Concanavalin A induced apoptosis in fibroblasts: the role of cell surface carbohydrates in lectin mediated cytotoxicity

Lectibodies as antivirals

Growing concerns regarding the emergence of highly transmissible viral diseases highlight the urgent need to expand the repertoire of antiviral therapeutics. For this reason, new strategies for neutralizing and inhibiting these viruses are necessary. A promising approach involves targeting the glycans present on the surfaces of enveloped viruses. Lectins, known for their ability to recognize specific carbohydrate molecules, offer the potential for glycan-targeted antiviral strategies. Indeed, numerous studies have reported the antiviral effects of various lectins of both endogenous and exogenous origins. However, many lectins in their natural forms, are not suitable for use as antiviral therapeutics due to toxicity, other unfavorable pharmacological effects, and/or unreliable manufacturing sources. Therefore, improvements are crucial for employing lectins as effective antiviral therapeutics. A novel approach to enhance lectins' suitability as pharmaceuticals could be the generation of recombinant lectin-Fc fusion proteins, termed "lectibodies." In this review, we discuss the scientific rationale behind lectin-based antiviral strategies and explore how lectibodies could facilitate the development of new antiviral therapeutics. We will also share our perspective on the potential of these molecules to transcend their potential use as antiviral agents.

Concanavalin A promotes angiogenesis and proliferation in endothelial cells through the Akt/ERK/Cyclin D1 axis

CONTEXT

Concanavalin A (Con A) exhibited multiple roles in cancer cells. However, the role of Con A in endothelial cells was not reported.

OBJECTIVE

Our present study investigated the potential angiogenic role of Con A in endothelial cells and ischaemic hind-limb mice.

MATERIALS AND METHODS

Human umbilical vein endothelial cells and Ea.hy926 cells were employed to determine the effect of Con A (0.3, 1, and 3 μg/mL) or vehicle on angiogenesis and cell proliferation with tube formation, ELISA, flow cytometry, EdU, and western blot. Hind-limb ischaemic mice were conducted to determine the pro-angiogenic effect of Con A (10 mg/kg) for 7 days.

RESULTS

Con A promoted tube formation to about three-fold higher than the control group and increased the secretion of VEGFa, PDGFaa, and bFGF in the medium. The cell viability was promoted to 1.3-fold by Con A 3 μg/mL, and cell cycle progression of G0G1 phase was decreased from 77% in the vehicle group to 70% in Con A 3 μg/mL, G2M was promoted from 15 to 19%, and S-phase was from 7 to 10%. Con A significantly stimulated phosphorylation of Akt and ERK1/2 and expression of cyclin D1 and decreased the expression of p27. These effects of Con A were antagonised by the PI3K inhibitor LY294002 (10 μM) and MEK pathway antagonist PD98059 (10 μM). Moreover, Con A (10 mg/kg) exhibited a repair effect in ischaemic hind-limb mice.

DISCUSSION AND CONCLUSIONS

This study will provide a new option for treating ischaemic disease by local injection with Con A.

Impact of glycoengineering and antidrug antibodies on the anticancer activity of a plant-made lectin-Fc fusion protein

Plants are an efficient production platform for manufacturing glycoengineered monoclonal antibodies and antibody-like molecules. Avaren-Fc (AvFc) is a lectin-Fc fusion protein or lectibody produced in Nicotiana benthamiana, which selectively recognizes cancer-associated high-mannose glycans. In this study, we report the generation of a glycovariant of AvFc that is devoid of plant glycans, including the core α1,3-fucose and β1,2-xylose residues. The successful removal of these glycans was confirmed by glycan analysis using HPLC. This variant, AvFc^ΔXF , has significantly higher affinity for Fc gamma receptors and induces higher levels of luciferase expression in an antibody-dependent cell-mediated cytotoxicity (ADCC) reporter assay against B16F10 murine melanoma cells without inducing apoptosis or inhibiting proliferation. In the B16F10 flank tumour mouse model, we found that systemic administration of AvFc^ΔXF , but not an aglycosylated AvFc variant lacking affinity for Fc receptors, significantly delayed the growth of tumours, suggesting that Fc-mediated effector functions were integral. AvFc^ΔXF treatment also significantly reduced lung metastasis of B16F10 upon intravenous challenge whereas a sugar-binding-deficient mutant failed to show efficacy. Lastly, we determined the impact of antidrug antibodies (ADAs) on drug activity in vivo by pretreating animals with AvFc^ΔXF before implanting tumours. Despite a significant ADA response induced by the pretreatment, we found that the activity of AvFc^ΔXF was unaffected by the presence of these antibodies. These results demonstrate that glycoengineering is a powerful strategy to enhance AvFc's antitumor activity.

Concanavalin A as a promising lectin-based anti-cancer agent: the molecular mechanisms and therapeutic potential

Concanavalin A (ConA), the most studied plant lectin, has been known as a potent anti-neoplastic agent for a long time. Since initial reports on its capacity to kill cancer cells, much attention has been devoted to unveiling the lectin's exact molecular mechanism. It has been revealed that ConA can bind to several receptors on cancerous and normal cells and modulate the related signaling cascades. The most studied host receptor for ConA is MT1-MMP, responsible for most of the lectin's modulations, ranging from activating immune cells to killing tumor cells. In this study, in addition to studying the effect of ConA on signaling and immune cell function, we will focus on the most up-to-date advancements that unraveled the molecular mechanisms by which ConA can induce autophagy and apoptosis in various cancer cell types, where it has been found that P73 and JAK/STAT3 are the leading players. Moreover, we further discuss the main signaling molecules causing liver injury as the most significant side effect of the lectin injection. Altogether, these findings may shed light on the complex signaling pathways controlling the diverse responses created via ConA treatment, thereby modulating these complex networks to create more potent lectin-based cancer therapy.

Concanavalin A induces apoptosis in a dose-dependent manner by modulating thiol/disulfide homeostasis in C6 glioblastoma cells

Glioma is the most common brain tumor. C6 rat glioblastoma cells provide the possibility to the scientist to study brain cancer. Concanavalin A (Con A) has a lot of antitumoral effects, especially over oxidative stress. In the present study, it was aimed to decide the impacts of various doses of Con A on C6 glioblastoma cells regarding cytotoxicity, thiol/disulfide homeostasis, apoptosis, and inflammation. We detected the cytotoxic activity of Con A (from 7.8 to 500 µg/ml) in C6 cells by utilizing 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) and determined the toxic concentration of Con A. Once the optimal doses were found, the thiol-disulfide homeostasis, levels of total antioxidant and oxidant status (TAS and TOS), malondialdehyde (MDA) and glutathione (GSH), pro-inflammatory cytokines as tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6), apoptotic proteins as cytochrome c (CYCS), and caspase 3 (CASP3) were measured. Apoptotic and morphological changes in the C6 cells were examined with an inverted microscope and flow cytometry technique. Dose-dependent Con A triggered oxidative damage in the C6 cells, affecting the inflammatory pathway, so reducing proliferation with apoptotic proteins and morphological changes. But especially, Con A increased disulfide formation by disrupting the thiol/disulfide balance in C6 cells. This study revealed that Con A, known as carbohydrate-binding protein, generated oxidative damage, inflammation, and apoptosis in a dose-dependent manner by modulating thiol/disulfide homeostasis in C6 glioblastoma cells.

Bauhinia forficata lectin (BfL) induces cell death and inhibits integrin-mediated adhesion on MCF7 human breast cancer cells

BACKGROUND

Plant lectins have attracted great interest in cancer studies due to their antitumor activities. These proteins or glycoproteins specifically and reversibly bind to different types of carbohydrates or glycoproteins. Breast cancer, which presents altered glycosylation of cell surface glycoproteins, is one of the most frequent malignant diseases in women. In this work, we describe the effect of the lectin Bauhinia forficata lectin (BfL), which was purified from B. forficata Link subsp. forficata seeds, on the MCF7 human breast cancer cellular line, investigating the mechanisms involved in its antiproliferative activity.

METHODS

MCF7 cells were treated with BfL. Viability and adhesion alterations were evaluated using flow cytometry and western blotting.

RESULTS

BfL inhibited the viability of the MCF7 cell line but was ineffective on MDA-MB-231 and MCF 10A cells. It inhibits MCF7 adhesion on laminin, collagen I and fibronectin, decreases α1, α6 and β1 integrin subunit expression, and increases α5 subunit expression. BfL triggers necrosis and secondary necrosis, with caspase-9 inhibition. It also causes deoxyribonucleic acid (DNA) fragmentation, which leads to cell cycle arrest in the G2/M phase and a decrease in the expression of the regulatory proteins pRb and p21.

CONCLUSION

BfL shows selective cytotoxic effect and adhesion inhibition on MCF7 breast cancer cells.

GENERAL SIGNIFICANCE

Cell death induction and inhibition of cell adhesion may contribute to understanding the action of lectins in breast cancer.

Identification of compounds that augment the lectin-sensitivity of Chinese Hamster Ovary cells

Glycosylation is now recognized as one of the most important modifications of eukaryotic proteins. In cancer biology, alterations in cell surface glycosylation have been exploited as valuable biomarkers, and the relationship of this modification to the metastatic characteristics of cancer cells has also been well-documented. Chemicals that can alter cell surface glycosylation patterns will therefore become attractive lead compounds for controlling the metastatic characteristics of cancer cells, one of the critical factors in their malignancy and prognosis of the disease. In this study, we established a system for screening compounds that have the potential to alter cell surface glycosylation by taking advantage of the susceptibility of cells toward various lectins. Through our screening of a chemical library, we were able to identify two compounds that augment the sensitivity of Chinese Hamster Ovary (CHO-K1) cells against the L4-PHA lectin. Surprisingly, these compounds did not result in alterations in cell surface glycan structures. Instead, they appeared to render the cells to be more sensitive to various lectins with distinct carbohydrate specificities. These compounds promise to be valuable, not only as tools for providing insights into the intracellular signaling of lectin-mediated growth arrest, but also as potential lead compounds for use as therapeutic, anti-cancer drugs.

Concanavalin A: a potential anti-neoplastic agent targeting apoptosis, autophagy and anti-angiogenesis for cancer therapeutics

Concanavalin A (ConA), a Ca(2+)/Mn(2+)-dependent and mannose/glucose-binding legume lectin, has drawn a rising attention for its remarkable anti-proliferative and anti-tumor activities to a variety of cancer cells. ConA induces programmed cell death via mitochondria-mediated, P73-Foxo1a-Bim apoptosis and BNIP3-mediated mitochondrial autophagy. Through IKK-NF-κB-COX-2, SHP-2-MEK-1-ERK, and SHP-2-Ras-ERK anti-angiogenic pathways, ConA would inhibit cancer cell survival. In addition, ConA stimulates cell immunity and generates an immune memory, resisting to the same genotypic tumor. These biological findings shed light on new perspectives of ConA as a potential anti-neoplastic agent targeting apoptosis, autophagy and anti-angiogenesis in pre-clinical or clinical trials for cancer therapeutics.

Plant lectins: targeting programmed cell death pathways as antitumor agents

Lectins, a group of highly diverse, carbohydrate-binding proteins of non-immune origin that are ubiquitously distributed in plants, animals and fungi, are well-characterized to have numerous links a wide range of pathological processes, most notably cancer. In this review, we present a brief outline of the representative plant lectins including Ricin-B family, proteins with legume lectin domains and GNA family that can induce cancer cell death via targeting programmed cell death pathways. Amongst these above-mentioned lectins, we demonstrate that mistletoe lectins (MLs), Ricin, Concanavalin A (ConA) and Polygonatum cyrtonema lectin (PCL) can lead to cancer cell programmed death via targeting apoptotic pathways. In addition, we show that ConA and PCL can also result in cancer cell programmed death by targeting autophagic pathways. Moreover, we summarize the possible anti-cancer therapeutic implications of plant lectins such as ConA, Phaseolus vulgaris lectin (PHA) and MLs that have been utilized at different stages of preclinical and clinical trials. Together, these findings can provide a comprehensive perspective for further elucidating the roles of plant lectins that may target programmed cell death pathways in cancer pathogenesis and therapeutics. And, this research may, in turn, ultimately help cancer biologists and clinicians to exploit lectins as potential novel antitumor drugs in the future.

Fluctuation methods to study protein aggregation in live cells: concanavalin A oligomers formation

Prefibrillar oligomers of proteins are suspected to be the primary pathogenic agents in several neurodegenerative diseases. A key approach for elucidating the pathogenic mechanisms is to probe the existence of oligomers directly in living cells. In this work, we were able to monitor the process of aggregation of Concanavalin A in live cells. We used number and brightness analysis, two-color cross number and brightness analysis, and Raster image correlation spectroscopy to obtain the number of molecules, aggregation state, and diffusion coefficient as a function of time and cell location. We observed that binding of Concanavalin A to the membrane and the formation of small aggregates paralleled cell morphology changes, indicating progressive cell compaction and death. Upon protein aggregation, we observed increased membrane water penetration as reported by Laurdan generalized polarization imaging.

Lectin of Pisum arvense seeds induces in-vivo and in-vitro neutrophil migration

PAL is a glucose/mannose-specific lectin isolated from Pisum arvense seeds. Previously, we demonstrated the capacity of other leguminous lectins to induce oedema formation and neutrophil stimulation. To investigate the potential pro-inflammatory activity of PAL, we have studied its ability to induce neutrophil migration into peritoneal cavities of rats and neutrophil chemotaxis in-vitro. The role of resident cells and sugar residues on PAL activity was analysed. PAL or saline (control) were administered intraperitoneally to rats, and total and differential leucocyte (macrophages, neutrophils and mast cells) counts were performed. The role of resident cells on the PAL effect was evaluated using three strategies: reducing the total resident cell population by lavage of rat cavities with saline; increasing macrophage population by treating animals with thioglycolate; and depleting mast cell population by subchronic treatment of rats with compound 48/80. PAL induced in-vitro and in-vivo neutrophil migration. In-vivo, PAL (50, 100, 200 and 300 μg) significantly (P < 0.05) and dose-dependently increased neutrophil migration by 600, 740, 900 and 940%, respectively, showing maximal effect 4 h after injection. PAL induced mononuclear cell migration. The neutrophil stimulatory effect of PAL was potentiated in animals treated with both thioglycolate and compound 48/80. The indirect lectin chemotactic effect was shown in rats injected with supernatant from cultured macrophages stimulated by PAL. In conclusion, PAL was shown to exhibit in-vivo and in-vitro proinflammatory activity. The in-vivo effect seemed to occur by a dual mechanism that was independent, but also dependent, on resident cells.

Growth inhibitory activity of a novel lectin from Cliona varians against K562 human erythroleukemia cells

PURPOSE

In this study, the antitumoral potential of a novel lectin (CvL) purified from the marine sponge Cliona varians was studied in different cancer cell lines.

METHODS

CvL cytotoxicity was evaluated in mammalian tumor cells and in normal human peripheral blood lymphocytes by the MTT assay using the same range of concentrations (1-150 microg ml(-1)). The mechanisms involved in K562 cell death were investigated by confocal fluorescence microscopy, flow cytometry and immunoblot.

RESULTS

CvL inhibited the growth of human leukemia cells, with IC(50) values of 70 and 100 microg ml(-1) for K562 and JURKAT cells, respectively, but it was ineffective on blood lymphocytes and solid tumor cell lines. K562 cell death occurred 72 h after exposure to the lectin and with signs of apoptosis, as analyzed by DAPI and annexin V/PI staining. Investigation of the possible mediators of this process showed that cell death occurred via a caspase-independent pathway. Confocal fluorescence microscopy indicated a pivotal role for the lysosomal protease cathepsin B in mediating cell death. Accordingly, pre-incubation of K562 cells with the cathepsin inhibitor L-trans-epoxysuccinyl-L-leucylamido-(4-guanidino)butane (E-64) abolished CvL cytotoxic effect. Furthermore, we found upregulation of tumor necrosis factor receptor 1 (TNFR1) and down-modulation of p65 subunit of nuclear factor kappa B (NFkappaB) expression in CvL-treated cells. These effects were accompanied by increased levels of p21 and reduced expression of pRb, suggesting that CvL can induce cell cycle arrest.

CONCLUSIONS

Collectively, these findings indicate an antileukemic effect for CvL and suggest that cathepsin B acts as a death mediator in CvL-induced cytotoxicity possibly in an uncharacterized connection with the membrane death receptor pathway.

Novel function of PERK as a mediator of force-induced apoptosis

Induction of apoptosis by tensile forces is an important determinant of connective tissue destruction in osteoarthritis and periodontal diseases. We examined the role of molecular components of the unfolded protein response in force-induced apoptosis. Magnetic fields were used to apply tensile force through integrins to cultured fibroblasts bound with collagen-coated magnetite beads. Tensile force induced caspase 3 cleavage, DNA fragmentation, depolarization of mitochondria, and induction of CHOP10, all indicative of activation of apoptosis. Immunoblotting, immunocytochemistry, and release of Ca(2+) from the endoplasmic reticulum showed evidence for both physical and functional associations between bound beads and the endoplasmic reticulum. Force-induced apoptosis was not detected in PERK null cells, but reconstitution of wild-type PERK in PERK null cells restored the apoptotic response. Force-induced apoptosis did not require PKR, GCN2, eIF2alpha, or CHOP10. Furthermore, force more than 24 h did not activate other initiators of the unfolded protein response including IRE-1 and ATF6. However, force-induced activation of caspase 3 was dependent on caspase 9 but was independent of mitochondria. We conclude that force-induced apoptosis depends on a novel function of PERK that occurs in addition to its canonical role in the unfolded protein response.

Analysis of T-cell proliferation and cytokine secretion in the individuals exposed to arsenic

Over six million people in nine districts of West Bengal, India are exposed to very high levels of arsenic primarily through their drinking water. More than 300,000 people showed arsenic-induced skin lesions in these districts. This is regarded as the greatest arsenic calamity in the world. Chronic arsenicosis causes varied dermatological signs ranging from pigmentation changes, hyperkeratosis to non-melanocytic cancer of skin, and also malignancies in different internal organs. Higher incidences of opportunistic infections are found in the arsenic-exposed individuals, indicating that their immune systems may be impaired somehow. We have thus investigated the effect of arsenic on T-cell proliferation and cytokine secretion in 20 individuals with arsenic-induced skin lesions and compared the results with 18 arsenic-unexposed individuals. A marked dose-dependent suppression of Concanavalin A (Con A) induced T-cell proliferation was observed in the arsenic-exposed individuals compared with the unexposed ( P < 0.001) individuals. This correlated with a significant decrease in the levels of secreted cytokines by the T cells (TNF-α, IFN-γ, IL2, IL10, IL5, and IL4) in the exposed individuals ( P < 0.001). Thus it can be inferred that arsenic exposure can cause immunosuppression in humans.

Pouterin, a novel potential cytotoxic lectin-like protein with apoptosis-inducing activity in tumorigenic mammalian cells

In this study, the cytotoxicity of pouterin in tumorigenic and non-tumorigenic mammalian cell lines was investigated. We found that HeLa, Hep-2 and HT-29 tumor cells were highly sensitive to pouterin cytotoxicity in a dose-dependent manner, whereas non-tumorigenic Vero cells and human lymphocytes were relatively resistant to the protein. Among the tumor cell lines, HeLa cells showed the highest susceptibility to pouterin cytotoxicity, exhibiting a time-dependent increase in LDH leakage and an IC(50) value of 5mug/mL. Morphological alterations such as rounding, cell shrinkage and chromatin condensation, consistent with apoptotic cell death were observed. Apoptosis induction was demonstrated by DNA fragmentation as detected by terminal dUTP nick-end labeling (TUNEL). Furthermore, HeLa cells incubated with pouterin showed disruption of the actin cytoskeleton. Western blot analysis revealed that pouterin caused increased expression of p21, thus indicating cell cycle arrest. Subsequent studies provided evidence that apoptosis may be partially explained in the activation of the tumor necrosis factor receptor 1 (TNFR1) signaling. Interestingly, a time-dependent decrease of the expression of p65 nuclear factor kappa B (NFkappaB) subunit, concomitant with a downregulation of the inhibitor of apoptosis protein 1 (IAP1) was observed, suggesting that TNFR-mediated apoptosis is the predominant pathway induced by pouterin in HeLa cells.

Lectin-based three-color flow cytometric approach for studying cell surface glycosylation changes that occur during apoptosis

BACKGROUND

Changes in cell surface glycosylation that accompany apoptosis are thought to be involved in the recognition and removal of apoptotic cells by phagocytes, but in most instances these changes are ill defined. To improve our understanding of this phenomenon, we designed a trivariate flow cytometry procedure that allows direct comparison of cell surface glycosylation in apoptotic and viable cells.

METHODS

The annexin V/propidium iodide assay has been adapted for cell surface glycosylation analysis by combining the use of these two reagents with biotinylated lectins, and this has been used to investigate camptothecin-induced apoptosis in U-937 cells.

RESULTS

Although numerous lectins are potent inducers of apoptosis, we found that it is possible to determine lectin concentrations that produce interpretable data without inducing significant cytotoxicity even when using apoptogenic lectins. That apoptosis is associated with a marked decrease in cell surface sialylation was confirmed by using the sialic acid-specific lectins Maackia amurensis agglutinin and Sambucus nigra agglutinin. These observations were corroborated by lectin blotting analysis with the same lectins.

CONCLUSIONS

Species- and cell-dependent altered glycosylation patterns are likely to be associated with different modes of apoptosis. The easy and versatile method described in this report should be useful for exploring this field.

Differential effects of arsenic on intracellular free calcium levels and the proliferative response of murine mitogen-stimulated lymphocytes

This study examined the effects of sodium arsenite treatment on free [Ca(2+)]i and cell death in mitogen-activated murine lymphocytes. The main findings of this study were that simultaneous sodium arsenite treatment inhibited PHA- but not Con A-induced T cell proliferation, induced a higher increase in free [Ca(2+)]i and an early increase in the proportion of dead cells in PHA than in Con A activated cells. Sodium arsenite pre-treatment reduced both PHA- and Con A-induced T-cell proliferation. Phorbol myristate ester (PMA) did not prevent the inhibitory effects of both sodium arsenite treatments, suggesting that sodium arsenite did not significantly decreased PKC activation or that its effects occurred on events parallel to PKC activation. Both PHA and Con A increased free [Ca(2+)]i after stimulation, yet the effect was more pronounced in mitogen-activated cells simultaneously treated with sodium arsenite and particularly in those activated with PHA. The increase in free [Ca(2+)]i was in agreement with the early cell death induced by sodium arsenite in PHA-activated cells, a finding consistent with the inhibitory effects on PHA-induced proliferation. Sodium arsenite-induced cell death occurred faster in PHA-activated cells. Further studies are needed to ascertain the relationships between the effects of sodium arsenite on free [Ca(2+)]i levels and the type of cell death induced by sodium arsenite and their relevance for the proliferative response of T cells.

Cytoprotection against mechanical forces delivered through beta 1 integrins requires induction of filamin A

Cells in mechanically active environments can activate cytoprotective mechanisms to maintain membrane integrity in the face of potentially lethal applied forces. Cytoprotection may be mediated by expression of membrane-associated cytoskeletal proteins including filamin A, an actin-binding protein that increases the rigidity of the subcortical actin cytoskeleton. In this study, we tested the hypotheses that applied forces induce the expression of filamin A specifically and that this putative protective response inhibits cell death. Magnetically generated forces were applied to protein-coated magnetite beads bound to human gingival fibroblasts, cells with constitutively low basal levels of filamin A mRNA and protein. Forces applied through collagen or fibronectin, but not bovine serum albumin or poly-l-lysine-coated beads, increased mRNA and protein content of filamin A by 3-7-fold. Forces had no effect on the expression of other filamin isotypes or other cytoskeletal proteins. This effect was dependent on the duration of force and was blocked by anti-beta(1) integrin antibodies. Force also stimulated a 60% increase in expression of luciferase under the control of a filamin A promoter in transiently transfected Rat2 fibroblasts and was dependent on Sp1 transcription factor binding sites located immediately upstream of the transcription start site. Experiments with actinomycin D-treated cells showed that the increased filamin A expression after force application was due in part to prolongation of mRNA half-life. Antisense filamin oligonucleotides blocked force-induced filamin A expression and increased cell death by >2-fold above controls. The force-induced regulation of filamin A was dependent on intact actin filaments. We conclude that cells from mechanically active environments can couple diverse signals from forces applied through beta-integrins to up-regulate the production of cytoprotective cytoskeletal proteins, typified by filamin A.

Human cytomegalovirus pp65- and immediate early 1 antigen-specific HLA class I-restricted cytotoxic T cell responses induced by cross-presentation of viral antigens

Dendritic cells (DCs) play a pivotal role in the development of anti-viral CD8(+) CTL responses. This is straightforward if they are directly infected with virus, but is less clear in response to viruses that cannot productively infect DCS: Human CMV (HCMV) shows strain-specific cell tropism: fibroblast (Fb)-adapted laboratory strains (AD169) and recent clinical isolates do not infect DCs, whereas endothelial cell-adapted strains (TB40/E) result in productive lytic DC infection. However, we show here that uninfected DCs induce CD8(+) T cell cytotoxicity and IFN-gamma production against HCMV pp65 and immediate early 1 Ags following in vitro coculture with HCMV-AD169-infected Fbs, regardless of the HLA type of these FBS: CD8(+) T cell stimulation was inhibited by pretreatment of DCs with cytochalasin B or brefeldin A, indicating a phagosome/endosome to cytosol pathway. HCMV-infected Fbs were not apoptotic as measured by annexin V binding, and induction of apoptosis of infected Fbs in vitro did not augment CTL induction by DCs, suggesting a mechanism other than apoptosis in the initiation of cross-presentation. Furthermore, HCMV-infected Fbs provided a maturation signal for immature DCs during coculture, as evidenced by increased CD83 and HLA class II expression. Cross-presentation of HCMV Ags by host DCs enables these professional APCs to bypass some of the evasion mechanisms HCMV has developed to avoid T cell recognition. It may also serve to explain the presence of immediate early 1 Ag-specific CTLs in the face of pp65-induced inhibition of Ag presentation at the level of the infected cell.

The deletion of transforming growth factor-beta-induced myofibroblasts depends on growth conditions and actin organization

Myofibroblasts are important but transient mediators of normal wound contraction and are characterized phenotypically by their high levels of alpha-smooth-muscle actin (SMA). During wound maturation, these cells disappear. We have examined the mechanisms that lead to myofibroblast deletion in a fibroblast culture model. Transforming growth factor-beta (TGF-beta) was used to increase SMA content in gingival fibroblasts (three- to sixfold). After replating TGF-beta-induced cells at low density with serum, there was a fivefold decrease in SMA protein content, SMA protein synthesis, and SMA mRNA as cells proliferated. These reductions were due to reduced SMA mRNA stability. For TGF-beta-induced cells plated at high density without serum (ie, quiescent conditions), protein content was reduced by only 20% over 12 days. TGF-beta protected SMA-positive cells against apoptosis in serum-free cultures. Those cells that were protected against apoptosis exhibited well-developed stress fibers enriched in SMA. We conclude that, in quiescent myofibroblasts, SMA protein turnover is slow, and cells are long-lived. In proliferative conditions SMA protein and mRNA turn over quickly, and the myofibroblast phenotype dissipates. The reduced apoptosis of myofibroblasts in quiescent conditions is due in part to the organization of SMA into stress fibers.

The effect of orally administered viable probiotic and dairy lactobacilli on mouse lymphocyte proliferation

Four common Lactobacillus strains were screened for their effects on proliferation of mouse splenic lymphocytes. Mice received perorally 10(9) viable bacteria kg(-1) body weight for 7 days. Lactobacillus acidophilus treatment enhanced ex vivo basal proliferation (by 43%) and B-cell response at suboptimal and optimal concentrations of lipopolysaccharide (LPS) (by 27-28%). Conversely, Lactobacillus casei, Lactobacillus gasseri and Lactobacillus rhamnosus inhibited both basal proliferation (by 14-51%) and mitogen-stimulated lymphoproliferation, particularly at supra-optimal concentrations of concanavalin A (by 43-68%) and LPS (by 23-62%). Therefore, these Lactobacillus strains demonstrate strain-specific effects on B- and T-cells and may also alter the splenocyte sensitivity to the cytotoxic effects of mitogens.

Effects of orally administered viable Lactobacillus rhamnosus GG and Propionibacterium freudenreichii subsp. shermanii JS on mouse lymphocyte proliferation

Immunomodulation by probiotics is a subject of growing interest, but the knowledge of dose response and time profile relationships is minimal. In this study we examined the effects of Lactobacillus rhamnosus GG (LGG) and Propionibacterium freudenreichii subsp. shermanii JS (PJS) on the proliferative activity of murine lymphocytes ex vivo. Dose dependency was assessed by treating animals perorally with a low or a high dose (i.e., 10(9) or 10(12) viable bacteria/kg of body weight) for 7 days. The lower dose levels of each strain appeared to enhance T-cell proliferation at the optimal concanavalin A (ConA) concentration (by 69 to 84%) and B-cell proliferation at the optimal and supraoptimal concentrations of lipopolysaccharide (by 57 to 82%). B-cell proliferation was also enhanced by the high LGG dose (by 32 to 39%) but was accompanied by a marginal decrease in T-cell proliferation (by 8%) at the optimal ConA concentration. The time profiles of the immune responses were assessed after daily treatment with the higher dose for 3, 7, and 14 days. A significant decrease in basal lymphoproliferation (by 32 to 42%) was observed with PJS treatment after the 3- and 7-day periods; however, this activity returned to control levels after 14 days of treatment, which also resulted in significantly enhanced T-cell proliferation at optimal and supraoptimal ConA concentrations (by 24 to 80%). The 14-day LGG treatment also enhanced the latter activity (by 119%). In conclusion, LGG and PJS have specific dose- and duration-dependent immunomodulatory effects on the proliferative activity of B and T lymphocytes and may also reduce lymphocyte sensitivity to the cytotoxic effects of lectin mitogens.

Apoptosis observed in BALB/3T3 cells having ingested Staphylococcus aureus

Staphylococcus aureus was previously shown to be internalized by murine fibroblast. We examined the intracellular events of S. aureus ingested by BALB/3T3 cells. After uptake of strains A191 and A151, isolates from atopic lesion, and a laboratory strain, Cowan I, for 1 hr, BALB/3T3 cells were incubated with 1.25 microg/ml lysostaphin. Laddering of the DNA in multiples of approximately 180 bp occurred within 4 hr following bacterial addition in BALB/3T3 cells infected with A191 and within 18 hr in BALB/3T3 cells infected with A151: histochemical staining by the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling method revealed that the rate of the fragmentation of nucleic DNA in Cowan I-infected BALB/3T3 cells at 21 hr following bacterial addition was 0.52 +/- 0.25%, significantly higher than that in the control cells. Transmission electron micrographs of BALB/3T3 cells at 4 hr following A191 addition showed that the apoptotic features, including electron-dense nucleus and plasma membrane blebbing, occurred in some cells in which many staphylococci escaped the endosome and went on to cell division. At the same time, A151 organisms enclosed with endosome membrane were static in the intact BALB/3T3 cells. The significant increase of A191 was confirmed by counting intracellular live bacteria during 2- to 6-hr incubation. These results suggest that internalized S. aureus escapes the endosome, multiplies and induces apoptosis in the fibroblast cell.

Induction of T lymphocyte apoptosis by treatment with glycyrrhizin

The effect of glycyrrhizin (GL), a Chinese herbal drug extracted from licolice roots, on murine lymphocytes for inducing apoptotic cell death was studied. Addition of GL (25-400 micrograms/ml) to cultured splenocytes and thymocytes from BALB/c mice definitely promoted DNA fragmentation. A single injection of GL (100 micrograms/mouse) into BALB/c mice did not cause any detectable DNA fragmentation or cell death of splenocytes and thymocytes. Cytofluorometric analysis of these cells, however, demonstrated a reduction in mitochondrial transmembrane potentials (delta psi m). Repeated injections of GL (100 micrograms/mouse/day) into mice for 7 days actually resulted in induction of low grade DNA fragmentation selectively in splenocytes. Cell population analysis of viable lymphocytes suggested that both CD4+ Th lymphocytes and CD8+ Tc lymphocytes may have been relatively more sensitive than B220+ B lymphocytes for the apoptotic cell death. We concluded from these results that GL acts as a rather selective inducer of mature T lymphocyte apoptosis with a reduction in delta psi m potentially preceding lymphocyte death.

Shedding of membrane type 1 matrix metalloproteinase in a human breast carcinoma cell line

Membrane type 1 matrix metalloproteinase (MT1-MMP) with a transmembrane domain is a new member of the MMP gene family and is expressed on the cell surfaces of many carcinoma cells to activate the zymogen of MMP-2 (gelatinase A). We have previously reported that MT1-MMP is released into culture media in a complex form with tissue inhibitor of metalloproteinases 2 (TIMP-2) from a human breast carcinoma cell line, MDA-MB-231, treated with concanavalin A (Con A). In the present study, we further studied the release mechanism of MT1-MMP. Immunoblot analysis indicated that the amounts of MT1-MMP in culture media increase with the time of exposure and the concentration of Con A, and those in cell lysates conversely decrease in a similar way. Time- and dose-dependent release of MT1-MMP into the media was confirmed by a sandwich enzyme immunoassay specific to MT1-MMP. The molecular weight of the immunoreactive MTI-MMP in the media was Mr 56,000, which was 4,000-Mr smaller than that in the cell lysates. Northern blot analysis demonstrated that the mRNA expression level of MT1-MMP is about 3-fold enhanced after a 24 h-exposure to Con A and this is maintained up to 72-h exposure. The release of MT1-MMP from the Con A-treated cells was inhibited by metalloproteinase inhibitors such as EDTA and o-phenanthroline, but not by MMP inhibitors including TIMP-1, TIMP-2 and BB94 or other proteinase inhibitors of serine, cysteine and aspartic proteinases. During the Con A treatment of the cells, cell viability decreased time- and dose-dependently and dead cells reacted positively in the TdT-mediated dUTP Nick-End Labeling (TUNEL) method. Con A-treated MDA cells showed apoptotic morphology when stained with Hoechst dye and hematoxylin and eosin. DNA ladder formation was detected by electrophoresis of the DNA from Con A-treated MDA cells. These results suggest that MT1-MMP release from Con A-treated cells is due to shedding mediated by metalloproteinase(s) other than MMPs, and is associated with apoptosis.

A putative role for carbohydrates in sea urchin gastrulation

Many studies have examined the effects of lectins on embryonic development. Recently, it has been shown that lectins actually enter the blastocoel of sea urchin embryos without microinjection and bind to specific cell types. The present study was performed to examine the effects of lectins on sea urchin gastrulation. Strongylocentrotus purpuratus sea urchin embryos were incubated with several lectins at concentrations from 0.01 microgram/ml to 100 micrograms/ml at 15-28 h in the presence or absence of the preferential binding sugars. The most interesting findings were that the mannose specific lectins Lens culinaris agglutinin (LcH) which binds to secondary mesenchyme cells involved in archenteron anchoring and Pisum sativum (PSA) caused exogastrulation. Wheat germ agglutinin (WGA) which binds to primary mesenchyme cells involved in skeletogenesis caused defective skeletogenesis. Our findings suggest that D-mannose-like residues (LcH and PSA specific sugar) may function in archenteron development and anchoring, while N-acetyl-D-glucosamine-like groups (WGA specific sugar) may contribute to control of primary mesenchyme positioning and function. Specific carbohydrate-containing receptors may, therefore, be of importance in specific gastrulation events.

Lectins inhibit periodontal ligament fibroblast attachment, spreading and migration on laminin substrates

The ways in which carbohydrate signals from the extracellular matrix influence the responses of periodontal ligament (PDL) fibroblasts are essentially unknown. The purpose of this study, using video digital image analysis, was to examine the effects of lectins on the attachment, spreading and migrational responses of rat periodontal ligament fibroblasts on the highly glycosylated glycoprotein laminin (LN) in vitro. PDL fibroblasts were isolated from rat molar teeth and grown in culture. Bound LN and control substrates (bovine serum albumin and untreated plastic) were incubated with solutions of either wheatgerm agglutinin (WGA), specific for N-acetylglucosamine, or concanavalin A (ConA), specific for mannose, in 96-well plates. Unbound lectin was rinsed away and 10.0 x 10(3) cells were seeded per well and allowed to attach for 1.5 h. Incubation of LN substrates with WGA, prior to the addition of any cells, inhibited PDL fibroblast binding more than 5-fold. ConA, however, had no effect on cell binding but inhibited mean individual cell spreading nearly 3-fold under similar assay conditions. The effects could be prevented by adding each lectin's respective specific sugar. The lectins had no effects on the control substrates. In a 24-h cell migration assay WGA and ConA both significantly inhibited migration of PDL fibroblasts. It is likely that WGA inhibited cell attachment and cell migration, by binding to oligosaccharides and blocking access to adjacent polypeptide cell recognition sequences on LN. The results from the ConA experiments, where binding was allowed but spreading was severely inhibited, suggest a possible informational role for the carbohydrates present on LN.

Carbohydrate binding and resistance to proteolysis control insecticidal activity of Griffonia simplicifolia lectin II

Griffonia simplicifolia leaf lectin II (GSII), a plant defense protein against certain insects, consists of an N-acetylglucosamine (GlcNAc)-binding large subunit with a small subunit having sequence homology to class III chitinases. Much of the insecticidal activity of GSII is attributable to the large lectin subunit, because bacterially expressed recombinant large subunit (rGSII) inhibited growth and development of the cowpea bruchid, Callosobruchus maculatus (F). Site-specific mutations were introduced into rGSII to generate proteins with altered GlcNAc binding, and the different rGSII proteins were evaluated for insecticidal activity when added to the diet of the cowpea bruchid. At pH 5.5, close to the physiological pH of the cowpea bruchid midgut lumen, rGSII recombinant proteins were categorized as having high (rGSII, rGSII-Y134F, and rGSII-N196D mutant proteins), low (rGSII-N136D), or no (rGSII-D88N, rGSII-Y134G, rGSII-Y134D, and rGSII-N136Q) GlcNAc-binding activity. Insecticidal activity of the recombinant proteins correlated with their GlcNAc-binding activity. Furthermore, insecticidal activity correlated with the resistance to proteolytic degradation by cowpea bruchid midgut extracts and with GlcNAc-specific binding to the insect digestive tract. Together, these results establish that insecticidal activity of GSII is functionally linked to carbohydrate binding, presumably to the midgut epithelium or the peritrophic matrix, and to biochemical stability of the protein to digestive proteolysis.

Role of mitochondrial membrane potential in concanavalin A-induced apoptosis in human fibroblasts

Lectins induce apoptosis in a wide variety of cell types but the mechanisms of apoptotic induction are unknown. We examined the role of mitochondrial membrane potential (Psi m) in concanavalin A-induced apoptosis in human diploid fibroblasts. Cells were treated with Con A for 0.5, 1, 3, 5, and 24 h. Con A induced a time-dependent increase of the proportion of TUNEL+ ve cells over 24 h. Psi m was examined by staining cells with the mitochondria-specific fluorescent cationic dye JC-1. Comparison of JC-1 fluorescence within mitochondria by flow cytometry showed that after 3 h, Con A reduced Psi m in a subpopulation of apoptotic cells with smaller cell volume and with apoptotic nuclear morphology. In contrast, Psi m was unchanged in a separate population of viable cells with normal volume and normal nuclear morphology. Cyclosporin A protected cells against reduction of Psi m and also against nuclear condensation and morphological apoptosis. Measurement of intracellular calcium ion concentration ([Ca2+]i) by ratio fluorimetry of fura 2-loaded cells showed that Con A did not affect [Ca2+]i in viable cells but induced a progressive depletion of [Ca2+]i with generation of calcium oscillations in apoptotic cells. Assessment of Bcl-2 in Con A-treated cells demonstrated an initially strong increase in Bcl-2 protein and mRNA but the appearance of degraded Bcl-2 protein at 3 and 5 h after treatment, indicating an inadequate protective response to the Con A stimulation. Collectively, these data indicate that lectin-induced apoptosis in fibroblasts is associated with breakdown of Psi m, loss of [Ca2+]i homeostasis, and induced Bcl-2 expression.

Induction of apoptosis and tyrosine phosphorylation of cellular proteins in T cells and non-T cells by stimulation with concanavalin A

A high concentration (30 microg/ml or more) of Con A caused the death of not only thymocytes but also splenic cells of BALB/c mice, whereas a moderate concentration (3 microg/ml) of Con A induced proliferation of these cells. A high concentration of Con A also induced the death of splenic cells of athymic BALB/c-nu/nu mice and the bone marrow cells of BALB/c mice which mainly consist of non-T cells. However, any concentration (1-30 microg/ml) of Con A failed to induce the proliferation of these cells. Specific binding of tetrameric Con A to mannose-containing receptors was required for the induction of cell death. DNA fragmentation was observed by both laser flow cytometry and electrophoresis in Con A-stimulated T cells and non-T cells. This indicated that the mechanism of induction of apoptosis with Con A is not necessarily TCR-dependent. Con A induced tyrosine phosphorylation of a number of proteins in various types of cells. Interestingly, phosphorylation of the 40 kDa protein developed only in the thymocytes and spleen cells that contain T cells, whereas phosphorylation of the 80 and 120 kDa proteins appeared in both T cells and non-T cells. These results suggested that the Con A-induced apoptosis of T cells and non-T cells involves different but possibly mutually related protein tyrosine phosphorylation-linked signals.

Anti-Fas induces apoptosis and proliferation in human dermal fibroblasts: differences between foreskin and adult fibroblasts

Apoptosis, or programmed cell death, is a naturally occurring process mediated by extracellular signals. We studied anti-Fas (CD95/Apo-1) antibody-induced apoptosis in cultured human foreskin and adult dermal fibroblasts. Induction of apoptosis was identified by fluorescence in situ DNA end-labeling. Anti-Fas antibody induced apoptosis in fibroblasts in a dose- and time-dependent manner. Adult dermal skin fibroblasts were more susceptible to anti-Fas antibody-induced apoptosis than foreskin fibroblasts, with 21-52% dead cells in different strains. In foreskin fibroblasts, anti-Fas antibody (1.0 microg/ml) predominantly induced proliferation ([3H]thymidine incorporation increased to 115-165% of control level) and only low levels of apoptotic cell death after 48 hours of treatment. No induction of proliferation by anti-Fas was found in the adult fibroblasts. Addition of tumor necrosis factor-alpha (TNF-alpha) slightly augmented the anti-Fas antibody-induced apoptosis in both cell types. When we examined the levels of Fas expression using flow cytometry, we found two- to threefold higher Fas expression in adult fibroblasts. C6-ceramide treatment, which induces Fas-independent apoptosis, gave similar levels of cell death in both foreskin and adult fibroblasts. No proliferation was observed in C6-ceramide-treated fibroblasts. Thus, this difference in apoptosis between adult dermal and foreskin fibroblasts appears to be related to the level of Fas expression. When clones of foreskin fibroblasts were examined, there was heterogeneity of anti-Fas antibody-induced apoptosis and proliferation in the cloned fibroblast subpopulations, but this was not correlated with differences in Fas expression. Alterations in fibroblast populations during the process of differentiation and aging may result from selective loss of apoptosis-susceptible populations.

Increased apoptosis of adult rat lymphocytes after single neonatal vitamin A treatment (hormonal imprinting). A flow cytometric analysis

Newborn rats were treated with a single dose of vitamin A (retinol) and apoptosis of peripheral lymphocytes was studied by flow cytometry in adult age. Vitamin A treatment (hormonal imprinting) caused a moderate, however significant elevation in the number of apoptotic lymphocytes after three months. Dexamethasone or Concanavalin-A alone did not influence apoptosis significantly. However, in the neonatally retinol treated rats dexamathasone significantly elevated the quantity of apoptotic lymphocytes related to the control or Concanavalin-A treated control cells. The results call attention to the prolonged effect of hormonal imprinting in a new index and to the possible dangerous effects in human, neonatally treated with vitamin A.

Tamoxifen attenuates glucocorticoid actions on bone formation in vitro

Tamoxifen is a synthetic estrogen analog which may regulate osteogenesis in vivo by virtue of its antiglucocorticoid properties. We have examined tamoxifen regulation of glucocorticoid-induced osteogenesis in two different in vitro bone systems: the chicken periosteal osteogenesis model (CPO) and rat bone marrow stromal cells (RBMC). Hormone uptake studies were conducted with the osteosarcoma cell line, ROS 17/2.8. In the CPO model, alkaline phosphatase (AP) activity and collagen synthesis were stimulated by the glucocorticoid dexamethasone (Dex; 0.1 microM). These Dex-mediated effects were inhibited by increasing concentrations of tamoxifen (10-100 microM). Similarly, in the RBMC model, Dex-dependent (0.01 microM Dex) mineralized tissue formation and AP activity were blocked by tamoxifen (0.1 microM). Although tamoxifen inhibited Dex-mediated increases of AP activity in ROS 17/2.8 cells, it did not inhibit uptake of 3H-Dex or of 3H-estrogen. Northern analyses showed that tamoxifen did not affect messenger RNAs (mRNAs) for AP. Tamoxifen did seem to reduce mRNA for collagen type I, but not bone sialoprotein, osteopontin, and osteocalcin. Dex-induced increases for all proteins mRNAs in the RBMC model were not reduced by tamoxifen. Similarly, tamoxifen had no effects on cellular proliferation. We conclude that tamoxifen has no direct effect on gene expression of bone-related proteins of osteoblastic cells. Further, in the ROS 17/2.8 cell line, the antiglucocorticoid properties of tamoxifen do not appear to be mediated through either Dex or estrogen receptors.

Is fibroblast heterogeneity relevant to the health, diseases, and treatments of periodontal tissues?

There are wide variations of gene expression and strikingly different responses to extracellular signals among different fibroblast populations. This has prompted a large number of in vitro studies which suggest that fibroblasts are not homogeneous but instead comprise multiple subpopulations with extensive site-to-site and intra-site variations. Conceivably, either fibroblasts are not all created equal, or, alternatively, discrete subpopulations may emerge in development, inflammatory lesions, or wound healing. While the heterogeneous nature of cultured fibroblasts has been known for some time, are these variations relevant to our understanding of the biology of oral tissues, their involvement in disease, and their response to therapy? Since fibroblasts are the predominant cell type in soft connective tissue matrices, the regulation of their proliferative, synthetic, and degradative behavior is likely to be important in tissue physiology and pathology. In this review, we use the current literature to assess whether fibroblast subpopulations really make a difference in the health and disease of periodontal tissues. We address the following questions: (1) Is fibroblast heterogeneity a real in vivo phenomenon? (2) How can we advance our knowledge of phenotypic variations and the regulation of fibroblast differentiation? (3) Could a knowledge of fibroblast heterogeneity have an impact on the development of new approaches to pathogenesis and the treatment of periodontal tissues?

Role of integrins in regulation of collagen phagocytosis by human fibroblasts

Phagocytosis of collagen fibrils by fibroblasts is an important pathway for degradation of extracellular matrix in mature connective tissues. To study regulatory mechanisms in phagocytosis, 2-microns fluorescent beads coated with either collagen (COL) or bovine serum albumin (BSA) were incubated with human gingival fibroblasts in vitro. For these studies single cell suspensions were prepared by trypsinization, and bead internalization and collagen receptor expression were assessed by flow cytometry. After 3-h incubations, up to 8-fold more cells internalized COL beads than BSA-coated beads. Increased collagen coating concentration was associated with elevated proportions of cells that internalized COL beads, and was observed also in the presence of competing fibronectin-coated beads. The number of beads per cell and the percent of phagocytic cells increased proportionally with higher bead loadings. At > 4 beads per cell a maximum of approximately 80% of cells were phagocytic. Cells reacted with mAbs against the alpha 1, alpha 2, and alpha 3 integrin subunits were, respectively, 5%, 98% and 93% positively stained above background controls. All cells that internalized COL beads exhibited alpha 2 staining but there were large proportions of phagocytic cells that were not stained for alpha 1. In unfixed cells, bead internalization caused an immediate reduction of surface staining of membrane-bound alpha 2 by approximately 55% which returned to control levels within 3 h, indicating that cell-surface alpha 2 was internalized by phagocytosis. Preincubation of cells with up to 8 COL beads per cell reduced the proportion of phagocytic cells and the number of internalized beads after a second COL bead incubation 4 h later. To assess the relationship between the percent of phagocytic cells and alpha 2 integrin levels, serum starvation and cycloheximide experiments were conducted. Compared to controls, serum starvation for 24 h induced a 3.2-fold increase of cells internalizing COL beads but did not alter alpha 2 staining levels. In contrast, 3 h cycloheximide treatment reduced alpha 2 staining to 60% of control levels and this treatment also inhibited COL bead internalization. GRGDTP peptide as well as mAbs against the alpha 1 and alpha 2 subunits significantly reduced internalization of COL beads by 1.8 to 2.6-fold, whereas GRGESP peptide and alpha 3 mAb exerted no effect. Internalization of BSA beads was not affected by any of these treatments. Collectively, these data indicate that the alpha 2 integrin, along with other, as yet unidentified components, is likely involved in COL bead internalization. The alpha 2 integrin subunit is rapidly recycled or synthesized following a phagocytic load. In contrast, the alpha 1 integrin is not directly required for phagocytosis but may regulate the internalization step.