Transforming growth factor-beta induces expression of vascular endothelial growth factor in human retinal pigment epithelial cells: involvement of mitogen-activated protein kinases

Vascular endothelial growth factor (VEGF) is a major agent in choroidal and retinal neovascularization, events associated with age-related macular degeneration (AMD) and diabetic retinopathy. Retinal pigment epithelium (RPE), strategically located between retina and choroid, plays a critical role in retinal disorders. We have examined the effects of various growth factors on the expression and secretion of VEGF by human retinal pigment epithelial cell cultures (HRPE). RT-PCR analyses revealed the presence of three isoforms of mRNA corresponding to VEGF 121, 165, and 189 that were up regulated by TGF-beta1. TGF-beta1, beta2, and beta3 were the potent inducers of VEGF secretion by HRPE cells whereas bFGF, PDGF, TGF-alpha, and GM-CSF had no effects. TGF-beta receptor type II antibody significantly reversed induction of VEGF secretion by TGF-beta. In contrast activin, inhibin and BMP, members of TGF-beta super family, had no effects on VEGF expression in HRPE. VEGF mRNA levels and protein secretion induced by TGF-beta were significantly inhibited by SB203580 and U0126, inhibitors of MAP kinases, but not by staurosporine and PDTC, protein kinase C and NF-kappaB pathway inhibitors, respectively. TGF-beta also induced VEGF expression by fibroblasts derived from human choroid of eye. TGF-beta induction of VEGF secretion by RPE and choroid cells may play a significant role in choroidal neovascularization (CNV) in AMD. Since the secretion of VEGF by HRPE is regulated by MAP kinase pathways, MAP kinase inhibitors may have potential use as therapeutic agents for CNV in AMD.     

Expression patterns of PDGF-A, -B, -C and -D and the PDGF-receptors alpha and beta in activated rat hepatic stellate cells (HSC)

The platelet-derived growth factor (PDGF) family, which regulates many physiological and pathophysiological processes has recently been enlarged by two new members, the isoforms PDGF-C and -D. Little is known about the expression levels of these new members in hepatic fibrosis. We therefore investigated by quantitative real time PCR (Taqman) the mRNA expression profiles of all four PDGF isoforms in transdifferentiating primary cultured hepatic stellate cells (HSC), an in vitro model system of hepatic fibrogenesis, either with or without stimulation of the cells with PDGF-BB or TGF-beta1. All four isoforms were expressed in HSC transdifferentiating to myofibroblast-like cells (MFB) albeit with different profiles: while PDGF-A mRNA exhibited minor fluctuations only, PDGF-B was rapidly down-regulated. In contrast, both PDGF-C and -D mRNA were strongly induced: PDGF-C up to 5 fold from day 2 to day 8 and PDGF-D up to 8 fold from day 2 to day 5 of culture. Presence of PDGF-DD in activated HSC was confirmed at the protein level by immunocytochemistry. Stimulation of HSC and MFB with PDGF-BB led to down-regulation of the new isoforms, whereas TGF-beta1 upregulated PDGF-A only. We further show that PDGF receptor-beta (PDGFR-beta) mRNA was rapidly upregulated within the first day of culture and was constantly expressed from day 2 on while the expression profile of PDGFR-alpha mRNA was very similar to that of PDGF-A during transdifferentiation. Given the dramatic changes in PDGF-C and -D expression, which may compensate for down-regulation of PDGF-B, we hypothesize that the new PDGF isoforms may fulfil specific functions in hepatic fibrogenesis.     

Transforming growth factor-beta induces platelet-derived growth factor (PDGF) messenger RNA and PDGF secretion while inhibiting growth in normal human mammary epithelial cells

Platelet-derived growth factor (PDGF) is a potent mitogen in human serum which specifically stimulates the proliferation of mesenchymal cells. We have now examined normal human mammary epithelial cells (HMEC) derived from reduction mammaplasties and grown in a serum-free defined medium. Medium conditioned by HMEC contained a PDGF-like activity that competed with [125I]PDGF for binding to PDGF receptors in normal human fibroblasts. When conditioned media were incubated with antiserum specific for either PDGF-A or PDGF-B, only PDGF-A antiserum was capable of inhibiting binding of conditioned media to PDGF receptors. Using an RNase protection assay, mRNA from normal HMEC was probed for both the PDGF-A and PDGF-B chains. Little or no PDGF-B was found in HMEC strains, while a strong signal was seen with the PDGF-A probe. When HMEC were grown in the presence of transforming growth factor-beta (TGF beta) for 48 h, inhibition of growth was observed in association with a 20- to 40-fold stimulation of PDGF-B mRNA and a 2-fold stimulation of PDGF-A mRNA. This mRNA induction was extremely rapid (within 1 h), and secreted PDGF activity was induced 2- to 3-fold. Two other HMEC growth inhibitors and differentiating agents, sodium butyrate and phorbol ester 12-O-tetradecanoylphorbol-13-acetate, had no effect on PDGF mRNA regulation. The current study suggests that PDGF gene induction is an extremely rapid and specific indicator of TGF beta function regardless of whether TGF beta is acting in a growth stimulatory or inhibitory manner. Any role of PDGF-B in TGF beta modulation of differentiation of normal or malignant mammary gland remains to be determined.     

PKA and Epac cooperate to augment bradykinin-induced interleukin-8 release from human airway smooth muscle cells

Background

Platelet-derived growth factor A (PDGF-A) signals solely through PDGF-Rα, and is required for fibroblast proliferation and transdifferentiation (fibroblast to myofibroblast conversion) during alveolar development, because pdgfa-null mice lack both myofibroblasts and alveoli. However, these PDGF-A-mediated mechanisms remain incompletely defined. At postnatal days 4 and 12 (P4 and P12), using mouse lung fibroblasts, we examined (a) how PDGF-Rα correlates with ki67 (proliferation marker) or alpha-smooth muscle actin (αSMA, myofibroblast marker) expression, and (b) whether PDGF-A directly affects αSMA or modifies stimulation by transforming growth factor beta (TGFβ).

Methods

Using flow cytometry we examined PDGF-Rα, αSMA and Ki67 in mice which express green fluorescent protein (GFP) as a marker for PDGF-Rα expression. Using real-time RT-PCR we quantified αSMA mRNA in cultured Mlg neonatal mouse lung fibroblasts after treatment with PDGF-A, and/or TGFβ.

Results

The intensity of GFP-fluorescence enabled us to distinguish three groups of fibroblasts which exhibited absent, lower, or higher levels of PDGF-Rα. At P4, more of the higher than lower PDGF-Rα + fibroblasts contained Ki67 (Ki67+), and Ki67+ fibroblasts predominated in the αSMA + but not the αSMA- population. By P12, Ki67+ fibroblasts comprised a minority in both the PDGF-Rα + and αSMA+ populations. At P4, most Ki67+ fibroblasts were PDGF-Rα + and αSMA- whereas at P12, most Ki67+ fibroblasts were PDGF-Rα- and αSMA-. More of the PDGF-Rα + than - fibroblasts contained αSMA at both P4 and P12. In the lung, proximate αSMA was more abundant around nuclei in cells expressing high than low levels of PDGF-Rα at both P4 and P12. Nuclear SMAD 2/3 declined from P4 to P12 in PDGF-Rα-, but not in PDGF-Rα + cells. In Mlg fibroblasts, αSMA mRNA increased after exposure to TGFβ, but declined after treatment with PDGF-A.

Conclusion

During both septal eruption (P4) and elongation (P12), alveolar PDGF-Rα may enhance the propensity of fibroblasts to transdifferentiate rather than directly stimulate αSMA, which preferentially localizes to non-proliferating fibroblasts. In accordance, PDGF-Rα more dominantly influences fibroblast proliferation at P4 than at P12. In the lung, TGFβ may overshadow the antagonistic effects of PDGF-A/PDGF-Rα signaling, enhancing αSMA-abundance in PDGF-Rα-expressing fibroblasts.     

Spontaneous hepatic fibrosis in transgenic mice overexpressing PDGF-A

Platelet derived growth factor (PDGF) plays a central role in repair mechanisms after acute and chronic tissue damage. To further evaluate the role of PDGF-A in liver fibrogenesis in vivo, we generated transgenic mice with hepatocyte-specific overexpression of PDGF-A using the CRP-gene promoter. Transgenic but not wildtype mice showed expression of PDGF-A mRNA in the liver. Hepatic PDGF-A overexpression was accompanied by a significant increase in hepatic procollagen III mRNA expression as well as TGF-beta1 expression. Liver histology showed increased deposition of extracellular matrix in transgenic but not in wildtype mice. PDGF-A-transgenic mice showed positive sinusoidal staining for alpha-SMA indicating an activation of hepatic stellate cells. Since the profibrogenic effect of PDGF-A was accompanied by increased TGF-beta1 protein concentration in the liver of transgenic mice, it can be postulated that PDGF-A upregulates expression of TGF-beta1 which is a strong activator of hepatic stellate cells. Thus, these results point towards a fibrosis induction by PDGF-A via the TGF-beta1 signalling pathway. In conclusion, expression and functional analysis of PDGF-A in the liver of transgenic mice suggest a relevant profibrogenic role of PDGF-A via TGF-beta1 induction. Counteracting PDGF-A may therefore be one of the effects of tyrosine kinase inhibitors which showed protective effects in animal models of liver fibrosis.     

Comparison of infarct-derived and control ovine cardiac myofibroblasts in culture: response to cytokines and natriuretic peptide receptor expression profiles

This study investigated whether gene expression profiles of myofibroblasts derived from infarcted myocardium differ from normal cardiac fibroblasts. We compared the expression of cytoskeletal proteins in cultured ovine cardiac fibroblasts derived from infarcted (ID) and noninfarcted ovine myocardium (NID) and the levels of expression of the natriuretic peptide receptors (NPR)-A and NPR-B in response to treatment with transforming growth factor (TGF)-beta1 and/or platelet-derived growth factor (PDGF). Transformation of cultured cardiac fibroblasts to myofibroblasts, as indicated by alpha-smooth muscle actin and vimentin expression, was independent of the presence of TGF-beta1, PDGF, or cell origin. ID fibroblasts had higher basal levels than NID fibroblasts of NPR-A (ID: 58.0 +/- 32.2 arbitrary density units, NID: undetectable), NPR-B (ID: 780 +/- 155, NID: 330 +/- 38 arbitrary density units) and collagen I (ID: 17.2 +/- 0.5, NID: 10.5 +/- 1.7 pg mRNA/mug total RNA, P < 0.05) but lower levels of alpha-SMa expression (ID: 50.2 +/- 7.9, NID: 76.9 +/- 3.2 fluorescence units, P < 0.05). NPR-A mRNA in ID fibroblasts showed a rapid fourfold increase in response to TGF-beta1 and/or PDGF at 4 and 2 h, respectively, followed by a profound decline; in NID cells, NPR-A mRNA was undetectable. In ID fibroblasts, cytokines reduced NPR-B mRNA below control levels; in NID fibroblasts, TGF-beta1 and PDGF elicited prompt increments in expression: a fourfold increase with TGF-beta1 at 8 h and a twofold increase with PDGF at 4 h (P < 0.05). In summary, transformation of cardiac fibroblasts to myofibroblasts in culture is independent of cytokine treatment. Moreover, whether the cultured cardiac fibroblasts are from infarct tissue is a major determinant of NPR expression levels and cytokine responses, even after four to five passages.     

Modulation of PDGF-C and PDGF-D expression during bleomycin-induced lung fibrosis

PDGF isoforms are a family of polypeptides that bind to cell surface receptors and induce fibroblast proliferation and chemotaxis. The PDGF-A and -B chain isoforms have been implicated in fibroproliferative lung injury in animal models and in human disease. Two recently recognized PDGF polypeptides, PDGF-C and -D, differ from the PDGF-A and -B isoforms in that they require proteolytic cleavage before they can bind and activate the PDGF receptors. Our findings demonstrate that administration of bleomycin to murine lungs leads to a significant increase in PDGF-C mRNA expression and a significant decrease in PDGF-D mRNA expression. PDGF-C expression was localized to areas of lung injury by in situ hybridization, and PDGF-C expression was not upregulated in the lungs of BALB/c mice that are resistant to bleomycin-induced lung fibrosis. Moreover, there is in vivo phosphorylation of the PDGF-receptor that binds PDGF-C in response to bleomycin administration. These observations strongly suggest a role for PDGF-C in bleomycin-induced pulmonary fibrosis.     

Basic fibroblast growth factor modulates the mitogenic potency of the platelet-derived growth factor (PDGF) isoforms by specific upregulation of the PDGF alpha receptor in vascular smooth muscle cells.

Platelet-derived growth factor AA (PDGF AA), in contrast to PDGF AB and BB, is a poor mitogen for smooth muscle cells (SMC). However, together with basic fibroblast growth factor (bFGF) it acts synergistically on DNA synthesis of these cells. Northern blot analysis revealed that bFGF selectively increases the PDGF-receptor alpha subtype (PDGF-R alpha) mRNA level without a significant effect on the PDGF-R beta mRNA level. The amount of PDGF-R alpha protein is also selectively increased after stimulating SMC with bFGF as shown by immunoprecipitation of lysates from SMC with anti-PDGF-R alpha antibodies. The number of binding sites for 125I-PDGF AA is more than doubled after bFGF-treatment, whereas the specific binding for PDGF AB and BB increased only by approximately 30 and 20%, respectively. The increase in the number of PDGF-R alpha renders the SMC responsive for PDGF AA as demonstrated by the induction of the proto-oncogene c-fos as well as by an increased cell proliferation. The enhanced PDGF binding after bFGF treatment may in fact explain the observed synergistic behavior. These data are discussed with regard to a possible role of growth factor-induced transmodulation of receptor expression during atherogenesis.     

Differential binding, biological and biochemical actions of recombinant PDGF AA, AB, and BB molecules on connective tissue cells.

We have compared the biological and biochemical properties of recombinant PDGF AA, AB, and BB using three types of fibroblastic cells: NIH/3T3, human skin fibroblast, and fetal bovine aortic smooth muscle. PDGF binding, receptor autophosphorylation, phosphatidyl inositol hydrolysis, as well as chemotactic and mitogenic responses of the cells were analyzed. PDGF-AB and PDGF-BB showed similar receptor binding, receptor autophosphorylation, and potent biological activity for all three of the cell types tested. In contrast, PDGF-AA was biologically active only for the NIH/3T3 cells in which binding sites for PDGF-AA were abundant, but was inactive for bovine aortic smooth muscle cells and human skin fibroblasts in which binding sites for PDGF-AA were absent. PDGF-AA could not induce any biochemical changes in the human skin fibroblasts or smooth muscle cells. Western blot studies with anti-Type alpha and beta PDGF receptor antibodies indicate that the NIH/3T3 cells contained both PDGF alpha and beta receptors, whereas the human skin fibroblasts and bovine smooth muscle cells contained only detectable levels of beta receptors. These results indicate that cells possessing high levels of PDGF beta receptors only are capable of responding equally well to either PDGF AB or BB.     

Differential regulation of the expression of proteinases/antiproteinases in fibroblasts. Effects of interleukin-1 and platelet-derived growth factor.

We have previously reported that polypeptide growth factors had an anti-inflammatory effect by decreasing the cytokine-enhanced expression of factor B (FB), an activator of the alternative complement pathway, in human fibroblasts. To further characterize the role of cytokines and growth factors in the inflammatory/repair continuum, we have studied the effects of interleukin-1 (IL-1) and platelet-derived growth factor (PDGF) on the expression of metalloproteinases/antiproteinases of the extracellular matrix in cultured human fibroblasts. Co-incubation of IL-1 and PDGF synergistically increased the expression of stromelysin and interstitial collagenase to 23-fold (for both proteins) over background, while PDGF decreased the IL-1-enhanced expression of FB by 82%. PDGF, but not IL-1, alone or in combination, increased the synthesis of tissue inhibitor of metalloproteinases. RNA blot analysis indicated that the changes in protein synthesis were regulated at a pretranslational level. Cycloheximide treatment indicated that the effects of PDGF on the metalloproteinases/antiproteinases were not protein-dependent, in contrast to results obtained for FB. The effect of the three dimeric forms of PDGF (AA, AB, and BB) on the synthesis of metalloproteinases and FB was also analyzed. The effects were qualitatively similar for each of the dimeric forms; however, the BB and AB isoforms had considerably greater effects than PDGF-AA. It has been reported that the PDGF receptors found in human fibroblasts have higher binding affinity for the BB and AB isoforms of the growth factor. The results presented in this paper are in accord with the possibility that differences in the biological activity of the three isoforms of PDGF are due to differences in the number or affinity of the binding sites of the target cells, rather than to different activation pathways of the receptor. Thus, PDGF increased cytokine effects on metalloproteinases, while decreasing cytokine effects or complement activator FB. The net effect of these changes may be to decrease inflammation and enhance remodeling early in repair and to enhance matrix stability later in the repair process.     

Phospholipase C-gamma, a substrate for PDGF receptor kinase, is not phosphorylated on tyrosine during the mitogenic response to CSF-1.

Quiescent mouse NIH3T3 cells expressing a transduced human c-fms gene encoding the receptor for colony stimulating factor-1 (CSF-1) were stimulated with mitogenic concentrations of platelet-derived growth factor (PDGF) or CSF-1. Immunoprecipitated phospholipase C-gamma (PLC-gamma) was phosphorylated on tyrosine and calcium was mobilized following treatment of intact cells with PDGF. In contrast, only trace amounts of phosphotyrosine were incorporated into PLC-gamma and no intracellular calcium signal was detected after CSF-1 stimulation. Similarly, CSF-1 treatment did not stimulate phosphorylation of PLC-gamma on tyrosine in a CSF-1-dependent. SV40-immortalized mouse macrophage cell line that expresses high levels of the CSF-1 receptor. In fibroblasts, antiserum to PLC-gamma co-precipitated a fraction of the tyrosine phosphorylated form of the PDGF receptor (PDGF-R) after ligand stimulation, implying that phosphorylated PDGF-R and PLC-gamma were associated in a stable complex. Pre-treatment of cells with orthovanadate also led to tyrosine phosphorylation of PLC-gamma which was significantly enhanced by PDGF, but not by CSF-1. Thus, although the PDGF and CSF-1 receptors are structurally related and appear to be derived from a single ancestor gene, only PDGF-induced mitogenesis in fibroblasts correlated with tyrosine phosphorylation of PLC-gamma.     

Platelet-derived growth factor and transforming growth factor-beta enhance tissue repair activities by unique mechanisms

Platelet-derived growth factor (PDGF) and transforming growth factor-beta (TGF-beta) markedly potentiate tissue repair in vivo. In the present experiments, both in vitro and in vivo responses to PDGF and TGF-beta were tested to identify mechanisms whereby these growth factors might each enhance the wound-healing response. Recombinant human PDGF B-chain homodimers (PDGF-BB) and TGF-beta 1 had identical dose-response curves in chemotactic assays with monocytes and fibroblasts as the natural proteins from platelets. Single applications of PDGF-BB (2 micrograms, 80 pmol) and TGF-beta 1 (20 micrograms, 600 pmol) were next applied to linear incisions in rats and each enhanced the strength required to disrupt the wounds at 5 d up to 212% of paired control wounds. Histological analysis of treated wounds demonstrated an in vivo chemotactic response of macrophages and fibroblasts to both PDGF-BB and to TGF-beta 1 but the response to TGF-beta 1 was significantly less than that observed with PDGF-BB. Marked increases of procollagen type I were observed by immunohistochemical staining in fibroblasts in treated wounds during the first week. The augmented breaking strength of TGF-beta 1 was not observed 2 and 3 wk after wounding. However, the positive influence of PDGF-BB on wound breaking strength persisted through the 7 wk of testing. Furthermore, PDGF-BB-treated wounds had persistently increased numbers of fibroblasts and granulation tissue through day 21, whereas the enhanced cellular influx in TGF-beta 1-treated wounds was not detectable beyond day 7. Wound macrophages and fibroblasts from PDGF-BB-treated wounds contained sharply increased levels of immunohistochemically detectable intracellular TGF-beta. Furthermore, PDGF-BB in vitro induced a marked, time-dependent stimulation of TGF-beta mRNA levels in cultured normal rat kidney fibroblasts. The results suggest that TGF-beta transiently attracts fibroblasts into the wound and may stimulate collagen synthesis directly. In contrast, PDGF is a more potent chemoattractant for wound macrophages and fibroblasts and may stimulate these cells to express endogenous growth factors, including TGF-beta, which, in turn, directly stimulate new collagen synthesis and sustained enhancement of wound healing over a more prolonged period of time.     

Platelet-derived growth factor potentiates phorbol ester-induced neuronal differentiation of human neuroblastoma cells.

Previous studies have shown that platelet-derived growth factor (PDGF) and PDGF receptors are expressed in the mammalian central nervous system and that primary cultured neuroblasts from rat hindbrain have functional PDGF beta-receptors. Here, it is shown that cultured human neuroblastoma cells express PDGF alpha- and beta-receptors, but not PDGF-A and PDGF-B chain mRNA. In contrast to alpha-receptor expression, beta-receptor expression appears to be associated with a mature neuronal phenotype. Under serum-free growth conditions, PDGF-AA and -BB induce a trophic and weak mitogenic response in SH-SY5Y neuroblastoma cells, showing that the PDGF receptors in these cells are functional. In combination with 12-O-tetradecanoylphorbol-13-acetate, all three PDGF isoforms induce sympathetic neuronal differentiation of the SH-SY5Y cells, as shown by morphology and by increased expression of the genes coding for growth-associated protein 43 and neuropeptide tyrosine, respectively. This indicates a potential role for PDGF in the development of sympathetic neurons in particular and of the nervous system in general.     

Angiogenesis during human extraembryonic development involves the spatiotemporal control of PDGF ligand and receptor gene expression.

We have examined the role of platelet-derived growth factor (PDGF) ligand and receptor genes in the angiogenic process of the developing human placenta. In situ hybridization analysis of first trimester placentae showed that most microcapillary endothelial cells coexpress the PDGF-B and PDGF beta-receptor genes. This observation indicates that PDGF-B may participate in placental angiogenesis by forming autostimulatory loops in capillary endothelial cells to promote cell proliferation. Endothelial cells of macro blood vessels maintained high PDGF-B expression, whereas PDGF beta-receptor mRNA was not detectable. In contrast, PDGF beta-receptor mRNA was readily detectable in fibroblast-like cells and smooth muscle cells in the surrounding intima of intermediate and macro blood vessels. Taken together, these data suggest that the PDGF-B signalling pathway appears to switch from an autocrine to a paracrine mechanism to stimulate growth of surrounding PDGF beta-receptor-positive mesenchymal stromal cells. Smooth muscle cells of the blood vessel intima also expressed the PDGF-A gene, the protein product of which is presumably targeted to the fibroblast-like cells of the mesenchymal stroma as these cells were the only ones expressing the PDGF alpha-receptor. PDGF-A expression was also detected in columnar cytotrophoblasts where it may have a potential role in stimulating mesenchymal cell growth at the base of the growing placental villi. We discuss the possibility that the regulation of the PDGF-B and beta-receptor gene expression might represent the potential targets for primary angiogenic factors.     

Role of PDGF-A expression in the control of vascular smooth muscle cell growth by transforming growth factor-beta

Transforming growth factor-beta (TGF-beta) is a multifunctional regulatory peptide that can inhibit or promote the proliferation of cultured vascular smooth muscle cells (SMCs), depending on cell density (Majack, R. A. 1987. J. Cell Biol. 105:465-471). In this study, we have examined the mechanisms underlying the growth-promoting effects of TGF-beta in confluent SMC cultures. In mitogenesis assays using confluent cells, TGF-beta was found to potentiate the stimulatory effects of serum, PDGF, and basic fibroblast growth factor (bFGF), and was shown to act individually as a mitogen for SMC. In gene and protein expression experiments, TGF-beta was found to regulate the expression of PDGF-A and thrombospondin, two potential mediators of SMC proliferative events. The induction of thrombospondin protein and mRNA was density-dependent, delayed relative to its induction by PDGF and, based on cycloheximide experiments, appeared to depend on the de novo synthesis of an intermediary protein (probably PDGF-A). The relationship between PDGF-A expression and TGF-beta-mediated mitogenesis was investigated, and it was determined that a PDGF-like activity (probably PDGF-A) was the biological mediator of the growth-stimulatory effects of TGF-beta on confluent SMC. The effects of purified homodimers of PDGF-A on SMC replication were investigated, and it was determined that PDGF-AA was mitogenic for cultured SMC, particularly when used in combination with other growth factors such as bFGF and PDGF-BB. The data suggest several molecular mechanisms that may account for the ability of TGF-beta to promote the growth of confluent SMC in culture.     

Over-expression of PDGF-C using a lung specific promoter results in abnormal lung development

PDGF isoforms are a family of polypeptides that bind to cell surface receptors and induce fibroblast proliferation and chemotaxis. PDGF-A and -B chain isoforms have previously been shown to be involved in murine lung development. A new PDGF polypeptide, PDGF-C, was recently recognized and differs from the PDGF-A and -B isoforms in that it requires proteolytic cleavage before it can bind and activate the PDGF alpha receptor. In these studies PDGF-C was over-expressed during embryogenesis using the lung specific surfactant protein C promoter. PDGF-C transgenic pups died from respiratory insufficiency within minutes following birth. At E18.5, nontransgenic lungs exhibited lung morphology consistent with the saccular stage of lung development. In contrast, E18.5 transgenic lungs retained many features of the canalicular stage of lung development and had abundant numbers of large poorly differentiated mesenchymal cells. These results suggest that PDGF-C is activated during lung development and is a potent growth factor for mesenchymal cells in vivo.