIDD-deficient mice and examined its in vivo functions particularly in cell death and in antiviral innate immunity. The first major aim of the thesis is to determine whether or not PIDD is required in cell death. PIDD mice are developmentally normal and do not display a pronounced phenotype. Surprisingly, PIDD deficiency perturbed neither DNA damage-induced nor stress-induced cell death in a variety of cell types, suggesting that PIDD may not play a critical role in cell death. In addition, caspase-2 processing occurred normally in the absence of PIDD in response to ionizing irradiation or etoposide treatment, indicating that PIDD is dispensable in the cleavage of caspase-2. The second major aim is to examine the role of PIDD and RAIDD in LCMV-induced innate immunity. To study the role of PIDD and RAIDD in antiviral immune responses, I have generated PIDD/RAIDD double-deficient mice and challenged them with lymphocytic choriomeningitis virus (LCMV). Interestingly, I observed that ablation of both PIDD and RAIDD together resulted in defective viral clearance in the spleen, but not in other organs including the lung, liver, and kidney. In addition, the production of type I IFN was also decreased in the mice deficient in both PIDD and RAIDD. However, the cytotoxicity of the T lymphocytes was largely intact in the absence of both PIDD and RAIDD. Collectively, our results suggest that PIDD is dispensable in cell death, yet PIDD and RAIDD together have a synergistic effect in LCMV-induced antiviral innate immunity. The findings presented in this thesis provide a better understanding of the physiological role of PIDD and may ultimately contribute to the novel therapeutic strategies for the proper control of viral infection.

Programmed cell death (apoptosis) is the most common mechanism of cell death in eukaryotes. The ability of cancer cells to evade and inhibit apoptosis has become a hallmark feature of cancer. This is accomplished through a family of proteins known as the inhibitor of apoptosis proteins (IAPs). X-Linked inhibitor of apoptosis protein (XIAP) is one of the best characterized IAPs. XIAP suppresses apoptosis by forming complexes with cysteine-aspartic proteases (caspase), through one of its baculovirus IAP repeat (BIR) domains. Its activity is endogenously antagonized by a second mitochondria derived activator of caspase (Smac). The anti-apoptotic behaviour of XIAP and the critical role it plays in the apoptotic program makes the Smac-XIAP interaction an important drug target. To this end, our laboratory is interested in synthesizing biologically related Smac mimetics which can induce apoptosis in a MDA-MB-231 cell line. Efforts have focused on (1) understanding BIR domain binding sites which allow for this interaction, and (2) the design and synthesis of molecules which are much more effective at inducing apoptosis compared to other well known analogues. Through the synthesis and evaluation of various divalent Smac mimetics we have been able to support the hypothesis that the likely binding site on XIAP is the BIR3 domain. As well, through the synthesis of a library of novel compounds, as described in the thesis, we have been able to assess the nature of the linker which joins the two tetrapeptide units. In our effort to understand which domains Smac binds with, various divalent analogues were synthesized containing MeAVPI-linker-IPVMeA (forward-reverse) and MeAVPI-linker-MeAVPI (forward-forward) sequence, which incorporated linkers with varying degrees of flexibility. We hypothesized that the forward-forward divalent mimetics would have decreased activity compared to the peptides synthesized in a forward-reverse fashion. Lastly, information gathered from structure activity relationship (SAR) studies have shown that substituting the lysine (P2) and isoleucine residues (P4) in the AVPI protein can create more potent inducers of apoptosis than its native AVPI sequence. As one of the most potent Smac mimetic that has been previously made known contains an alkyne bridge at P2 and a large hydrophobic moiety at P4, we hypothesized that similar Smac mimetics containing a propargyl glycine residue at P2 and a bulky hydrophobic moiety at P4 will be much more potent in inducing apoptosis.

<p>Many pro-apoptotic signals trigger mitochondrial cytochrome c release, leading to caspase activation and ultimate cellular breakdown. Cell survival pathways, including the mitogen-activated protein kinase (MAPK) cascade, promote cell viability both by impeding mitochondrial cytochrome c release and by inhibiting subsequent activation of caspases. Cytosolic cytochrome c is directly responsible for initiating formation of the caspase-activating apoptosome, which, in many cell types, plays a crucial role in the apoptotic process. Given the important role of cytochrome c in dismantling the dying cell, we wanted to investigate the process of cytochrome c-induced apoptosis with the goal of understanding how this mechanism is altered in certain malignant conditions. </p> <p> First, we examined cytochrome c-induced caspase activation in normal and tumorigenic mammary epithelial cells. Although most tumor types have developed mechanisms for evading apoptosis, we surprisingly discovered that breast cancer cells were hypersensitive to cytochrome c when compared with their normal counterpart. Specifically, breast cancer cells show increased binding of caspase-9 to the Apaf-1 caspase recruitment domain. This altered apoptosome formation is mediated by overexpression of the protein PHAPI in the malignant mammary epithelial cells. Immunoblot analysis demonstrated that protein levels of PHAPI are also elevated in human breast tumors. These results suggest a novel paradigm where breast cancer cells are refractory to cytochrome c release in response to certain stimuli, but they are quite sensitive to apoptosis downstream of the mitochondria. </p> <p> Secondly, we describe a mechanism for the inhibition of cytochrome c-induced caspase activation by MAPK signaling, identifying a novel mode of apoptotic regulation exerted through Apaf-1 phosphorylation by the 90-kDa ribosomal S6 kinase (Rsk). This Apaf-1 phosphorylation results in impaired apoptosome formation, thereby inhibiting caspase activation. The Rsk effect on Apaf-1 is antagonized by protein phosphatase 1 (PP1), which promotes Apaf-1 dephosphorylation. High endogenous levels of Rsk in PC3 prostate cancer cells leads to Apaf-1 phosphorylation and renders them relatively insensitive to cytochrome c, suggesting a role for Rsk signaling in the apoptotic resistance of certain cancers. These results identify a novel locus of apoptosomal regulation wherein MAPK signaling promotes direct Rsk-catalyzed phosphorylation of Apaf-1, resulting in decreased cellular responsiveness to cytochrome c. Collectively, this work provides insight into novel mechanisms of regulation for cytochrome c-induced apoptosis.</p> / Dissertation

<p>Apoptosis is a form of programmed cellular "suicide" which is activated in response to a variety of pro-death stimuli. Apoptotic cell death is orderly and energy-dependent, and cellular constituents are packaged into membrane-bound vesicles for consumption by phagocytes. Toxic intracellular signals are never exposed to neighboring cells or to the extracellular environment, and a host inflammatory response does not occur. Apoptosis is executed by the coordinated activation of caspase family proteins. Caspase-2 is an apical protease in this family, and promotes cell death after receipt of cues from intracellular stressor signals. Caspase-2 helps to initiate apoptosis by responding to cellular death stimuli and signaling for downstream cytochrome c release and executioner caspase activation.</p><p> Several years ago our lab determined that Xenopus laevis oocyte death is partly controlled by the activation of caspase-2. In the setting of oocyte or egg extract nutrient depletion, caspase-2 was observed to be activated upstream of mitochondrial cytochrome c. In fact, caspase-2 is suppressed in response to the nutrient status of the oocyte: nutrient-replete oocytes with healthy pentose phosphate pathway flux and abundant NADPH production are able to inhibit caspase-2 via S135 phosphorylation catalyzed by calcium/calmodulin-dependent protein kinase II. Phosphorylation of caspase-2 at S135 is critical in preventing oocyte cell death, and a caspase-2 mutant unable to be phosphorylated loses its ability to respond to suppressive NADPH signals. </p><p> In this dissertation we examine the converse mechanism of metabolically-regulated caspase-2 activation in the Xenopus egg extract. We now show that caspase-2 phosphorylated at S135 binds the interactor 14-3-3 zeta, thus preventing caspase-2 dephosphorylation. Moreover, we determined that S135 dephosphorylation is catalyzed by protein phosphatase-1, which directly binds caspase-2. Although caspase-2 dephosphorylation is responsive to metabolism, neither PP1 activity nor binding is metabolically regulated. Rather, release of 14-3-3 zeta from caspase-2 is the point of metabolic control and allows for caspase-2 dephosphorylation. Accordingly, a caspase-2 mutant unable to bind 14-3-3 zeta is highly susceptible to activation. Although this mechanism was initially established in Xenopus, we now demonstrate similar control of murine caspase-2 by phosphorylation and 14-3-3 binding in mouse eggs. </p><p> In the second part of this dissertation we examine the paradigm of caspase-2 metabolic regulation in a mammalian somatic cell context. We observed that mammalian caspase-2 is a metabolically-regulated phosphoprotein in somatic cells, and that the site of regulation is caspase-2 S164. Phosphorylation at S164 appears to inhibit mammalian caspase-2 by preventing its induced proximity oligomerization, thus also preventing procaspase-2 autocatalytic processing. We further identify some of the molecular machinery involved in S164 phosphorylation and demonstrate conservation with the validated Xenopus regulators. Interestingly, we extend the findings of caspase-2 phosphorylation to a study of ovarian cancer, and show that caspase-2 S164 phosphorylation might be involved in determining cancer cell chemosensitivity. We further provide evidence that chemosensitivity can be modulated by the cellular metabolic status in a caspase-2-dependent manner. Thus, we have identified a novel phosphorylation site on mammalian caspase-2 in somatic cells, and are working further to understand the implications of caspase-2 signaling in the context of cancer cell responsiveness to chemotherapeutic treatments.</p> / Dissertation