Platelet-derived growth factor signaling in human malignancies

Platelet-derived growth factors (PDGFs) and their receptors were identified and purified decades ago. PDGFs are important during normal development and in human cancers. In particular, autocrine PDGF signaling has been implicated in various types of malignancies such as gliomas and leukemia. In contrast, paracrine signaling was found in cancers that originate from epithelial cells, where it may be involved in stromal cell recruitment, metastasis, and epithelial-mesenchymal transition. This editorial briefly...     

The characteristics of astrocytomas and oligodendrogliomas are caused by two distinct and interchangeable signaling formats

Chronic platelet-derived growth factor (PDGF) signaling in glial progenitors leads to the formation of oligodendrogliomas in mice, whereas chronic combined Ras and Akt signaling leads to astrocytomas. Different histologies of these tumors imply that the pathways activated by these two oncogenic stimulations are different, and that the apparent lineage of the tumor cells may result from specific signaling activity. Therefore, we have investigated the signaling effects of PDGF in culture and in gliomas in vivo. In culture, PDGF transiently activates ERK1/2 and Akt, and subsequently elevates p21 and PCNA expression similar to chronic PDGF autocrine signaling in cultured astrocytes and PDGF-induced oligodendrogliomas in vivo. Culture experiments show that autocrine PDGF stimulation, and combined active Ras and Akt generate signaling patterns that are in some ways mutually exclusive. Furthermore, forced Akt activity in the context of chronic PDGF stimulation results in cells with an astrocytic differentiation pattern both in culture and in vivo. These data imply that these two interconvertible signaling motifs are distinct in mice and lead to gliomas resembling the two major glioma histologies found in humans. The ability of signaling activity to convert tumor cells from one lineage to another presents a mechanism for the development of tumors apparently comprised of cells from multiple lineages.     

Dependence of autocrine growth factor stimulation in platelet-derived growth factor-B-induced mouse brain tumor cells

In human gliomas, platelet-derived growth factor (PDGF) ligand and receptor mRNA are often co-expressed, which suggests the presence of an autocrine loop. To further investigate the significance of PDGF stimulation in brain tumors, we used a previously developed mouse tumor model, in which malignant brain tumors of neuroepithelial origin were induced by injecting a murine retrovirus containing the human PDGF B-chain gene into the brains of neonatal mice. In the present investigation, we have characterized a cell line established from such an experimentally induced tumor in an INK4a-/- mouse. Cultured tumor cells expressed nestin and NG2 chondroitin sulfate proteoglycan and are thus most likely derived from an oligodendrocyte precursor cell. Tumor cells produced PDGF-B protein and displayed constitutively activated PDGF alpha receptors. Autocrine receptor activation could be blocked with the specific PDGF receptor tyrosine kinase inhibitor CGP 57148B, which led to almost complete inhibition of cell proliferation, which was much less affected by a PDGF B-chain aptamer that inhibits binding of PDGF-B to PDGF receptors and is unlikely to be able to pass through the plasma membrane. Our results imply an important role for PDGF autocrine stimulation in both initiation and progression of a subtype of gliomas.     

PDGF和PDGFR与肿瘤的关系及其靶向治疗研究进展

血小板衍生生长因子(platelet-derived growth factor, PDGF)是从人的血小板中分离出来的促血管生成因子。血小板衍生生长因子受体(platelet-derived growth factor receptor, PDGFR)是酪氨酸蛋白激酶家族成员, 能够促进细胞的趋化、分裂与增殖; 在机体生长发育、创伤修复等生理过程中起着积极重要的作用。PDGF/PDGFR作为血管生成因子之一与肿瘤的发生发展有密切关系, 肿瘤细胞释放的PDGF能诱导血管内皮细胞、平滑肌细胞和肿瘤细胞的增殖和迁移, 并抑制其凋亡。以PDGF/PDGFR为靶点的靶向治疗也已取得了较大进展。本文就PDGF/PDGFR在肿瘤发生、发展中的作用及靶向治疗方面的进展加以综述。      

Platelet-derived growth factor and its receptors in human glioma tissue: expression of messenger RNA and protein suggests the presence of autocrine and paracrine loops.

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.     

神经胶质瘤中肿瘤相关巨噬细胞研究进展

肿瘤相关巨噬细胞（TAM）是异质性细胞群体,是肿瘤微环境中炎性细胞的主要成分。这些细胞在神经胶质瘤中部分源自中枢神经系统小胶质细胞和循环单核细胞,并与神经胶质瘤的血管生成、免疫抑制、肿瘤进展和侵袭有关。文章综述了TAM通过各种途径促进神经胶质瘤发展的潜在机制,为神经胶质瘤的靶向治疗提供了新的可能性。     

PDGF BB induces VEGF secretion in ovarian cancer

We identified the platelet derived growth factor receptor (PDGFR) as a potential target in epithelial ovarian carcinoma (EOC). This led us to test whether inhibition of the PDGFR affects ovarian cancer cell proliferation and survival and regulates other processes critical to tumor growth and metastasis. We postulated that there is a correlation between the PDGF-PDGFR axis and the secretion of VEGF in EOC. VEGF secretion in ovarian tumors, cancer cells, serum and ascites fluid was measured by IHC, Western Blot and ELISA. We found increased VEGF expression and secretion in most ovarian tumors (by IHC), in EOC malignant ascites and in the conditioned media of primary ovarian cancer cells (quantified by ELISA). In malignant ascites, the levels of secreted PDGF BB and VEGF were strongly correlated (Pearson coefficient of correlation R = 0.728), suggesting that the two pathways interconnect. In PDGFR expressing immortalized ovarian cancer cells, PDGF potently induced VEGF secretion, while imatinib mesylate (Gleevec), a partially selective PDGFR inhibitor, reduced PDGF stimulated VEGF production to basal state. In ovarian cancer cells overexpressing constitutively active Akt, imatinib inhibited partially VEGF secretion, suggesting that the PI3K/Akt pathway is implicated in PDGF-stimulated VEGF secretion. In summary, these results suggest a correlation between the PDGF and VEGF networks in ovarian cancer cells and tumors. The effects of imatinib on VEGF secretion in tumor cells may affect the tumor microenvironment in a manner detrimental to tumor progression.     

Platelet-derived growth factor receptor-β, carrying the activating mutation D849N,accelerates the establishment of B16 melanoma

Background  

Platelet-derived growth factor (PDGF)-BB and PDGF receptor (PDGFR)-β are mainly expressed in the developing vasculature, where PDGF-BB is produced by endothelial cells and PDGFR-β is expressed by mural cells, including pericytes. PDGF-BB is produced by most types of solid tumors, and PDGF receptor signaling participates in various processes, including autocrine stimulation of tumor cell growth, recruitment of tumor stroma fibroblasts, and stimulation of tumor angiogenesis. Furthermore, PDGF-BB-producing tumors are characterized by increased pericyte abundance and accelerated tumor growth. Thus, there is a growing interest in the development of tumor treatment strategies by blocking PDGF/PDGFR function. We have recently generated a mouse model carrying an activated PDGFR-β by replacing the highly conserved aspartic acid residue (D) 849 in the activating loop with asparagine (N). This allowed us to investigate, in an orthotopic tumor model, the role of increased stromal PDGFR-β signaling in tumor-stroma interactions.     

Vasohibin‐1 expression inhibits advancement of ovarian cancer producing various angiogenic factors

Vasohibin‐1 (VASH1) is a negative feedback regulator of angiogenesis, the first to be discovered, and was identified in vascular endothelial growth factor (VEGF)‐stimulated vascular endothelial cells. Vasohibin‐1 inhibits abnormal vascularization induced by various angiogenic factors including fibroblast growth factor and platelet‐derived growth factor (PDGF), in addition to VEGF. By focusing on this characteristic of VASH1, we investigated the antitumor effects of VASH1 expression on ovarian cancer cells that produce different angiogenic factors. By using a high VEGF‐producing ovarian cancer cell line, SHIN‐3, and a high PDGF‐producing ovarian cancer cell line, KOC‐2S, the cells were transfected with either a VEGF antagonist, soluble VEGF receptor‐1 (sVEGFR‐1, or sFlt‐1), or VASH1 genes to establish their respective cellular expression. The characteristics of these transfectants were compared with controls. We previously reported that the expression of sFlt‐1 inhibited tumor vascularization and growth of high VEGF‐producing ovarian cancer cells, reduced peritoneal dissemination and ascites development, and prolonged the survival time of the host. However, in the current study, the expression of sFlt‐1 had no such effect on the high PDGF‐producing ovarian cancer cells used here, whereas VASH1 expression inhibited tumor vascularization and growth, not only in high VEGF‐producing cells, but also in high PDGF‐producing cells, reduced their peritoneal dissemination and ascites, and prolonged the survival time of the host. These results suggest that VASH1 is an effective treatment for ovarian cancer cells that produce different angiogenic factors.     

Combined anti‐PDGFRα and PDGFRβ targeting in non‐small cell lung cancer

Activation of the platelet‐derived growth factor (PDGF)‐receptors is critically involved into various stromal cell functions including recruitment of stromal cells and vascular endothelial growth factor (VEGF) induction in tumor and perivascular cells. To evaluate the effects of combined PDGFRα and ‐β inhibition in a non‐small cell lung cancer model, we stably transfected A549 lung cancer cells with the PDGF‐A mutant PDGF‐0. PDGF‐0 has been generated by substituting amino acids in the binding region of PDGF‐A with the corresponding VEGF‐A region, leading to a decreased receptor‐binding affinity and activation. Compared with control vector transfected cells, transfection with PDGF‐0 had no impact on monolayer growth and apoptosis in vitro, but significantly impaired the number of colony formation in soft agar. After subcutaneous injections, all mice developed tumors within 5 days. While control vector transfected A549 cells were characterized by constant tumor growth, PDGF‐0 transfected A549 revealed a reduced tumor mass (p < 0.001) with no further growth beyond 14 days (2 months observation time) and complete regressions in 7 of 13 cases. Immunohistochemical analyses revealed that PDGF‐0 transfected tumors demonstrated decreased recruitment of periendothelial cells, while the tumor invasion zone was similar to control vector transfectants. Similarly, conditioned medium from PDGF‐0 transfected cells induced significantly less migration of smooth muscle cells and fibroblasts in vitro. Interestingly, in PDGF‐0 transfectants, neither total vessel count nor VEGF expression were significantly altered. These studies demonstrate that combined inhibition of PDGFRα and ‐β results in markedly decreased tumor growth in vivo because of impaired recruitment of periendothelial cells. © 2008 Wiley‐Liss, Inc.     

Inhibition of PDGF receptor signaling in tumor stroma enhances antitumor effect of chemotherapy

Lowering of tumor interstitial hypertension, which acts as a barrier for tumor transvascular transport, has been proposed as a general strategy to enhance tumor uptake and therapeutic effects of anticancer drugs. The tyrosine kinase platelet-derived growth factor (PDGF) beta-receptor is one mediator of tumor hypertension. The effects of PDGF antagonists on chemotherapy response were investigated in two tumor models that display PDGF receptor expression restricted to the tumor stroma, and in which PDGF antagonists relieve tumor hypertension. Inhibitory PDGF aptamers and the PDGF receptor tyrosine kinase inhibitor STI571 enhanced the antitumor effect of Taxol on s.c. KAT-4 tumors in SCID mice. Treatment with only PDGF antagonists had no effect on tumor growth. Taxol uptake in tumors was increased by treatment with PDGF antagonists. Cotreatment with PDGF antagonists and Taxol was not associated with antiangiogenic effects, and PDGF antagonists did not enhance the Taxol effect on in vitro growth of KAT-4 cells. STI571 also increased the antitumor effects of 5-fluorouracil on s.c. PROb tumors in syngeneic BDIX rats, without increasing the effect of 5-fluorouracil on cultured PROb cells. Expression of PDGF receptors in tumor stroma, as well as tumor hypertension, occurs in most common solid tumors. Therefore, our results have implications for treatment regimens for large patient groups and merit clinical testing. In conclusion, our study identifies inhibition of PDGF signaling in tumor stroma as a novel, possibly general strategy for enhancement of the therapeutic effects chemotherapy.     

Growth inhibition of dermatofibrosarcoma protuberans tumors by the platelet-derived growth factor receptor antagonist STI571 through induction of apoptosis

Dermatofibrosarcoma protuberans (DFSP) and giant cell fibroblastoma (GCF) are recurrent, infiltrative skin tumors that presently are treated with surgery. DFSP and GCF tumors are genetically characterized by chromosomal rearrangements fusing the collagen type Ialpha1 (COLIA1) gene to the platelet-derived growth factor B-chain (PDGFB) gene. It has been shown that the resulting COL1A1/PDGF-B fusion protein is processed to mature PDGF-BB. Autocrine PDGF receptor stimulation has therefore been predicted to contribute to DFSP and GCF tumor development and growth. Here we demonstrate presence of activated PDGF receptors in primary cultures derived from six different DFSP and GCF tumors. Three of the primary cultures were further characterized; their in vitro growth displayed an increased sensitivity to treatment with the PDGF receptor tyrosine kinase inhibitor STI571, as compared with normal fibroblasts. Transplantable tumors, displaying a DFSP-like histology, were established from one of the DFSP primary cultures. Treatment of tumor-bearing severe combined immunodeficient mice with STI571 reduced tumor growth. The growth-inhibitory effects in vitro and in vivo occurred predominantly through induction of tumor cell apoptosis. Our study demonstrates growth-inhibitory effects of PDGF receptor antagonists on human DFSP- and GCF-derived tumor cells and demonstrates that autocrine PDGF receptor stimulation provides antiapoptotic signals contributing to the growth of these cells. These findings suggest targeting of PDGF receptors as a novel treatment strategy for DFSP and GCF.     

Nestin, PDGFRbeta, CXCL12 and VEGF in glioma patients: different profiles of (pro-angiogenic) molecule expression are related with tumor grade and may provide prognostic information

Angiogenesis is a key event in the natural progression of gliomas. Nestin, a marker for multipotential neuroepithelial stem cells, is detected in neuroepithelial tumors and in proliferating endothelial cells (ECs) and is involved in the early stages of lineage commitment, proliferation and differentiation. Nestin expression is correlated with proangiogenic chemokines (CXCL12 and its receptor CXCR4) and growth factors (VEGF, PDGF-B and its receptor PDGFRbeta). VEGF expression upregulates CXCR4 on endothelial cells, binding the chemokine SDF1/CXCL12 (Stromal Derived Factor) that has a role on angiogenesis and chemotaxis of endothelial cells; PDGF (platelet-derived growth factor) and PDGFRbeta are also crucial by increasing the expression of VEGF. We performed a retrospective study on the presence and role of nestin-expressing cells in 102 patients with glioma, relating the findings to VEGF, CXCL12, PDGFRbeta expression and to clinical outcome (time to tumor progression-TTP and survival time-ST). Our results suggest that in gliomas the detection of proliferating ECs expressing nestin correlates to histological malignancy grade and clinical outcome. Also, the expression of CXCL12 in low-grade gliomas was the only factor associated with a significantly shorter TTP, suggesting a role of this chemokine in angiogenic shift and/or disease progression.     

Survivin promotes glioma angiogenesis through vascular endothelial growth factor and basic fibroblast growth factor in vitro and in vivo

Survivin is involved in multiple signaling mechanisms in tumor maintenance, and accumulated studies elucidate that knockdown of survivin in endothelial cells could inhibit angiogenesis; however, the role of survivin in tumor cells to regulate tumor-derived angiogenesis remains largely unclear. In the present study 80 cases of brain glioma were chosen and protein expressions of survivin, vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), and platelet-derived growth factor (PDGF) in glioma cells were investigated by immunohistochemistry (IHC). Human umbilical vein endothelial cells (HUVEC) were cocultured with human glioma U251 wild-type cells, U251 cells survivin silenced, SHG44 wild-type cells, and SHG44 survivin-overexpressing cells, respectively. The proliferation and migration of HUVEC were evaluated by MTT assay and transwell chamber assay. The microvessels density (MVD) marked by CD31 expression in vascular endothelial cells in glioma xenografts in nude mice was detected by IHC. VEGF, bFGF, and PDGF in the aforementioned cells were detected by quantitive PCR (qPCR), Western blot, ELISA, and IHC in vitro and in vivo. The results showed that VEGF immunoreactivity score (IRS), bFGF IRS, and PDGF IRS were all positively correlated with survivin IRS in gliomas, respectively (P < 0.01). Survivin in human glioma cells could significantly promote the proliferation and migration of HUVEC and increase MVD, which could be contributed to survivin-dependent burst of VEGF and bFGF expression, followed by increase of tumor growth and proliferation. In summary, survivin, through upregulation of VEGF and bFGF, plays an essential role during glioma angiogenesis.     

PDGF/PDGFR effects in osteosarcoma and the “add-on” strategy

New treatment options for advanced osteosarcoma have remained limited. The platelet-derived growth factor (PDGF)/platelet-derived growth factor receptor (PDGFR) pathway plays an important role in the development and metastasis of osteosarcoma, via either direct autocrine stimulation of tumor cells, or paracrine stimulation on tumor stromal cells. It promotes angiogenesis to overcome hypoxia in the tumor microenvironment, and modulates tumor interstitial fluid pressure to control the influx and efflux of other agents. Targeting the PDGF/PDGFR pathway is a promising therapeutic method to overcome drug resistance and improve patients’ outcome in osteosarcoma. Further evidence is needed to define the detailed mechanism. Results from clinical trials using PDGF/PDGFR inhibitor as a single agent were disappointing, both in osteosarcoma and soft tissue sarcoma. However, when combined with other agents, named as “add-on” strategy, a synergistic antitumor effect has been confirmed in soft tissue sarcoma, and should be attempted in osteosarcoma.     

Inhibition of platelet-derived growth factor signalling induces autophagy in malignant glioma cells

Malignant gliomas highly coexpress platelet-derived growth factor (PDGF) and its receptor, suggesting the presence of an autocrine loop. Therefore, disruption of PDGF ligand/receptor complex represents a promising strategy for the treatment of malignant gliomas. However, the mechanisms of the antitumour effect exerted by the inhibition of PDGF-mediated cell growth remain unclear. In the present study, using anti-PDGF neutralising antibody, we investigated the effect of the inhibition of PDGF signalling on malignant glioma U87-MG, D54, and T98G cells with high levels of PDGF-A and -B. As a control, normal fibroblast MRC5 cells expressing low levels of PDGF-A and -B were used. Treatment with anti-PDGF neutralising antibody did not affect the expressions of PDGF-A, PDGF-B, and Akt, but suppressed the level of phosphorylated Akt in tumour cells, indicating the inhibition of PDGF signalling. The cell viability of all malignant glioma cells tested in this study was significantly inhibited in a time-dependent manner following the treatment compared to that of fibroblast cells (P<0.02 to <0.05). The antitumour effect of anti-PDGF antibody was suppressed by the activation of Akt and enhanced by the downregulation of Akt. Interestingly, the inhibition of PDGF signalling induced the development of acidic vesicular organelles and the autophagosome membrane association of the microtubule-associated protein light chain 3, which are characteristic of autophagy, in malignant glioma cells, while apoptotic cell death was not observed. Together these findings imply a new concept of autophagy for PDGF autocrine inhibition in malignant gliomas.     

CD70/CD27 ligand, a member of the TNF family, is expressed in human brain tumors

CD70 (CD27 ligand) has been implicated in proapoptotic signals mediated by its receptor CD27 in lymphocytes as well as in proliferative effects induced by reverse signaling in CD70-positive hematopoetic tumor cells. We were able to show that CD70 is expressed at the mRNA and protein level in human meningioma cells, glioma cells from solid human gliomas as well as glioma cell lines and 1 ependynoma. The intensity of CD70 expression varies considerably between different samples from 1 tumor type. In the U343 glioma cell line, CD70 was preferentially localized in the cell processes. Thus, our studies identify CD70 as a new marker molecule in brain tumors that are of nonhematopoetic origin.     

Platelet-derived growth factor as a mediator of normal and neoplastic cell proliferation

Human platelet-derived growth factor is the major mitogen in serum for connective-tissue-derived cells in culture. The factor is 30,000 mol. wt protein composed of two disulphide-linked polypeptide chains, named A and B. The B-chain is virtually identical to part of the transforming protein of simian sarcoma virus (SSV), implying that SSV-transformation is mediated by a PDGF-like growth factor. This notion is supported by the finding that specific as well as nonspecific inhibitors of PDGF-action (PDGF antibodies and suramin, respectively) are efficient inhibitors of SSV-transformation and revert the transformed phenotype of SSV-transformed cells. Expression of the genes encoding the PDGF subunits and production of PDGF-like growth factors is a common feature of human sarcoma cell lines, suggesting a role of PDGF in the pathogenesis of sarcomas, although direct support in favor of this notion is lacking. An involvement of PDGF in autocrine and paracrine stimulation of normal cell growth is suggested by the finding that responsive (arterial smooth muscle cells and placental cytotrophoblasts) as well as nonresponsive (endothelial cells and macrophages) cells produce PDGF-like growth factors. In conclusion, PDGF-like growth factors may be widely implicated in normal as well as neoplastic growth processes.     

Platelet-derived growth factor receptor (PDGFR) expression in primary spinal cord gliomas

Abnormal signaling through the platelet-derived growth factor receptor (PDGFR) has been proposed as a possible mechanism of spinal cord glioma initiation and progression. However, the extent of PDGFR expression in human spinal cord gliomas remains unknown. In this study we perform immunohistochemical analysis of PDGFRα expression in a series of 33 primary intramedullary spinal cord gliomas of different types and grades. PDGFRα was seen to be expressed in a significant subset of these tumors across all major glioma types including ependymoma, oligodendroglioma, pilocytic astrocytoma, astrocytoma, and glioblastoma. These results support the hypothesis that growth factor signaling through the PDGFR may be important for the development of at least a subset of human spinal cord gliomas. Further studies investigating the prognostic significance of PDGFR expression as well as the role of PDGF signaling on the development of intramedullary spinal cord gliomas are warranted.