‘Heated’ Debates in Apoptosis

Hippocrates’ assertion that ‘what the lance does not heal, fire will’ underscores the fact that for thousands of years heat has been used to treat a variety of diseases, including cancer. Indeed, spontaneous tumor remission has been observed in patients following feverish infection [1], and expression of activated oncogenes, such as Ras, can render tumor cells sensitive to heat compared with normal cells [2, 3]. In the past, a primary drawback to the use of heat as a clinical therapy was the inability to selectively focus heat to tumors in situ. Of late, however, several approaches have been devised to deliver heat more precisely, including the use of heated nanoparticles, making hyperthermia a more clinically tractable treatment option [4, 5]. Despite these practical advances, the mechanisms responsible for heat shock-induced cell death remain controversial and ill-defined. In this Visions and Reflections we discuss recent findings surrounding the initiation of heat shock-induced apoptosis, and propose future areas of research. Received 17 March 2007; received after revision 25 April 2007; accepted 22 May 2007     

The <Emphasis Type="Italic">Adhesion</Emphasis> GPCRs: A unique family of G protein-coupled receptors with important roles in both central and peripheral tissues

G protein-coupled receptors (GPCRs) are a diverse superfamily of membrane-bound receptors. The second largest subgroup of GPCRs, the Adhesion GPCRs, has 33 members in humans. Phylogenetic analysis of the entire repertoire of the seven transmembrane- domain (7TM) regions of GPCRs shows that the Adhesion GPCRs form a distinct family. Adhesion GPCRs are characterised by (1) long N termini with multiple functional domains often found in other proteins such as tyrosine kinases, integrins and cadherins, (2) highly complex genomic structure with multiple introns and splice variants and (3) a 7TM region that has no clear similarities with 7TM from other GPCRs. Several Adhesion GPCRs are known to have a role in the immune system but it is becoming more evident that many have important roles in the CNS. We speculate that the overall structural construction of the Adhesion GPCRs allows them to participate in different types of cell guidance. Received 8 February 2007; received after revision 21 March 2007; accepted 25 April 2007     

On the structural definition of amyloid fibrils and other polypeptide aggregates

Amyloid fibrils occur inside the human body, associated with ageing or a group of diseases that includes, amongst others, Alzheimer’s disease, atherosclerosis and type II diabetes. Many natural polypeptide chains are able to form amyloid fibrils in vivo or in vitro, and this ability has been suggested to represent an inherent consequence of the chemical structure of the polypeptide chain. Recent literature has provided a wealth of information about the structure of aggregates, precipitates, amyloid fibrils and other types of fibrillar polypeptide assemblies. However, the biophysical meaning associated with these terms can differ considerably depending on the context of their usage. This overview presents a structural comparison of amyloid fibrils and other types of polypeptide assemblies and defines amyloid fibrils, based on structural considerations, as fibrillar polypeptide aggregates with a cross-β conformation. Received 1 March 2007; received after revision 15 March 2007; accepted 25 April 2007     

Role of heregulin in human cancer

Heregulin (HRG) is a soluble secreted growth factor, which, upon binding and activation of ErbB3 and ErbB4 transmembrane receptor tyrosine kinases, is involved in cell proliferation, invasion, survival and differentiation of normal and malignant tissues. The HRG gene family consists of four members: HRG-1, HRG-2, HRG-3 and HRG-4, of which a multitude of different isoforms are synthesized by alternative exon splicing, showing various tissue distribution and biological activities. Disruption of the physiological balance between HRG ligands and their ErbB receptors is implicated in the formation of a variety of human cancers. The general mechanisms involved in HRG-induced tumorigenesis is discussed. Received 8 March 2007; received after revision 6 May 2007; accepted 9 May 2007     

Aspirin: recent developments

Aspirin exerts anti-thrombotic action by acetylating and inactivating cyclooxygenase-1, preventing the production of thromboxane A ₂ in platelets. Through this inhibition of platelet function, aspirin is considered as a preventative of ischemic diseases such as coronary and cerebral infarction. However, many studies have revealed that aspirin has other beneficial actions in addition to its anti-platelet activity. For example, aspirin may confer some benefit against colorectal cancer. Here, we discuss the involvement of inflammation in atherosclerosis and how aspirin exerts its beneficial actions in atherosclerotic diseases and cancer. Received 30 September 2007; received after revision 31 October 2007; accepted 6 November 2007     

Epigenetic mechanisms in the context of complex diseases

Complex diseases arise from a combination of heritable and environmental factors. The contribution made by environmental factors may be mediated through epigenetics. Epigenetics is the study of changes in gene expression that occur without a change in DNA sequence and are meiotically or mitotically heritable. Such changes in gene expression are achieved through the methylation of DNA, the post-translational modifications of histone proteins, and RNA-based silencing. Epigenetics has been implicated in complex diseases such as cancer, schizophrenia, bipolar disorder, autism and systemic lupus erythematosus. The prevalence and severity of these diseases may be influenced by factors that affect the epigenotype, such as ageing, folate status, in vitro fertilization and our ancestors’ lifestyles. Although our understanding of the role played by epigenetics in complex diseases remains in its infancy, it has already led to the development of novel diagnostic methods and treatments, which augurs well for its future health benefits. Received 6 December 2006; received after revision 29 January 2007; accepted 15 March 2007     

Angiotensin-converting enzyme 2 in acute respiratory distress syndrome

Angiotensin-converting enzyme (ACE) and ACE2 are highly homologous metalloproteases that provide essential catalytic functions in the renin-angiotensin system (RAS). Angiotensin II is one key effector peptide of the RAS, inducing vasoconstriction and exerting multiple biological functions. ACE cleaves angiotensin I to generate angiotensin II, whereas ACE2 reduces angiotensin II levels. Accumulating evidence has demonstrated a physiological and pathological role of ACE2 in the cardiovascular systems. Intriguingly, the SARS coronavirus, the cause of severe acute respiratory syndrome (SARS), utilizes ACE2 as an essential receptor for cell fusion and in vivo infections. Moreover, recent studies have demonstrated that ACE2 protects murine lungs from acute lung injury as well as SARS-Spike protein-mediated lung injury, suggesting a dual role of ACE2 in SARS infections and protection from ARDS. Received 18 May 2006; received after revision 12 March 2007; accepted 24 April 2007     

Unveiling molecular mechanisms of bacterial surface proteins: <Emphasis Type="Italic">Streptococcus pneumoniae</Emphasis> as a model organism for structural studies

Bacteria present a variety of molecules either on their surface or in a cell-free form. These molecules take part in numerous processes in the interactions with their host, with its tissues and other molecules. These molecules are essential to bacterial pathogenesis either during colonization or the spread/invasion stages, and most are virulence factors. This review is focused on such molecules using Streptococcus pneumoniae, a Gram-positive bacterium, as an example. Selected surface proteins are introduced, their structure described, and, whenever available, their mechanisms of function on an atomic level are explained. Such mechanisms for hyaluronate lyase, pneumococcal surface protein A, pneumolysin, histidine-triad and fibronectin-binding proteins are discussed. Elucidation of molecular mechanisms of virulence factors is essential for the understanding of bacteria and their functional properties. Structural biology appears pivotal for these studies, as structural and mechanistic insights facilitate rational approach to the development of new treatments. Received 12 March 2007; received after revision 28 June 2007; accepted 18 July 2007     

Targeted inhibition of Livin resensitizes renal cancer cells towards apoptosis

Cancer cells are typically characterized by apoptosis deficiency. In order to investigate a possible role for the anti-apoptotic livin gene in renal cell cancer (RCC), we analyzed its expression in tumor tissue samples and in RCC-derived cell lines. In addition, we studied the contribution of livin to the apoptotic resistance of RCC cells by RNA interference (RNAi). Livin gene expression was detected in a significant portion of RCC tumor tissue specimens (13/14, 92.9%) and tumor-derived cell lines (12/15, 80.0%). Moreover, targeted inhibition of livin by RNAi markedly sensitized RCC cells towards proapoptotic stimuli, such as UV irradiation or the chemotherapeutic drugs etoposide, 5-fluorouracil, and vinblastine. These effects were specific for livin expressing tumor cells. We conclude that livin can contribute significantly to the apoptosis resistance of RCC cells. Targeted inhibition of livin could represent a novel therapeutic strategy to increase the sensitivity of renal cancers towards pro-apoptotic agents. Received 30 November 2006; received after revision 22 February 2007; accepted 20 March 2007     

The corticotropin-releasing factor (CRF) family of peptides as local modulators of adrenal function

Corticotropin-releasing factor (CRF), also termed corticotropin-releasing hormone (CRH) or corticoliberin, is the major regulator of the adaptive response to internal or external stresses. An essential component of the adaptation mechanism is the adrenal gland. CRF regulates adrenal function indirectly through the central nervous system (CNS) via the hypothalamic-pituitary-adrenal (HPA) axis and via the autonomic nervous system by way of locus coeruleus (LC) in the brain stem. Accumulating evidence suggests that CRF and its related peptides also affect the adrenals directly, i.e. not through the CNS but from within the adrenal gland where they form paracrine regulatory loops. Indeed, CRF and its related peptides, the urocortins (UCNs: UCN1, UCN2 and UCN3), their receptors CRF type 1 (CRF₁) and 2 (CRF₂) as well as the endogenous pseudo-receptor CRF-binding protein (CRF-BP) are all expressed in adrenal cortical, medullary chromaffin and resident immune cells. The intra-adrenal CRF-based regulatory system is complex and depends on the balance between the local concentration of CRF ligands and the availability of their receptors. Received 19 December 2006; received after revision 20 February 2007; accepted 26 March 2007     

Tumor suppressor CYLD: negative regulation of NF-κB signaling and more

CYLD is a protein with tumor suppressor properties which was originally discovered associated with cylindromatosis, an inherited cancer exclusively affecting the folicullo-sebaceous-apocrine unit of the epidermis. CYLD exhibits deubiquitinating activity and acts as a negative regulator of NF-κB and JNK signaling through its interaction with NEMO and TRAF2. Recent data suggest that this is unlikely to be its unique function in vivo. CYLD has also been shown to control other seemingly disparate cellular processes, such as proximal T cell receptor signaling, TrkA endocytosis and mitosis. In each case, this enzyme appears to act by regulating a specific type of polyubiquitination, K63 polyubiquitination, that does not result in recognition and degradation of proteins by the proteasome but instead controls their activity through diverse mechanisms. Received 6 October 2007; received after revision 2 November 2007; accepted 23 November 2007     

Molecular mechanisms of nitrosative stress-mediated protein misfolding in neurodegenerative diseases

Nitrosative and oxidative stress, associated with the generation of excessive reactive oxygen or nitrogen species, are thought to contribute to neurodegenerative disorders. Many such diseases are characterized by conformational changes in proteins that result in their misfolding and aggregation. Accumulating evidence implies that at least two pathways affect protein folding: the ubiquitin-proteasome system (UPS) and molecular chaperones. Normal protein degradation by the UPS can prevent accumulation of aberrantly folded proteins. Molecular chaperones – such as protein-disulfide isomerase, glucose-regulated protein 78, and heat shock proteins – can provide neuroprotection from aberrant proteins by facilitating proper folding and thus preventing their aggregation. Our recent studies have linked nitrosative stress to protein misfolding and neuronal cell death. Here, we present evidence for the hypothesis that nitric oxide contributes to degenerative conditions by S-nitrosylating specific chaperones or UPS proteins that would otherwise prevent accumulation of misfolded proteins. Received 5 December 2006; received after revision 7 February 2007; accepted 15 March 2007     

Xanthorhodopsin: Proton pump with a carotenoid antenna

Retinal proteins function as photoreceptors and ion pumps. Xanthorhodopsin of Salinibacter ruber is a recent addition to this diverse family. Its novel and distinctive feature is a second chromophore, a carotenoid, which serves as light-harvesting antenna. Here we discuss the properties of this carotenoid/retinal complex most relevant to its function (such as the specific binding site controlled by the retinal) and its relationship to other retinal proteins (bacteriorhodopsin, archaerhodopsin, proteorhodopsin and retinal photoreceptors of archaea and eukaryotes). Antenna addition to a retinal protein has not been observed among the archaea and emerged in bacteria apparently in response to environmental conditions where light-harvesting becomes a limiting factor in retinal protein functioning. Received 2 April 2007; received after revision 14 May 2007; accepted 16 May 2007     

Anticonvulsant,antiepileptogenic, and antiictogenic pharmacostrategies

Pharmacological concepts tailored to status epilepticus, to epileptogenesis following acquired brain insults, and to ictogenesis in established epilepsy vary considerably and should ideally be directed at those pathophysiological mechanisms that presumably underly these conditions. Currently known important molecular targets include voltage-gated sodium and calcium channels, the γ-aminobutyric acid (GABA) system and ionotropic glutamate receptors. Metabotropic glutamate receptors, potassium channels, and neurotransmitters such as acetylcholine, glycine, and monoamines are beyond the scope of this review. In status epilepticus, immediate failure of GABAergic inhibition occurs, and administration of benzodiazepines and barbiturates displays the pharmacostrategic mainstay. In epileptogenesis within limbic structures, the most important underlying pathophysiological mechanisms currently discussed are transient loss of inhibition and aberrant mossy fiber sprouting. Both processes may be facilitated by N-methy-d-aspartat (NMDA) receptor regulation. NMDA antagonists may exhibit antiepileptogenic properties in experimental animals, but reliable data in humans are lacking. In established epilepsy, voltage-gated ion channels and impairment of GABAergic functions contribute to mechanisms facilitating ictogenesis. Blockade of sodium and calcium channels and enhancement of GABAergic inhibition are currently the most important tools to prevent the occurrence of seizures. Received 16 January 2007; received after revision 7 March 2007; accepted 17 April 2007     

Antimalarial drugs: recent advances in molecular determinants of resistance and their clinical significance

Molecular determinants of antimalarial drug resistance are useful and informative tools that complement phenotypic assays for drug resistance. They also guide the design of strategies to circumvent such resistance once it has reached levels of clinical significance. Established resistance to arylaminoalcohols such as mefloquine and lumefantrine in SE Asia is mediated primarily by gene amplification of the P. falciparum drug transporter, pfmdr1. Single nucleotide polymorphisms in pfmdr1, whether assessed in field isolates or transfection experiments, are associated with changes in IC₅₀ values (to arylaminoalcohols and chloroquine), but not of such magnitude as to influence clinical treatment outcomes. Recently described emerging in vitro resistance to artemisinins in certain areas correlates with mutations in the SERCA-like sequence PfATP6 and supports PfATP6 as a key target for artemisinins. Received 13 February 2006; revised after revision 7 March 2006; accepted 29 March 2006     

Molecular genetics of RecQ helicase disorders

The RecQ helicases belong to the Superfamily II group of DNA helicases, and are defined by amino acid motifs that show sequence similarity to the catalytic domain of Escherichia coli RecQ. RecQ helicases have crucial roles in the maintenance of genome stability. In humans, there are five RecQ helicases and deficiencies in three of them cause genetic disorders characterised by cancer predisposition, premature aging and/or developmental abnormalities. RecQ helicase-deficient cells exhibit aberrant genetic recombination and/or DNA replication, which result in chromosomal instability and a decreased potential for proliferation. Here, we review the current knowledge of the molecular genetics of RecQ helicases, focusing on the human RecQ helicase disorders and mouse models of these conditions. Received 9 March 2007; received after revision 26 April 2007; accepted 2 May 2007     

Mechanisms of sperm-egg interactions emerging from gene-manipulated animals

Untangling the molecular nature of sperm-egg interactions is fundamental if we are to understand fertilization. These phenomena have been studied for many years using biochemical approaches such as antibodies and ligands that interact with sperm or with eggs and their vestments. However, when homologous genetic recombination techniques were applied, most of the phenotypic factors of the gene-manipulated animals believed “essential” for fertilization were found to be dispensable. Of course, all biological systems contain redundancies and compensatory mechanisms, but as a whole the old model of fertilization clearly requires significant modification. In this review, we use the results of gene manipulation experiments in animals to propose the basis for a new vision. Received 26 January 2007; received after revision 7 March 2007; accepted 17 April 2007     

tRNase Z: the end is not in sight

Although the enzyme tRNase Z has only recently been isolated, a plethora of data has already been acquired concerning the enzyme. tRNase Z is the endonuclease that catalyzes the removal of the tRNA 3′ trailer, yielding the mature tRNA 3′ end ready for CCA addition and aminoacylation. Another substrate cleaved by tRNase Z is the small chromogenic phosphodiester bis(p-nitrophenyl)phosphate (bpNPP), which is the smallest tRNase Z substrate known so far. Hitherto the biological function as tRNA 3′-end processing enzyme has been shown only in one prokaryotic and one eukaryotic organism, respectively. This review summarizes the present information concerning the two tRNase Z substrates pre-tRNA and bpNPP, as well as the metal requirements of tRNase Z enzymes. Received 29 March 2007; received after revision 15 May 2007; accepted 21 May 2007     

Progesterone regulation of implantation-related genes: new insights into the role of oestrogen

Genomic profiling was performed on explants of late proliferative phase human endometrium after 24-h treatment with progesterone (P) or oestradiol and progesterone (17β-E₂+P) and on explants of menstrual phase endometrium treated with 17β-E₂+P. Gene expression was validated with real-time PCR in the samples used for the arrays, in endometrium collected from early and mid-secretory phase endometrium, and in additional experiments performed on new samples collected in the menstrual and late proliferative phase. The results show that late proliferative phase human endometrium is more responsive to progestins than menstrual phase endometrium, that the expression of several genes associated with embryo implantation (i.e. thrombomodulin, monoamine oxidase A, SPARC-like 1) can be induced by P in vitro, and that genes that are fully dependent on the continuous presence of 17β-E₂ during P exposure can be distinguished from those that are P-dependent to a lesser extent. Therefore, 17β-E₂ selectively primes implantation-related genes for the effects of P. H. Dassen, C. Punyadeera: These authors contributed equally. Received 18 December 2006; received after revision 6 February 2007; accepted 8 March 2007     

Human and yeast ζ-crystallins bind AU-rich elements in RNA

ζ-crystallins constitute a family of proteins with NADPH:quinone reductase activity found initially in mammalian lenses but now known to be present in many other organisms and tissues. Few proteins from this family have been characterized, and their function remains unclear. In the present work, ζ-crystallins from human and yeast (Zta1p) were expressed, purified and characterized. Both enzymes are able to reduce ortho-quinones in the presence of NADPH but are not active with 2-alkenals. Deletion of the ZTA1 gene makes yeast more sensitive to menadione and hydrogen peroxide, suggesting a role in the oxidative stress response. The human and yeast enzymes specifically bind to adenine-uracil rich elements (ARE) in RNA, indicating that both enzymes are ARE-binding proteins and that this property has been conserved in ζ-crystallins throughout evolution. This supports a role for ζ-crystallins as trans-acting factors that could regulate the turnover of certain mRNAs. Received 21 February 2007; received after revision 16 April 2007; accepted 23 April 2007 M. R. Fernández, S. Porté: These authors contributed equally to this work.