Detection of c-sis transcripts and synthesis of PDGF-like proteins by human osteosarcoma cells

Absence of Expression of c-sis and Transforming Growth Factor-β mRNA in Malignant Fibrous Histiocytoma

Total RNA was extracted from five malignant fibrous histiocytomas and two benign fibrohistiocytic lesions and assayed for mRNA expressions for transforming growth factor beta (TGF-β) and c-sis by Northern blot analysis. Production of both of these has been associated with cells of monocyte-macrophage lineage, and these factors have been shown to be important in physiologic mesenchymal cell proliferation. No mRNA expression of either TGF-β or c-sis was identified in any of the fibrohistiocytic tumor samples. The lack of expression of TGF-β and c-sis may be consistent with a nonhistiocytic origin of malignant fibrous histiocytoma, or may reflect transformation- associated loss of the normal molecular mechanisms of mesenchymal proliferation. The absence of c-sis mRNA expression can be reconciled with the prior immunohisto chemical demonstration of platelet-derived growth factor in tumor cells of malignant fibrous histiocytoma. Int J Surg Pathol(2):117-122, 1993

Overexpression of PDGF-BB decreases colorectal and pancreatic cancer growth by increasing tumor pericyte content

We hypothesized that overexpression of PDGF-BB in colorectal cancer (CRC) and pancreatic cancer cells would result in increased pericyte coverage of ECs in vivo, rendering the tumor vasculature more resistant to antiangiogenic therapy. We stably transfected the cDNA for the PDGF-B into HT-29 human CRC and FG human pancreatic cancer cells. Surprisingly, when HT-29 or FG parental and transfected cells were injected into mice (subcutaneously and orthotopically), we observed marked inhibition of tumor growth in the PDGF-BB-overexpressing clones. In the PDGF-BB-overexpressing tumors, we observed an increase in pericyte coverage of ECs. Treatment of PDGF-BB-overexpressing tumors with imatinib mesylate (PDGFR inhibitor) resulted in increased growth and decreased total pericyte content compared with those in untreated PDGF-BB-overexpressing tumors. In vitro studies demonstrated the ability of VSMCs to inhibit EC proliferation by approximately 50%. These data show that increasing the pericyte content of the tumor microenvironment inhibits the growth of angiogenesis-dependent tumors. Single-agent therapy targeting PDGF receptor must be used with caution in tumors when PDGFR is not the target on the tumor cell itself.

Platelet-derived growth factor induces interleukin-6 transcription in osteoblasts through the activator protein-1 complex and activating transcription factor-2

Platelet-derived growth factor (PDGF) BB, a mitogen that stimulates bone resorption, increases the expression of interleukin-6 (IL-6), a cytokine that induces osteoclast recruitment. The mechanisms involved in IL-6 induction by PDGF BB are poorly understood. We examined the effect of PDGF BB on IL-6 expression in cultures of osteoblasts from fetal rat calvariae (Ob cells). PDGF BB increased IL-6 mRNA and heterogeneous nuclear RNA levels, the rate of transcription, and the activity of base pairs (bp) -2906 to +20 IL-6 promoter fragments transiently transfected into Ob cells. Deletion analysis revealed two responsive regions, one containing an activator protein-1 (AP-1) site located between bp -276 and -257, and a second, less well defined, downstream of -257. Targeted mutations of a cyclic AMP-responsive element (CRE), and nuclear factor-IL-6 and nuclear factor-kappaB binding sites in a bp -257 to +20 IL-6 construct that was transfected into Ob cells, revealed that the CRE also contributed to IL-6 promoter induction by PDGF BB. Electrophoretic mobility shift assay revealed AP-1 and CRE nuclear protein complexes that were enhanced by PDGF BB. Supershift assays revealed binding of Jun and Fos to AP-1 and CRE sequences and binding of activating transcription factor-2 to CRE. In conclusion, PDGF BB induces IL-6 transcription in osteoblasts by regulating nuclear proteins of the AP-1 complex and activating transcription factor-2.

Distraction bone healing versus osteotomy healing: a comparative biochemical analysis

This study investigates the biochemical changes in a canine tibia lengthening model in comparison with a nonlengthened osteotomy model. The lengthened and the osteotomized callus and a contralateral corresponding segment were analyzed for their mineralization profile, collagen content, osteocalcin, insulin-like growth factor I (IGF-I), and transforming growth factor beta1 (TGF-beta1). Examinations of bone samples were performed using specimens excised at different time intervals (respectively at 3, 5, 7, 9, and 13 weeks postoperatively). Several serum parameters (alkaline phosphatase [ALP], osteocalcin, IGF-I, and TGF-beta1) were also measured during the experimental period. A progressive increase in mineral parameters was noticed in both the lengthened and the osteotomized areas. A higher level of hydroxyproline and TGF-beta1 was observed in the lengthened area compared with the osteotomized area. IGF-I showed a significant increase in both the lengthened and contralateral control area at the later stage of the experimental period in the lengthened group. In serum, a high level of TGF-beta1 and a progressively increasing osteocalcin concentration were observed in the lengthened dogs in comparison with the osteotomized dogs. Serum ALP was significantly increased in both models during the experimental period. Serum IGF-I was increased in the lengthened models during the distraction period and decreased in the osteotomized models at the early stage of the experimental period. These results suggest that the mechanical strain induced by the Ilizarov distraction procedure stimulates osteoblast proliferation and promotes biosynthesis of bone extracellular matrix in distracted callus. Our data furthermore show that this process is different compared with normal fracture healing.

Distribution of platelet-derived growth factor (PDGF) A chain mRNA, protein, and PDGF-alpha receptor in rapidly forming human bone

Platelet-derived growth factors (PDGFs) are potent bone cell mitogens which stimulate the proliferation of osteoblastic cells, may also be involved in the regulation of osteoclastic bone resorption, and indirectly induce vascular endothelial cell proliferation and angiogenesis. In view of the established relationship between angiogenesis and osteogenesis, the production of PDGFs by both osteoblastic and vascular endothelial cells suggests that they may play a role in bone formation during skeletal development. We have used two human models of rapid bone formation, heterotopic bone and osteophytic bone, to investigate the expression of PDGF-A mRNA and protein and the PDGF-alpha receptor protein in vivo using in situ hybridization and immunohistochemistry. PDGF-A mRNA and protein were widely distributed throughout heterotopic and osteophytic bone. Within the cartilaginous tissue PDGF-A mRNA and protein were most strongly expressed by mature chondrocytes with decreased expression in the hypertrophic zone and almost no staining in the mineralizing and mineralized zones. PDGF mRNA and protein were also expressed in cells of small blood vessels within fibrous and cartilaginous tissue. In contrast, PDGF-alpha receptor expression was restricted to a minority of hypertrophic chondrocytes and sites of vascular invasion. Within the bone and fibrous tissue the growth factor and the receptor were widely distributed, being detected on most cells at sites of bone formation or in remodeling sites; no receptor was detected on osteoclasts. These data demonstrate the widespread expression of PDGF-A and its receptor in forming human bone and indicate that this growth factor may exert autocrine and paracrine effects to regulate osteogenesis during skeletal development.

Transcriptional regulation of the SIS/PDGF-B gene in human osteosarcoma cells by the Sp family of transcription factors

Expression of PDGF-B, the gene encoding the platelet-derived growth factor B chain, has been implicated as a participant in an autocrine growth loop in the human osteosarcoma cell line U2-OS. In previous work, we identified a primary site in the PDGF-B promoter, the SIS proximal element (SPE), which is critical for transcription of the PDGF-B gene in U2-OS cells. We also identified Sp1 as one of the SPE-binding proteins in U2-OS nuclear extracts. In the present work, we have identified another SPE-binding protein to be Sp3. Gel mobility shift assays showed that both Sp1 and Sp3 require the CACCC motif within the SPE for binding. In vitro transcription assays showed that Sp1 or/and Sp3 is necessary for transcription of the PDGF-B gene. Cotransfection experiments functionally demonstrated that Sp1 and Sp3 can independently or additively activate the PDGF-B promoter through the SPE as well as a synthetic promoter. However, the CACCC motif within the SPE is not the only site within the minimal PDGF-B promoter through which Sp1/Sp3 acts; additional nested deletion analyses showed that multiple cis-acting elements within the minimal promoter are required for full level transcription of the PDGF-B gene in U2-OS cells.

Regulation of bone formation by bone morphogenetic proteins and other growth factors

Bone formation in adult humans is a complex and closely regulated process. It usually occurs at sites of previous osteoclastic bone resorption. It also may occur in the growing long bones during endochondral bone formation, and appositional bone formation also can appear during growth and adolescence without prior local resorption, particularly on periosteal surfaces. The cellular events involved in bone formation include chemotaxis of osteoblast precursors; proliferation of committed osteoblast precursors; differentiation, including expression of growth regulatory factors and the structural proteins of bone, such as osteocalcin, osteopontin, and Type I collagen; and mineralization. It is clear that these cellular events must be under very tight regulatory control. They may all be modulated by systemic hormones, including the calciotropic hormones, parathyroid hormone and 1,25 dihydroxyvitamin D and other systemic hormones, such as the pituitary and thyroid hormones and sex steroids, but probably are modulated predominantly by local factors or cytokines generated in the bone cell microenvironment.

ErbB-2 expression is correlated with poor prognosis for patients with osteosarcoma

BACKGROUND

It has been reported that the c-erbB-2 protooncogene is frequently amplified and overexpressed in many types of cancers, except sarcomas and hematological malignancies.

METHODS

Expression of ErbB-2 in the tumors of 26 patients with conventional osteosarcoma was evaluated by immunoblotting. DNA from osteosarcoma tissues that expressed ErbB-2 were analyzed by Southern blot hybridization to examine gross rearrangement of the gene. The DNA was also surveyed for the presence of genetic mutation in the transmembrane domain of ErbB-2 by polymerase chain reaction-single-stranded DNA conformation polymorphism analysis. In addition, possible correlation of ErbB-2 expression with gender, age, histopathologic subtype, and response to chemotherapy was analyzed. Survival analysis was performed by the Kaplan-Meier test using the approximate chi-square statistic for the log-rank test.

RESULTS

The ErbB-2 protein was detected in 11 of 26 osteosarcoma tissues (42%) by immunoblot analysis. Expression of ErbB-2 was confirmed by immunohistochemical studies using specific anti-ErbB-2 monoclonal antibody. However, neither amplification of the c-erbB-2 gene nor evidence of significant genetic mutation was found in these osteosarcomas. Expression of ErbB-2 examined by immunoblotting was most strongly correlated with early pulmonary metastases (P < 0.05). Among the entire group of 26 patients in this study, Kaplan-Meier life table survival of the patients with apparent ErbB-2 expression was significantly worse than that of the patients with little ErbB-2 expression (P < 0.01).

CONCLUSIONS

In 42% of the osteosarcomas, the tumor cells expressed ErbB-2. Expression of ErbB-2 was strongly correlated with early pulmonary metastasis and poor survival rate for the patient. These data suggest that ErbB-2 plays a significant role in aggressive tumor growth and in the promotion of metastatic potential in osteosarcomas. ErbB-2 in the osteosarcoma tissues would be a useful prognostic marker for patients.

Cytokine regulation of bone cell differentiation

Systemic hormones and cytokines play important roles in regulating both osteoblast and osteoclast activity. These cytokines can have either positive or negative effects on the growth and differentiation of bone cells. These effects appear to be dependent on the model systems use to assess them, as well as the species tested. In the near future, other autocrine-paracrine factors will be identified that enhance osteoblast and osteoclast activity, and model systems should be available to further delineate their effects on cells in the osteoblast lineage. Use of transgenic mice with genes targeted to the osteoblast and osteoclast may further reveal the mechanisms responsible for the growth and differentiation of these cells, as well as produce immortalized cell lines that more accurately reflect the cell biology of the osteoclast and osteoblast in vivo.

Signals controlling the expression of PDGF

PDGF is an important polypeptide growth factor that plays an essential role during early vertebrate development and is associated with tissue repair and wound healing in the adult vertebrate. Moreover, PDGF is thought to play a role in a variety of pathological phenomena, such as cancer, fibrosis and atherosclerosis. PDGF is expressed as a dimer of A and/or B chains, the precursors of which are encoded by two single copy genes. Although the PDGF genes are expressed coordinately in a number of cell types, they are independently expressed in a majority of cell types. The expression of either PDGF gene can be affected by very diverse extracellular stimuli and the type of response is dependent on the cell type that is exposed to the stimulus. Expression of the PDGF chains can be modulated at every imaginable level: by regulating accessibility of the transcription start site, by varying the transcription initiation rate, by using alternative transcription start sites, by alternative splicing, by using alternative polyadenylation signals, by varying mRNA decay rates, by regulating efficiency of translation, by protein modification, and by regulating secretion. Even upon secretion, the activity of PDGF can be modulated by non-specific or specific PDGF-binding proteins. This review provides an overview of the cell types in which the PDGF genes are expressed, of the factors that are known to affect the expression of PDGF, and of the various levels at which the expression of PDGF genes can be regulated.

A novel human c-sis mRNA species is transcribed from a promoter in c-sis intron 1 and contains the code for an alternative PDGF B-like protein

The human platelet-derived growth factor (PDGF) B chain precursor is usually translated from a 3.5 kb c-sis/PDGF B gene transcript. The first exon of the c-sis/gene contains the code for the signal peptide of the PDGF B chain precursor, preceded by a 1 kb long untranslated sequence with potent translation inhibitory activity. In this paper we show that a novel 2.6 kb c-sis mRNA present in the human choriocarcinoma cell line JEG-3 initiates at an alternative exon 1, which we refer to as exon 1a. The 90 bp long exon 1a is located in the center of the first intron of the gene. It coincides with a very pronounced DNase-I-hypersensitive site and is preceded by a functional promoter. Of the three ATG codons present in exon 1a, the third one perfectly matches the criteria of a consensus start codon. It initiates an open reading frame that is continuous with the code for the PDGF B chain precursor but lacks the code for a signal peptide. We conclude that this novel 2.6 kb c-sis mRNA species lacks the strong translation inhibitory potential of the regular exon 1 and contains the code for a PDGF B-like protein that may be targeted to the cell nucleus.

The effects of cytokines and growth factors on osteoblastic cells

In this short review, some regulatory mechanisms that are involved in the control of normal bone formation are proposed, based on several in vivo and in vitro models our group has utilized recently to study osteoblast differentiation and mineralized bone matrix formation. Of course, these proposals must be assessed in the light of the limitations of the models, which probably represent a simplification of the complex and different ways in which normal mammalian bone is formed at different sites. Nevertheless, it is likely that the same general types of control mechanisms are active in each of the different types of bone formation. In adult humans, bone formation predominantly occurs by remodeling, the process by which bone which has recently been resorbed by osteoclasts is replaced by teams of osteoblasts. Other types of bone formation such as endochondral bone formation and appositional bone formation are also important, particularly during growth and adolescence. The end results of each of these processes are the same, namely a complex mineralized proteinaceous bone matrix. These processes are modulated by systemic hormonal influences, which are particularly important with respect to pituitary hormones and sex steroids during growth and adolescence, and by local cellular microenvironmental differences. The former will not be discussed here. Rather, we will concentrate on the local events and factors which are likely involved in the bone formation process occurring during normal bone remodeling.

The expression of platelet-derived growth-factor gene in Dupuytren contracture

Dupuytren contracture is a disease of the palmar fascia characterized by nodular fibroblastic proliferation; its etiology and pathogenesis are poorly understood. Growth factors are polypeptides that regulate cell growth and differentiation and extracellular matrix production. Platelet-derived growth factor is known to cause fibroblastic proliferation, and it may be involved in the pathogenesis of Dupuytren contracture. The purpose of this study was to determine if the gene for the B chain of platelet-derived growth factor is expressed in Dupuytren contracture. Tissue from patients who had Dupuytren disease was examined immunohistochemically with the 5B5 antibody, which is a marker for fibroblasts. Polymerase chain reaction, gel electrophoresis, Southern blotting, and in situ hybridization were also used to study gene expression in the tissue as well as in normal fascia, A172 cells, and MRC5 cells. Total cellular RNA was extracted from tissue and cells. Polymerase chain reaction was done with oligonucleotide primers complementary to a portion of the platelet-derived growth-factor-B and platelet-derived growth-factor-receptor genes. The platelet-derived growth-factor-B gene was expressed in all six specimens from the patients who had Dupuytren contracture as well as in the A172 cells, but not in the normal fascia lata or the MRC5 cells. These results were confirmed with Southern blotting of the products of the reaction with a platelet-derived growth-factor-B probe. The gene for the platelet-derived growth-factor receptor was expressed by all tissues and cells studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of multiple oncogenes in human esophageal carcinomas

To study the oncogenesis of human esophageal carcinoma, the expression of a variety of oncogenes was studied in 10 esophageal carcinoma cell lines and 16 pairs of tumor and nontumor tissues removed from patients with esophageal carcinoma. Northern blot analyses using 11 different oncogene probes revealed that 5 oncogenes, i.e. c-myc, c-H-ras, c-sis, c-raf, and c-fos, were expressed. Among them, a variant c-sis mRNA transcript of 2.7 kilobase (kb) was expressed in 7 of 10 cell lines and in 9 of 16 tumor tissues. Furthermore, an overexpression and an amplification of c-myc gene was observed in some cell lines. These results suggest that multiple oncogene expression may be required for the induction, maintenance, and progression of esophageal carcinoma. The expression of a 2.7-kb transcript, of c-sis and overexpression of c-myc gene may play some role in the carcinogenesis of esophageal carcinoma.

Localization and functional analysis of DNase-I-hypersensitive sites in the human c-sis/PDGF-B gene transcription unit and its flanking regions

We studied the regulation of the expression of the human c-sis/PDGF-B gene in the following panel of cell lines: K562 cells, in which expression is inducible by phorbol esters; cytotrophoblast-derived cell lines JEG-3 and JAR; carcinoma-derived cell lines PC3, T24 and HeLa, which show extensive differences in c-sis mRNA content; dermal fibroblasts, which do not express the gene. We demonstrate that the wide variety of levels of c-sis mRNA in these cells is mainly determined at the transcription level. Extensive gene rearrangements or amplifications, or significant differences in the stability of the c-sis transcript could not be found. In fibroblasts and placenta cell lines, inaccessibility of the c-sis promoter, rather than the absence of transcription factors that activate it, inhibits expression of the endogenous gene. Examination of the chromatin structure of the transcription unit and immediate flanking regions revealed several cell-type-specific DNase-I-hypersensitivity (DH) sites. Functional analysis of genomic fragments harbouring one or more DH sites showed the presence of negative regulatory elements within intron 1, and of an activating element downstream of the gene. A DH site, located immediately downstream of the promoter in dermal fibroblasts, may regulate accessibility of the promoter by means of specific nucleosome phasing.

Three human osteosarcoma cell lines exhibiting different phenotypic expressions

to elucidate the differences in histological features and biological behavior of osteosarcomas, three human osteoblastic-type osteosarcomas were studied in vitro and in nude mice. The secretory processing and extracellular fiber formation of type I collagen proved to be the most important factor in bone formation in the osteosarcomas. Alkaline phosphatase also seemed to be important. However, we were unable to find any particular protooncogene abnormalities which could have been implicated in the occurrence or biological behavior of these osteosarcomas.

Development and characterization of a rapidly proliferating, well-differentiated cell line derived from normal adult human osteoblast-like cells transfected with SV40 large T antigen

A new bone cell line was established by transfecting normal adult human osteoblast-like (hOB) cells, derived from a 68-year-old woman, with the plasmid pSV3 neo. The plasmid included coding sequences and promotors for the large and small T antigens of the SV40 virus as well as resistance to the antibiotics neomycin and G418. A single antibiotic-resistant colony was located and cloned. Large tumor antigen production in the clonal cell line was confirmed by indirect immunofluorescence study. Treatment with 1,25-dihydroxy-vitamin D3 increased steady-state concentrations of protein and mRNA for osteocalcin and for alkaline phosphatase. Northern blot analyses also demonstrated the presence of mRNAs for alpha(I)-procollagen, osteopontin 1a, transforming growth factor beta, and interleukin-1 beta. The plasma membrane calcium pump and osteonectin were identified by immunocytochemical analysis. These cells produced a matrix that mineralized when beta-glycerophosphate was added to their cultures. As assessed by functional receptor assays, both estrogen and androgen receptors were present and functional, although at low concentrations. Treatment with parathyroid hormone did not stimulate adenylate cyclase activity. Thus, these cells are a well-differentiated, steroid-responsive clonal cell line that closely approximates the phenotype of the mature osteoblast. They should serve as an excellent model for the study of osteoblast biology.

Malignant transformation of NIH 3T3 fibroblasts by human c-sis is dependent upon the level of oncogene expression

High-level expression of the c-sis oncogene, which encodes the beta chain of platelet-derived growth factor, transforms immortalized rodent fibroblasts in vitro to a malignant phenotype. c-sis gene expression has been demonstrated in a variety of human tumors, although generally at levels much lower than those shown to transform cells in vitro. We examined the effect of lower levels of c-sis expression on the phenotype of NIH 3T3 fibroblasts. Clones with various levels of c-sis expression were generated by transfecting NIH 3T3 cells with a plasmid that expressed the human c-sis cDNA and the TN5 neomycin-resistance gene. G418-resistant clones, which expressed the c-sis cDNA, were selected and characterized. Alterations in the phenotype of the clones that expressed c-sis ranged from increased growth in soft agar to malignant tumor formation in nude and syngeneic mice. Increased levels of c-sis cDNA expression correlated with the acquisition of features of transformation in a dose-dependent manner and altered the cellular phenotype in a manner consistent with the progression of cells towards malignancy. These data support a model in which low levels of sis gene expression in tumors contribute to the acquisition of some features of transformation but require complementation by other genes or factors to produce a fully malignant phenotype.

PDGF: a multifunctional growth factor

hPDGF is the major growth factor of human blood serum. In vivo, it is apparently synthesized by megakaryocytes and is transported in blood stored in the alpha granules of platelets. hPDGF is a heterodimer of two homologous polypeptide chains (PDGF-1(A) and PDGF-2(B] linked together by disulphide bonds. The PDGF-1(A) chain is encoded by a gene localized in chromosome 7 and the PDGF-2(B) chain is encoded by the c-sis proto-oncogene localized in chromosome 22. The hPDGF heterodimer and its two isoforms, the PDGF-1(A) and PDGF-2(B) homodimers, are potent mitogens and chemoattractants for target cells such as diploid fibroblasts, osteoblasts, arterial smooth muscle cells and brain glial cells. The PDGF-1(A) homodimer binds only to its specific receptor alpha, and the hPDGF heterodimer and PDGF-2(B) homodimer bind to both receptors a and b. In addition to their mitogenic action, PDGF stimulates important cellular metabolic activities, including protein, lipid and prostaglandin synthesis. It appears to be an important factor in early development and in vivo appears to modulate tissue regeneration and remodelling during wound healing and osteogenesis. The inappropriate expression of PDGF genes and their mitogenic products has been linked to several proliferative disorders such as fibrosis, atherosclerosis and neoplasia.

Glioma-derived PDGF-related protein presents as 17 kd intracellularly and assembled form induces actin reorganization

We have electrophoretically obtained platelet-derived growth factor (PDGF)-related protein from human glioma (glioma derived PDGF-related protein: GD-PDGF) and produced rabbit antiserum against the monomer of GD-PDGF. By methods of immunoaffinity chromatography and Western blotting, we analyzed GD-PDGF in cultured human glioma cells and conditioned medium. The intracellular GD-PDGF was only detected at 17 kd molecular weight by the purified rabbit antibody. When the intracellular 17 kd monomer was purified by the IgG-coupled immunoaffinity chromatography, the eluted protein was not detected at 17 kd but at 52 kd. The 52 kd GD-PDGF was spontaneously and immediately converted to 56 kd, which was partly degraded to 32 and 35 kd within 24 hours. On the other hand, in the conditioned media of glioma cell lines GD-PDGF presents mainly as 56 kd. The assembled forms of GD-PDGF exhibited a powerful activity to induce membrane ruffle formation and reorganization of actin filaments in cultured glial cells and glioma cells. These results indicated that GD-PDGF is intracellularly stored as 17 kd monomer and exists extracellularly as assembled forms, which may act as an autocrine and paracrine effect on the surrounding cells.

Production of platelet-derived growth factor by human lung cancer

The production of platelet-derived growth factor (PDGF) was studied in small cell lung carcinoma, lung squamous carcinoma and lung adenocarcinoma cell lines, and in seven human lung tissues obtained from each type of lung cancer. By indirect immunofluorescence, PDGF was detected in all the cell lines. Likewise, five out of seven biopsies derived from patients with adenocarcinoma and squamous carcinoma, and four out of seven specimens with small cell carcinoma displayed a positive pattern for PDGF. In addition, lung carcinoma cell lines expressed both PDGF-A and PDGF-B/sis genes, as judged by Northern blot analysis. Biologically active, serum-free conditioned media obtained from all three cell lines stimulated the incorporation of [3H]thymidine into quiescent BALB/c-3T3 cells. This effect was abolished when IgG-PDGF antiserum was used. These findings suggest that an abnormal expression of PDGF occurs in the three more frequent types of lung cancer, which can play a potential role in neoplastic transformation and uncontrolled cell growth.

Human osteoblasts synthesize and respond to platelet-derived growth factor

Platelet-derived growth factor (PDGF) is a potent mitogen for cells of mesenchymal origin. We previously demonstrated that PDGF is produced by osteogenic sarcoma cells. We report here that normal human bone-derived cells produce PDGF and that these cells have an osteoblastic phenotype. This was demonstrated by use of a double immunofluorescent technique and by examining cloned human adult osteoblasts. Northern blot analysis indicates that PDGF production is accounted for by expression of the PDGF-A gene. PDGF-AA and PDGF-BB generally stimulated thymidine incorporation in normal human bone explants to a similar extent. All of the cloned human osteoblasts responded to PDGF-BB while the response to PDGF-AA varied. Similarly, five cloned osteoblastic cell populations were shown to produce PDGF while one did not. This result supports the hypothesis that there are different osteoblastic cell populations that differ in their growth factor responses or in the production of growth factors. Our results suggest that PDGF-BB has the potential to act as a paracrine factor for normal human osteoblasts because all of the osteoblastic cell populations responded to PDGF-BB. None of the osteoblastic cell populations expressed the PDGF-B gene, indicating that it would not act as an autocrine factor. Although not definitive, our results suggest that PDGF-AA has the potential to act as an autocrine factor because osteoblastic bone cell populations were shown to express the PDGF-A gene and respond to PDGF-AA.

Relative binding and biochemical effects of heterodimeric and homodimeric isoforms of platelet-derived growth factor in osteoblast-enriched cultures from fetal rat bone

Platelet-derived growth factor (PDGF) exists as a homodimer or a heterodimer comprising either PDGF-A or PDGF-B subunits, and each isoform occurs in various tissues, including bone. Although the stimulatory effects of PDGF-BB have been studied in cultures of bone cells and intact bone fragments, the influence of other isoforms that may arise locally or systematically in vivo, has not been reported. Therefore recombinant human PDGF-BB, PDGF-AB, and PDGF-AA were evaluated in osteoblast-enriched cultures from fetal rat bone. Within 24 hours these factors produced a graded response in bone cell DNA and protein synthesis, with half-maximal effects at approximately 0.6, 2.1, and 4.8 nM PDGF-BB, PDGF-AB, and PDGF-AA, respectively. Increases in collagen and noncollagen protein synthesis were abrogated when DNA synthesis was blocked with hydroxyurea. Furthermore, each factor reduced alkaline phosphatase activity, PDGF-BB being the most inhibitory. Binding studies with 125I-PDGF-BB or 125I-PDGF-AA and each unlabeled PDGF isoform produced discrete ligand binding and displacement patterns: 125I-PDGF-BB binding was preferentially displaced by PDGF-BB (Ki approximately 0.7 nM), less by PDGF-AB (Ki approximately 2.3 nM) and poorly by PDGF-AA. In contrast, 125I-PDGF-AA binding was measurably reduced by PDGF-AA (Ki approximately 4.0 nM), but was more effectively displaced by PDGF-BB or PDGF-AB (each with Ki approximately 0.7 nM). These studies indicate that each PDGF isoform produces biochemical effects proportional to binding site occupancy and suggest that receptors that favor PDGF-B subunit binding preferentially mediate these results in osteoblast-enriched bone cell cultures.

A monoclonal antibody against PDGF B-chain inhibits PDGF-induced DNA synthesis in C3H fibroblasts and prevents binding of PDGF to its receptor

A monoclonal antibody (MAb 6D11) against platelet-derived growth factor (PDGF) was studied. We found that the MAb 6D11 in concentrations equimolar to PDGF blocked the [3H]thymidine incorporation in C3H/10T1/2 C18 fibroblasts stimulated by PDGF B-B and PDGF A-B. This inhibition was overcome by high doses of PDGF. The [3H]thymidine incorporation stimulated by other growth factors (aFGF, bFGF and bombesin) was not inhibited by the antibody. The MAb 6D11 blocked receptor binding of PDGF B-B, but not PDGF A-A. These findings suggest that the MAb 6D11 abolishes PDGF-induced DNA synthesis by blocking PDGF receptor binding. In this communication we demonstrate an isoform-specific monoclonal antibody against PDGF.

Human meningiomas co-express platelet-derived growth factor (PDGF) and PDGF-receptor genes and their protein products

The present studies investigated the expression of the platelet-derived growth factor (PDGF) and PDGF-receptor genes in human meningiomas. Northern blot analysis demonstrated that all meningiomas examined expressed both the c-sis/PDGF-2 proto-oncogene and the PDGF-receptor gene. In situ hybridization localized the c-sis mRNA and the PDGF-receptor mRNA in the tumor cells of the meningioma tissues. Control pachymeninges derived from adult individuals, without meningiomas, expressed only PDGF-receptor mRNA but not the c-sis mRNA. Immunocytochemistry studies detected both the c-sis and the PDGF-receptor protein products in meningioma tissues but only the PDGF-receptor protein products in control pachymeninges. These findings indicate the presence of an autocrine mechanism in human meningiomas based on the co-expression of the c-sis/PDGF-2 proto-oncogene and PDGF-receptor gene and their protein products. This co-expression of a potent mitogen and its receptor may contribute to the growth and maintenance of human meningiomas.

Coexpression of platelet-derived growth factor (PDGF) and PDGF-receptor genes by primary human astrocytomas may contribute to their development and maintenance

The present studies investigated the expression of the two PDGF genes (c-sis/PDGF-2 and PDGF-1) and the PDGF-receptor b gene (PDGF-R) in 34 primary human astrocytomas. Northern blot analysis demonstrated the coexpression of the c-sis/PDGF-2 protooncogene and the PDGF-R gene in all astrocytomas examined. The majority of the tumors also expressed the PDGF-1 gene. There was no correlation between the expression of the two PDGF genes. Nonmalignant human brain tissue expressed the PDGF-R and PDGF-1 genes but not the c-sis/PDGF-2 protooncogene. In situ hybridization of astrocytoma tissue localized the expression of the c-sis and PDGF-R mRNA's in tumor cells. Capillary endothelial cells also expressed c-sis mRNA. In contrast, nonmalignant human brain tissue expressed only PDGF-R mRNA but not c-sis/PDGF-2 mRNA. The coexpression of a potent mitogenic growth factor protooncogene (c-sis) and its receptor gene in astrocytoma tumor cells suggests the presence of an autocrine mechanism that may contribute to the development and maintenance of astrocytomas. The expression of c-sis mRNA in tumor cells but not in nonmalignant brain cells may serve as an additional diagnostic criterion for the detection of astrocytomas in small tissue specimen using in situ hybridization for the detection of c-sis mRNA and/or immunostaining for the recognition of its protein product.

Activation of cancer cell proteases and cytotoxicity by EGF and PDGF growth factors

The biological effects of EGF and PDGF growth factors on A172 and hEGFr-3T3 cell lines were studied using RBC induced cytolysis and polyacrylamide-gelatin gel electrophoresis assays. The authors report that growth factor-induced cytotoxicity in these cells is mediated by proteolytic enzymes. Treatment of A172 cells with either EGF or PDGF resulted in marked increase of their cytotoxicity (Release Index = 150%). Similarly, RBC induced release index by hEGFr-3T3 cells was elevated to 420% in the presence of 3.4 pM of EGF. However, in A172 cells, PDGF did not have a significant effect on DNA and protein synthesis indicating that stimulation of proteolytic activity is independent of the growth factor signaling pathway. Growth factor induced cytotoxicity was significantly reduced by protease inhibitors in both cell lines. Using EDTA and leupeptin several proteolytic species were identified and localized to cellular membranes as evidenced by polyacrylamide-gelatin electrophoresis assay. These data suggest that growth factors regulate the activation or secretion of proteolytic enzymes in cancer cells and may mediate the invasive and metastatic behavior of these cells.

The molecular biology of radiation-induced carcinogenesis: thymic lymphoma, myeloid leukaemia and osteosarcoma

In mice, external X- or gamma-irradiation may induce thymic lymphomas or myeloid leukaemias, while bone-seeking alpha-emitters may induce osteosarcomas and, to a lesser extent, acute myeloid leukaemia. The present paper aims to review briefly some of the experimental data with respect to the molecular mechanisms underlying these radiation-induced carcinogenic processes. Thymic lymphomagenesis proceeds through an indirect mechanism. Recombinant proviruses often occur in the tumour cell DNA, favouring the idea that they might be involved. However, there are indications that they might mediate tumour growth rather than induction. It is plausible that activation of ras oncogenes by somatic point mutations might play a role in the carcinogenic process, although at a yet undetermined stage. Myeloid leukaemogenesis is characterized by a very early, putative initiating event, consisting of non-random rearrangements and/or deletions of chromosome 2. These may be related to deletions in the developmentally important homeobox gene clusters and to rearrangements of the sequences flanking the IL-1 beta gene. Either a gene of the homeobox family or IL-1 beta might be considered as potentially involved in the induction process. Osteosarcomagenesis in mice is often associated with the expression of proviruses, and the tumours often contain somatically acquired proviruses. These viruses may contribute to tumour development by affecting various growth-suppressor genes. Viruses isolated from bone tumours, although non-sarcomagenic, induce osteopetrosis, osteomas and lymphomas upon infection of newborn mice. Osteogenic tumours frequently display amplification of a region on mouse chromosome 15, which encompasses c-myc and Mlvi-1 sequences. Enhanced transcription of various oncogenes is found in individual tumours, but no specificity for osteosarcomas has been identified. In vitro systems of skeletoblast differentiation are being developed to study tumour induction in vitro.

Expression of transferrin and vitamin D-binding protein genes in an osteogenic sarcoma cell line

Expression of genes encoding transferrin and the vitamin D-binding protein is described in a cell line, U-2 OS, derived from a human osteogenic sarcoma. The mRNA transcripts of transferrin and vitamin D-binding protein were shown to be the lengths of those found in normal human liver. The cells synthesize and secrete the transferrin and vitamin D-binding proteins, in addition to human albumin and ceruloplasmin. The U-2 OS cells were successfully transfected with chimeric genes carrying 670 bp of the 5' regulatory sequence of the human transferrin gene fused to a reporter chloramphenicol acetyltransferase gene. These data indicate that the appropriate transcriptional factors required for expression of four plasma proteins are produced by U-2 OS nuclei and that the U-2 OS cell line will be useful for studies analyzing regulation of these genes.

Two distinct cell lines derived from a human osteosarcoma

Two cell lines were established from a human osteosarcoma transplanted into athymic nude mice after the second (O9N2) and fifth passages (HuO9). Both cell lines expressed 1,25(OH)2D3-responsive alkaline phosphatase activity and produced tumors in the dorsum of nude mice that were histologically similar to the original tumor. However, the morphological and growth characteristics of the two cell lines differed. O9N2 cells were large and polygonal, whereas HuO9 cells showed spindle shapes. HuO9 cells had a higher growth rate and saturation density than O9N2 cells. The c-myc oncogene was amplified 4- to 8-fold in HuO9 cells but not in O9N2 cells. Both cell lines had a homozygous internal deletion, lacking the 7.4-kb HindIII fragment in the Rb gene. The results suggest the importance of the c-myc oncogene in the growth and morphological control of human osteosarcoma cells and of the Rb gene in the pathogenesis of the tumor.

Identification of monocyte chemotactic activity produced by malignant cells

Human malignant cells secrete low molecular size proteins that attract peripheral blood monocytes and may be responsible for the accumulation of tumor-associated macrophages observed in vivo. Similar chemotactic proteins are secreted by cultured vascular smooth muscle cells. The predominant monocyte chemoattractants produced by tumor cells of differing origin were demonstrated to be related to smooth muscle cell-derived chemotactic factor. Thus, a single class of chemotactic proteins is produced by different cell types, which suggests a common mechanism for the recruitment of monocytes and macrophages. These results are significant in view of the potential of macrophages to affect tumor growth.

Increased expression of PDGF-B (c-sis) mRNA in rat lung precedes DNA synthesis and tissue repair during chronic hyperoxia

Several aspects of tissue response to injury, including cell proliferation, cell migration, and deposition of extracellular matrix, have been attributed to platelet-derived growth factor (PDGF)-like cytokines. Because these responses play key roles in lung injury, PDGF-B (c-sis) gene expression was measured by Northern blot analysis of lung total RNA prepared after oxidant injury was induced by chronic exposure of rats to 85% oxygen for zero, 1, 3, and 7 days. Constitutive but low levels of PDGF-B mRNA (4.0 kb) were observed in the lungs of control animals exposed to 21% oxygen. Steady-state levels of PDGF-B mRNA in lung were elevated 2.5-fold by day 3 of hyperoxia and remained so up to at least day 7. The early increase in PDGF-B mRNA expression after 3 days of hyperoxic exposure preceded several other aspects of the reparative response. DNA synthesis measured by in vivo incorporation of [3H]thymidine into lung DNA was unchanged at day 3 but markedly elevated by day 7. A similar increase in extractable lung RNA implies a quantitative or qualitative change in extractable RNA at this later phase of tissue injury. Subtle changes in actin mRNA expression were also noted late in the course of lung injury. The content of cytoplasmic (beta,gamma) actin mRNA (2.1 kb) in lung was doubled after 7 days of hyperoxia (P less than 0.05). In addition, increased expression of an actin cDNA-hybridizing mRNA, which co-migrates with muscle-specific alpha-actin mRNA (1.7 kb), was detected on day 7, suggesting hyperplasia of smooth muscle and myofibroblasts. These data show that PDGF-B transcripts are constitutively expressed in rat lung tissue. The expression of PDGF-B mRNA increases early in the course of hyperoxic lung injury and precedes an increase in DNA synthesis and other responses that reflect tissue remodeling. These results suggest that the production of PDGF-like cytokines by cells within the lung itself initiates or modulates various aspects of lung injury and repair.

Retinal pigment epithelial cells produce PDGF-like proteins and secrete them into their media

Human retinal pigment epithelial cells at confluence were used to condition serum-free Dulbecco's modified Eagle's medium. Conditioned media were exhaustively dialyzed against 0.5 N acetic acid, lyophilized, and subjected to Western blot analysis, using as primary antibody an IgG fraction prepared from goat antiserum directed against human platelet-derived growth factor. Native platelet-derived growth factor was resolved as a band with Mr of 30 kDa under non-reducing conditions, while bands with Mr of 36-38 kDa and 18.5 kDa were resolved from retinal pigment epithelial cell-conditioned media. Acid extracts of retinal pigment epithelial cells also contained bands at 36-38 kDa and media conditioned for 48 hr exhibited much denser bands than media conditioned for 24 hr. No bands were detected when non-immune goat IgG fractions were substituted for primary antibody and when conditioned media were prepared from several human fibroblast lines in the same manner as those prepared from retinal pigment epithelial cells, no detectable bands or only a faint shadow at 36 kDa were seen. Retinal pigment epithelial cell-conditioned media prepared in the presence of [35S]methionine were loaded on an anti-platelet-derived growth factor IgG affinity column, eluted, and subjected to SDS-polyacrylamide gel electrophoresis. Bands with Mr slightly less than 36 kDa and 18 kDa were visualized by autoradiography, demonstrating that the platelet-derived growth factor-like proteins in retinal pigment epithelial cell-conditioned media are newly synthesized. Two fractions eluted from the column also markedly stimulated fibroblast chemotaxis and incorporation of [3H]thymidine, both of which were neutralized by soluble anti-platelet-derived growth factor IgG. These data suggest that retinal pigment epithelial cells in culture produce platelet-derived growth factor-like proteins and secrete them into their media where they are capable of stimulating fibroblast chemotaxis and proliferation.

Regulation of expression of the c-sis proto-oncogene

Regulation of expression of platelet derived growth factor polypeptide B encoded by the c-sis proto-oncogene is important in a number of physiological and pathological conditions. Sequences in the 1028 nucleotide long 5' untranslated region of the c-sis mRNA were found to inhibit protein synthesis. The inhibition is relieved by deletion of nucleotides 154-378 or 398-475. Sequences within 375 nucleotides upstream of the RNA initiation site are important for transcriptional activity. Sequences in two portions of this region, between -375 and -235 nucleotides and between -235 and -99 nucleotides relative to the RNA CAP site are important for full activity. A transcriptional enhancer activity is demonstrated by its ability to increase the activity of the human T lymphotropic virus type (HTLV) I promoter at a distance and in an orientation-independent manner. Furthermore, sequences upstream of the c-sis RNA CAP site respond to the HTLV I transactivator protein to increase RNA synthesis from either the c-sis or HTLV I promoter.

Oncogene-related growth factors and growth factor receptors in human malignant glioma-derived cell lines

Oncogenes induce malignant transformation of cells. Two oncogenes are closely related to genes coding for a mitogenic growth factor (v-sis to the PDGF gene) and a receptor for a mitogenic growth factor (v-erb B to the EGF receptor gene). We studied the possibility that cells derived from malignant gliomas produce mitogenic factors that bind to cell surface receptors, the activation of which could lead to excessive stimulation of cell proliferation. All six cell lines tested secrete into their medium factors that stimulate DNA synthesis. The factor secreted by one cell line was characterized and found to resemble PDGF. Six of 11 cell lines had receptors for PDGF demonstrable by binding and receptor autophosphorylation assays. Six of six cell lines tested had EGF receptors demonstrable by binding and receptor autophosphorylation experiments. The extremely high levels of EGF receptor in one cell line may reflect excessive expression of the erb B oncogene associated with abnormalities of chromosome 7 that occur in this cell line.

The potential role of platelet-derived growth factor as an autocrine or paracrine factor for human bone cells

Platelet-derived growth factor, PDGF, is a potent mitogen for cells of mesenchymal origin such as fibroblasts, smooth muscle cells and glial cells. PDGF is thought to have the potential to act as both a paracrine and an autocrine factor. Studies described here extend these observations to human bone-derived cells. Exogenous PDGF induces biologic activity in two human osteogenic sarcoma cell lines and in one of these, the two PDGF genes, PDGF-1 and PDGF-2/c-sis are expressed. In addition, PDGF stimulates proliferation of normal osteoblastic cells derived from adult human cancellous bone. The expression of the PDGF-1 gene but not the PDGF-2/c-sis gene is demonstrated in normal human adult bone-derived cells by Northern blot analysis and synthesis of PDGF is shown by immunoprecipitation with PDGF antisera. These studies indicate that PDGF has the potential to act as a paracrine or autocrine regulator of bone cells.

Characterization of a high-molecular-weight protein immunoprecipitated by platelet-derived growth factor antisera

We previously demonstrated that a high-molecular-weight glycoprotein could be immunoprecipitated from metabolically labeled U-2 OS cells with platelet-derived growth factor (PDGF) antiserum and that it appears to be derived from a different precursor than is the 30 kD PDGF-like mitogen produced by these cells. These findings were unexpected, since the molecular weight of this glycoprotein is too large to be encoded by the PDGF structural genes. From experiments with metabolically labeled U-2 OS human osteosarcoma, fibroblasts, and NRK cells, we report here that a 185 kD protein immunoprecipitated with PDGF antiserum has the following characteristics. 1) It is a PDGF binding protein that is unrelated to alpha 2-macroglobulin. 2) It is phosphorylated in response to PDGF stimulation. 3) It is immunoprecipitated by phosphotyrosine antibodies. 4) It is not a substrate of epidermal growth factor-induced tyrosine kinase activity. These studies indicate that high-molecular-weight proteins immunoprecipitated by antiserum to PDGF represent a complex between PDGF and a binding protein capable of being phosphorylated by a PDGF-induced tyrosine kinase. These characteristics are identical to those of the PDGF receptor.