Cardiac fibrosis is characterized by net accumulation of extracellular matrix proteins in the cardiac interstitium, and contributes to both systolic and diastolic dysfunction in many cardiac pathophysiologic conditions. This review discusses the cellular effectors and molecular pathways implicated in the pathogenesis of cardiac fibrosis. Although activated myofibroblasts are the main effector cells in the fibrotic heart, monocytes/macrophages, lymphocytes, mast cells, vascular cells and cardiomyocytes may also contribute to the fibrotic response by secreting key fibrogenic mediators. Inflammatory cytokines and chemokines, reactive oxygen species, mast cell-derived proteases, endothelin-1, the renin/angiotensin/aldosterone system, matricellular proteins, and growth factors (such as TGF-β and PDGF) are some of the best-studied mediators implicated in cardiac fibrosis. Both experimental and clinical evidence suggests that cardiac fibrotic alterations may be reversible. Understanding the mechanisms responsible for initiation, progression, and resolution of cardiac fibrosis is crucial to design anti-fibrotic treatment strategies for patients with heart disease.

Macrophages are supposed to play a key role in inflammatory and tumor angiogenesis. Their importance derives from (1) their ubiquitous presence in normal and especially inflamed tissues, (2) their potential to become activated in response to appropriate stimuli, and (3) their repertoire of secretory products. By release of proteases, growth factors (bFGF, GM-CSF, TGF-alpha, IGF-I, PDGF, VEGF/VPF, TGF-beta), and other monokines (IL-1, IL-6, IL-8, TNF-alpha, substance P, prostaglandins, interferons, thrombospondin 1), activated macrophages have the capability to influence each phase of the angiogenic process, such as alterations of the local extracellular matrix, induction of endothelial cells to migrate or proliferate, and inhibition of vascular growth with formation of differentiated capillaries. This review describes macrophage physiology and the influence of macrophage secretory products on the different phases of angiogenesis in vitro and in vivo.

Neurons and oligodendrocytes are produced in the adult brain subventricular zone (SVZ) from neural stem cells (B cells), which express GFAP and have morphological properties of astrocytes. We report here on the identification B cells expressing the PDGFRalpha in the adult SVZ. Specifically labeled PDGFRalpha expressing B cells in vivo generate neurons and oligodendrocytes. Conditional ablation of PDGFRalpha in a subpopulation of postnatal stem cells showed that this receptor is required for oligodendrogenesis, but not neurogenesis. Infusion of PDGF alone was sufficient to arrest neuroblast production and induce SVZ B cell proliferation contributing to the generation of large hyperplasias with some features of gliomas. The work demonstrates that PDGFRalpha signaling occurs early in the adult stem cell lineage and may help regulate the balance between oligodendrocyte and neuron production. Excessive PDGF activation in the SVZ in stem cells is sufficient to induce hallmarks associated with early stages of tumor formation.

A mouse platelet-derived growth factor A chain (PDGF-A) null allele is shown to be homozygous lethal, with two distinct restriction points, one prenatally before E10 and one postnatally. Postnatally surviving PDGF-A-deficient mice develop lung emphysema secondary to the failure of alveolar septation. This is apparently caused by the loss of alveolar myofibroblasts and associated elastin fiber deposits. PDGF alpha receptor-positive cells in the lung having the location of putative alveolar myofibroblast progenitors were specifically absent in PDGF-A null mutants. We conclude that PDGF-A is crucial for alveolar myofibroblast ontogeny. We have previously shown that PDGF-B is required in the ontogeny of kidney mesangial cells. The PDGFs therefore appear to regulate the generation of specific populations of myofibroblasts during mammalian development. The two PDGF null phenotypes also reveal analogous morphogenetic functions for myofibroblast-type cells in lung and kidney organogenesis.

Fibrosis is one of the largest groups of diseases for which there is no therapy but is believed to occur because of a persistent tissue repair program. During connective tissue repair, "activated" fibroblasts migrate into the wound area, where they synthesize and remodel newly created extracellular matrix. The specialized type of fibroblast responsible for this action is the alpha-smooth muscle actin (alpha-SMA)-expressing myofibroblast. Abnormal persistence of the myofibroblast is a hallmark of fibrotic diseases. Proteins such as transforming growth factor (TGF)beta, endothelin-1, angiotensin II (Ang II), connective tissue growth factor (CCN2/CTGF), and platelet-derived growth factor (PDGF) appear to act in a network that contributes to myofibroblast differentiation and persistence. Drugs targeting these proteins are currently under consideration as antifibrotic treatments. This review summarizes recent observations concerning the contribution of TGFbeta, endothelin-1, Ang II, CCN2, and PDGF and to fibroblast activation in tissue repair and fibrosis and the potential utility of agents blocking these proteins in affecting the outcome of cardiac fibrosis.

Platelet-derived growth factor (PDGF) isoforms play a major role in stimulating the replication, survival, and migration of myofibroblasts during the pathogenesis of fibrotic diseases. During fibrogenesis, PDGF is secreted by a variety of cell types as a response to injury, and many pro-inflammatory cytokines mediate their mitogenic effects via the autocrine release of PDGF. PDGF action is determined by the relative expression of PDGF alpha-receptors (PDGFRalpha) and beta-receptors (PDGFRbeta) on the surface of myofibroblasts. These receptors are induced during fibrogenesis, thereby amplifying biological responses to PDGF isoforms. PDGF action is also modulated by extracellular binding proteins and matrix molecules. This review summarizes the literature on the role of PDGF and its receptors in the development of fibrosis in a variety of organ systems, including lung, liver, kidney, and skin.

Affinity-purified bovine brain phosphatidylinositol 3-kinase (PI3-kinase) contains two major proteins of 85 and 110 kd. Amino acid sequence analysis and cDNA cloning reveals two related 85 kd proteins (p85 alpha and p85 beta), which both contain one SH3 and two SH2 regions (src homology regions). When expressed, these 85 kd proteins bind to and are substrates for tyrosine-phosphorylated receptor kinases and the polyoma virus middle-T antigen/pp60c-src complex, but lack PI3-kinase activity. However, an antiserum raised against p85 beta immunoprecipitates PI3-kinase activity. The active PI3-kinase complex containing p85 alpha or p85 beta and the 110 kd protein binds to PDGF but not EGF receptors. p85 alpha and p85 beta may mediate specific PI3-kinase interactions with a subset of tyrosine kinases.

Novel mechanisms, and molecular mediators, of the pro-tumorigenic effects of cancer-associated fibroblasts (CAFs) have been identified. These include CXCL12/SDF-1-mediated recruitment of bone marrow-derived endothelial precursor cell and pro-metastatic effects of CCL5. Co-culture experiments also suggest that CAFs can influence the drug-sensitivity of cancer cells. Comparisons of CAFs from different tumors have started to identify tumor-type specific differences in CAF gene expression and marker protein profiling indicates the existence of multiple distinct co-existing CAF-subsets. Studies in animal models have demonstrated that CAFs can be derived from bone marrow-derived cells or from epithelial or endothelial cells undergoing mesenchymal transition. The genetic status of CAFs remains controversial following conflicting findings. Meanwhile, analyses of CAFs from human tumors have revealed consistent epigenetic changes. An increasing number of translational studies have emphasized the prognostic significance of different CAF-related tumor characteristics. Clinical studies aiming at CAF-targeting can now be envisioned based on findings from experimental intervention studies with agents targeting, for example FAP or PDGF-, TGF-beta- or hedgehog-signaling.

The expression of platelet-derived growth factor (PDGF) and its receptors was analyzed in 14 gliomas of various degrees of malignancy and compared with three gliosis cases by in situ hybridization and immunohistochemistry techniques. Expression of both PDGF A- and B-chains was higher in glioblastomas than in astrocytomas. The PDGF A-chain mRNA was predominantly found in cell-rich areas in glioblastomas. The cognate PDGF-alpha receptor (PDGFR-alpha) mRNA was heterogeneously distributed in gliomas of all grades, and PDGFR-alpha expression was higher in gliomas than in gliosis. Within some glioblastomas probed with PDGFR-alpha complementary RNA, cells heavily loaded with grains were intermingled with others containing low or moderate signals. The heavily labeled cells were often found in the vicinity of proliferating capillaries. Immunostaining with an anti-PDGF antibody and an affinity-purified antiserum against the PDGFR-alpha showed strong staining of most tumor cells with both antibodies in glioblastoma. In addition, the PDGFR-alpha antibodies yielded a strong staining of scattered cells, and the anti-PDGF antibody yielded staining of a few cells within the astrocytoma. Furthermore, high levels of the PDGF-beta receptor (PDGFR-beta) and PDGF B-chain mRNA as well as the beta receptor protein were found in hyperplastic capillaries. These results suggest the presence of autocrine and paracrine loops in glioma, activating the PDGFR-alpha in glioma cells and the PDGFR-beta in endothelial cells.

This review has summarized some of the evidence suggesting that cytokines may play an important role in mediating pathophysiologic events in RA. However, these proteins are capable of mediating both stimulatory (agonist) and inhibitory (antagonist) effects in the rheumatoid synovium. GM-CSF, IL-1, TNF alpha, and PDGF are all produced in the rheumatoid synovium and may function to induce inflammation, enzyme release, fibroblast proliferation, and tissue destruction. Local release of IL-6 may alter the effects of IL-1 and TNF alpha, as well as induce Ig production and hepatic synthesis of acute-phase proteins. However, specific inhibitors of IL-1 and TNF alpha exist, which, if also released into the synovium, may antagonize the proinflammatory effects of these cytokines. In addition, IL-1 may have antiinflammatory effects, such as the induction of the synthesis of collagen and enzyme inhibitors by chondrocytes and synovial fibroblasts. Stimulation of these latter cells by TGF beta also may result in decreased matrix degradation and increased formation of scar tissue. The developing scenario is one of cell-cell interactions that are influenced in positive and negative manners by the local release of various mediators. A further understanding of cytokines and cytokine inhibitors in the rheumatoid synovium may lead to the development of more specific and effective therapeutic agents.

Idiopathic pulmonary fibrosis is a progressive and fatal fibrotic disease of the lungs with unclear etiology. Prior efforts to treat idiopathic pulmonary fibrosis that focused on anti-inflammatory therapy have not proven to be effective. Recent insight suggests that the pathogenesis is mediated through foci of dysregulated fibroblasts driven by profibrotic cytokine signaling. TGF-beta and PDGF are 2 of the most potent of these cytokines. In the current study, we investigated the role of TGF-beta-induced fibrosis mediated by activation of the Abelson (Abl) tyrosine kinase. Our data indicate that fibroblasts respond to TGF-beta by stimulating c-Abl kinase activity independently of Smad2/3 phosphorylation or PDGFR activation. Moreover, inhibition of c-Abl by imatinib prevented TGF-beta-induced ECM gene expression, morphologic transformation, and cell proliferation independently of any effect on Smad signaling. Further, using a mouse model of bleomycin-induced pulmonary fibrosis, we found a significant inhibition of lung fibrosis by imatinib. Thus, Abl family members represent common targets for the modulation of profibrotic cytokine signaling.

Chronic myelomonocytic leukemia (CMML) is a myelodysplastic syndrome characterized by abnormal clonal myeloid proliferation and by progression to acute myelogenous leukemia (AML). CMML thus offers an opportunity to study early genetic events in the transition to AML. A recently recognized subgroup of CMML has a t(5;12)(q33;p13) balanced translocation. We report that the consequence of the t(5;12) translocation is expression of a fusion transcript in which the tyrosine kinase domain of the platelet-derived growth factor receptor beta (PDGFR beta) on chromosome 5 is coupled to a novel ets-like gene, tel, on chromosome 12. The tel-PDGFR beta fusion demonstrates the oncogenic potential of PDGFR beta and may provide a paradigm for early events in the pathogenesis of AML.

Tendon healing is a complex and highly-regulated process that is initiated, sustained and eventually terminated by a large number and variety of molecules. Growth factors represent one of the most important of the molecular families involved in healing, and a considerable number of studies have been undertaken in an effort to elucidate their many functions. This review covers some of the recent investigations into the roles of five growth factors whose activities have been best characterised during tendon healing: insulin-like growth factor-I (IGF-I), transforming growth factor beta (TGFbeta), vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), and basic fibroblast growth factor (bFGF). All five are markedly up-regulated following tendon injury and are active at multiple stages of the healing process. IGF-I has been shown to be highly expressed during the early inflammatory phase in a number of animal tendon healing models, and appears to aid in the proliferation and migration of fibroblasts and to subsequently increase collagen production. TGFbeta is also active during inflammation, and has a variety of effects including the regulation of cellular migration and proliferation, and fibronectin binding interactions. VEGF is produced at its highest levels only after the inflammatory phase, at which time it is a powerful stimulator of angiogenesis. PDGF is produced shortly after tendon damage and helps to stimulate the production of other growth factors, including IGF-I, and has roles in tissue remodelling. In vitro and in vivo studies have shown that bFGF is both a powerful stimulator of angiogenesis and a regulator of cellular migration and proliferation. This review also covers some of the most recent studies into the use of these molecules as therapeutic agents to increase the efficacy and efficiency of tendon and ligament healing. Studies into the effects of the exogenous application of TGFbeta, IGF-I, PDGF and bFGF into the wound site singly and in combination have shown promise, significantly decreasing a number of parameters used to define the functional deficit of a healing tendon. Application of IGF-I has been shown to increase in the Achilles Functional Index and the breaking energy of injured rat tendon. TGFbeta and PDGF have been shown separately to increase the breaking energy of healing tendon. Finally, application of bFGF has been shown to promote cellular proliferation and collagen synthesis in vivo.

Platelet-derived growth factors (PDGFs) were discovered more than two decades ago. Today the PDGF family of growth factors consists of five different disulphide-linked dimers built up of four different polypeptide chains encoded by four different genes. These isoforms, PDGF-AA, PDGF-AB, PDGF-BB, PDGF-CC and PDGF-DD, act via two receptor tyrosine kinases, PDGF receptors alpha and beta. The classic PDGFs, PDGF-A and PDGF-B, undergo intracellular activation during transport in the exocytic pathway for subsequent secretion, while the novel PDGFs, PDGF-C and PDGF-D, are secreted as latent factors that require activation by extracellular proteases. The classical PDGF polypeptide chains, PDGF-A and PDGF-B, are well studied and they regulate several physiological and pathophysiological processes, mainly using cells of mesenchymal or neuroectodermal origin as their targets. The discovery of two additional ligands for the two PDGF receptors suggests that PDGF-mediated cellular signaling is more complex than previously thought.

We compared the transcriptomes of marrow-derived mesenchymal stem cells (MSCs) with differentiated adipocytes, osteocytes, and chondrocytes derived from these MSCs. Using global gene-expression profiling arrays to detect RNA transcripts, we have identified markers that are specific for MSCs and their differentiated progeny. Further, we have also identified pathways that MSCs use to differentiate into adipogenic, chondrogenic, and osteogenic lineages. We identified activin-mediated transforming growth factor (TGF)-beta signaling, platelet-derived growth factor (PDGF) signaling and fibroblast growth factor (FGF) signaling as the key pathways involved in MSC differentiation. The differentiation of MSCs into these lineages is affected when these pathways are perturbed by inhibitors of cell surface receptor function. Since growth and differentiation are tightly linked processes, we also examined the importance of these 3 pathways in MSC growth. These 3 pathways were necessary and sufficient for MSC growth. Inhibiting any of these pathways slowed MSC growth, whereas a combination of TGF-beta, PDGF, and beta-FGF was sufficient to grow MSCs in a serum-free medium up to 5 passages. Thus, this study illustrates it is possible to predict signaling pathways active in cellular differentiation and growth using microarray data and experimentally verify these predictions.

Tumor-infiltrating blood vessels deviate morphologically and biochemically from normal vessels, raising the prospect of selective pharmacological targeting. Current antiangiogenic approaches focus mainly on endothelial cells, but recent data imply that targeting pericytes may provide additional benefits. Further development of these concepts will require deeper insight into mechanisms of pericyte recruitment and function in tumors. Here, we applied genetic tools to decipher the function of PDGF-B and PDGF-Rbeta in pericyte recruitment in a mouse fibrosarcoma model. In tumors transplanted into PDGF-B retention motif-deficient (pdgf-b(ret/ret)) mice, pericytes were fewer and were partially detached from the vessel wall, coinciding with increased tumor vessel diameter and hemorrhaging. Transgenic PDGF-B expression in tumor cells was able to increase the pericyte density in both WT and pdgf-b(ret/ret) mice but failed to correct the pericyte detachment in pdgf-b(ret/ret) mice. Coinjection of exogenous pericytes and tumor cells showed that pericytes require PDGF-Rbeta for recruitment to tumor vessels, whereas endothelial PDGF-B retention is indispensable for proper integration of pericytes in the vessel wall. Our data support the notion that pericytes serve an important function in tumor vessels and highlight PDGF-B and PDGF-Rbeta as promising molecular targets for therapeutic intervention.

TGFbeta acts as a tumor suppressor in normal epithelial cells and early-stage tumors and becomes an oncogenic factor in advanced tumors. The molecular mechanisms involved in the malignant function of TGFbeta are not fully elucidated. We demonstrate that high TGFbeta-Smad activity is present in aggressive, highly proliferative gliomas and confers poor prognosis in patients with glioma. We discern the mechanisms and molecular determinants of the TGFbeta oncogenic response with a transcriptomic approach and by analyzing primary cultured patient-derived gliomas and human glioma biopsies. The TGFbeta-Smad pathway promotes proliferation through the induction of PDGF-B in gliomas with an unmethylated PDGF-B gene. The epigenetic regulation of the PDGF-B gene dictates whether TGFbeta acts as an oncogenic factor inducing PDGF-B and proliferation in human glioma.

Human umbilical vein endothelial (HUVE) cells have been previously reported to express the genes for the A and B chains of PDGF and to secrete PDGF-related factors into culture media. Antihuman PDGF IgG affinity chromatography was used to purify PDGF-related activity from HUVE cell-conditioned media. Immunoblot analysis of the affinity-purified proteins with anti-PDGF IgG and antibodies specific for the A or B chain peptides of PDGF combined with chemotactic and mitogenic assays revealed that the major PDGF immunorelated molecule secreted by HUVE cells is a monomer of approximately 36-38 kD and that less than 10% of the purified biologically active molecules are PDGF A or B chain peptides. Screening of an HUVE cell cDNA library in the expression vector lambda gtl 1 with the anti-PDGF antibody resulted in the cloning and sequencing of a cDNA with an open reading frame encoding a 38-kD cysteine-rich secreted protein which we show to be the major PDGF-related mitogen secreted by human vascular endothelial cells. The protein has a 45% overall homology to the translation product of the v-src-induced CEF-10 mRNA from chick embryo fibroblasts. We have termed this new mitogen connective tissue growth factor.

Several platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF) family members display C-terminal protein motifs that confer retention of the secreted factors within the pericellular space. To address the role of PDGF-B retention in vivo, we deleted the retention motif by gene targeting in mice. This resulted in defective investment of pericytes in the microvessel wall and delayed formation of the renal glomerulus mesangium. Long-term effects of lack of PDGF-B retention included severe retinal deterioration, glomerulosclerosis, and proteinuria. We conclude that retention of PDGF-B in microvessels is essential for proper recruitment and organization of pericytes and for renal and retinal function in adult mice.

The establishment of functional and stable vascular networks is essential for angiogenic therapy. Here we report that a combination of two angiogenic factors, platelet-derived growth factor (PDGF)-BB and fibroblast growth factor (FGF)-2, synergistically induces vascular networks, which remain stable for more than a year even after depletion of angiogenic factors. In both rat and rabbit ischemic hind limb models, PDGF-BB and FGF-2 together markedly stimulated collateral arteriogenesis after ligation of the femoral artery, with a significant increase in vascularization and improvement in paw blood flow. A possible mechanism of angiogenic synergism between PDGF-BB and FGF-2 involves upregulation of the expression of PDGF receptor (PDGFR)-alpha and PDGFR-beta by FGF-2 in newly formed blood vessels. Our data show that a specific combination of angiogenic factors establishes functional and stable vascular networks, and provides guidance for the ongoing clinical trials of angiogenic factors for the treatment of ischemic diseases.

Recent evidence indicates that vascular progenitor cells may be the source of smooth muscle cells (SMCs) that accumulate in atherosclerotic lesions, but the origin of these progenitor cells is unknown. To explore the possibility of vascular progenitor cells existing in adults, a variety of tissues from ApoE-deficient mice were extensively examined. Immunohistochemical staining revealed that the adventitia in aortic roots harbored large numbers of cells having stem cell markers, e.g., Sca-1(+) (21%), c-kit(+) (9%), CD34(+) (15%), and Flk1(+) cells (4%), but not SSEA-1(+) embryonic stem cells. Explanted cultures of adventitial tissues using stem cell medium displayed a heterogeneous outgrowth, for example, islands of round-shaped cells surrounded by fibroblast-like cell monolayers. Isolated Sca-1(+) cells were able to differentiate into SMCs in response to PDGF-BB stimulation in vitro. When Sca-1(+) cells carrying the LacZ gene were transferred to the adventitial side of vein grafts in ApoE-deficient mice, beta-gal(+) cells were found in atherosclerotic lesions of the intima, and these cells enhanced the development of the lesions. Thus, a large population of vascular progenitor cells existing in the adventitia can differentiate into SMCs that contribute to atherosclerosis. Our findings indicate that ex vivo expansion of these progenitor cells may have implications for cellular, genetic, and tissue engineering approaches to vascular disease.

Upon ligand-induced tyrosine phosphorylation, the platelet-derived growth factor (PDGF) receptor (PDGFR) beta subunit associates with PLC-gamma 1, RasGAP, P13K, and a 64 kd protein. To determine the relative role of each of these associated proteins in PDGFR signaling, we constructed a PDGFR mutant (F5) unable to bind any of them and a panel of "add-back" mutants that could bind only one of the receptor-associated proteins. F5 PDGFR failed to activate PLC-gamma 1, P13K, or Ras and was unable to trigger DNA synthesis. Permitting association of F5 PDGFR with either PLC-gamma 1 or P13K restored Ras activation and a mitogenic response. Surprisingly, even though binding of the 64 kd protein almost fully restored Ras activation, it did not rescue the receptor's ability to trigger DNA synthesis. Thus Ras activation is insufficient to trigger PDGF-dependent DNA synthesis, and PLC-gamma 1 and P13K are independent downstream mediators of PDGF's mitogenic signal.

OBJECTIVE

To investigate the role of microRNA (miRNA) as posttranscriptional regulators of profibrotic genes in systemic sclerosis (SSc).

METHODS

MicroRNA, which target collagens, were identified by in silico analysis. Expression of miRNA-29 (miR-29) was determined by TaqMan real-time polymerase chain reaction analysis of skin biopsy and fibroblast samples from SSc patients and healthy controls as well as in the mouse model of bleomycin-induced skin fibrosis. Cells were transfected with precursor miRNA (pre-miRNA)/anti-miRNA of miR-29 using Lipofectamine. Collagen gene expression was also studied in luciferase reporter gene assays. For stimulation, recombinant transforming growth factor beta (TGFbeta), platelet-derived growth factor B (PDGF-B), or interleukin-4 (IL-4) was used. The effects of inhibiting PDGF-B and TGFbeta signaling on the levels of miR-29 were studied in vitro and in the bleomycin model.

RESULTS

We found that miR-29a was strongly down-regulated in SSc fibroblasts and skin sections as compared with the healthy controls. Overexpression in SSc fibroblasts significantly decreased, and accordingly, knockdown in normal fibroblasts increased, the levels of messenger RNA and protein for type I and type III collagen. In the reporter gene assay, cotransfection with pre-miR-29a significantly decreased the relative luciferase activity, which suggests a direct regulation of collagen by miR-29a. TGFbeta, PDGF-B, or IL-4 reduced the levels of miR-29a in normal fibroblasts to those seen in SSc fibroblasts. Similar to human SSc, the expression of miR-29a was reduced in the bleomycin model of skin fibrosis. Inhibition of PDGF-B and TGFbeta pathways by treatment with imatinib restored the levels of miR-29a in vitro and in the bleomycin model in vivo.

CONCLUSIONS

These data add the posttranscriptional regulation of collagens by miR-29a as a novel aspect to the fibrogenesis of SSc and suggest miR-29a as a potential therapeutic target.

Growth factors, defined as polypeptides that stimulate cell proliferation, are major growth-regulatory molecules for cells in culture and probably also for cells in vivo. Nontransformed cells show an absolute requirement for growth factors for proliferation in culture and generally more than one growth factor is required. Under usual culture conditions, growth factors are more rapidly depleted than other media components and thus become rate limiting for proliferation. The loss of or decreased requirement for specific growth factors is a common occurrence in neoplastically transformed cells and may lead to a growth advantage, a cardinal feature of cancer cells. Recent work with transforming growth factors, the platelet-derived growth factor, and oncogenes has produced some insight into the mechanisms through which alterations in growth factor-receptor-response pathways could lead to a growth advantage. Evidence has been derived for autocrine secretion in which the cell produces its own growth factor. Many transformed mesenchymal cells produce PDGF (the product of the c-sis proto-oncogene) and certain transformed cells both produce and respond in a growth-stimulatory manner to TGF beta. With TGF beta, which is a growth inhibitor for certain epithelial and other cell types, the loss of the normal inhibitory response in transformed cells could have the same result as the activation of a growth-stimulatory response. Two proto-oncogenes, erbB and fms, encode growth factor receptors. In the erbB case, the viral erbB aberrant receptor produced is truncated and appears to be constitutively activated without the need for a growth factor. Recent studies suggest that the p21 product of the ras oncogene may be an obligatory intermediate in transducing the growth factor signal. Activation of ras may, therefore, activate the growth factor pathway without the need for either a growth factor or its receptor. The transcription of myc and fos is induced by growth factor stimulation of quiescent cells. The protein products of both are nuclear associated and conceivably could be involved in regulating other genes important in the control of cell proliferation. Activation or inappropriate expression of either myc or fos could produce the same end result as stimulation of a growth factor pathway leading to a growth advantage. Study of the molecular mechanism(s) of growth factor action has just begun. The excitement and attention focused on cellular oncogenes in recent years is now turning toward growth factors, not only as they concern the control of normal cell growth but also the involvement of growth factor-initiated pathways in the etiology of cancer.(ABSTRACT TRUNCATED AT 400 WORDS)