Post-translational modification of apolipoprotein B by transglutaminases

Uncovering protein polyamination by the spermine-specific antiserum and mass spectrometric analysis

The polyamines spermidine and spermine, and their precursor putrescine, have been shown to play an important role in cell migration, proliferation, and differentiation. Because of their polycationic property, polyamines are traditionally thought to be involved in DNA replication, gene expression, and protein translation. However, polyamines can also be covalently conjugated to proteins by transglutaminase 2 (TG2). This modification leads to an increase in positive charge in the polyamine-incorporated region which significantly alters the structure of proteins. It is anticipated that protein polyamine conjugation may affect the protein-protein interaction, protein localization, and protein function of the TG2 substrates. In order to investigate the roles of polyamine modification, we synthesized a spermine-conjugated antigen and generated an antiserum against spermine. In vitro TG2-catalyzed spermine incorporation assays were carried out to show that actin, tubulins, heat shock protein 70 and five types of histone proteins were modified with spermine, and modification sites were also identified by liquid chromatography and linear ion trap-orbitrap hybrid mass spectrometry. Subsequent mass spectrometry-based shotgun proteomic analysis also identified 254 polyaminated sites in 233 proteins from the HeLa cell lysate catalyzed by human TG2 with spermine, thus allowing, for the first time, a global appraisal of site-specific protein polyamination. Global analysis of mouse tissues showed that this modification really exists in vivo. Importantly, we have demonstrated that there is a new histone modification, polyamination, in cells. However, the functional significance of histone polyamination demands further investigations.

Extracellular modifications of HDL in vivo and the emerging concept of proteolytic inactivation of preβ-HDL

PURPOSE OF REVIEW

Both quantity and quality of the circulating HDL particle matter for the optimal antiatherogenic potential of HDL. This review summarizes various mechanisms capable of inducing extracellular modifications of HDL and reducing the function of HDL subclasses as cholesterol acceptors. Special emphasis is laid on the proteolytic inactivation of lipid-poor preβ-migrating HDL (preβ-HDL).

RECENT FINDINGS

HDL particles can undergo functional inactivation in vivo. During atherogenesis, different cell types in the arterial intima release enzymes into the intimal fluid, potentially capable of causing structural and chemical modifications of the various components present in the lipid core or in the polar surface of the HDL particles. Enzymatic oxidation, lipolysis and proteolysis, and nonenzymatic glycosylation are among the HDL modifications that adversely affect HDL functionality. Proteolysis of preβ-HDL by various proteases present in the arterial intima has emerged as a potential mechanism that impairs the efficiency of HDL to promote cholesterol efflux from macrophage foam cells, the mast cell-derived neutral protease chymase being a prime example of such impairment. A paradigm of proteolytic inactivation of preβ-HDL in vivo is emerging.

SUMMARY

Several extracellular enzymes present in the arterial intima may compromise various cardioprotective functions of HDL. Observations on proteolysis of specific lipid-poor HDL subpopulations in vivo constitute the basis for future studies evaluating the actual impact of proteolytic microenvironments on the initiation and progression of atherosclerotic lesions.

Polyamines as clinical laboratory tools

Since their discovery by Antoni van Leeuwenhoek in 1678 until the recent development of transgenic mice expressing proteins altering polyamine levels in a tissue-specific manner, polyamines have been the object of intense research efforts which have shed light on several biological and pathological processes. From the discovery of a particular form of proteasome regulation of the catabolism of the key regulatory enzyme in their synthetic pathway, to the experimental cancer treatment or prevention with polyamine antagonists or inhibitors of the latter enzyme, a whole spectrum of interests can be revealed. Still, many aspects of their functions remain elusive and difficulties inherent in their analysis, which relies on sophisticated high-performance liquid chromatographic (HPLC) methods, and the lack of standardization; have hampered the transit from the research realm to the standard clinical laboratory domain. Their assay in biological fluids has been used for cancer diagnosis and for monitoring anticancer treatment. In this article, we attempt to provide an overview of polyamine structure, nutritional value, metabolism, and physiological roles. Next, we will summarize the main analytical methods on which we count, and finally we will address their role in diagnosis of cancer as well their proposed role as antioxidant and antiglycation agents.

Tissue transglutaminase: an enzyme with a split personality

Tissue transglutaminase (tTG) belongs to the family of transglutaminase enzymes that catalyze the posttranslational modification of proteins via Ca(2+)-dependent cross-linking reactions. The catalytic action of tTG results in the formation of an isopeptide bond that is of great physiological significance since it is highly resistant to proteolysis and denaturants. Although tTG-mediated cross-linking reactions have been implicated to play a role in diverse biological processes, the precise physiological function of the enzyme remains unclear. Recent data, however, suggest that the protein polymers resulting from tTG-catalyzed reactions may play a role in commitment of cells to undergo apoptosis. On the same token, tTG-mediated formation of insoluble protein aggregates may underlie the markers of numerous pathological conditions, such as the senile plaques in Alzheimer's disease and the Lewy bodies in Parkinson's disease. In addition to catalyzing Ca(2+)-dependent cross-linking reactions, tTG can also bind and hydrolyze guanosine triphosphate and adenosine triphosphate. By virtue of this ability, tTG has been identified as a novel G-protein that interacts and activates phospholipase C following stimulation of the alpha-adrenergic receptor. The ability of tTG to mediate signal transduction may contribute to its involvement in the regulation of cell cycle progression. The following review summarizes the important features of this multifunctional enzyme that have emerged as a result of recent work from different laboratories.

Tumour growth modifies intravascular polyamine transport by plasma lipoproteins in the mouse

Polyamines are polycationic compounds which are implicated in cell division and tumor growth. We have evaluated the potential role of plasma lipoproteins in the transport of major polyamines, spermine, spermidine and putrescine, and the effect of tumor growth on such transport. Plasmas of healthy male BL6/DBA2 mice and of mice bearing Lewis lung carcinoma (3LL) were fractionated by isopycnic density gradient ultracentrifugation, and polyamine content determined in lipoprotein fractions. Spermidine was the most abundant polyamine in the lipoproteins of both control and tumor-bearing mice and was principally associated with HDL (d: 1.046-1.136 g/ml); approx. 40% of total plasma polyamines was lipoprotein-associated in control mice and 60% in cancerous mice. Only minor amounts were transported by LDL (< 10% of total lipoprotein-associated polyamines), while VLDL were devoid of these substances. Marked elevations of circulating levels of LDL were found in 3LL grafted mice: in these particles however, the contents of spermidine and spermine were significantly reduced. A preferential uptake of polyamines by red blood cells could in part explain this marked reduction of LDL polyamine content, but the consequence of this reduction on the net electrical charge and biochemical function of LDL remains unknown. Elevations of plasma LDL and HDL levels in 3LL-grafted mice underlie the finding that only minor modification was detected in the putrescine content of these particles. However, it is evident that elevated total amounts of putrescine were present in the plasma of such animals. Finally, the density profile of polyamines was modified in cancerous mice in which a shift to transport in lighter apo.AI-containing HDL particles was observed for spermidine; an even more marked shift was found for spermine. In conclusion, our data demonstrate that HDL particles constitute the major plasma vehicle for polyamine transport in both control and in tumor-bearing mice.

Increase in epsilon(gamma-glutamyl)lysine crosslinks in atherosclerotic aortas

Portions of aortas from normal and atherosclerotic rabbits and from human autopsy subjects were washed and separated into layers which were subjected to exhaustive proteolytic digestion. The digests were assayed for epsilon(gamma-glutamyl)lysine crosslinks by a two-stage high performance liquid chromatography (HPLC) procedure. Crosslink concentrations in intima-media from rabbits where more than 15% of the aorta lumen surface was lesioned are greater than in normal aortas or aortas with less than 15% of the surface lesioned. Higher crosslink concentrations occur in fibrolipid plaques from human aortas than in intima-media layers of equal thickness from non-lesioned areas of the same aortas. Much of the crosslink in fibrolipid plaques occurs in the proteins which float at d < 1.18 g/ml. Non-lesioned areas of intima-media from aortas with fatty streaks or plaques have higher crosslink concentrations than intima-media from aortas with no lesions. In normal and lesioned intimas thinner than 0.2 mm, the concentration of the crosslink is lower than in the subjacent media. These findings indicate that increased epsilon(gamma-glutamyl)lysine crosslinking occurs in the atherosclerotic aorta and is associated principally with smooth muscle cells. It is suggested that the crosslinked products may be involved in retention of lipoproteins and the increase in collagen production.

High levels of transglutaminase expression in doxorubicin-resistant human breast carcinoma cells

Tissue type II transglutaminase (TGase) is a member of the TGase family that catalyzes Ca(2+)-dependent covalent cross-linking of several amines to the gamma-carboxamide group of protein-bound glutamine residues. The degree of therapeutic efficacy or toxicity of drugs may be related to their ability to serve as a substrate for TGase and their covalent linkage to glutamine residues of regulatory proteins through the catalytic action of this enzyme. Here, doxorubicin (adriamycin)-resistant human breast carcinoma MCF-7ADR cells exhibited 40- to 6C-fold higher TGase activity than control drug-sensitive MCF-7WT cells. The same was observed in vivo: a small proportion of tumor cells became positive for TGase after administration of adriamycin-based chemotherapy to patients with breast carcinoma. Similarly, continuous culture of MCF-7WT cells in the presence of adriamycin led to the appearance of the drug-resistant phenotype that was in turn associated with increased expression of TGase. This increase in TGase was specific for adriamycin resistance. Like most known TGase, MCF-7ADR TGase was completely dependent on the presence of Ca2+ for its catalytic activity. Based on its immunoreactivity, the TGase in MCF-7ADR cells was identified as an 85-kDa tissue-type TGase and was present only in the soluble form. Immunoblot analysis revealed that the increase in TGase activity was due to accumulation of the protein. Two cytosolic proteins of approximately 20 and 30 kDa in MCF-7 cells served as suitable acyl donor substrates in TGase-catalyzed reactions.

Identification of a novel transglutaminase from the filarial parasite Brugia malayi andits role in growth and development

Recently, we reported the presence of a putative transglutaminase in adult female worms of Brugia malayi [1]. The enzyme activity was shown to be essential for in utero growth and development of microfilariae. Here, we demonstrate that adult worms of B. malayi have a large amount of epsilon-(gamma-glutamyl)lysine isopeptide bonds, a product of physiologically active transglutaminase. A 25-kDa immunoreactive band detected in female worm extracts by a monospecific monoclonal antibody (CUB 7401) against guinea pig liver transglutaminase was associated with the enzymatic activity. Unlike the mammalian enzyme, the parasite enzyme did not require Ca2+ for its catalytic activity. Furthermore, in utero developing embryos, especially during early stages of development, contained very high amounts of this enzyme. Adult female worms contained several proteins that could serve as suitable substrates for the enzyme. Inhibition of the enzyme activity by an enzyme-specific pseudosubstrate, monodansylcadaverine, led to a time- and dose-dependent inhibition of microfilariae production and release by gravid female worms. The inhibition of microfilariae production was due to the inhibition of transglutaminase-catalyzed crosslinking of parasite proteins that in turn seemed to be essential for in utero growth and development of the embryos. The results suggest that transglutaminase-catalyzed reactions may play an important role during early development of embryos to mature microfilariae inside the adult female worms of filarial parasites.

Lipoprotein binding of crosslinked type III collagen aminopropeptide and fractions of its antigen in blood

When [125I] labelled bovine type III collagen aminopropeptide (PIIIP) is incubated with tissue transglutaminase (TGase) mixed with hyperlipemic rabbit plasma and subjected to ultracentrifugation the labelled fraction with density less than 1.2 g/ml is larger than when either lipoprotein or TGase is omitted. Chromatography of the fraction with density less than 1.2 g/ml shows the presence of peaks which are not present in the denser material. Since their elution positions indicate that they have higher molecular weights than PIIIP it is concluded that they consist of [125I]PIIIP which had been crosslinked by TGase and bound to lipoprotein. Low concentrations of similar low density, high molecular weight PIIIP antigens were found in normal human plasma and pooled sera from angiography subjects. In two out of seven infarct patients an unusually large fraction of the PIIIP antigen in the serum was found in a very high molecular weight peak containing low density material. It is speculated that this may arise from atherosclerotic lesions.