Co-localization of galectin-1 with GM1 ganglioside in the course of its clathrin- and raft-dependent endocytosis

Mammalian galectin-1 (Gal-1), a beta-galactoside-binding lectin has a prominent role in regulating cell adhesion, cell growth and immune responses. Downregulation of these biological functions may occur via internalization of Gal-1. In the present study we have investigated the mechanism and possible mediator(s) of Gal-1 endocytosis. We show that internalization occurs at a temperature higher than 22 degrees C in an energy dependent fashion. After one hour incubation Gal-1 localizes in the Golgi system within the cells, and then disappears without accumulation in degradation compartments, such as lysosomes. Based on their strong intracellular co-localization, two glycoconjugates, GM1 ganglioside and CD7 are implicated in the sorting of internalized Gal-1 into Golgi. Other known Gal-1 binding glycoproteins on T cells (CD2, CD3, CD43 and CD45) do not cointernalize with the lectin. Internalization of Gal-1 depends on its lectin activity and follows dual pathways involving clathrin-coated vesicles and raft-dependent endocytosis.     

Epigenetic mechanisms in mammals

DNA and histone methylation are linked and subjected to mitotic inheritance in mammals. Yet how methylation is propagated and maintained between successive cell divisions is not fully understood. A series of enzyme families that can add methylation marks to cytosine nucleobases, and lysine and arginine amino acid residues has been discovered. Apart from methyltransferases, there are also histone modification enzymes and accessory proteins, which can facilitate and/or target epigenetic marks. Several lysine and arginine demethylases have been discovered recently, and the presence of an active DNA demethylase is speculated in mammalian cells. A mammalian methyl DNA binding protein MBD2 and de novo DNA methyltransferase DNMT3A and DNMT3B are shown experimentally to possess DNA demethylase activity. Thus, complex mammalian epigenetic mechanisms appear to be dynamic yet reversible along with a well-choreographed set of events that take place during mammalian development.     

Mcl-1 is a potential therapeutic target in multiple types of cancer

Resistance to apoptosis is a common challenge in human malignancies contributing to both progress of cancer and resistance to conventional therapeutics. Abnormalities in a variety of cell intrinsic and extrinsic molecular mechanisms cooperatively promote tumor formation. Therapeutic approaches that specifically target components of these molecular mechanisms are getting widespread attention. Mcl-1 is a highly expressed pro-survival protein in human malignancies and its cellular expression is tightly regulated via multiple mechanisms. Mcl-1 differs from other members of the Bcl-2 family in having a very short half-life. So inhibition of its expression and/or neutralization of its anti-apoptotic function will rapidly make Mcl-1-dependent cells more susceptible to apoptosis and provide an opportunity to combat several types of cancers. This review summarizes the current knowledge on the regulation of Mcl-1 expression and discusses the alternative approaches targeting Mcl-1 in human cancer cells whose survivals mainly depend on Mcl-1. Received 6 October 2008; received after revision 21 October 2008; accepted 10 November 2008     

The Role of the Agouti-Related Protein in Energy Balance Regulation

The Agouti-Related Protein (AgRP) is a powerful orexigenic peptide that increases food intake when ubiquitously overexpressed or when administered centrally. AgRP-deficiency, on the other hand, leads to increased metabolic rate and a longer lifespan when mice consume a high fat diet. In humans, AgRP polymorphisms have been consistently associated with resistance to fatness in Blacks and Whites and resistance to the development of type-2 diabetes in African Blacks. Systemically administered AgRP accumulates in the liver, the adrenal gland and fat tissue while recent findings suggest that AgRP may also have inverse agonist effects, both centrally and peripherally. AgRP could thus modulate energy balance via different actions. Its absence or reduced functionality may offer a benefit both in terms of bringing about negative energy balance in obesigenic environments, as well as leading to an increased lifespan.     

Bile Acids: Chemistry, Pathochemistry, Biology, Pathobiology, and Therapeutics

Bile acids and bile alcohols in the form of their conjugates are amphipathic end products of cholesterol metabolism with multiple physiological functions. The great variety of bile acids and bile alcohols that are present in vertebrates are tabulated. Bile salts have an enterohepatic circulation resulting from efficient vectorial transport of bile salts through the hepatocyte and the ileal enterocyte; such transport leads to the accumulation of a pool of bile salts that cycles between the liver and intestine. Bile salt anions promote lipid absorption, enhance tryptic cleavage of dietary proteins, and have antimicrobial effects. Bile salts are signaling molecules, activating nuclear receptors in the hepatocyte and ileal enterocyte, as well as an increasing number of G-protein coupled receptors. Bile acids are used therapeutically to correct deficiency states, to decrease the cholesterol saturation of bile, or to decrease the cytotoxicity of retained bile acids in cholestatic liver disease.     

Myosin I: From yeast to human

Myosin I is a non-filamentous, single-headed, actin-binding motor protein and is present in a wide range of species from yeast to man. The role of these class I myosins have been studied extensively in simple eukaryotes, showing their role in diverse processes such as actin cytoskeleton organization, cell motility, and endocytosis. Recently, studies in metazoans have begun to reveal more specialized functions of myosin I. It will be a major challenge in the future to examine the physiological functions of each class I myosin in different cell types of metazoans.     

Progress in understanding the neuronal SNARE function and its regulation

Vesicle budding and fusion underlies many essential biochemical deliveries in eukaryotic cells, and its core fusion machinery is thought to be built on one protein family named soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE). Recent technical advances based on site-directed fluorescence labelling and nano-scale detection down to the single-molecule level rapidly unveiled the protein and the lipid intermediates along the fusion pathway as well as the molecular actions of fusion effectors. Here we summarize these new exciting findings in context with a new mechanistic model that reconciles two existing fusion models: the proteinaceous pore model and the hemifusion model. Further, we attempt to locate the points of action for the fusion effectors along the fusion pathway and to delineate the energetic interplay between the SNARE complexes and the fusion effectors. Received 01 July 2008; received after revision 29 August 2008; accepted 23 September 2008     

Prolactin inducible protein in cancer, fertility and immunoregulation: structure, function and its clinical implications

Prolactin inducible protein (PIP) is a 17- kDa single polypeptide chain, known by various names due to its versatile nature and function in human reproductive and immunological systems. It is expressed in several exocrine tissues such as the lacrimal, salivary, and sweat glands. Its expression is up regulated by prolactin and androgens, and estrogens down regulate it. Due to its over-expression in metastatic breast and prostate cancer, presently PIP is considered as a prognostic biomarker. Moreover, its aspartyl-proteinase nature suggests its role in tumor progression. PIP has unique features because it is small in size and plays multiple important functions. Its ability to bind potentially with CD4-T cell receptor, immunoglobulin G (IgG), actin, zinc α2-glycoprotein (ZAG), fibronectin and enamel pellicle, reveals its important biological functions. This is the first comprehensive review on the structure and functional analysis of PIP and its clinical applications. Received 04 August 2008; received after revision 09 September 2008; accepted 15 September 2008     

Regulation of chondrocyte differentiation by ADAMTS-12 metalloproteinase depends on its enzymatic activity

ADAMTS-12, a metalloproteinase that belongs to ADAMTS family, is strongly upregulated during chondrogenesis and demonstrates prominent expression in the growth plate chondrocytes. ADAMTS-12 potently inhibits chondrocyte differentiation, as revealed by altered expression of both early and later genes critical for chondrogenesis. In addition, ADAMTS-12-mediated inhibition of chondrogenesis depends on its enzymatic activity, since its point mutant lacking enzymatic activity completely loses this activity. Furthermore, the C-terminal four thrombospondin motifs known to bind COMP substrate is necessary for its full proteolytic activity and inhibition of chondrocyte differentiation. Mechanism studies demonstrate that ADAMTS-12 induces PTHrP, whereas it inhibits IHH during chondrogenesis. Furthermore, PTHrP induces ADAMTS-12 and ADAMTS-12 is hardly detectable in PTHrP-/-growth plate chondrocytes. Importantly, knocking down ADAMTS-12 mRNA levels or blocking ADAMTS-12 activity almost abolishes the PTHrP-mediated inhibition of type X collagen expression. Collectively, these findings demonstrate that ADAMTS-12, a downstream molecule of PTHrP signaling, is a novel regulator of chondrogenesis. X. H. Bai, D.W. Wang: These two authors contributed equally to this work.     

A novel member of the Rhomboid family, RHBDD1, regulates BIK-mediated apoptosis

Rhomboid family members are widely conserved and found in all three kingdoms of life. They are serine proteases and serve important regulatory functions. In the present study, a novel gene highly expressed in the testis, RHBDD1, is shown to be a new member of the Rhomboid family, participating in the cleavage of BIK, a proapoptotic member of the Bcl-2 family. The RHBDD1-involved proteolytic modification is upstream of the BIK protein degradation pathway. Mutagenesis studies show that the amino acid residues glycine142 and serine144 of RHBDD1 are crucial for its activity in cleaving BIK at a site located in the transmembrane region. Overexpression or knock-down of RHBDD1 in HEK 293T cells can reduce or enhance BIK-mediated apoptosis, respectively. The present findings suggest that, by acting as a serine protease, RHBDD1 modulates BIK-mediated apoptotic activity. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Received 31 July 2008; received after revision 16 September 2008; accepted 19 September 2008     

Interaction of galectin-1 with caveolae induces mouse embryonic stem cell proliferation through the Src, ERas, Akt and mTOR signaling pathways

Galectins have the potential to provide a promising alternative for unveiling the complexity of embryonic stem (ES) cell self-renewal, although the mechanism by which galectins maintain ES cell self-renewal has yet to be identified. Galectin-1 increased [³H]-thymidine incorporation as well as cyclin expression and decreased p27^kip1 expression. Src and caveolin-1 phosphorylation was increased by galectin-1, and phospho-caveolin-1 was inhibited by PP2. In addition, inhibition of caveolin-1 by small interfering RNA and methyl-β-cyclodextrin (Mβ-CD) decreased galectin-1-induced cyclin expression and [³H]-thymidine incorporation. Galectin-1 caused Akt and mTOR phosphorylation, which is involved in cyclin expression. Galectin-1-induced phospho-Akt and -mTOR was inhibited by PP2, ERas siRNA, caveolin-1 siRNA and Mβ-CD. Furthermore, mTOR phosphorylation was decreased by LY294002 and Akt inhibitor. Galectin-1-induced increase in cyclin expression and decrease in p27^kip1 was blocked by Akt inhibitor and rapamycin. In conclusion, galectin-1 increased DNA synthesis in mouse ES cells via Src, caveolin-1 Akt, and mTOR signaling pathways. Received 30 October 2008; received after revision 18 February 2009; accepted 24 February 2009     

Histone methylation and ubiquitination with their cross-talk and roles in gene expression and stability

Methylation of lysine residues of histones is associated with functionally distinct regions of chromatin, and, therefore, is an important epigenetic mark. Over the past few years, several enzymes that catalyze this covalent modification on different lysine residues of histones have been discovered. Intriguingly, histone lysine methylation has also been shown to be cross-regulated by histone ubiquitination or the enzymes that catalyze this modification. These covalent modifications and their cross-talks play important roles in regulation of gene expression, heterochromatin formation, genome stability, and cancer. Thus, there has been a very rapid progress within past several years towards elucidating the molecular basis of histone lysine methylation and ubiquitination, and their aberrations in human diseases. Here, we discuss these covalent modifications with their cross-regulation and roles in controlling gene expression and stability. Received 24 September 2008; received after revision 21 November 2008; accepted 28 November 2008