Modulation of phenotypic expression of fibroblasts by alteration of the cytoskeleton

Involvement of a cysteine protease in the secretion process of human xylosyltransferase I

Xylosylation of core proteins takes place in the Golgi-apparatus as the transfer of xylose from UDP-xylose to specific serine residues in proteoglycan core proteins. This initial and rate-limiting step in glycosaminoglycan biosynthesis is catalyzed by human xylosyltransferase I (XT-I). XT-I is proteolytically cleaved from the Golgi surface and shed in its active form into the extracellular space. The secreted, circulating glycosyltransferase represents a serum biomarker for various diseases with an altered proteoglycan metabolism, whereas a physiological function of secreted XT-I is still unknown. To shed light on the secretion process of XT-I and on its biological function, the cleavage site was examined and the group of proteases involved in the cleavage was identified in this study. The peptide mass fingerprint from partly purified secreted XT-I revealed the cleavage site to be localized in the aminoterminal 231 amino acids. The addition of a cysteine protease inhibitor cocktail to cells recombinantly expressing XT-I led to a concentration-dependent shift of enzyme activity towards the cell lysates attended by consistent total intracellular and extracellular XT-I activities. In conclusion, our findings provide a first insight into the XT-I secretion process regulated by a cysteine protease and may contribute to understanding the biological and pathological role of this process.

Osteogenic cell contact with biomaterials influences phenotype expression

Relationship between (1) osteoblast adhesion and spreading, and (2) phenotype expression was investigated. Cellular adhesion and spreading were estimated after short time (24 h), whereas proliferation and other osteoblast functions--after 7 days. Primary human osteogenic cells were seeded on the samples of titanium (T), surgical steel (S) and tissue culture polystyrene (PS), and incubated at 37 degrees C. After 24 h a number of samples were stained with crystal violet and Hoechst; the average single cell area (spreading) and adhering cell number was measured on each sample. The remaining cultures were supplemented with dexamethasone (10 nM) and beta-glycerophosphate (5 mM), and incubation was continued for 7 days. The cells on each sample were counted and the following tests were performed: XTT mitochondrial activity assay, total protein content, alkaline phosphatase activity (ALP), Sirius Red test for collagen, osteocalcin and calcium concentration. After 24 h significantly greater cell spreading (p < 0.05) and number (p < 0.05) were on T than on S. After 7 days significantly higher on T than on S were: ALP activity (p < 0.000001), collagen (p < 0.0015) and calcium concentration (p < 0.03). XTT results were bigger on S than on T. In control - XTT results were higher than on the metals; collagen and ALP were lower than on T, and calcium level was significantly lower than on T and S (p < 0.025). After 7 days there were no differences in cell number between T and S. Cell number (24 h) correlated with ALP activity (7 days) on steel (coefficient of correlation, CC = 0.866) and titanium (CC = 0.742). The spreading correlated on steel and on titanium with calcium concentration (CC = 0.645 on S, CC = 0.696 on T) and collagen level (CC = -0.638 on S, CC = -0.69 on T).

CONCLUSIONS

Better conditions for osteoblast phenotype expression on T after 7 days of culture coincided with greater adhesion and spreading of cells after 24 h on T, as compared with S. The initial contact of cells with underlying surface may influence osteoblast functions and possibly, bone regeneration and implant osteointegration in vivo. Early cell spreading may be an indicator of further expression of osteoblast phenotype and may be important for application of osteogenic cells in reconstructive surgery.

Serum xylosyltransferase: a new biochemical marker of the sclerotic process in systemic sclerosis

UDP-D-xylose:proteoglycan core protein beta-D-xylosyltransferase (EC2.4.2.26) is the initial enzyme in the biosynthesis of chondroitin sulfate and dermatan sulfate proteoglycans in fibroblasts and chondrocytes. Secretion of xylosyltransferase into the extracellular space was determined in cultured human dermal fibroblasts. A more than 6-fold accumulation of xylosyltransferase activity in cell culture supernatant was observed (day 1, 0.6 microU per 106 cells; day 9, 4.1 microU per 106 cells); however, intracellular xylosyltransferase activity remained at a constant level (0.4 microU per 106 cells). Exposure of human chondrocytes to colchicine led to a 3-fold decreased level of xylosyltransferase and chondroitin-6-sulfate concentration in cell culture. Specific xylosyltransferase activity and chondroitin-6-sulfate concentration decreased in a concentration-dependent manner and in parallel in culture medium and accumulated 5-fold in cell lysates indicating that xylosyltransferase is secreted simultaneously into the extracellular space with chondroitin sulfate proteoglycans. Xylosyltransferase activities were determined in serum samples of 30 patients with systemic sclerosis. Xylosyltransferase activities in female (mean value 1.28 mU per liter, 90% range 1.10-1.55 mU per liter) and male patients (mean 1.39 mU per liter, 90% range 1.16-1. 57 mU per liter) with systemic sclerosis were significantly increased in comparison with blood donors of a corresponding age. Furthermore, xylosyltransferase activity was correlated with the clinical classification of systemic sclerosis. Female patients with diffuse cutaneous systemic sclerosis showed higher serum xylosyltransferase activities than patients with limited systemic sclerosis. These results confirm that the increase of proteoglycan biosynthesis in sclerotic processes of scleroderma is closely related to an elevated xylosyltransferase activity in blood and demonstrate the validity of xylosyltransferase as an additional diagnostic marker for determination of sclerotic activity in systemic sclerosis.

Comparative effects of TGFbeta on proliferation of 7- and 14-day-old chick embryo fibroblasts and lack of involvement of the ODC/PA system in the TGFbeta signaling pathway

The growth regulatory activity of transforming growth factor beta (TGFbeta) on chick embryo skin fibroblasts was compared in two developmental ages, days 7 and 14. The time course of 3H-thymidine incorporation, an S-phase marker of replication, was determined during 36 hr of TGFbeta treatment. Seven-day-old cells showed a prereplicative phase of 6 hr, and 14-day-old cells showed a prereplicative phase of 12 hr. DNA synthesis peaked at 24 hr in 7-day-old fibroblasts and was 10 times higher than that in 14-day-old fibroblasts. Ornithine decarboxylase (ODC) activity and content of the natural polyamines spermine (Spm), spermidine (Spd), and putrescine (Put) differed during cell cycle. ODC activity peaked at 12 hr in 7-day-old cells and at 6 hr in 14-day-old cells. Its level was two times higher at day 7 and was associated with a greater content of ODC mRNA. The maximum of polyamine (PA) concentration was determined after 12 hr of treatment in 7-day-old cells and after 36 hr in 14-day-old cells. These findings indicate that the TGFbeta proliferative response of embryo fibroblasts changes during development and is associated with activation of the ODC/PA system. Cotreatment with alpha-difluoromethylornithine, an enzyme-activated irreversible inhibitor of ODC, did not reduced growth rate. Inhibition of ODC resulted in levels of Put and Spd comparable to that of quiescent fibroblasts, whereas Spm concentration remained higher. Because an altered ODC metabolism does not convey the effects of TGFbeta on DNA synthesis, the ODC/PA system may not play a role in the pathway of TGFbeta signaling.

Microtubule assembly is regulated by externally applied strain in cultured smooth muscle cells

Mechanical forces clearly regulate the development and phenotype of a variety of tissues and cultured cells. However, it is not clear how mechanical information is transduced intracellularly to alter cellular function. Thermodynamic modeling predicts that mechanical forces influence microtubule assembly, and hence suggest microtubules as one potential cytoskeletal target for mechanical signals. In this study, the assembly of microtubules was analyzed in rat aortic smooth muscle cells cultured on silicon rubber substrates exposed to step increases in applied strain. Cytoskeletal and total cellular protein fractions were extracted from the cells following application of the external strain, and tubulin levels were quantified biochemically via a competitive ELISA and western blotting using bovine brain tubulin as a standard. In the first set of experiments, smooth muscle cells were subjected to a step-increase in strain and the distribution of tubulin between monomeric, polymeric, and total cellular pools was followed with time. Microtubule mass increased rapidly following application of the strain, with a statistically significant increase (P<0.05) in microtubule mass from 373+/-32 pg/cell (t=0) to 514+/-30 pg/cell (t=15 minutes). In parallel, the amount of soluble tubulin decreased approximately fivefold. The microtubule mass decreased after 1 hour to a value of 437+/-24 pg/cell. In the second set of experiments, smooth muscle cells were subjected to increasing doses of externally applied strain using a custom-built strain device. Monomeric, polymeric, and total tubulin fractions were extracted after 15 minutes of applied strain and quantified as for the earlier experiments. Microtubule mass increased with increasing strain while total cellular tubulin levels remained essentially constant at all strain levels. These findings are consistent with a thermodynamic model which predicts that microtubule assembly is promoted as a cell is stretched and compressional loads on the microtubules are presumably relieved. Furthermore, these data suggest microtubules are a potential target for translating changes in externally applied mechanical stimuli to alterations in cellular phenotype.

Amphiphysin I antisense oligonucleotides inhibit neurite outgrowth in cultured hippocampal neurons

Amphiphysin I is an SH3 domain-containing neuronal protein, enriched in axon terminals, which was reported to act as a physiological binding partner for dynamin I in synaptic vesicle endocytosis. Rvs167 and Rvs161, the yeast homologs of amphiphysin I, have been implicated in endocytosis, actin function, and cell polarity. Now we have explored the possibility that amphiphysin I also may have a role in actin dynamics and cell polarity by testing the effect of amphiphysin I suppression on neurite outgrowth. Freshly plated hippocampal neurons were exposed to antisense oligonucleotides via a new delivery system based on a polycationic amphipathic polymer, PS980. Western blot analysis revealed that amphiphysin I levels steadily increased with neuronal differentiation, whereas in antisense-treated cultures amphiphysin I levels were reduced to approximately 10% of control levels at 48 hr. Concomitantly, a collapse of growth cones and a severe inhibition of neurite outgrowth and axon formation were observed. A similar effect was observed previously after dynamin I suppression in the same culture system (). We also have found that amphiphysin I and dynamin I colocalize in developing neurons at all developmental stages and that a pool of both proteins is colocalized with actin patches at the leading edge of growth cones. Our findings suggest a conserved role of the amphiphysin protein family in the dynamics of the cortical cell cytoskeleton and provide new evidence for a close functional link between amphiphysin I and dynamin I.

Nuclear accumulation of G-actin in isolated rat hepatocytes by adenine nucleotides

Extracellular ATP induces bleb formation in isolated rat hepatocytes. We examined the effect of extracellular ATP on the actin cytoskeleton of these hepatocytes. Exposure to 100 microM ATP caused pronounced nuclear accumulation of G-actin. ADP, AMP, adenosine, and dibutyryl-cAMP induced the same effect. Adenosine deaminase could inhibit both ATP- and adenosine-induced nuclear accumulation. The P2-receptor agonists, UTP and 2' & 3'-O-(4-benzoylbenzoyl)-adenosine 5'-triphosphate, did not induce this redistribution of G-actin. Phalloidin, which prevents depolymerisation of F-actin filaments to G-actin monomers, inhibited adenosine-induced nuclear accumulation of G-actin. These observations suggest that nuclear accumulation of G-actin is mediated by adenosine receptors.

Regulation of parathyroid hormone-related protein secretion and mRNA expression in normal human keratinocytes and a squamous carcinoma cell line

Parathyroid hormone-related protein (PTHrP) has been identified as a causative factor in the pathogenesis of humoral hypercalcemia of malignancy (HHM). The regulation and mechanisms of PTHrP secretion in most normal and malignant cells are unknown. PTHrP secretion, mRNA expression, and transcription were measured in neoplastic human squamous carcinoma cells (A253) and normal human foreskin keratinocytes (NHFK) by radioimmunoassay, RNase protection assay, and transient transfections of the 5'-flanking region of human PTHrP in a luciferase expression vector. Mechanisms of PTHrP secretion were investigated using chemicals (monensin, colchicine, cytochalasin B, guanosine 5'-[gamma-thio]triphosphate (GTPgammaS)) that interfere with or facilitate intracellular transport. Monensin inhibited PTHrP secretion in both NHFK and A253 cells. Ultrastructurally, monensin caused dilatation of rough endoplasmic reticulum and the formation of numerous cytoplasmic secretory vacuoles in both cell lines. Colchicine decreased PTHrP production in NHFK cells and stimulated PTHrP production and mRNA levels in A253 cells. Colchicine also stimulated transcription of the PTHrP-luciferase reporter gene. Cytochalasin B stimulated PTHrP secretion and mRNA expression in A253 cells, but had no effect in NHFK cells. GTPgammaS had no effect on PTHrP secretion in either cell line. It was concluded that PTHrP secretion is dependent on the constitutive movement of secretory vesicles to the cytoplasmic membrane and regulation of PTHrP secretion and mRNA expression are altered in squamous carcinoma cells compared to normal human keratinocytes in vitro.