Current Pediatric Reviews - Volume 7, Issue 4, 2011

Apoptosis (Programmed Cell Death) in Pediatric Disorders One of the biggest challenges in modern medicine is the comprehension of programmed cell death (apoptosis) in the context of pediatric diseases. Apoptosis is a highly regulated process that is critically important for cellular self-destruction in a variety of normal and disease situations. The term apoptosis was coined by Kerr and colleagues in 1972, after the Greek word meaning leaves falling from a tree in the autumn, to describe a tightly regulated cell suicide program under physiological conditions [1]. Apoptotic cell is recognized by characteristic features [2] including chromatin condensation and nuclear fragmentation (pyknosis), plasma membrane blebbing, and cell shrinkage. The apoptotic cells break into apoptotic bodies, which are engulfed by phagocytosis without inflammatory response [3] (Fig. 1). Many of apoptotic changes are caused by the activation of a family of intracellular cysteine proteases called caspases [4]. Now, almost four decades later, we understand much about the control of this machinery but need to translate this knowledge into clinical practice and drug discovery. Apoptosis eliminates unwanted or unnecessary cells and thus modulates pathological contexts [5]. Deregulation of apoptosis may cause diseases either by insufficient or excessive programmed cell death [5-7]. The emerging advances in developmental biology, genomics & genetics, and cell immunology & biology, combined with the development of appropriate animal models, have offered tremendous support for the proposed roles for programmed cell death in pediatric disorders. Future studies using both animal models and clinical specimens hold the promise of scientific groundwork for medical interventions [8-17]. Although advances in elucidating the regulation of apoptosis have laid the foundation for a deeper understanding of the pathophysiology of many pediatric diseases, we still need to know much about how the apoptotic machinery is connected to many aspects of developmental and disease pathways in the human body [18-20]. Apoptotic cell death occurs via tightly controlled and well-established extrinsic and intrinsic cellular diverse cascades [21-23]. There are now growing lists of both upstream and downstream mediators of both extrinsic and intrinsic apoptosis. The extrinsic pathway of apoptosis is initiated by the binding of death ligands of the tumor necrosis factor (TNF) superfamily, e.g. Fas ligand, TNF alpha or TNF-related apoptosis-inducing ligand (TRAIL), to their corresponding receptors on the surface of the cell, such as CD95 (Fas) or the TNF receptor [24-28]. This recruits the intracellular death inducing signaling complex (DISC). The DISC activates the initiator caspases, particularly caspase-8, leading to activation of downstream caspases, like caspases 9 and 3, and eventually to apoptotic cell death. It is now well established that the extrinsic pathway plays important role in controlling growth of cancer cells. The intrinsic (mitochondrial) pathway of apoptosis is regulated by pro and anti-apoptotic proteins of the Bcl-2 (B-cell lymphoma-2) family proteins [29, 30]. Upon apoptosis induction, cytochrome C is released from the mitochondria into the cytosol where it promotes the assembly of a caspase-activating complex termed the apoptosome [31- 33]. The apoptosome is a multimeric protein complex containing Apaf-1, cytochrome c, and caspase-9. Upon binding to the apoptosome, caspase-9 is activated, and subsequently activates the downstream effector caspases, leading to proteolysis and apoptotic cell death. There are now various strategies that can be used to prevent or induce cell death or to develop drugs that can block pro- or antiapoptotic factors. Furthermore, the promising new pharmaceutical strategies to treat deregulated gene-directed processes may provide advances in the control of various pediatric diseases including cancer, congenital malformations, immune system diseases, metabolic disorders and other diseases of various systems. Thus, the contribution of apoptosis to the pathogenesis of various diseases and abnormalities is likely to be modulated by targeting specific factors involved in the entire process.....

Resistance to undergo apoptosis is a characteristic feature of human malignancies, including childhood leukemia. Apoptosis, or programmed cell death, is the most common intrinsic cell death programs that plays a critical role in maintaining tissue homeostasis, for example in the hematopoietic system. Accordingly, too little apoptosis can promote tumor formation and also treatment resistance, since the anti-leukemic activity of most current treatment approaches, i.e. chemo-, radio- or immunotherapy critically depends on intact apoptosis signaling pathways in leukemia cells. Thus, defective apoptosis programs confer resistance to anti-leukemic therapy. The elucidation of the signaling components mediating apoptosis in leukemia cells has provided multiple targets for therapeutic intervention. These targets can be exploited to develop novel treatment strategies better directed at selective intervention points in the apoptotic machinery. Several of these approaches have already been translated into a clinical application. Thus, the exploitation of apoptosis signaling pathways for leukemia therapy opens new perspectives for effective treatment protocols for childhood leukemia.

Nephropathic cystinosis is a lethal autosomal recessive lysosomal storage disease that destroys kidney function by ten years of age. It results from failure to transport cystine from lysosomes, leading to large accumulations of cystine within the lysosomes of almost all tissues. How the failure of function of cystinosin and the attendant lysosomal cystine accumulation effects the severe phenotype is now being investigated on several fronts, chief among which is the increase in the rate of apoptosis observed in cultured cells and renal tissue whose lysosomes are cystine-laden.

An estimated 17.6 per 100,000 new cases of childhood cancer are diagnosed each year in the United States. The major subtypes of childhood cancer include leukemia, central nervous system tumors, lymphoma, neuroblastoma, rhabdomyosarcoma, osteosarcoma, Ewing sarcoma, Wilms tumor, Germ cell tumors, and other rare tumors. Despite improvements in the diagnosis and treatment of these tumors over the last 30 years, subsets of children still have poor outcomes and many others have significant morbidity. Dysregulation of apoptotic pathways has been shown to contribute to tumor formation as well as resistance to therapy in both pediatric and adult malignancies. Survivin, the smallest member of the inhibitor of apoptosis protein (IAP) family, is highly expressed in diverse cancers and correlates with decreased patient survival. Here, we review the current literature on Survivin expression in pediatric cancer, its relationship to clinical outcome and potential therapeutic options to target this protein in pediatric cancer.

Apoptosis plays an important role in normal lung development as well as repair after lung injury and contributes to the pathophysiology of many lung diseases. Bronchopulmonary dysplasia (BPD) is a major cause of neonatal pulmonary and non-pulmonary morbidity and is characterized by an arrest in alveolar development. Currently there is no specific treatment for BPD. Mechanical ventilation, exposure to high concentrations of oxygen and inflammation are important risk factors for the development of BPD. Emerging evidence links the pathophysiology of BPD to an imbalance between anti-apoptotic and pro-apoptotic signaling pathways. Different apoptotic signaling pathways have been implicated, including Fas/FasL, caspase-dependent and -independent pathways, pro-survival Akt, transforming growth factor-β and p53. To what extent these pathways are involved in the pathogenesis of BPD is a hot topic of research. The aim of this review is to describe the timing and apoptotic events during lung development and the pathogenesis of BPD with particular focus upon apoptotic pathways activated by mechanical ventilation, hyperoxia and inflammation. An appreciation of these apoptotic pathways is essential for understanding the aetiology of and the development of treatments for pulmonary diseases such as BPD.

Cell death occurs physiologically in the mammalian brain during the period of the growth spurt. In humans, this period starts in the 3rd trimester of gestation and ends by the third year of life. Environmental factors can interact with programmed cell death mechanisms to pathologically increase the numbers of neurons undergoing self elimination (apoptosis) and potentially lead to brain injury. It has been shown that classes of drugs which block glutamate N-methyl-D-aspartate (NMDA) receptors, activate γ- aminobutyric-acid (GABAA) receptors or block voltage gated sodium channels, when administered to immature rodents during susceptible developmental periods, trigger profound apoptotic cell death in the brain. Sedative, anesthetic and anticonvulsant drugs utilize these mechanisms to exert their actions. In addition, short exposures to non-physiologic oxygen levels can also trigger apoptotic cell death in the brains of infant rodents. Pathomechanisms involved in the neurotoxic actions of sedatives, anesthetics, anticonvulsants and oxygen include decreased expression of neurotrophins, inactivation of survival signaling proteins, activation of inflammatory cytokines as well as oxidative stress. These findings raise concerns regarding treatment of pregnant women, infants and young children with sedatives, anesthetics and anticonvulsants and premature infants with oxygen. Modified approaches should be developed for patients within these vulnerable age groups.

Longitudinal bone growth is a complex and tightly regulated process. A precise balance between proliferation, differentiation and cell death in growth plate chondrocytes is a key to normal bone growth in children. Indeed, decrease in proliferation/differentiation and increase in undesired cell death in growth plate cartilage are directly associated with impaired bone growth. Proteasome inhibitors are currently undergoing phase I and II clinical trials to evaluate their efficacy in the treatment of various childhood cancers. Effects of proteasome inhibitors on bone growth in children have not yet been reported. However, recent preclinical observations suggest that proteasome inhibitors may selectively target essential cell populations in growth plate cartilage, causing significant growth failure; an effect associated with increased apoptosis and decreased proliferation of chondrocytes. The observed effects on apoptosis and proliferation in growth plate chondrocytes appear to be mediated by several proteins including p53, apoptosis inducing factor (AIF), caspases, NF-κB and β-catenine. These observations could have important implications for the use of proteasome inhibitors in the treatment of childhood diseases.

Keratins, a major component of epithelial cell intermediate filaments, provide structural support to the cell and are important for the maintenance of structural integrity. Beyond its role of structural integrity in hepatocytes, keratin 18 (K18) is a known marker of apoptosis and has been proposed as an indicator of progression in chronic liver diseases such as nonalcoholic fatty liver disease (NAFLD). NAFLD is the most common cause of chronic liver disease in children and adolescents in the United States and throughout the world and comprises a wide spectrum of diseases ranging from simple steatosis (fatty liver) to nonalcoholic steatohepatitis (NASH) and cirrhosis. While simple steatosis is typically benign in nature, NASH is a more serious condition that may progress to end-stage liver disease and liver failure. Currently, liver biopsy is considered the most reliable method of assessing the histological severity of disease and differentiating between simple steatosis and NASH. Because biopsy is invasive in nature, expensive, and subject to sampling error and/or variability in interpretation, it is not suitable as a screening test. Therefore, it is necessary to examine known mechanisms associated with the progression of liver disease, such as hepatocellular apoptosis, and identify potential biomarkers that could be used as a diagnostic tool in NASH. This review will focus on the 1) the role of keratins in health and disease, 2) K18 as a potential non invasive biomarker in liver disease, 3) overview of pediatric NAFLD and issues with diagnosis and 4) potential role for K18 in assessing pediatric nonalcoholic fatty liver disease.

The Wilms' tumor 1 (WT1) gene encodes a transcription factor that was among the first tumor suppressor genes to be identified. Dependent on the splice variant, some WT1 isoforms can function as transcriptional regulators, whereas other WT1 proteins are presumably involved in RNA processing. The mechanisms by which WT1 regulates transcription and the identification of bona fide target genes have been difficult to study, which is partially due to the complex nature of the gene and its context specific functions. While the role of WT1 as a tumor suppressor in Wilms' tumor is widely accepted, considerable evidence points to an oncogenic function in other tumors. Recent studies have provided new insights into the underlying mechanisms that lead to the development of Wilms' tumor. In addition, a conditional Wt1 knockout mouse model and RNAi-mediated screening approaches have uncovered new functions for WT1 in development and tumorigenesis.

Idiopathic neutropenia (IN) is a disorder that can lead to severe, life-threatening infections. IN in children is usually characterized by decreased neutrophil counts (<1500/ μl) and can result in reduced monocyte count and phagocyte function. Since no current therapies exist and the pathophyisiology is not fully understood, we sought to investigate immune mechanisms of IN in children. We investigated the role of circulating Fas Ligand in mediating IN by comparing IN patients to healthy control (HC) patients. Our results suggest that high levels of FasL contribute to IN pathology. Additionally, the plasma from acute IN patients was found to have higher levels of soluble FasL than chronic IN patients. When incubating HC neutrophils with IN patient plasma, higher levels of apoptosis were seen. The plasma-derived factor in inducing apoptosis was found to be preferentially specific for neutrophils. Addition of anti-sFasL antibodies to IN patient plasma resulted in a significant decrease in neutrophil apoptosis. In summary, these data suggest that sFasL in IN patient plasma may reduce neutrophil cell death and that the Fas/FasL apoptotic pathway may play a role in the pathology of idiopathic neutropenia.

Apoptosis is a tightly regulated physiologic process of programmed cell death that occurs in both normal and pathologic tissues. Numerous in vitro or in vivo studies have indicated that cardiomyocyte death through apoptosis and necrosis is a primary contributor to the progression of anthracycline-induced cardiomyopathy. There are now several pieces of evidence to suggest that activation of intrinsic and extrinsic apoptotic pathways contribute to anthracyclineinduced apoptosis in the heart. Novel strategies were developed to address a wide variety of cardiotoxic mechanisms and apoptotic pathways by which anthracycline influences cardiac structure and function. Anthracycline-induced apoptosis provides a very valid representation of cardiotoxicity in the heart, an argument which has implications for the most appropriate animal models of damaged heart plus diverse pharmacological effects. In this review we describe various aspects of the current understanding of apoptotic cell death triggered by anthracycline. Differences in the sensitivity to anthracycline-induced apoptosis between young and adult hearts are also discussed.

Dynamic equilibrium between apoptosis and mitosis facilitates growth and development of the intestinal mucosa in mammalian neonates. Groups of enterocytes observed dying together on the villi in the gut of the neonates suggest involvement of the paracrine factors in the propagation of the death signalling. The most potent death-inducing, paracrine factor for the enterocytes is the TGF-β. It is secreted from the macrophages and epithelial cells upon uptake of apoptotic bodies. Through its receptor complex (TGF-R I and II) and SMAD cascade it regulates the intracellular balance of the pro- and antiapoptotic proteins from the Bcl-2 family, sensitising enterocytes for other death signals (i. e. TNFα) and directing them towards apoptosis. Just before and after birth mitosis-to-apoptosis ratio shifts towards proliferation. First ingestion of colostrum, initiates the major remodelling of the gut mucosa. Apoptosis is enhanced, facilitating the removal of foetal-type enterocytes and closing of the gut barrier. Second major remodelling stage occurs after weaning, when the adaptation to the ingestion of solid foods takes place. Proliferation and cell death in the intestinal mucosa are under the direct control of a variety of growth factors (EGF, IGFs, TGF-βs) and tissue hormones (leptin, ghrelin, GLP, CCK, etc.) provided either by mothers colostrum and milk or produced in the neonate organism. Finally, uncontrolled apoptosis may lead to the weakening of the gut barrier, which results in the increased susceptibility to the intestinal disorders, pathogen infiltration and may lead to the development of the necrotising enterocolitis in the infants.

Congenital urinary tract obstruction is an important cause of renal disease in children. Early animal studies have shown a relationship between the timing of obstruction and the different pathological entities that collectively comprise renal maldevelopment. A multitude of molecular factors through both in-vivo and in-vitro studies have been identified that are involved in apoptosis and/or interstitial fibrosis as a result of urinary tract obstruction. These molecular factors have been found to work in parallel and also occasionally divergent pathways. Apoptosis is a prominent feature of early in-utero urinary tract obstruction and a major factor in the loss of renal mass and renal dysfunction. Furthermore, there is an increasing body of evidence to suggest that apoptosis is an inciting event in the development of progressive interstitial fibrosis which further contributes to renal demise. Clearly, a better understanding of the mechanisms of apoptosis and the subsequent development of targeted therapies directed against apoptosis may prove beneficial in the prevention of renal dysfunction associated with urinary tract obstruction.