Browsing by Subject "Health Sciences, Pathology"

The liver is a primary target for toxicants and/or their metabolites. Selected fish species now serve as model organisms for laboratory investigations of toxic responses in the liver. One such model is the Japanese medaka (Oryzias latipes), a small freshwater teleost with a robust history of usage in liver and biliary toxicity studies. The structural components of the medaka hepatobiliary system have been well-described by recent studies in two- and three-dimensional contexts, but efforts to characterize the molecular mechanisms underlying critical medaka liver functions during normalcy remain sparse. This dearth of information makes it difficult to definitively characterize toxic responses in this model organism. A crucial transcription factor underlying proper hepatobiliary function in both mammalian and non-mammalian species is the farnesoid X receptor alpha (FXRα), a member of the nuclear receptor superfamily that plays a key role in bile acid homeostasis. This dissertation describes the function of medaka fxrα during both normalcy and toxicity.

To achieve this overall objective, in vitro techniques were first employed to study the function of medaka fxrα. Two isoforms of fxrα that differ in the AF1 domain, Fxrα1 and Fxrα2, were isolated from liver cDNA and are the result of alternative splicing of one gene locus. Fxrα2 responded significantly to C24 bile acids and the synthetic FXRα agonist GW4064. On the other hand, Fxrα1, despite having an identical ligand-binding domain to that of Fxrα2, showed no response to any agonists tested by transient transactivation assays. Furthermore, Fxrα2 interacted with nuclear receptor coactivators PGC-1α and SRC-1 in mammalian two-hybrid assays while Fxrα1 did not. These findings point to a significant importance of the AF1 domain to overall receptor structure and function.

Following in vitro functional characterization, in vivo experiments using medaka larvae were performed to determine fxrα's function during normalcy. Quantitative, real-time PCR data demonstrated that Fxrα1 is highly expressed in adult liver, while Fxrα2 is expressed predominantly in gut. Fxrα1's expression was higher than Fxrα2 in embryos and larvae at all developmental timepoints tested. In vivo exposures of medaka hatchlings to GW4064 at various doses significantly altered expression of defined FXRα targets, including: bile salt export protein (BSEP), small heterodimer partner (SHP), and cytochrome P450 7A1 (CYP7A1). Surprisingly, numerous sublethal hepatic alterations to hepatocytes and bile preductular epithelial cells (BPDECs) were observed following exposure to GW4064; alterations included: lipid accumulation, glycogen depletion, mitochondrial swelling and rupture of mitochondrial membranes, disruption of endoplasmic reticulum, and apoptosis. Significant lipid accumulation, as revealed by oil red O whole mount staining of larvae, was also noted at lower doses of GW4064. These findings were the first observations of sublethal hepatotoxicity of GW4064; to date, no studies in the mammalian literature reported alterations following its administration.

Because of studies in the mammalian literature that demonstrated alleviation of cholestatic injury induced via the classic hepatotoxicant α-naphthylisothiocyanate (ANIT) by GW4064, it was originally hypothesized that a similar finding would be observed in medaka coexposed to these compounds. However, because of GW4064's ability to induce sublethal hepatic alterations in medaka, it was anticipated that its coadministration with ANIT would result in enhanced toxicity rather than alleviation as described in rodent models. However, despite the sublethal alterations induced by 1 uM GW4064, alleviation of toxicity following exposure to 15 uM ANIT was observed. Surprisingly, reduction of GW4064's toxicity was also observed in larvae exposed to both compounds. These investigations of fxrα function are an important and essential component in furthering our understanding of hepatobiliary toxicity in small aquarium fish models of human liver disease. These collective findings have created molecular underpinnings necessary for understanding medaka hepatobiliary function during normalcy and toxicity.

Glucose metabolism remains an intensely explored topic of cancer biology since the initial discoveries of Otto Warburg nearly 80 years ago. Many solid tumors metabolize glucose primarily to lactate despite the availability of oxygen, revealing a dependence on glycolysis that may serve as a basis for targeted therapy. In particular, a diet devoid of carbohydrate may minimize the growth capabilities of glucose-dependent cancers. As our interests lie in prostate cancer, we examined whether a ketogenic diet devoid of carbohydrate (NCKD) would reduce the growth rate of tumors derived from human prostate cancer cell lines in a murine xenograft model.

Our initial experiments utilized the LAPC-4 cell line, a human androgen-sensitive prostate cancer cell line, in a SCID-mouse xenograft model to determine the effects of an NCKD on tumor growth and animal survival relative to two other diets: (1.) a Western-type diet (WD) reflecting consumptions patterns of men diagnosed with prostate-cancer in the Western world and (2.) a low-fat diet (LFD) representing the present standard of care. Following this study, we conducted a second study utilizing a different human prostate cancer cell line (LNCaP) in order to assess whether our initial observations were robust across multiple prostate cancer tumor models and to also further explore the molecular underpinnings of our observations. Both studies revealed the NCKD leads to a reduction in tumor growth rate and greater overall mouse survival relative to the WD. In addition, the NCKD was equivalent in these parameters to the LFD. We also observed key associations between survival and extent of urinary ketosis as well as favorable changes in insulin and insulin-like growth factor-1 (IGF-1) and gene expression that would be predictive of prolonged survival in mice consuming the NCKD.

We believe these data provide compelling evidence to consider a potential therapeutic role for dietary carbohydrate restriction in prostate cancer. We hope these results ultimately serve as a basis to conduct future clinical trials assessing whether dietary carbohydrate restriction, either alone or in combination with more conventional therapies, provides clinicians with an additional weapon against prostate cancer.

Thymus atrophy is highly inducible by stress and prolonged thymus atrophy can contribute to T cell deficiency or inhibit immune recovery after acute peripheral T cell depletion. Little is known regarding the mechanisms driving thymic involution or thymic reconstitution after acute stress. Leptin deficiency in mice results in chronic thymic atrophy, suppressed cell-mediated immunity, and decreased numbers of total lymphocytes, suggesting a role for leptin in regulating thymopoiesis and overall immune homeostasis. Exogenous leptin administration during stress has been shown to protect against thymic damage, yet the mechanisms governing these thymostimulatory effects are currently undefined. Studies herein define the impact of endotoxin-induced thymic damage in the stromal and lymphoid compartment of the thymus and systemic glucocorticoid and cytokine responses in the animal. We report here the novel finding that leptin receptor expression is restricted to medullary thymic epithelial cells in the normal thymus. Using a model of endotoxin-induced acute thymic involution and recovery, we have demonstrated a role for the metabolic hormone leptin in protection of medullary thymic epithelial cells from acute endotoxin-induced damage. We also demonstrated that systemic leptin treatment decreased endotoxin-induced apoptosis of double positive thymocytes and promoted proliferation of double negative thymocytes in vivo through a leptin receptor isoform b-specific mechanism. Leptin treatment increased thymic expression of IL-7, an important soluble thymocyte growth factor produced by medullary thymic epithelial cells. We also found leptin to inhibit systemic glucocorticoid and pro-inflammatory cytokine responses. Using leptin-deficient and leptin receptor-deficient mice in our stress model, we found that endotoxin-induced thymic atrophy was exacerbated in the absence of leptin, despite an inability to mount a proper pro-inflammatory cytokine response. Together, these data support a model in which leptin can function to protect the thymus gland from stress-induced acute damage in part by reduction of systemic corticosteroid and pro-inflammatory cytokine responses, and intrathymically through a mechanism orchestrated by medullary thymic epithelial cells and their soluble mediators (e.g. IL-7). Taken together, these studies suggest a physiological role for leptin signaling in the thymus for maintaining healthy thymic epithelium and promoting thymopoiesis, which is revealed when thymus homeostasis is perturbed by stress.

The receptor-recognized form of α₂-macroglobulin (α₂M*) targets antigens (Ag) to professional Ag-presenting cells (APCs) for rapid internalization, processing, and presentation. When employed as an Ag delivery vehicle, α₂M* amplifies major histocompatibility complex (MHC) class II presentation as demonstrated by increased antibody (Ab) titers. Recent evidence, however, suggests that α₂M*-encapsulation may also enhance Ag-specific cytotoxic T lymphocyte (CTL) immunity. In these studies, we demonstrate that α₂M*-delivered Ag (ovalbumin, OVA) enhances the production of specific in vitro and in vivo CTL responses.

Murine splenocytes expressing a transgenic T cell receptor (TCR) specific for CTL peptide OVA_257-264 (SIINFEKL) demonstrated up to 25-fold greater IFN-γ and IL-2 secretion when treated in vitro with α₂M*-OVA compared to soluble OVA. The frequency of IFN-γ -producing cells was increased ~15-fold as measured by ELISPOT. Expansion of the OVA-specific CD8+ T cells, as assayed by tetramer binding and [3H]thymidine incorporation, and cell-mediated cytotoxicity, as determined by a flow cytometric assay, were also significantly enhanced by α₂M*-OVA. Furthermore, CTL responses were observed at Ag doses tenfold lower than those required with OVA alone.

We also observed enhanced humoral and CTL responses by naïve mice following intradermal immunization with α₂M*-OVA. These α₂M*-OVA-immunized mice displayed increased protection against a subcutaneously implanted OVA-expressing tumor, as demonstrated by delayed tumor growth and prolonged animal survival. The anti-tumor response observed with α₂M*-mediated Ag delivery was comparable to that of an accepted vaccine adjuvant (CpG 1826) and appeared superior to a cell-based vaccine technique.

To further understand the mechanism underlying this enhanced CTL immunity, the subsets of professional APCs capable of cross-presenting α₂M*-encapsulated Ag were investigated. Although both dendritic cells (DCs) and macrophages appear to stimulate some degree of cross-priming in response to α₂M*-encapsulated Ag, CD8+CD4- and CD8-CD4+ DCs appear to do so with the greatest efficiency. The implications of this finding to the ongoing debate regarding the relative contributions of APC subsets to Ag cross-presentation and the determinants of which cells cross-present with high efficiency are discussed.

These observations demonstrate that α₂M*-mediated Ag delivery promotes cross-presentation resulting in enhanced Ag-specific CTL immunity. Considered in the context of previous work, these results support α₂M* as an effective Ag delivery system that may be particularly useful for vaccines based on weakly immunogenic subunits or requiring dose sparing.

In addition to being the natural genetic information carrier, DNA can also serve as a versatile material for construction of nanoscale objects. By using the base-pairing properties of DNA, we have been able to mass-produce nano-scale structures in a variety of different shapes, upon which patterns of other molecules can be further specified. The diversity of molecules and materials that can be attached to DNA and the capability of providing precise spatial positioning considerably enhance the attractiveness of DNA for nano-scale construction. A further challenge remains to use these DNA based structures for biomedical applications.

As proof-of-concept, a DNA-based nanodevice for multivalent thrombolytic delivery is designed, which intends to employ DNA nanostructures as carriers for the delivery of tissue plasminogen activator (tPA) and plasminogen. Universal modular adapter molecules that can simultaneously bind "down" to the DNA structures and "up" to these thrombolytic drugs are further proposed. We begin with exploring the molecular recognition properties provided by biotin-avidin and aptamer-ligand pairs, and are able to achieve site-specific display of certain protein targets along the DNA nanostructure scaffold. Yet for both of these approaches, only biotinylated or specially selected proteins can be patterned. We further propose to develop single-chain diabodies (scDb) as the adapter molecules. This scDb approach is highly modular and can be extended to assemble virtually any proteins and therapeutic molecules of interests, which at the same time will greatly enhance our molecular toolbox for nanoscale construction.

Sepsis is a major cause of morbidity and mortality in the United States, and Staphylococcus aureus (S. aureus) is the bacteria most commonly cultured from septic patients. In severe sepsis, the relationship between the systemic inflammatory response and the resulting mitochondrial and metabolic dysfunction is not fully understood, especially with respect to the mechanisms of mitochondrial damage resolution. The process of mitochondrial biogenesis, which leads to the restoration of metabolic and anti-oxidative functions in damaged or stressed cells and tissues, is pro-survival and is a critical protective response in sepsis. Mitochondrial biogenesis requires the coordinated expression of multiple regulatory proteins, including the PPARgamma-coactivator (PGC) family of proteins. Previous work in sepsis has focused on mitochondrial biogenesis in response to late signals of mitochondrial damage; however, for acute sepsis, we have hypothesized a direct and early link between the innate immune response and the transcriptional activation of mitochondrial biogenesis. Since the Toll-like receptors (TLRs) are a major part of the innate immune response, we hypothesized that they could activate mitochondrial biogenesis in bacterial sepsis. Earlier work showed that TLR4 (which responds to components of Gram-negative bacteria) was necessary for mitochondrial biogenesis induction in response to heat-killed E. coli challenge. For this work, the objective was to investigate whether signaling by TLR2 (which responds to components of Gram-positive bacteria) would activate mitochondrial biogenesis in response to S. aureus sepsis in mice. The sepsis model was initially characterized in wild-type (WT) mice by PCR analysis of hepatic RNA, in which the up-regulation of several regulatory proteins for mitochondrial biogenesis, including all three PGC family members, was observed. In contrast, in both TLR2-/- and TLR4-/- mice, the mitochondrial biogenesis response was deficient and delayed. In addition, PGC-1alpha and PGC-1beta were differentially regulated in WT, TLR2-/-, and TLR4-/- mice. To identify the mechanisms involved in this induction pattern, the known TLR signaling pathways were systematically probed for activation using several strains of genetic knockout mice. These data demonstrated that the differential regulation of the PGC family is independent of the MyD88 adapter protein and is caused in part by IRF7 signaling. IRF7 is a pro-inflammatory transcription factor that is normally involved in the interferon response; in this case, IRF7 was found to be necessary but not sufficient for PGC-1alpha/beta induction. In addition, a second level of regulation was identified in the microRNA mmu-mir-202-3p, which is inversely correlated with the expression of PGC-1alpha and PGC-1beta mRNA in WT, TLR2-/-, and TLR4-/- mice and was shown to functionally decrease PGC-1alpha mRNA. If these observations are confirmed in humans, IRF7 and mir-202-3p may be potential therapeutic targets for the up-regulation of PGC-1alpha/beta levels in the clinical setting of sepsis and impaired mitochondrial biogenesis.