HAb18G/CD147-mediated calcium mobilization and hepatoma metastasis require both C-terminal and N-terminal domains

HAb18G/CD147 is a heavily glycosylated protein containing two immunoglobulin superfamily domains. Our previous studies have indicated that overexpression of HAb18G/CD147 enhances metastatic potentials in human hepatoma cells by disrupting the regulation of store-operated Ca²⁺ entry by nitric oxide (NO)/cGMP. In the present study, we investigated the structure-function of HAb18G/CD147 by transfecting truncated HAb18G/CD147 fragments into human 7721 hepatoma cells. The inhibitory effect of HAb18G/CD147 on 8-bromo-cGMP-regulated thapsigargin-induced Ca²⁺ entry was reversed by the expression of either C or N terminus truncated HAb18G/CD147 in T7721C and T7721N cells, respectively. The potential effect of HAb18G/CD147 on metastatic potentials, both adhesion and invasion capacities, of hepatoma cells was abolished in T7721C cells, but not affected in T7721N cells. Release and activation of matrix metalloproteinases (MMPs), MMP-2 and MMP-9, were found to be enhanced by the expression of HAb18G/CD147, and this effect was abolished by both truncations. Thapsigargin significantly enhanced release and activation of MMPs (MMP-2 and MMP-9) in non-transfected 7721 cells, and this effect was negatively regulated by SNAP. However, no effects of thapsigargin or SNAP were observed in T7721 cells, and expression of HAb18G/CD147 enhanced secretion and activation of MMPs at a stable and high level. Taken together, these results suggest that both ectodomain and intracellular domains of HAb18G/CD147 are required to mediate the effect of HAb18G/CD147 on the secretion and activation of MMPs and metastasis-related processes in human hepatoma cells by disrupting the regulation of NO/cGMP-sensitive intracellular Ca²⁺ mobilization although each domain may play different roles.Received 1 April 2004; received after revision 15 June 2004; accepted 22 June 2004     

Oxygen intervention in the regulation of gene expression: the photosynthetic bacterial paradigm

The means by which oxygen intervenes in gene expression has been examined in considerable detail in the metabolically versatile bacterium Rhodobacter sphaeroides. Three regulatory systems are now known in this organism, which are used singly and in combination to modulate genes in response to changing oxygen availability. The outcome of these regulatory events is that the molecular machinery is present for the cell to obtain energy by means that are best suited to prevailing conditions, while at the same time maintaining cellular redox balance. Here, we explore the dangers associated with molecular oxygen relative to the various metabolisms used by R. sphaeroides, and then present the most recent findings regarding the features and operation of each of the three regulatory systems which collectively mediate oxygen control in this organism.Received 26 June 2003; received after revision 30 July 2003; accepted 8 August 2003     

The molecular mechanisms of congenital hypofibrinogenaemia

Congenital hypofibrinogenaemia is characterized by abnormally low levels of fibrinogen and is usually caused by heterozygous mutations in the fibrinogen chain genes (, and ). However, it does not usually result in a clinically significant condition unless inherited in a homozygous or compound heterozygous state, where it results in a severe bleeding disorder, afibrinogenaemia. Various protein and expression studies have improved our understanding of how mutations causing hypo- and afibrinogenaemia affect secretion of the mature fibrinogen molecule from the hepatocyte. Some mutations can perturb chain assembly as in the 153 Cys Arg case, while others such as the B Leu Arg and the B414 Gly Ser mutations allow intracellular hexamer assembly but inhibit protein secretion. An interesting group of mutations, such as 284 Gly Arg and 375 Arg Trp, not only cause hypofibrinogenaemia but are also associated with liver disease. The nonexpression of these variant chains in plasma fibrinogen is due to retention in the endoplasmic reticulum, which in turn leads to hypofibrinogenaemia.Received 17 December 2003; received after revision 19 January 2004; accepted 21 January 2004     

Ubiquitin-proteasome pathway as a primary defender against TRAIL-mediated cell death

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces apoptotic cell death as well as expression of proinflammatory genes such as CXCL8 in malignant human astrocytoma cells. However, the molecular mechanisms that determine the fate of cells are not yet understood. The ubiquitin (Ub)-proteasome pathway regulates a wide range of cellular functions through degradation of various regulatory proteins; given this, we hypothesized that this pathway may play a central role in TRAIL-mediated signaling. We demonstrate here that inhibition of the Ub-proteasome pathway enhanced TRAIL-mediated cell death of human astrocytoma CRT-MG cells within hours by blocking degradation of active caspase-8 and -3. Proteasome inhibitors suppressed TRAIL-mediated activation of NF-B; however, inhibition of the NF-B pathway alone was not sufficient to enhance TRAIL-mediated cell death. Collectively, these results suggest that the Ub-proteasome pathway may play an important role as an antiapoptotic surveillance system by eliminating activated caspases as well as mediating NF-B-dependent signals.Received 30 December 2003; received after revision 9 February 2004; accepted 13 February 2004     

Novel aspects of glypican glycobiology

Mutations in glypican genes cause dysmorphic and overgrowth syndromes in men and mice, abnormal development in flies and worms, and defective gastrulation in zebrafish and ascidians. All glypican core proteins share a characteristic pattern of 14 conserved cysteine residues. Upstream from the C-terminal membrane anchorage are 3–4 heparan sulfate attachment sites. Cysteines in glypican-1 can become nitrosylated by nitric oxide in a copper-dependent reaction. When glypican-1 is exposed to ascorbate, nitric oxide is released and participates in deaminative cleavage of heparan sulfate at sites where the glucosamines have a free amino group. This process takes place while glypican-1 recycles via a nonclassical, caveolin-1-associated route. Glypicans are involved in growth factor signalling and transport, e.g. of polyamines. Cargo can be unloaded from heparan sulfate by nitric oxide-dependent degradation. How glypican and its degradation products and the cargo exit from the recycling route is an enigma.Received 27 November 2003; received after revision 8 January 2004; accepted 13 January 2004     

What’s new in the renin-angiotensin system?

The angiotensin AT(4) receptor was originally defined as the specific, high-affinity binding site for the hexapeptide angiotensin IV (Ang IV). Subsequently, the peptide LVV-hemorphin 7 was also demonstrated to be a bioactive ligand of the AT(4) receptor. Central administration of Ang IV, its analogues or LVV-hemorphin 7 markedly enhance learning and memory in normal rodents and reverse memory deficits observed in animal models of amnesia. The AT(4) receptor has a broad distribution and is found in a range of tissues, including the adrenal gland, kidney, lung and heart. In the kidney Ang IV increases renal cortical blood flow and decreases Na(+) transport in isolated renal proximal tubules. The AT(4) receptor has recently been identified as the transmembrane enzyme, insulin-regulated membrane aminopeptidase (IRAP). IRAP is a type II integral membrane spanning protein belonging to the M1 family of aminopeptidases and is predominantly found in GLUT4 vesicles in insulin-responsive cells. Three hypotheses for the memory-potentiating effects of the AT(4) receptor/IRAP ligands, Ang IV and LVV-hemorphin 7, are proposed: (i) acting as potent inhibitors of IRAP, they may prolong the action of endogenous promnestic peptides; (ii) they may modulate glucose uptake by modulating trafficking of GLUT4; (iii) IRAP may act as a receptor, transducing the signal initiated by ligand binding to its C-terminal domain to the intracellular domain that interacts with several cytoplasmic proteins.     

Oncogenic protein tyrosine kinases

Signals through Kit receptor tyrosine kinase are essential for development of erythrocytes, melanocytes, germ cells, mast cells and interstitial cells of Cajal (ICCs). Mice and rats with a double gene dose of loss-of-function mutations of Kit show depletion of these cells. Although human homozygotes with loss-of-function mutations of Kit have not been reported, gain-of-function mutations of Kit result in development of tumors from mast cells, germ cells and ICCs in humans. The ICC tumors are called gastrointestinal stromal tumors (GISTs), and GISTs are a good target for the Kit inhibitor imatinib mesylate. The interrelationship between the type of Kit gain-of-function mutations and the therapeutic effect of imatinib mesylate has been well characterized in GISTs. Kit is interesting from both a biological and clinical view-point.     

Oncogenic protein tyrosine kinases

HER2 (human epidermal growth factor receptor-2; also known as erbB2) and its relatives HER1 (epidermal growth factor receptor; EGFR), HER3 and HER4 belong to the HER family of receptor tyrosine kinases. In normal cells, activation of this receptor tyrosine kinase family triggers a rich network of signaling pathways that control normal cell growth, differentiation, motility and adhesion in several cell lineages. The first tumor studied for an alteration of the HER2 oncogene is breast carcinoma, and so far the majority of studies have been performed on this oncotype. Although involvement of HER2 as a cause of human cell transformation needs to be further investigated, overexpression of the HER2 oncogene in human breast carcinomas has been associated with a more aggressive course of disease. It has been suggested that this association depends on HER2-driven proliferation, vessel formation and/or invasiveness; however, poor prognosis may not be directly related to the presence of the oncoprotein on the cell membrane but instead to the breast carcinoma subset identified by HER2 overexpression and characterized by a peculiar gene expression profile, as recently identified. HER2-positive tumors were recently shown to benefit from anthracyclin treatment and to be resistant to endocrine therapy. Despite the fact that many pathways interacting with HER2 are still not fully understood, this tyrosine kinase receptor is, to date, a promising molecule for targeted therapy.