Postepy Biologii Komorki, vol. 32, 2005, issue 1

The Role of Peroxisome Proliferator-Activated Receptors Gamma (PPARg) in Adipogenesis, Obesity and Type 2 Diabetes

Summary: The fat tissue is very dynamic and in the result of hyperplasia, which is the aftermath of proliferation and the differentiation of preadipocytes, can have an affect on the development of obesity. This process is complexly regulated by many specific genes. The one of the most noteworthy is the peroxisome proliferator-activated receptor gamma (PPARg), predominantly expressed in adipose tissue. It is the key factor in the adipogenesis process and is responsible for the differentiation of preadipocytes into adult cells, adipocytes. Moreover, it regulates the lipids metabolism in the fat tissue. Several data indicate that the activation of the PPARg receptors increases the uptake of free fatty acids made during lipolysis or circulating excessively in the blood by adipocytes and subsequently it facilitates their storage and/or beta-oxidation. The role of PPARg receptors in the battle with metabolic symptoms of type 2 diabetes caused by obesity is invaluable. The administration of synthetic agonist PPARg receptors from the tiazolidinedione (TZD) group to individuals diagnosed with type 2 diabetes causes changes in the gene expression involved in the metabolism of glucose and lipids in the fat tissue. Drugs from the TZD group enhance the gene expression involved in: trans-membrane glucose transport, the lipolysis, lipogenesis, lipid storage and fatty acid mitochondrial oxidation processes. The long-term PPARg activation by TZD has an effect on the size change of the adipocytes in favour of the smaller, more sensitive to insulin. All of these biochemical processes in the fat tissue, where the PPARg receptor is involved in, testifies the exceptionally crucial role of this receptor, frequently labelled as the lipid sensor, which protects the muscle tissue and liver from the high lipid and glucose levels.

Key words: fat tissue, adipocytes, adipogenesis, obesity, type 2 diabetes, PPARg receptors

Symplasmic Communication: Terminology, Fluorochromes and Arabidopsis thaliana Embryogenesis

Summary: Plasmodesmata (PD) are channels within the walls between neighbouring cells linking the cytoplasm of these cells. It is postulated that PD evolved to facilitate communication between cells [61]. Symplasmic communication via plasmodesmata is an important element in the system of information exchange between cells during plant growth and development [10, 24, 28, 31]. The upper limit of the size of molecules that can freely diffuse through PD changed during the development because the PD diameter can be changed temporally, spatially and physiologically [61]. It means that in different developmental stages the same molecule can or can not diffuse between cell. As we know now, molecules which can be exchange between neighbouring cells through PD are not only ions, hormones, minerals, amino acids and sugars but also proteins, transcriptional factors and different classes of RNA including mRNA which are known as a regulatory molecules [26, 36, 46, 61]. All information mentioned above indicate that plasmodesmata can regulate cell-to-cell movement and in this way participate in the regulation and co-ordination of plant development. It is known that PD play an important roles during morphogenesis. In this article the cell-to-cell communication was described on the example of Arabidopsis thaliana embryogenesis which is the most important developmental process in plants. Studies of the role of symplasmic communication during embryogenesis were based on the analysis of the movement of fluorochromes or GFP between embryo cell in different stages of embryo development. It was shown that appearance of symplasmic domains precedes the appearance of morphologically different structures during Arabidopsis embryogenesis. It appeared that Arabidopsis embryo is one symplasmic domain up to the mid-torpedo stage [28]. From that moment of development embryo is no longer single symplast and movement of symplasmic transport tracers of different molecular weights is restricted to different symplasmic domains and subdomains which correlate with development of primary tissues [28]. It means that downregulation of PD as the embryo develops is important for normal embryogenesis [61]. Studies mentioned above also indicate that disturbance in normal permeability of PD lead to the disorder in Arabidopsis development [28]. The changes in PD permeability took place also when embryo change the development from radial to bilateral symmetry [31]. Detailed analysis of the GFP movement between cells revealed also the existence of subdomains which correspond to establishing the apical-basal axis of Arabidopsis embryo [29]. These results clearly showed that regulation of embryogenesis is based (among others) on changes in symplasmic transport between embryo cells and revealed temporal and spatial correlation between stages of embryo development and formation of symplasmic domains and subdomains [28, 29, 30, 31, 51]. All results mentioned above support the general concept that PD in younger tissues are more dilated then PD in older tissues [30]. In connection with presented topic, terminology and definitions concerned the symplasmic communication and characteristic of symplasmic transport fluorochromes were also described. It is proposed that for the description of communication between cells through PD the term "symplasmic" instead of "symplastic" should be used because the second one according to definition presented by Priestley [44] is reserved for description of events taking place in the cell wall and not in the symplasm. Definition of symplasmic domains both permanent and transient, subdomains and symplasmic fields are also presented. Differences between Size Exclusion Limit (SEL) which is a maximal molecular size of the substances transported through PD and Molecular Exclusion Limit (MEL) where apart from the size, the shape and charge of the molecule is taken into consideration were described. Fluorochromes used for investigations of symplasmic transport between cells such as HPTS (8-hydroxypyrene-1,3,6-trisulfonic acid), fluorescein, fluorescein diacetate, CFDA (5-(and-6)-Carboxyfluorescein diacetate), LYCH (Lucifer Yellow CH) and CB (Cascade Blue hydrazide) were described and their main physical and chemical characteristic was presented. Fluorochromes description (among others) concerns their properties for plasma membrane penetration, methods for application and spectral characteristic. Dextrans labeled with FITC (Fluorescein Isothiocyanate), GFP (Green Fluorescent Protein) and caged fluorescein (CMNB-Caged Fluorescein; bis-(5-carboxymethoxy-2-NitroBenzyl) ether) as a new tool for analysis of symplasmic communication between plant cells were also described. The similarity of the role of PD in plant organisms and gap junctions in animal organisms was also shortly presented.

Key words: embryogenesis, symplasm, Arabidopsis, fluorochromes, symplasmic fields and domains, differentiation, development, plasmodesmata

Platelets – Endothelial Cells Interactions in Inflammation
Part I. Adhesive Receptors of Platelets, Endothelial Cells, and Microparticles in Hemostasis and Inflammation

Summary: It becomes increasingly evident that blood platelets do not only exert important functions in hemostasis and thrombus formation but are also involved in atherosclerotic vascular disease. The intact, nonactivated endothelium normally prevents platelet adhesion to the arterial wall. Inflamed endothelial cells become adhesive for platelets. During the adhesion process, platelets become activated and release potent inflammatory and mitogenic substances into the local microenvironment, thereby altering chemotactic, adhesive, and proteolytic properties of endothelial cells and supporting leukocytes recruitment into inflamed vascular wall. Activation of platelets, endothelial cells and leukocytes result in the formation of increased levels of microparticles, highly proinflammatory and proatherosclerotic bodies distributing substantial amounts of immunologically active substances between cells promoting inflammation. The evolving inflammatory reactions are instrumental in the initiation of atherosclerotic plaques and their destabilization. Atherosclerotic plaque develops in response to a localized inflammatory reaction in the vessel wall. Inflammation is crucial at all stages of atherosclerosis, when the endothelial cells and platelets are activated and express cytokines and adhesion molecules leading to monocyte/lymphocyte recruitment and infiltration into subendothelium. The purpose of this review is to bring together the current information concerning the role of activated platelets and activated endothelial cells in the development and progression of inflammation and atherosclerosis. The part one of this review focuses on multicellular adhesive interaction in the vasculature, with particular attention to the interplay between adhesive receptors of endothelial cells and platelets. This review summarizes also the present understanding of the role of microvesicles derived from blood cells in the progression of inflammation.

Key words: platelets, endothelial cells, adhesion, adhesion receptors, hemostasis, inflammation

Platelets – Endothelial Cells Interactions in Inflammation
Part II. Platelet-Derived Mediators and Regulatory Pathways of Signal Transduction in Endothelial Cells in Inflammation

Summary: Megakaryocytes and platelets synthesize inflammatory mediators, which are stored in a granules. Although platelets do not ordinarily bind to endothelial cells, pathological interactions between platelets and arterial endothelial cells may contribute to the adhesion of platelets to endothelial cells. During the adhesion process platelets become activated and release an arsenal of potent inflammatory and mitogenic substances into the local environment. The released substances trigger autocrine and paracrine activation processes that lead to leukocyte recruitment into the vascular wall. Activation of platelets is crucial for platelet function in the development of inflammation leading to atherosclerotic lesions. The second part of the review highlights the contribution of the products released from activated platelets to atherosclerotic events. Mechanisms responsible for the induction of expression of proinflammatory proteins on platelets and endothelial cells are also described. Finally, emphasis is placed on important stimulatory and inhibitory signaling pathways used by endothelial cells to initiate and accelerate inflammation and atherosclerosis.

Key words: platelets, endothelial cells, inflammatory mediators, signal transduction

Summary: he plant hormone auxin regulates a wide variety of growth and developmental processes including apical dominance, differentiation of vascular tissues, adventitious and lateral root formations, embryogenesis, phyllotaxis and tropic responses. The cellular response to auxin involves rapid events at plasma membrane and changes in the expression of genes. Identification of receptors involved in auxin perception is a key step towards understanding the molecular basis of hormone action. The experimental evidence suggests that there are multiple hormone receptors, and hence auxin affects such a wide range of physiological processes. At present, two auxin receptors functioning in independent mechanisms of auxin perception are under consideration: the auxin binding protein (ABP1), with a known three-dimensional structure but still elusive physiological function and the transport inhibitor response protein (TIR1) involving in auxin-induced gene expression. The extracellular auxin receptor ABP1 mediates in electrophysiological responses at the plasma membrane and protoplast swelling. Furthermore, ABP1 is essential for early embryonic development, but no evidence has been obtained to its involvement in auxin-regulated transcriptional changes that are clearly responsible for many auxin responses. The mature ABP1 is a dimer and has no hydrophobic regions. ABP1 contain both a signal peptide and an KDEL endoplasmic reticulum (ER) retention motif. The most of auxin binding protein 1 resides in the ER lumen as a soluble protein and a small part of it is secreted at the plasma membrane surface through the Golgi secretory pathway. ABP1 probably interacts at the surface of plasma membrane directly with ion channels and H⁺-ATPase or via transmembrane "docking" protein (yet to be identified) with downstream transduction signal pathway elements. The intracellular auxin receptor TIR1 contains F-box domain, suggesting involvement in ubiquitin-mediated protein degradation. It is a part of a multimeric Skp1-Culin-F-box protein (SCF) ubiquitin ligase complex localized in nucleus. The F-box domain mediates interaction with the SCF complex via the SKP1/ASK subunit. TIR1 represents a new type of the receptor that regulates the expression of auxin-responsive genes by the poliubiquitinylation and subsequent degradation of tran-scriptional repressor proteins, Aux/IAA. The crystal structure of Arabidopsis thaliana TIR 1, both in free form and in complex with auxin has been known and the first structural model of a plant hormone receptor has been established. This structure shows that the leucine-rich repeat domain of TIR1 is involved in auxin perception and illustrates precisely how auxin is perceived. According to this model, auxin probably enhances TIR1-Aux/IAA interactions by acting as a "molecular glue", helping TIR1 bind to the Aux/IAA proteins. Under low auxin concentrations, auxin response factors (ARFs) consists with Aux/IAA repressors proteins as heterodimers and repress genes expression. In addition to TIR1 protein, three AUXIN SIGNALING F-BOX proteins, AFB1, AFB2 and AFB3, have been found to exhibit auxin-depending binding to Aux/IAA. These proteins are highly related to TIR1 and they are functional auxin receptors that mediate the effect of auxin on gene expression during growth and development of plants. In this paper the recent advances in studies on the molecular structure, binding site models and activity of auxin receptors are presented.

Key words: auxin, auxin binding protein 1, ubiquitin ligase SCFTIR1, proteolysis, TIR1, ubiquitin

Summary: Anthracycline antibiotics are extensively used in conventional cancer chemotherapy of solid tumors and hematological malignancies. The clinical efficacy of these drugs, mainly doxorubicin and daunorubicin is however, limited by severe side effects, particularly the cardiotoxicity. Despite the development of numerous compounds which improve antitumour activity or reduce toxicity, only a very limited number of anthracycline derivatives is commercially available. One of these new drugs with reduced cardiac toxicity in comparison to anthracycline of the first generation is aclarubicin, produced by Streptomyces galilaeus. Although, aclarubicin is used less intensively than doxorubicin or daunorubicin in conventional anticancer protocols, it is beneficial in clinical trials of acute non-lymphocytic leukaemia in patients, who are resistant to the first-line chemotherapy, because of lack of cross-resistance to others anthracyclines. Molecules of aclarubicin are consisted of a planar polyaromatic ring system named aclavinone which contains a quinone moiety. This structure is linked by a O-glycosidic bond to three saccharides (rhodosamine, 2-deoxyfucose and cinerulose A). In comparison to the other anticancer antibiotics, there is little information, in respect of the mechanism of ACL antineoplastic efficacy. The mechanisms of action of this trisaccharide anthracycline are different from the classical monosaccharide doxorubicin and daunorubicin. Fluorescence microscopic observations indicate that aclarubicin localizes mainly in the cytoplasm of the cells in contrast to the anthracyclines of first generation which are observed mainly in the nucleus. Extension of the time of incubation with aclarubicin does not change its localization in the cell. This review presents also the current knowledge about interaction of aclarubicin with cell membrane, its transport across the membrane on the way of flip-flop mechanism and role of this structure in multidrug resistance. Aclarubicin is a strong DNA intercalating agent that prevents the binding of topoisomerase II to DNA. Recent studies have shown that aclarubicin also inhibits topoisomerase I in a concentration-dependent manner. Exposure of cell to aclarubicin is accompanied by the occurrence of DNA damage as determined by the single-cell microgel assay (comet assay). However, the primary intracellular effect of aclarubicin, is more likely to be an inhibition of RNA synthesis. Aclarubicin exerts also a potent inhibitory effect on migration and invasion of the cancer cells. Drug generates also reactive oxygen species but substantially less, in comparison to other anthracyclines. Several studies have indicate that reactive oxygen species are capable of damaging not only cellular macromolecules but they also cause cell death either by apoptosis or necrosis. Anthracycline antibiotics induce cell death with mitochondrial or receptor apoptosis pathway. Ceramide, p53 protein or Bcl-2 protein family may function as mediators of apoptosis. It has been observed p53 protein increase in ACL -treated cells. Numerous studies also have shown that anthracyclines of first generation induce both apoptosis and necrosis. By contrast ACL -treated cells died prevalently by apoptosis. Apoptosis and necrosis mode of cell death depends on cell types, their sensitivity to ACL and the time of incubation.

The Role of Connective Tissue Growth Factor (CTGF) in Fibroproliferative Processes and Tissues Fibrosis

Key words: connective tissue growth factor, CTGF, transforming growth factor ß (TGF-ß), CCN family, fibrosis

Summary: Phospholipases A₂ constitute the group of enzymes, which catalyze the hydrolysis of the ester bond at sn-2 position of glycerophospholipids, what generates a free fatty acid and lysophospholipids. Free fatty acids and lysophospholipids may act as the second messengers in CNS. There are some data suggesting, that protein kinase C (PKC), MAP kinases, protein kinase G (PKG) and other protein kinases, take part in activation of cPLA₂. Under physiological conditions, these enzymes regulate the turnover of free fatty acids in membrane phospholipids, assuring membrane stability, fluidity, and thereby may participate in regulation of transport processes through neuron's membrane. The generation of superoxide radicals during the metabolism of arachidonic acid is likely to play an important role in the toxic events which may result in cell death. An increased level of AA concentration, is associated with an induction of COX-2 (cyclooxygenase-2) enzyme, higher production of prostaglandins. That may contribute to neurodegenerative disorders including Parkinson disease.