Retinoic acid alters the proportion of high mannose to complex type oligosaccharides on fibronectin secreted by cultured chondrocytes

Transcriptional suppression of Sox9 expression in chondrocytes by retinoic acid

SOX9 is a transcription factor that is expressed in chondrocytes and regulates expression of chondrocyte phenotype related genes. Expression of these genes is known to be suppressed by retinoic acid (RA). We, therefore, examined whether the Sox9 gene expression is regulated by RA in chondrocytes. RA treatment suppressed Sox9 mRNA expression in primary chondrocytes prepared from newborn mouse rib cartilage within 12 h and this suppression lasted at least up to 24 h. The RA suppression of Sox9 mRNA levels was dose-dependent starting at 0.5 microM with a maximum at 1 microM. Nuclear run-on assays revealed that RA reduced the rate of transcription of Sox9 gene. Finally, Western blot analysis indicated that RA suppressed SOX9 protein levels in these chondrocytes. Furthermore, overexpression of SOX9 reversed RA suppression of Col2a1 enhancer activity. These observations indicate that RA suppresses Sox9 gene expression in chondrocytes at least in part through transcriptional events. J. Cell. Biochem. Suppl. 36: 71-78, 2001.

Effects of retinoic acid on N-glycosylation and mRNA stability of the liver/bone/kidney alkaline phosphatase in neuronal cells

Alkaline phosphatase (ALP) is a glycoenzyme that is highly expressed during carcinogenesis and is induced by retinoic acid (RA) in various cells. We investigated the effects of RA on N-linked glycosylation of the tissue nonspecific liver/bone/kidney- type of ALP (L/B/K ALP), on ALP transcripts, and on total protein glycosylation in two neuronal cell lines, P19 and NG108CC15, and in primary cultures of neonatal rat brain. ALP activity was determined in cell extracts and found to be induced by RA. Tunicamycin was used at various concentrations to inhibit protein N-glycosylation. After treatment of cells with low concentrations (0.1 and 0.125 microgram/ml) of tunicamycin for 48 h, uninduced and RA-induced ALP activity declined while incubation with a protease inhibitor restored activity, indicating that the L/B/K ALP bear N-linked oligosaccharide chains important for maintaining enzymatic activity. Interestingly, ALP activity in RA-treated cultures was less inhibited by tunicamycin compared to untreated controls suggesting that RA may have an impact on ALP N-glycosylation. To investigate effects of RA on ALP glycosylation further, incorporation of [(14)C]mannose and [(35)S]methionine into ALP protein was determined in the presence or absence of RA. The ratio of mannosylation and biosynthesis demonstrate that incubation of cells with RA increased [(14)C]mannose incorporation into ALP molecules. Also, the release of free [(14)C]mannose from ALP molecules relative to the amount of protein by N-Glycanase was increased in RA-treated cultures. In addition, mannosylation of total protein was found to be induced in cells after exposure to RA. Analysis of biosynthesized ALP monomers revealed that RA increased glycosylation of the polypeptides. Furthermore, tunicamycin decreased the stability of ALP mRNA, an effect that was reduced by cotreatment with RA. Thus, the degree of N-glycosylation of the L/B/K ALP as well as mRNA and protein levels of this enzyme are affected by RA. The P19 cell line provides a useful model system to study the molecular mechanism(s) underlying the action of RA on glycosylation during neuronal differentiation further.

Involvement of alpha5beta1 integrin in matrix interactions and proliferation of chondrocytes

Integrins are cell surface receptors involved in cellular processes including adhesion, migration, and matrix assembly. In the present study, we analyzed the possible involvement of alpha 5 beta 1 integrin in the regulation of chondrocyte adhesion, spreading, and proliferation. We found that rabbit growth plate chondrocytes were able to attach to substrates coated with type I collagen, type II collagen, or fibronectin within 24 h of culture. During this time period, attachment to fibronectin appeared to be dependent on alpha 5 beta 1 integrin, whereas adhesion to collagens was not. By day 3 of culture, chondrocytes spread onto all the substrates tested. We found that regardless of the nature of the substrate, cell spreading was reversed by treatment with RGD peptide or antibodies against alpha 5 beta 1 or fibronectin, indicating that cell spreading involved alpha 5 beta 1 and fibronectin endogenously produced and deposited by the chondrocytes themselves. Colony formation by chondrocytes in soft agar was inhibited by treatment with RGD peptides or BIIG2, an antibody that interferes with alpha 5 beta 1 integrin-ligand interactions. Furthermore, DNA content was decreased by treatment with anti-fibronectin antibody in micromass culture of chondrocytes. Immunohistochemical analysis on tissue sections revealed that the alpha 5 subunit was particularly abundant in the proliferative and hypertrophic zones of growth plate. The results of the study indicate that alpha 5 beta 1 integrin plays multiple roles in chondrocyte behavior and function and appears to be involved in the regulation of both chondrocyte-matrix interactions and proliferation.

Peritrophic matrix proteins of Anopheles gambiae and Aedes aegypti

Little is known about the composition and function of the mosquito peritrophic matrix (PM), a physical barrier that pathogens must traverse to complete their life cycles. Anopheles gambiae and Aedes aegypti PM proteins induced by blood or by a protein-free meal have been characterized by the use of 2-D gel electrophoresis and lectin-binding affinity assays. More than forty proteins have been identified in both species. Over half of the PM proteins of both mosquitoes migrate identically. Many PM proteins appear to be glycosylated, primarily by high mannose N-linked glycosyl groups.

Modulation of oligosaccharide structure of a pro-urokinase derivative (pro-UK delta GS1) by changing culture conditions of a lymphoblastoid cell line Namalwa KJM-1 adapted to serum-free medium

Pro-UK delta GS1 was designed as a long-life and thrombin-resistant derivative of pro-urokinase (pro-UK) by deleting the growth factor domain of pro-UK and introducing a glycosylation site near the thrombin cleaving site for thrombin-resistance using site-directed mutagenesis. An expression plasmid for pro-UKDGS1, pIH1UK delta GS1SEd1-5 was constructed and introduced into Namalwa KJM-1, a lymphoblastoid cell line adapted to serum-free medium, and cells resistant to G418 and Methotrexate (MTX) were obtained. Amongst them, the highest pro-UK delta GS1 producer (resistant to 200 nM of MTX), clone 2-9, was selected and used for further studies. Under the conventional conditions, i.e. at 37 degrees C in serum-free ITPSGF medium (based on RPMI-1640 medium), the oligosaccharide structure of pro-UK delta GS1 produced by clone 2-9 mainly consisted of fucose (Fuc)-containing biantennary complex-type oligosaccharide. Addition of dexamethasone (Dex), changed the carbohydrate contents in the media, and a shift down of incubation temperature caused a change in oligosaccharide structure of pro-UK delta GS1 from mainly Fuc-containing biantennary to mainly Fuc-containing tri- and tetraantennary complex-type oligosaccharide. The modulated pro-UK delta GS1 showed superior in vivo activity for a canine femoral thrombosis formed by inserting a copper-coil.

Retinoic acid enhances adhesiveness, laminin and integrin beta 1 synthesis, and retinoic acid receptor expression in F9 teratocarcinoma cells

The teratocarcinoma-derived F9 cells respond to retinoic acid (RA) and RA plus dibutyrylcyclic adenosine monophosphate (dcAMP) by differentiating into endoderm cells, which elaborate a laminin and type IV collagen-rich matrix. We found that the induction of differentiation is accompanied by a small but consistent increase in cell adhesiveness to a variety of substrates, including laminin. Therefore we investigated biochemical mechanisms involved in this phenomenon. Endoglycosidase treatment showed that laminin contains complex and hybrid oligosaccharide structures. RA enhanced general biosynthesis of laminin without a specific increase in galactose incorporation: this sugar was mainly in polylactosamine structures in the A chain of laminin and as terminal galactose alpha 1,3 galactose in the B chain. Laminin receptor analysis showed that RA decreased laminin binding protein-37 (LBP-37) but increased the amount of beta 1 integrin, suggesting the involvement of beta 1 integrin in the attachment process. Northern blot analysis showed increased expression of retinoid receptors within hours of RA exposure. These studies demonstrate that RA increases cell to substrate interactions by increasing the biosynthesis of laminin and beta 1 integrin. These effects are most likely subsequent to the RA-induced biosynthesis of the retinoid receptors.

Regulation of expression of the chondrocytic phenotype in a skeletal cell line (CFK2) in vitro

We have examined in vitro the spontaneous and regulated expression of phenotypic characteristics associated with differentiated chondrocytes in an established skeletal cell line (CFK2) derived from fetal rat calvariae. Extended culture of CFK2 cells resulted in the appearance of glycosaminoglycans and type II collagen in the cell layer in association with the formation of focal nodes of cells. In addition, induction of mRNA-encoding link protein, cartilage-specific proteoglycan core protein, and thrombospondin was observed in the differentiated population (dCFK2 cells). The expression of these mRNAs was present for at least two passages after subculturing the dCFK2 cells. The dCFK2 cells also demonstrated enhanced parathyroid hormone (PTH)-stimulated adenylate cyclase activity. Proliferation of CFK2 cells was stimulated by the peptide regulatory factors EGF and PTH and inhibited by the steroidal agents dexamethasone and retinoic acid. EGF and retinoic acid inhibited the formation of cell foci and glycosaminoglycan deposition and the expression of mRNA-encoding link protein. In contrast, PTH and dexamethasone enhanced the formation of focal cellular nodes and augmented matrix deposition and link protein mRNA expression. These studies therefore show that the CFK2 cell line can serve as a nontransformed model of rat chondrocytic cells in which both induction and regulation of the expression of cartilaginous matrix components can be observed. This line thereby provides a unique renewable source of chondrocytic precursor cells and an excellent in vitro model for evaluating temporal and environmental control of chondrocyte differentiation and cartilage matrix production.

Effect of retinyl acetate on the assembly of the fibronectin extracellular matrix and the processing of the fibronectin receptor beta subunit of confluent C3H/10T1/2 mouse embryo fibroblasts

The mouse embryo fibroblast cell line, C3H/10T1/2, synthesized and deposited a large amount of fibronectin especially in the pericellular matrix. Confluent cultures of these cells cultured in the presence of 0.3 micrograms/ml of retinyl acetate released cell surface fibronectin and the extracellular matrix fibronectin fibrils were disorganized. The immunoblot analysis demonstrated that the number of the fibronectin receptor was decreased in the prolonged culturing of retinyl acetate-treated cells. Immunoprecipitation of 35S-methionine pulse-chase labeled cell extracts by antifibronectin receptor antibody indicated that about one-half of the pre-beta subunit was processed and converted to the mature form in control cells, and only about one-fourth of the pre-beta subunit was processed in the retinyl acetate-treated confluent cells. 1-deoxymannojirimycin (MNJ), which is an inhibitor of oligosaccharide processing, induced disorganization of the extracellular matrix fibronectin assembly similar to that observed with retinyl acetate. The results of this study suggest that a mechanism of action of retinyl acetate is inhibition of the glycosylation during processing of the fibronectin receptor, a step necessary for fibronectin binding and for assembly of the extracellular matrix.

Etretinate or cyclosporin-A treatment normalizes the enhanced respiratory burst of polymorphonuclear leukocytes in psoriasis

During a therapeutic trial to treat psoriasis with either etretinate or cyclosporin A (CyA) we measured the respiratory burst activity of polymorphonuclear leukocytes (PMN). Six patients received 0.5-0.75 mg/kg etretinate and 14 patients 2.5-5.0 mg/kg CyA over a period of 10 weeks. The extent of psoriasis was graded by the psoriasis area-and-severity index (PASI score). The respiratory burst of PMN isolated from the peripheral blood was measured employing luminol-enhanced chemiluminescence at weeks 0, 3 and 10 and compared with that of 26 healthy control individuals. PMN were stimulated with zymosan particles, aggregated immunoglobulin (aggIg) and concanavalin A (ConA). Both treatment regimens improved psoriasis; at 10 weeks there was an approximate 40% PASI score reduction under etretinate and an 80% improvement under CyA. Before treatment the respiratory burst was abnormally high under stimulation with the three stimuli in patients (p = 0.021 to less than 0.0001). After 3 to 10 weeks PMN activity normalized in all patients and even tended to drop below values correlating with an improvement in skin lesions. We conclude that the elevated respiratory burst of PMN in psoriasis normalizes under treatment with both etretinate and CyA.

N-glycosylation of membrane glycoproteins in retinol-deficient rat liver

The effect of vitamin A deficiency on N-linked oligosaccharides of membrane glycoproteins was studied in rat liver in order to evaluate the suggested role of retinol in protein N-glycosylation. First, oligosaccharides of newly synthesized glycoproteins from rough endoplasmic reticulum of vitamin A deficient liver were compared with that of pair-fed controls. Oligosaccharides were metabolically labelled with D-[2-3H]mannose, released from the glycoproteins with endoglycosidase H, purified by reversed phase HPLC and ion exchange chromatography, and were reduced with sodium borohydride. HPLC fractionation of the oligosaccharide alditols showed that the glycoproteins carried mainly four oligosaccharide species, Glc1Man9GlcNAc2, Man9GlcNAc2, Man8GlcNAc2 and Man7GlcNAc2, in identical relative amounts in the vitamin A deficient and the control tissue. In particular, no increase in the proportion of short chain oligosaccharides was noted in vitamin A deficient liver. Second, the number of N-linked oligosaccharides was estimated in dipeptidylpeptidase IV (DPP IV), a major glycoprotein constituent of the hepatic plasma membrane, comparing the newly synthesized glycoprotein from rough endoplasmic reticulum and the mature form of DPP IV from the plasma membrane. No evidence was obtained that retinol deficiency caused incomplete glycosylation of this membrane glycoprotein. From these data, the suggested role of retinol as a cofactor involved in the synthesis of N-linked oligosaccharides of glycoproteins must be questioned.

Retinoids enhance lectin binding to gp130, a glycoprotein of NIH-3T3 cells: correlation with cell growth and adhesion

Our previous work has shown that retinoic acid (RA) enhances fibroblast cell attachment to plastic and to laminin. The treatment of NIH-3T3 cells with RA for 2 days also caused a reproducible increase in the binding of the lectin Phaseolus vulgaris leukoagglutinin (PHA-L) to a glycoprotein of molecular weight 130,000 (gp130) as judged by SDS-PAGE analysis. This finding is consistent with an increased number of beta-1,6-linked N-acetylglucosaminyl residues on gp130. Of the 11 additional lectins tested Ricinus communis agglutinin I (RCA), Phaseolus vulgaris erythroagglutinin (PHA-E), soybean agglutinin (SBA), and succinylated wheat germ agglutinin (sWGA) showed a significant increase in binding specifically to gp130. Similar to RA, 13-cis-RA and 3,5-di-tert-butyl-4-chalcone carboxylic acid, a synthetic retinoid, also increased PHA-L binding to gp130; they also enhanced cell adhesiveness and inhibited cell growth. N-(4-Hydroxyphenyl)-all-trans-retinamide and thyroxine failed to influence adhesion and did not increase PHA-L binding to gp130. Moreover these compounds also failed to inhibit cell growth and to alter the morphology of the cultured cells. Since trypsin is utilized to remove cells from the culture dishes before they are used in the attachment assay to laminin, we studied the effect of this trypsinization step on PHA-L binding to gp130. Trypsin reduced PHA-L binding thus suggesting cell surface localization of gp130. After trypsin treatment RA-treated cells still showed enhanced PHA-L binding compared to dimethyl sulfoxide (DMSO) control. In conclusion RA-induced cell adhesiveness and growth inhibition are accompanied by an increase in the PHA-L, PHA-E, SBA, RCA, and sWGA binding to gp130. The sensitivity of gp130 to trypsin suggests that it is a cell surface glycoprotein.

Pregnancy-associated changes in oligomannose oligosaccharides of human and bovine uromodulin (Tamm-Horsfall glycoprotein)

The urinary glycoprotein uromodulin (Tamm-Horsfall glycoprotein) exhibits a pregnancy-associated ability to inhibit antigen-specific T cell proliferation, and the activity is associated with a carbohydrate moiety [Muchmore and Decker (1985) Science 229:479-81; Hession et al., (1987) Science 237:1479-84; Muchmore, Shifrin and Decker (1987) J Immunol 138:2547-53]. We report here that the Man6(7)GlcNAc2-R glycopeptides derived from uromodulin inhibit antigen-specific T cell proliferation by 50% at 0.2-2 microM, and further studies, reported elsewhere, confirm that oligomannose glycopeptides from other sources are also inhibitory, with Man9GlcNAc2-R the most inhibitory of those tested [Muchmore et al., J Leukocyte Biol (in press)]. In this work, we have extended the observation of pregnancy-associated inhibitory activity to a second species, and have compared the oligomannose profile of Tamm-Horsfall glycoprotein (nonpregnant) with that of uromodulin (pregnant) derived from both human and bovine sources. Surprisingly, there was a pregnancy-associated decrease in the total content of oligomannose chains due predominantly to a reduction in Man5GlcNAc2-R and Man6GlcNAc2-R. Man7GlcNAc2-R, which did not decrease with pregnancy, comprised a significantly greater proportion of the total oligomannose chains in pregnant vs. nonpregnant samples from both species (human; 34.6% vs. 25.9%: bovine; 14.4% vs. 7.2%).

Environmental effects on protein glycosylation

Cultured mammalian cells are being used to produce proteins for therapeutic and diagnostic use because of their ability to perform complex post-translational modifications, including glycosylation. The oligosaccharide moieties can play an important role in defining several biological properties of glycoproteins, including clearance rate, immunogenicity, and biological specific activity. There is a growing interest in defining the factors that influence glycosylation, including the cell culture environment. In this review we organize the published data from in vitro cell culture and tissue culture studies that demonstrate direct effects of the culture environment on N-linked glycosylation.

Synthetic retinoids inhibit the antigen presenting properties of epidermal cells in vitro

The clinical efficacy of retinoids in benign and malignant skin diseases involving immune mechanisms suggests that they affect the immunologic functions of the epidermis. However, these effects have yet to be demonstrated. The action of vitamin A (retinol) and the synthetic retinoids, isotretinoin, etretinate, acitretin, and arotinoid-free acid have been studied on the lymphocyte proliferation induced by phytohemagglutinin (PHA), by the mixed lymphocyte reaction (MLR), and the mixed epidermal cell-lymphocyte reaction (MECLR). The results for PHA-induced proliferations were highly variable for all the retinoids. However, in MECLR, the synthetic retinoids consistently reduced the proliferation by 20%-30%. This occurred at therapeutic drug concentrations of about 10(-7)M. In MLR, a minor decrease of 10%-15% was only found for higher concentrations (10(-5)M). Retinol induced no effect in either reaction. Further analysis of acitretin on MECLR showed that it reduced lymphocyte proliferation in a dose-dependent fashion. This reduction was combined with a decrease in cytotoxic T-lymphocyte induction (CTL). Addition of 10(-6)M acitretin at various times also revealed that its presence at cell culture initiation was necessary to inhibit proliferation significantly. Furthermore, cell treatments prior to MECLR showed that exposure of epidermal cells to acitretin was essential to produce this inhibition, suggesting that it acts directly on epidermal cells. Consequently, it is suggested that the specific inhibitory effect of synthetic retinoids on lymphocyte activation in MECLR may partly account for their therapeutic action on the skin.

An established rat cell line expressing chondrocyte properties

Chondrocytes express a well-characterized set of marker proteins making these cells useful for studies on differentiation and regulation of gene expression. Because of the inherent instability of primary rat chondrocytes in culture, and because several rat chondrocyte genes have been cloned and characterized (including the collagen II promoter and enhancer), a rat chondrocyte cell line would be especially useful. To obtain this line we infected primary fetal rat costal chondrocytes with a recombinant retrovirus (NIH/J-2) carrying the myc and raf oncogenes, which have been shown to have an "immortalizing" function. Following infection, a rapidly proliferating clonal line was isolated that maintained a stable phenotype through 45 passages (11/2 year in culture). This line, termed IRC, grows in suspension culture as multicellular aggregates and in monolayer culture as polygonal cells which accumulate an alcian blue-stainable matrix. IRC cells synthesize high levels of cartilage proteoglycan core protein, and link protein, but show reduced collagen II expression. In addition, the cells express virally derived myc mRNA and protein, but do not express v-raf. Retinoic acid, which is a known modulator of chondrocyte phenotype, down-regulates expression of chondrocyte marker proteins, while stimulating v-myc expression by IRC cells. These data suggest that v-myc expression by chondrocytes results in rapid cell division and maintenance of many aspects of the differentiated phenotype. These "immortalized" cells, however, remain responsive to agents such as retinoic acid which modulate cell phenotype. The potential exists for development of chondrocyte cell lines from diseased cartilage, as well as from human cartilage.

Enhancement of sialyltransferase in two melanoma cell lines that are growth-inhibited by retinoic acid results in increased sialylation of different cell-surface glycoproteins

Previous studies have demonstrated the ability of retinoic acid (RA) to inhibit the growth of two spontaneous murine melanoma cell lines (B16-F1 and S91-C2) and to augment both sialyltransferase activity and the sialylation of an Mr 160,000 cell-surface glycoprotein. The present study examined the effects of RA on an ultraviolet irradiation-induced murine melanoma cell line K-1735P. Like the two spontaneous melanomas, the uv-induced melanoma exhibited susceptibility to the growth-inhibitory action of RA. Both the anchorage-dependent and the anchorage-independent growths of the K-1735P cells were suppressed by RA, with IC50 values of 5 X 10(-9) and 3 X 10(-12) M, respectively. Sialyltransferase activity in both S91-C2 and K-1735P cells treated with 10(-6) or 10(-5) M RA increased two- and three-fold, respectively, as compared with untreated cells. In contrast, cell-surface sialo- and galactoglycoproteins, revealed by labeling with periodate and tritiated borohydrate or with neuraminidase, galactose oxidase, and tritiated borohydrate, respectively, varied between the S91-C2 and the K-1735P cells, and each cell line's modulation by RA was also distinct. These findings suggest that although RA can increase the activity of sialyltransferase in different melanoma cells, this increased activity may, in turn, result in an increased sialylation of distinct cell-surface glycoproteins.

Modulation of oligosaccharide processing in an exocrine secretory glycoprotein of rat parotid cells by beta-adrenoreceptor activation

Such stimulation of rat parotid acinar cells in vitro modulated the rate of processing of N-linked oligosaccharides in a high-molecular weight (220 kdalton) secretory glycoprotein. Conversion of polymannose-type oligosaccharides to complex-type oligosaccharides was evaluated by sensitivity to endoglucosaminidase H and alpha-mannosidase, and with a specific inhibitor of glucosidases I/II. Oligosaccharide maturation in the 220 kdalton glycoprotein required one-third to half less time in cells exposed to the beta-adrenergic agonist isoproterenol than in controls.

Commitment of adipocyte differentiation in 3T3 cells is inhibited by retinoic acid, and the expression of lipogenic enzymes is modulated through cytoskeleton stabilization

Retinoic acid (RA), at 1-10 microM, inhibited adipose conversion of 3T3-F442A cells as determined by the activities of lipogenic enzymes, glycerophosphate dehydrogenase (GPD) and malic enzyme. This inhibition was reversible by RA removal, but the increase in lipogenic enzyme activities was considerably delayed in a dose-dependent manner. The onset of the two lipogenic enzyme activities could be regulated somewhat independently, suggesting that expression of the two enzymes is subject to noncoordinated regulation. The RA-inhibited cells had a more flattened and elongated shape, suggesting cytoskeletal changes. Cytochalasin B (CB) did not prevent RA inhibition and did not promote adipose conversion in cultures supplemented with nonadipogenic medium. Reversion of inhibition was accelerated if cells were cultured for 3 days with adipogenic medium containing CB. The drug promoted an early increase in lipogenic enzyme activities. On the other hand, cells cultured on fibronectin-coated dishes, a condition that stabilizes actin cytoskeleton, do not undergo adipocyte differentiation. However, we show here that cells cultured on fibronectin and changed to nonadipogenic medium containing insulin underwent adipose conversion; in contrast, cells treated with RA and then supplemented with nonadipogenic medium containing insulin, but without the retinoid, did not undergo differentiation. We conclude that RA blocks adipose conversion probably before commitment to differentiation, and modulates lipogenic enzyme expression in a noncoordinated manner through changes in cytoskeletal elements, whereas fibronectin blocks phenotype expression in differentiating cells.

The role of cartilage and fibronectin during respecification of pattern induced in the regenerating amphibian limb by retinoic acid

When retinoic acid (RA) is applied to the regenerating limb the positional information of blastemal cells is respecified and extra limb segments develop. We are trying to elucidate the molecular basis of the action of RA and report here experiments focused on the role that fibronectin (FN) might play in the process. The FN distribution in stump tissues, regeneration blastemas and RA-treated blastemas was investigated by immunocytochemistry. Two effects of RA were observed. Firstly, excessive dedifferentiation of the severed cartilage at the amputation plane, resulting in lumps of FN-positive matrix being released into the blastema; secondly, blastemal cells tend to aggregate together into FN-positive accumulations. Excessive dedifferentiation of the cartilage plays no role in the RA-induced respecification of pattern, because we show that extra segments are still produced in RA-treated limbs from which all the cartilage has been removed. The effect on blastemal cell FN distribution was investigated in several ways. Axolotl plasma FN and cellular FN were characterised on immunoblots, and no obvious change was observed after RA treatment; neither were there changes in amounts of FN detected by ELISA. Levels of FN synthesis were measured by [35S]-methionine labelling and again no change observed after RA treatment. We conclude that the change in FN distribution observed by immunocytochemistry after RA treatment may be due to the retention of FN on the surface of the blastemal cells rather than to any effect on the levels of synthesis of this molecule.

Retinoic acid rapidly reduces cartilage matrix synthesis by altering gene transcription in chondrocytes

Retinoic acid can alter the differentiation of a variety of cell types, including chondrocytes. This action may explain the high incidence of craniofacial and limb defects resulting from exposure to retinoic acid during development, and may be the basis for the compound's inhibition of a chondrosarcoma tumor in vivo. In order to understand the mechanism of action of retinoic acid, we studied the expression of chondrocyte-specific proteins as well as other proteins that indicate a shift in the differentiated phenotype of the cell following exposure to retinoic acid. After 48 hr of exposure to retinoic acid chondrocytes stopped synthesizing the chondrocyte-specific pro alpha 1 (II) chain of collagen II and a 370-kDa precursor protein of a cartilage-specific proteoglycan. Instead, the cells synthesized increased amounts of fibronectin and the pro alpha 1 chain of collagen III. These changes could be detected as early as 12 hr after treatment. In addition, the steady-state levels of specific mRNA transcripts coding for these differentiation markers correlated with their protein synthesis levels. Also, nuclear runoff experiments indicated that retinoic acid down regulated transcription of the collagen II gene, while stimulating collagen III gene transcription. These observations suggest that retinoic acid may alter the expression of the chondrocyte phenotype by selectively changing the normal pattern of gene expression.

Current ideas on the significance of protein glycosylation

Carbohydrate has been removed from a number of glycoproteins without major effect on the structure or enzyme activity of the protein. Thus carbohydrate has been suggested to underly a non-primary function for proteins, such as in relatively non-specific interactions with other carbohydrates or macromolecules, stabilization of protein conformation, or protection from proteolysis. This non-specific concept is consistent with both the general similarity in carbohydrate structure on very diverse glycoproteins and the frequent structural microheterogeneity of carbohydrate chains at given sites. The concept is supported in a general sense by the viability of cells whose glycosylation processes have been globally disrupted by mutation or pharmacological inhibitors. In contrast to the above observations, other studies have revealed the existence of specific, selective receptors for discrete oligosaccharide structures on glycoproteins which seem to be important for compartmentalization of the glycoprotein, or the positioning of cells on which the glycoprotein is concentrated. Sometimes multivalency in the carbohydrate-receptor interaction is crucial. There are additional possible roles for carbohydrate in the transduction of information upon binding to a receptor. The possibility of specific roles for carbohydrate is supported by the existence of numerous unique carbohydrate structures, many of which have been detected as glycoantigens by monoclonal antibodies, with unique distributions in developing and differentiated cells. This article attempts to summarize and rationalize the contradictory results. It appears that in general carbohydrate does in fact underlie only roles secondary to a protein's primary function. These secondary roles are simple non-specific ones of protection and stabilization, but often also satisfy the more sophisticated needs of spatial position control and compartmentalization in multicellular eukaryotic organisms. It is suggested that there are advantages, evolutionarily speaking, for the shared use of carbohydrate for non-specific roles and for specific roles primarily as luxury functions to be executed during the processes of cell differentiation and morphogenesis.

Synthesis of cellular and extracellular glycoproteins by cultured human keratinocytes and their response to retinoids

The glycoproteins synthesized by human keratinocytes cultured on 3T3 feeder layers were studied by metabolic labelling. Keratinocytes freed of feeder cells synthesized a complex pattern of cellular and extracellular glycoproteins that was distinct from that of 3T3 cells, dermal fibroblasts and epidermal melanocytes. The effect of low concentrations of all-trans-retinoic acid and arotinoid ethyl ester on glycoprotein synthesis was examined in keratinocyte cultures depleted of vitamin A. Treatment with either retinoid resulted in a 2-3-fold increase in the amount of D-[3H]glucosamine-labelled material in the culture medium. Gel electrophoresis revealed increased incorporation of D-[3H]glucosamine into extracellular glycoproteins of Mr 245,000, 170,000, 140,000, 130,000, 120,000 and 105,000 as well as into glycosaminoglycans in retinoid-treated cultures. The labelling of extracellular glycoproteins with L-[3H]leucine and L-[35S]methionine was also increased by retinoids suggesting increased synthesis of these components rather than an effect on their glycosylation. The Mr 245 000 glycoprotein was identified as keratinocyte-derived fibronectin by immunoblotting, immunoprecipitation and specific binding to gelatin. The results show that retinoids increase the synthesis of glycoprotein as well as glycosaminoglycan components of the extracellular matrix in human keratinocyte cultures. It is suggested that retinoids select for a population of cells that synthesize relatively large amounts of glycosaminoglycan, fibronectin and other as yet unidentified extracellular glycoproteins.

Independence of cell shape and loss of cartilage matrix production during retinoic acid treatment of cultured chondrocytes

Retinoic acid has been shown to cause chondrocytes in culture to flatten and to inhibit the synthesis of cartilage specific components. Since the biochemical expression of chondrocytes is considered to be dependent on cell shape, it has been proposed that retinoic acid acts on these cells primarily by causing a change in cell morphology. This hypothesis was tested by culturing chick sternal chondrocytes suspended in methyl cellulose, which prevents cell flattening. Cultures were labeled with [35S]methionine and differentiation was assessed by polyacrylamide gel electrophoresis. The results showed that retinoic acid-treated chondrocytes in suspension remained rounded but synthesized proteins characteristic of flattened or dedifferentiated chondrocytes. Chondrocytes exposed to retinoic acid in suspension became fibroblastic when placed in monolayer culture in the absence of retinoic acid. This effect was irreversible after 2 weeks of culture. These results suggest that retinoic acid has a direct molecular or biochemical effect on the chondrocyte and that the cell shape change is secondary.

Some thoughts on the relationship between vitamin A and cancer

Retinoids clearly show both prophylactic and therapeutic effects against many kinds of neoplasm. Vitamin A deficiency and vitamin A excess, which are very different states clinically and metabolically, should be separately considered relative to carcinogenesis. The anticancer effects of retinoids and related compounds may be more closely related to their chemical structure than to their similarity to the structure and function of vitamin A. Retinoids act on cells and tissues in a number of ways. Although many interesting ideas have been proposed to clarify their mode of action, no single hypothesis adequately explains their many molecular interactions and responses. Investigation of early molecular interactions between retinoids and cells and the subsequent metabolism of retinoids in neoplastic and normal cells may aid in the clarification of their action as antineoplastic agents.

Superoxide production and chemiluminescence induced in differentiated HL-60 cells by the chemoattractant formyl-methionyl-leucyl-phenylalanine

Superoxide production and chemiluminescence induced in differentiated HL-60 cells by the chemoattractant formylmethionyl-leucyl-phenylalanine: In order to study the generation of oxidative metabolites in relation to cell differentiation, dimethyl sulfoxide (DMSO) and retinoic acid (RA) differentiated HL-60 cells were stimulated with the chemotactic peptide formylmethionyl-leucyl-phenylalanine (FMLP). The oxidative response was measured as luminol-dependent chemiluminescence, lucigenin-dependent chemiluminescence, and cytochrome c reduction. Cells grown in the presence of DMSO or RA progressively expressed morphological changes, and when the mature cells were exposed to FMLP the cells produced oxidative metabolites. Quantitatively the HL-60 cells grown in the presence of DMSO gave rise to the most pronounced response. No correlation was obtained between superoxide production, luminol-chemiluminescence and lucigenin-dependent chemiluminescence, indicating that different aspects of the oxidative response are elucidated by the three different methods. Furthermore, the experiments show that DMSO and RA-induced differentiation of HL-60 cells leads to granulocyte-like cells with different abilities to produce oxidative metabolites, possibly due to differences in receptor function.