Protein biosynthetic activity of polymorphonuclear leukocytes in inflammatory arthropathies. Increased synthesis and release of fibronectin

We have investigated protein synthesis and release by polymorphonuclear leukocytes (PMN) to compare the protein biosynthetic activity of peripheral blood PMN and inflammatory synovial fluid (SF) PMN from patients with inflammatory arthropathies. We analyzed and compared the protein profiles produced by these cells, using patient matched peripheral blood and SF PMN as well as peripheral blood PMN from normals. Twenty-five patients with either rheumatoid arthritis, psoriatic arthritis or gout were studied. Fluorographs of SDS-polyacrylamide slab gels, performed using cell supernatants from metabolically labelled cells, revealed an increased release of de novo synthesized proteins by inflammatory SF PMN compared to peripheral blood PMN. Under reducing conditions, 4 clearly distinguishable high molecular mass products were observed (Mr 230,000, 185,000, 170,000 and 95,000). Two of the protein bands were found to be gelatin binding (Mr 230,000 and Mr 95,000). By Western blot, the Mr 230,000 protein was found to be fibronectin and the Mr 95,000 protein was shown to be identical to a recently described gelatinase. Thus, the activation of PMN in inflammation is accompanied by an increased release of a number of de novo synthesized proteins, including fibronectin. Our studies directly pertain to the in vivo inflammatory process since the PMN were not activated artificially in vitro.     

Association of La and Ro antigens with intracellular structures in HEp-2 carcinoma cells.

Monoclonal antibodies were raised against homogeneous Ro and La antigens, two proteins associated with Ro and La ribonucleoproteins (RNPs). The specificity of the monoclonal antibodies was proven by immunoblot analysis and by immunoprecipitation. The anti-Ro antibody reacted with a Mr 95,000 protein in a mouse lymphoma cell extract and with a Mr 60,000 polypeptide in extracts from human spleen. The anti-La antibody recognized a Mr 50,000 polypeptide in the mouse L5178y cell extract. The two monoclonal antibodies precipitated RNPs that contained the typical RNA species of Ro or La RNPs. The localization of Ro and La antigen was performed by direct immunofluorescence microscopy. It was found that the anti-Ro antibody reacted with a fibrous network that behaves like cytokeratin, one of the intermediate filament systems. The anti-La antibody reacted with nuclear structures that gave a speckled-type pattern.     

Oncoplacental protein SP1--a constitutive and inducible late differentiation marker of the human myelomonocytic lineage

The oncoplacental protein SP1 is found in large quantities in human placenta, amniotic fluid, and pregnancy serum. Low levels have been reported in association with malignancy but also in healthy nonpregnant individuals. By indirect immunofluorescence, fluorescence-activated cell sorting, and immunoprecipitation we here demonstrate the presence of SP1 both on the surface and in the cytoplasm of human granulocytes but not in earlier myeloid progenitor cells in bone marrow. Lymphocytes did not contain the protein, and only trace amounts could be found in the cytoplasm of blood monocytes. A major glycoprotein with an apparent mol wt of 90,000 was obtained by immunoprecipitation of surface-labeled granulocytes. Cultivated blood monocytes, while adhering to surfaces or forming multinuclear giant cells, displayed a strong membrane and cytoplasmic expression of SP1. Treatment of the myeloid leukemia cell line ML-2 with tetraphorbol acetate (TPA) strongly induced SP1 in the membrane and cytoplasm as revealed by immunofluorescence and polyacrylamide gel electrophoresis (PAGE) of immunoprecipitates from lysates of surface radiolabeled cells. The induction of synthesis of SP1 in TPA-treated cells was confirmed by immunoprecipitation from lysates of cells metabolically labeled with 35S-methionine. Human lymphoblastoid and erythroleukemic cell lines did not express SP1 either before or after induced differentiation. Thus SP1 provides a late differentiation marker for the myelomonocytic lineage and is strongly induced during macrophage differentiation or by TPA treatment of ML-2 cells.     

Immune cells and hepatocytes express glycosylphosphatidylinositol-anchored ceruloplasmin at their cell surface

BackgroundCeruloplasmin is a positive acute-phase protein with both anti- and pro-oxidant activities, thus having still unclear physiological functions in inflammatory processes. Importantly, ceruloplasmin has been implicated in iron metabolism due to its ferroxidase activity, assisting ferroportin on cellular iron efflux. Ceruloplasmin can be expressed as a secreted or as a membrane glycosylphosphatidylinositol-anchored protein (GPI-ceruloplasmin), this latter one being reported as expressed mostly in the brain.Design and methodsWe studied the expression of both ceruloplasmin isoforms in human peripheral blood lymphocytes, monocytes, mouse macrophages and human hepatocarcinoma cell line HepG2, using immunofluorescence and immunoblotting techniques. Co-localization of ceruloplasmin and ferroportin was also investigated by immunofluorescence in mouse macrophages.ResultsCeruloplasmin was detected by immunoblotting and immunofluorescence in membrane and cytosol of all cell types. The cell surface ceruloplasmin was identified as the GPI-isoform and localized in lipid rafts from monocytes, macrophages and HepG2 cells. In macrophages, increased expression levels and co-localization of ferroportin and GPI-ceruloplasmin in cell surface lipid rafts were observed after iron treatment. Such iron upregulation of ceruloplasmin was not observed in HepG2.ConclusionsOur results revealed an unexpected ubiquitous expression of the GPI-ceruloplasmin isoform in immune and hepatic cells. Different patterns of regulation of ceruloplasmin in these cells may reflect distinct physiologic functions of this oxidase. In macrophages, GPI-ceruloplasmin and ferroportin likely interact in lipid rafts to export iron from cells. Precise knowledge about ceruloplasmin isoforms expression and function in various cell types will help to clarify the role of ceruloplasmin in many diseases related to iron metabolism, inflammation and oxidative biology.     

P47 phosphoprotein of blood platelets (pleckstrin) is a major target for phorbol ester-induced protein phosphorylation in intact platelets, granulocytes, lymphocytes, monocytes and cultured leukaemic cells: absence of P47 in non-haematopoietic cells

Aggregating agents including phorbol esters which activate protein kinase C induce the rapid phosphorylation of a Mr = 47,000 cytosolic protein in blood platelets (P47 or pleckstrin). This protein is well resolved by analytical 16-BAC----SDS two-dimensional PAGE and was purified from platelets by preparative 16-BAC----SDS PAGE. Polyclonal antibodies were raised to the protein in mice and rabbits. These antisera detected a single protein with the migration of P47 on Western blots of platelet extracts, and the rabbit antisera immunoprecipitated 32P-labelled P47 from platelet cytosol. The presence of P47 in other haematopoietic cells was determined by prelabelling them with 32P and observing increased 32P incorporation into the location of P47 on autoradiographs of 16-BAC----SDS analytical PAGE of cells exposed to phorbol ester. The identity of the phosphoprotein found in this location was further established by probing Western blots of SDS PAGE gels of cultured cell lines with the P47 antisera. P47 was detected in peripheral blood lymphocytes, monocytes and granulocytes (including the granulocytes of two unrelated patients with X-linked chronic granulomatous disease). P47 was also found in HL-60 promyelocytes (especially after differentiation with retinoic acid), U937 histiocytes, HEL leukaemia cells, and Raji 'B' lymphoblasts. It was not detected in normal erythrocytes, K562 leukaemic cells, MOLT-3 'T' lymphoblasts, or in wide range of non-haematopoietic cell lines. We conclude that P47 is a major target for the action of phorbol ester induced phosphorylation in platelets, normal leucocytes and some haematopoietic cell lines. These cells have as their common feature the ability when stimulated to develop adhesive functions on their plasma membranes.     

Fas and Fas Ligand Expression on Human Peripheral Blood Leukocytes

Objectives : Study of Fas and Fas ligand(Fas-L) expression, as well as sFas–L release, by fresh human peripheral blood leukocytes. Methods : Flow cytometry, cytotoxicity, immunofluorescence staining of fresh smears, Western blotting, Results : Granulocytes and monocytes express a low level of Fas receptor, but no Fas–L. These cells, as well as NK cells, contain presynthesized depots of Fas–L which they express following activation by brief storage (60 min) at room temperature or during separation from whole blood. Such activation also leads to Fas receptor upregulation. NK cells do not express Fas receptor. Once expressed on blood leukocytes, fully functional Fas–L can be released from the membrane and can be detected in plasma–free cell supernatants. Conclusion : Human peripheral blood granulocytes, monocytes and NK cells contain intracellular presynthesized Fas–L which they readily express following blood anticoagulation, blood storage or cell separation. Soluble Fas–L is released from those cells and can be detected in protein–free supernatants by immunoblotting.     

Poly(ADP-ribose) synthesis, a marker of granulocyte differentiation.

By using an indirect immunofluorescence technique, the distribution of poly(ADP-ribose) synthesis in human blood cells was investigated. The antibody used was reactive with poly(ADP-ribose) larger than trimers. The specific immunofluorescence of poly(ADP-ribose) synthesized in situ from NAD+ was observed in nuclei of lymphocytes and monocytes in normal peripheral blood. No immunofluorescence, however, was detected in granulocytes and erythrocytes. In agreement with this finding, no incorporation of radioactivity from [adenine-14C]NAD+ into acid-insoluble material was detectable in nuclei isolated from granulocytes. In normal bone marrow, the immunofluorescence of poly(ADP-ribose) was not observed in myelocytes or in their descendants but was observed in nuclei of lymphocytes and erythroblasts. Myelocytes and mature granulocytes in peripheral blood as well as in bone marrow of patients with chronic myelocytic leukemia were totally negative in the polymer-specific immunofluorescence. In marked contrast, prominent fluorescence was observed in nuclei of myeloblasts that appeared in peripheral blood as well as in bone marrow of patients with acute myeloblastic leukemia. Myeloblasts appearing in peripheral blood of patients in blastic crisis of chronic myelocytic leukemia also showed the nuclear immunofluorescence. These results suggest that the capacity for synthesizing poly(ADP-ribose) serves as a marker of differentiation of granulocytes, and its immunohistochemical analysis may be useful for differential diagnosis of leukemias, especially in blastic crisis.     

Cell-surface glycoproteins of human sarcomas: differential expression in normal and malignant tissues and cultured cells

Normal differentiation and malignant transformation of human cells are characterized by specific changes in surface antigen phenotype. In the present study, we have defined six cell-surface antigens of human sarcomas and normal mesenchymal cells, by using mixed hemadsorption assays and immunochemical methods for the analysis of cultured cells and immunohistochemical staining for the analysis of normal tissues and greater than 200 tumor specimens. Differential patterns of F19 (Mr, 120,000/95,000 glycoprotein), F24 (Mr, 95,000 glycoprotein), G171 (Mr, 75,000 glycoprotein), G253 (Mr, 90,000 glycoprotein), S5 (Mr, 120,000 glycoprotein), and Thy-1 (Mr, 25,000 glycoprotein) antigen expression were found to characterize (i) subsets of cultured sarcoma cell lines, (ii) cultured fibroblasts derived from various organs, (iii) normal resting and activated mesenchymal tissues, and (iv) sarcoma and nonmesenchymal tumor tissues. These results provide a basic surface antigenic map for cultured mesenchymal cells and mesenchymal tissues and permit the classification of human sarcomas according to their antigenic phenotypes.     

Coxsackievirus B3 infection in human leukocytes and lymphoid cell lines

Although coxsackie B viruses (CBVs) are known to cause viremia during acute infection, the role of the blood cells as a target for virus replication is poorly understood. We have analyzed the susceptibility of human peripheral blood mononuclear cells (PBMCs), granulocytes, bone marrow (BM) cells, and lymphoid cell lines to coxsackievirus B3 infection. Lymphoid cell lines with B- and T-cell characteristics (Raji and Molt-4, respectively) supported virus replication to high titers and virus protein synthesis was detected by metabolic labeling and immunoprecipitation. CBV3 synthesis in the U937 cell line with mononuclear phagocytic characteristics was very limited. The virus was able to infect a small proportion of leukocytes and BM cells, and intracellular virus antigens were detected by immunofluorescent staining. However, only a diminutive amount of infectious virus was produced in isolated PBMCs and granulocytes, and no virus protein synthesis was detected by metabolic labeling and immunoprecipitation in these cells.     

Igf-I inhibits spontaneous apoptosis in human granulocytes

Granulocytes are key cells in inflammatory processes that are recruited to sites of inflammation by chemoattractants such as IL-8 produced by neutrophils and monocytes. Programmed cell death (apoptosis) of granulocytes and subsequent recognition and phagocytosis by macrophages is a crucial mechanism for resolution of inflammation. Because IGF-I is a potent antiapoptotic factor, we addressed the effects of IGF-I on in vitro apoptosis of human peripheral blood granulocytes. We detected 1390 +/- 467 IGF-I receptors with a dissociation constant of 2.3 +/- 0.9 nM on purified granulocytes. Using microscopical analysis, annexin V binding assays to detect relocation of phosphatidylserine to the cell surface, and DNA fragmentation assays, we showed that IGF-I inhibits spontaneous apoptosis of granulocytes in serum-free culture by 32-45%. IGF-I did not modulate the secretion of IL-6, TNF alpha, and IL-8 by granulocytes, but IL-8 secretion by peripheral blood mononuclear cells was enhanced by 40%. These observations indicate that IGF-I may promote granulocyte functions by increasing granulocyte longevity.     

Anti-granulocyte opsonic activity in sera from patients with systemic lupus erythematosus

Neutropenia is common in patients with systemic lupus erythematosus (SLE) but mechanisms of cell depletion remain obscure. To investigate the possible autoimmune aetiology of neutropenia in SLE, sera from 31 patients with this disorder were tested for anti-granulocyte activity. Granulocyte-binding immunoglobulins were detected by indirect immunofluorescence, and the ability of patient sera to opsonize granulocytes was determined by measuring the chemiluminescent response of human monocytes to granulocytes sensitized by test sera. Sera from 22 of the 31 patients bound IgG to granulocyte cell membranes and/or to nuclei, but only membrane-binding antibodies opsonized the cells for recognition by monocytes. There was no correlation between neutrophil count and the level of granulocyte-binding IgG as measured by indirect immunofluorescence. In contrast, opsonic activity and neutrophil count were inversely correlated (r = 0.5; P less than 0.05). However, opsonic activity was present in sera from most non-neutropenic patients. In patients with SLE, impaired reticuloendothelial system function may allow sensitized granulocytes to remain in the circulation.     

The bone marrow in human cerebral malaria: parasite sequestration within sinusoids

Bone marrow aspirates from patients with cerebral malaria were studied with the light and electron microscopes. Various abnormalities were found including: (1) an increase in plasma cells and macrophages, sometimes to a marked degree; (2) phagocytosis of parasitized red cells by macrophages and of merozoites by neutrophil metamyelocytes, neutrophil granulocytes and macrophages; (3) an increase in the proportion of eosinophil granulocytes and their precursors; (4) the presence of giant metamyelocytes; and (5) morphological abnormalities of erythroblasts, particularly irregularly-shaped nuclei and karyorrhexis. A high percentage of the red cells within marrow sinusoids were parasitized and the parasitized cells were attached to the endothelium. Some marrow sinusoids were packed with and completely obstructed by parasitized cells. Strands of electron-dense material were sometimes found connecting the knobs of parasitized red cells to endothelial cells or to the knobs of adjacent parasitized red cells. A striking finding was a complex interdigitation between cytoplasmic processes developed by some of the parasitized red cells and those developed by the endothelial cells to which they were attached. Occasionally, cytoplasmic processes arising from marginated parasitized red cells completely penetrated the endothelial cell and emerged extravascularly. Several parasitized red cells were also found extravascularly between haemopoietic cells. Sequestration of parasitized red cells within small blood vessels may play a part in the pathogenesis not only of the encephalopathy of cerebral malaria but also of the bone marrow dysfunction in severe malaria.     

Myeloperoxidase: its structure and expression during myeloid differentiation

Myeloperoxidase (MPO) is a major protein present in myeloid cells and is used by these cells to help kill microbes. The human promyelocytic HL-60 line can be induced to differentiate to granulocytes or macrophagelike cells. Poly (A) containing RNA was isolated from HL-60 granulocytes, HL-60 macrophages, HL-60 blasts, and normal human granulocytes. The mRNA was translated in a reticulocyte lysate system in the presence of 35S-methionine. The MPO was precipitated from the lysate with rabbit IgG antiserum to human MPO. The resulting precipitate from HL-60 blasts gave a major band of radioactivity of approximately 77,000 daltons and another band at approximately 46,000 daltons on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The MPO identity of the labeled bands was confirmed by cold competition. The relative mRNA activity expressed as a percentage of radioactivity incorporated into MPO (77,000-dalton band) as compared with total trichloracetic acid (TCA) precipitable radioactivity was 0.2%. Negligible mRNA activity for MPO was present in HL-60 granulocytes, HL-60 macrophages, and normal human granulocytes. Pulse- chase experiments showed that MPO was an approximate 75,000-dalton major band and 77,000-dalton minor band of radioactivity after HL-60 blasts were labeled for 1/2 hour with 35S-methionine and the cell lysate immunoprecipitated and subjected to SDS-PAGE. The chase experiments (one to 24 hours) showed that the 77,000- and 75,000-dalton bands of radioactivity were replaced with two major bands (55,000 and 15,000 daltons) and one minor band (approximately 39,000 daltons) of radioactivity. Six-hour 35S-methionine labeling experiments showed that the relative rate of MPO synthesis compared with total TCA precipitable radioactivity was 0.5% in HL-60 blasts and almost negligible in HL-60 macrophages and granulocytes, normal human granulocytes, and B- lymphocytes. The KG-1 myeloblasts and KG-1a early myeloblasts synthesized a small amount of the 75,000-dalton MPO protein. Although HL-60 cells no longer synthesized MPO after differentiation, HL-60 granulocytes and HL-60 macrophages continued to contain MPO as measured by enzyme activity.(ABSTRACT TRUNCATED AT 400 WORDS)     

Monoclonal antibody to a human osteogenic sarcoma cell line

Monoclonal antibodies against a human osteogenic sarcoma cell line were prepared by production of a somatic cell hybrids between the spleen cells from U-393OS--immunized mice and the mouse myeloma cells SP2/0. From 7 producing and well-growing clones only one--B-0S12--produced antibodies, reactive preferentially with osteosarcoma cells as identified by binding second antibodies and 125I-labeled Protein A. This antibody was tested against a panel of normal and tumor cell targets to determine the pattern of the antigen detected. The monoclonal antibody reacted strongly against U-3930S cells and another human sarcoma in vitro and more weakly against human fibroblasts, peripheral lymphocytes, red blood cells and was negative against mouse fibroblasts. When tested against a panel of unrelated human tumor cell lines, B-0S12 antibody was positive with melanoma cells and negative with cells from bladder, cervix and mammary carcinoma. These cross reactions suggested, that the antibody is reactive with a protein, expressed on different tumor types. This protein is not expressed on the cell surface and is probably associated with cytoskeleton, as revealed by immunofluorescence experiments. Western-blot analysis of a cytoskeletal preparation of U-3930S cells suggests, that B-0S12 antibody recognizes a protein with Mr 55 kD. Further studies are needed to characterize the molecules, carrying the epitope, identified by this monoclonal antibody.     

Vascular endothelium as a regulator of granulopoiesis: production of colony-stimulating activity by cultured human endothelial cells

Colony-stimulating activity is a regulatory factor(s) that promotes differentiation of hemopoietic stem cells to mature granulocytes and macrophages; in man it has been found that blood monocytes, lymphocytes, and tissue macrophages produce it. In an effort to identify other potenitally physiologic tissue sources of colony- stimulating activity, we have studied the capacity of primary cultures of human vascular endothelial cells to produce colony-stimulating activity. Medium conditioned by incubation with endothelial cultures contained activity that promoted granulocyte-macrophage colony formation of nonadherent human and murine marrow cells. Exposure of endothelial cultures to 0.1-5.0 microgram/ml S. typhosa endotoxin for 6- 72 hr enhanced colony-stimulating activity production. Similarly, incubation of endothelial cells with lysates of human blood granulocytes, or cocultivation with intact granulocytes, resulted in increased colony-stimulating activity levels. In 7-14 day cultures, freshly isolated endothelial cells, incorporated into agar underlayers, consistently stimulated more colony formation by nonadherent human marrow cells than comparable numbers of blood monocytes. These data indicate that: (1) cultured human endothelial cells are a potent source of colony-stimulating activity; (2) they respond to endotoxin and granulocytes and their contents by producing increased amounts of CSA; and (3) they produce morea colony-stimulating activity, than human blood monocytes under standardized conditions in vitro. These observations suggest that the vascular endothelium may play a role in the physiologic regulation of granulopoiesis.     

Transformation-enhancing activity of gelatin-binding fragments of fibronectin.

Studies have established that cryoprecipitates of the plasma of tumor patients contain a biological activity enhancing morphological cell transformation (transformation-enhancing factor; TEF) in cultures of chicken embryo fibroblasts infected with temperature-sensitive mutants of Rous sarcoma virus. We report here that similar TEF activity is effected by defined fragments of human plasma fibronectin obtained by limited digestion with major humoral or tissue proteinases. TEF activity was obtained from plasminolytic fragments of fibronectin and from cathepsin G-treated fibronectin. No activity was recorded from intact dimeric fibronectin or its reduced and alkylated subunits, from fibrinogen or its plasminolytic fragments, or from plasmin (EC 3.4.21.7) or cathepsin G (EC 3.4.21.20) treated or untreated with proteinase inhibitors. All of the TEF activity of the proteolytic fragments of fibronectin was located on the gelatin-binding peptides. The minimum effective doses in the TEF assay were 750 ng/ml of plasmin-treated fibronectin, 100 ng/ml of gelatin-binding plasminolytic fibronectin (enriched in Mr 180,000--190,000 polypeptides), and 100 ng/ml of gelatin-binding fragments of cathepsin G-treated fibronectin (enriched in a Mr 30,000 fragment). TEF activity of proteinase-treated fibronectin was inhibited by gelatin and by intact dimeric fibronectin. The potent TEF activity of proteolytic fragments of fibronectin raises the possibility that they may have a role in malignant transformation.     

Identification of human megakaryocyte coagulation factor V

Specific monoclonal and polyclonal antibody reagents and a double antigen indirect immunofluorescence microscopy technique were used to visualize coagulation factor V in human bone marrow. Marrow aspirates were smeared directly on glass slides, or washed and cytospun onto glass slides, or processed and plated into a plasma/methylcellulose cell culture system. Morphologically identifiable colonies of megakaryocytes, erythrocytes, granulocytes, or monocytes/macrophages were removed from 14- to 18-day marrow culture dishes by micropipette and streaked onto glass slides. Smears of marrow cell preparations were air-dried, fixed, washed, and incubated sequentially with primary IgG antibody reagents and with secondary anti-IgG antibody reagents conjugated with either fluorescein or rhodamine. Preparations were examined and photographed through a microscope suitably equipped for two-color fluorescence and phase contrast analysis. Cells of megakaryocytic lineage were identified by their immunofluorescent reactivity with murine monoclonal antibody HP1-1D, specific for human platelet plasma membrane glycoprotein IIb/IIIa (GP IIb/IIIa), or by their immunofluorescent reactivity with monoclonal or polyclonal antibodies specific for von Willebrand factor (vWF) or for platelet factor 4 (PF4). Coagulation factor V in bone marrow was detected by simultaneous immunofluorescent staining with polyclonal burro anti-human factor V antibody or with a panel of murine monoclonal anti-human factor V antibodies. The double antigen immunofluorescence staining technique, incorporating appropriate controls, revealed that coagulation factor V was principally located in marrow cells simultaneously identified as megakaryocytes by antibodies to GP IIb/IIIa, vWF, or PF4. The specific immunofluorescence of factor V in megakaryocytes and platelets was eliminated when excess purified factor V antigen was preincubated with anti-factor V antibody. Our observations establish the presence of human megakaryocyte coagulation factor V, confirm the presence of human platelet factor V, and indicate that human megakaryocyte/platelet coagulation factor V is a lineage-associated protein.     

Occurrence of caldesmon (a calmodulin-binding protein) in cultured cells: comparison of normal and transformed cells.

Caldesmon is a calmodulin-binding and F-actin-binding protein originally purified from chicken gizzard smooth muscle. This protein binds to F-actin filaments in a Ca2+- and calmodulin-dependent "flip-flop" fashion, thereby regulating the function of actin filaments. Here we report that various lines of cultured cells contain a Mr 77,000 protein that specifically reacts with the affinity-purified caldesmon antibody raised against chicken gizzard caldesmon . Among the fibroblast proteins that had been pulse-labeled with [35S]methionine, the Mr 77,000 protein was the only protein band detected on the NaDodSO4 gel that reacted with the anticaldesmon . The subcellular distribution of the Mr 77,000 protein was investigated by the indirect immunofluorescence technique using the anticaldesmon . In all fibroblast cell lines examined, the immunofluorescence localized along the cellular stress fibers and in leading edges of the cell. In Rous sarcoma virus-transformed cells (S7-1), however, the distribution of the fluorescence changed to a diffuse and blurred appearance. These staining patterns of anticaldesmon obtained with the normal and transformed cells coincided with those of antiactin in the corresponding states, strongly suggesting the functional linkage between the Mr 77,000 protein and actin filaments. We propose to refer to this Mr 77,000 protein as caldesmon 77. The cellular level of caldesmon 77 in transformed S7-1 cells decreased to about one-third of that in their normal counterparts (cell line no. 7). Essentially the same result was obtained with normal rat kidney cells infected with the temperature-sensitive transformation mutant Schmidt-Ruppin strain of Rous sarcoma virus (68 N2 clone). The cellular level of caldesmon 77 observed at a permissive temperature (35 degrees C) was about one-third of that at a nonpermissive temperature (38.5 degrees C). These changes of caldesmon 77 in transformed cells may correlate with the loss of Ca2+ regulation in the transformed state.