Abstract P4-08-01: AMPK facilitates breast cancer cell survival by modulating microenvironmental stress

Recurrent and metastatic breast cancers are responsible for the majority of breast-cancer related deaths. These cancer cells are able to adapt to stressors within the tumor microenvironment including hypoxia, low nutrient levels, and chemotherapy-induced toxicities. Breast cancer cells can respond to these microenvironmental stressors through a variety of mechanisms, including cell cycle inhibition and metabolic alteration. Tumor cell survival is dependent on the ability to alter these mechanisms in response to stress. AMPK (AMP-activated protein kinase) is the main metabolic sensor of the cell, and both its expression and activity have been reported to be altered in breast cancer. Moreover, there are two isoforms of the catalytic subunit (α1 and α2), and differential functionality of these isoforms has been suggested. Using estrogen receptor-positive human breast cancer cell lines, we investigated the effect of differential AMPKα isoform expression on breast cancer cell survival. We found that over-expression of AMPKα2 in MCF-7 cells resulted in decreased ATP production in response to low glucose levels, while the knockdown of AMPKα2 in HCC1500 cells ablated this response to low glucose conditions. A similar difference in response was also seen when the cells were treated with a combination of nutrient stress and the estrogen receptor alpha (ERα) inhibitor, ICI182780. In response to this finding, we compared the glycolytic and oxygen consumption rates of our MCF-7 GFP and MCF-7 AMPKα2 cells. We found that in response to low glucose stress, AMPKα2 expressing MCF-7 cells maintained both a higher glycolytic rate and a higher oxygen consumption rate as compared to GFP cells. Furthermore, these cells seem to alter their cellular signaling in response to metabolic stress faster than GFP cells. To evaluate this differential response to microenvironmental stress in vivo, MCF-7 cells expressing either GFP or AMPKα2 were injected into athymic nude mice previously implanted with slow-release estradiol pellets. After one week, the estradiol pellets were removed to induce cellular dormancy for thirty days. Analysis of tumors at this time indicated that more of the AMPKα2 expressing cells survived estradiol deprivation than did the control cells. Analysis of proliferation by Ki67 staining indicated that the GFP cells maintained proliferation during deprivation, while AMPKα2 cells were largely negative for proliferation. ApoTag staining revealed a similar trend for apoptotic cells. This suggests that an inability to control cell cycle resulted in a decreased survival of the GFP cells under estradiol deprivation. Following the deprivation period, estradiol pellets were re-implanted and residual dormant tumors resumed growth. AMPKα2 tumors grew to roughly double the size of GFP tumors. Interestingly, AMPKα2 tumors had a higher number of mitotic events than did GFP tumors as visualized by Ki67 staining. This could be due to more viable cells being present following estradiol deprivation. We conclude that the expression of AMPKα2 promotes long-term breast cancer survival in estrogen-sensitive cells, due to their increased ability to sense and respond to changes in their microenvironment, which therefore increases their chances for survival.

Citation Format: Sullivan KL, Kopsiaftis S, Phoenix KN, Fox MM, Tsurutani N, Vella AT, Claffey KP. AMPK facilitates breast cancer cell survival by modulating microenvironmental stress. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P4-08-01.

The Role of Adenosine Monophosphate-Activated Kinase α2 as a Putative Tumor Suppressor in Breast Cancer

Background: The evaluation of adenosine monophosphate-activated kinase (AMPK) in tumor cell survival has revealed a critical interface between metabolism and cancer biology. As a major metabolic regulator, AMPK protects the cell from damage when nutrients are restricted. Therapeutic activation of AMPK using metformin has been shown to reduce the growth of most breast cancer cells but also promote the growth and metastasis of ERα negative tumor models. These contradictory findings indicate multiple roles for AMPK in breast cancer initiation and progression. Materials and Methods: AMPKα1 and AMPKα2 protein expression was quantified by immunohistochemistry in forty patient-matched primary human breast cancer and non-tumor margin samples; in addition ten normal breast reduction samples were evaluated. AMPKα2 was overexpressed in the ERα negative MDA-MB-231 breast cancer cell line and stable knockdown of AMPKα2 was performed with shRNA in the ERα positive T47D breast cancer cell line. Proliferation, viability, and soft agar colony growth assays were performed on all cell lines. Mouse embryonic fibroblasts (MEFs) from AMPKα2 knockout or wild-type C57/B6 mice were transformed with retrovirus containing oncogenic v-Ras and analyzed for changes in proliferation, soft agar growth, and tumorigenesis in athymic nude mice. The methylation status of the human AMPKα2 promoter region was investigated using quantitative methylation-specific PCR (qMSP) in MCF-7 and MDA-MB-231 breast cancer cell lines and primary breast tumors. Cell lines were treated with hypomethylation agents and evaluated for induced AMPKα2 expression by immunoblot. Results: AMPKα2 is the predominantly expressed alpha isoform in normal breast ductal epithelial tissue. In primary breast cancer, however, AMPKα2 was significantly repressed while AMPKα1 remained unchanged. Transient overexpression of AMPKα2 in breast cancer cell lines induced apoptosis within 48h. Silencing AMPKα2 revealed little change in proliferation rate in monolayer culture but exhibited increased soft agar colony growth. The tumorigenic capacity of v-Ras transformed AMPKa2 null MEFs was demonstrated by increased proliferation rates in vitro, soft agar growth, and tumor growth in nude mice. The promoter region of primary human breast cancer and breast cancer cell lines with low endogenous AMPKα2 levels were found to be methylated using qMSP. The same cell lines were able to re-establish AMPKα2 expression following treatment with the DNMT1 inhibitor, 5-aza-2-deoxycytidine. Discussion: AMPKα2 suppression in primary breast cancer indicates that its loss may provide a growth advantage for cancer cells. Overexpression of AMPKα2 in breast cancer cell lines induces apoptosis while the knockdown and knockout of the protein promotes tumorigenesis in vitro and in vivo. The loss of AMPKα2 function likely permits cell cycle progression under metabolic stress. The lack of proliferation control by AMPKa2 is hypothesized to increase the likelihood of acquiring genomic damage and mutational events. These results suggest that AMPKα2 may function as a tumor suppressor in human breast cancer.

Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 3153.

Expression of the Cancer Stem Cell Marker ALDH1A1 in Primary Breast Cancer: A Mechanism for Chemotherapy Resistance

Background: Aldehyde dehydrogenase 1 (ALDH1) has been identified as a marker of breast cancer stem cells, and ALDH1 expression in primary breast tumors has been associated with resistance to chemotherapy and poor prognosis. ALDH1 is an enzyme that can detoxify reactive aldehyde compounds; however the mechanism of ALDH1 chemotherapy resistance and its promotion of the breast cancer stem cell phenotype is unclear. Materials and Methods: Illumina cDNA array was performed on primary breast tumors that were snap-frozen, and RNA was isolated from dense, viable tumor regions. Samples were normalized by rank-invariant method and gene expression was analyzed. Immunohistochemistry was performed for ALDH1A1 on initial and post-neoadjuvant tumor samples from patients who have undergone six cycles of chemotherapy. Clinical data including age, tumor pathology, lymph node status, and response to neoadjuvant therapy were available for analysis. Human breast cancer cell lines (MCF-7 and MDA-MB-468) were treated with increasing doses of retinoic acid (RA) with and without the retinoic acid receptor alpha antagonist, Ro 41-5253. Effects on cell proliferation and signal transduction pathways were assessed. Results: The expression of ALDH1A1 and/or ALDH1A3 isoform mRNA was found to be present in primary human breast tumors. The majority of these cases were noted as triple negative or Her-2 positive tumors. Evaluation of RA receptor expression revealed repressed RAR beta levels in all tumors, whereas RAR alpha expression was retained. Observations of patient tumor samples (pre and post neoadjuvant chemotherapy treatment) demonstrated high ALDH1A1 expression in tumor and stromal cell compartments. The expression was predominant in partial and non-responders to the neoadjuvant therapy. Interestingly, the tumor from a partial responder was converted to extremely high expresser of ALDH1A1 post-neoadjuvant therapy indicating a positive survival selection. Since ALDH1A1 can convert retinaldehyde to retinoic acid, breast cancer cell lines were treated with all-trans RA and responded with a significant decrease in cell proliferation. This effect was partially attenuated by pre-treatment with the RAR alpha antagonist, Ro 41-5253. Importantly, RA treatment of MCF-7 cells repressed MAPK signaling, which was also attenuated by pre-treatment with Ro 41-5253. Discussion: The observations from this study suggest that the expression of the breast cancer stem cell marker, ALDH1A1 on tumor cells or in tumor-associated stromal cells can provide a mechanism for survival of cancer during chemotherapy treatment. The conversion of therapeutic oxidant compounds to retinoic acid by ALDH1 may lead to increased RAR alpha signaling and suppression of proliferation. The suppression of proliferation correlates well with chemotherapy resistance in these neoadjuvant cases. This data demonstrates a possible mechanism for ALDH1 promotion of tumor cell survival under chemotherapy and suggests that treatment with RAR alpha antagonists may promote tumor cell susceptibility to current treatment modalities and improve the therapeutic window for primary, metastatic and chemo-refractory breast cancer.

Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 1135.

The correlation of tumor vascularity as imaged by near-infrared technology with tissue distribution of angiogenic and growth promoters

Abstract #907

Background: Angiogenesis is necessary for solid tumors to grow. Rapidly growing tissues, increased metabolic demands as well as the presence of hypoxia and ischemia promote angiogenesis by stimulation of factors like vascular endothelial cell growth factor (VEGF), a key angiogenic factor. Many studies have found that angiogenesis is an independent prognostic factor in both node positive and node negative breast cancer. We have developed a novel technique, ultrasound guided optical tomography in the near-infrared wavelength (NIR-US), that accurately localizes tumor lesion with ultrasound and then images tumor angiogenesis. We have now collected images and tissue from 12 patients treated with preoperative chemotherapy. These tumors are large and heterogeneous with respect to vascular images. We have begun to correlate the tissue expression of a variety of pro-angiogenic proteins with the extent, distribution and pattern of vascularity demonstrated by our NIR-US. Methods: Using NIR-US images, the quantification of vascular content was performed by measuring the total hemoglobin content X volume of distribution of that hemoglobin content. This measure is termed the Blood Volume Index (BVI). In 12 patients studied, the level of tumor vascularity as measured by the initial BVI did not correlate with tumor grade, mitotic index, or estrogen receptor status or most importantly with response to treatment. We analyzed tissue specimens already collected with known patterns of vascular distribution on NIR-US and with known MVD to understand what might be driving the vascular response. Histologic sections were prepared and stained for; endothelial adhesion marker PECAM/CD31, VEGF, the activated stress response kinase P-AMPK, the cell proliferation marker Ki67, and SMA (smooth muscle actin) a marker of vasculature support cells and glucose transporter-1. Each case was quantified using at least 5 40X fields and pixel-by-pixel summation using Image Pro Plus software (Media cybernetics, Inc.) represented as average +/- SD. Results: The tumor BVI as defined by NIR-US did not correlate with either CD31 or SMA expression and was inversely correlated with VEGF levels. The higher VEGF, the lower the BVI. Of particular interest, the highest CD31 signals correlated with high VEGF levels in two, HER-2 positive patients, but not with BVI. Proliferation rate (Ki67), glucose transporter-1 and P-AMPK expression did not correlate with vascular content as measured by NIR-US.
 Conclusion: Study of the above growth promoters in tissue imaged by NIR-US did not reveal an association between their expression and the vascular content of the tissue. Of interest, VEGF expression varied greatly within the tumor specimens and showed no correlation to BVI.
 Financial support provided by a Research Grant from the CT Breast Health Initiative.

Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 907.

Effective metabolic intervention of breast cancer progression and metastasis

Abstract #6024

Background: Aggressive breast cancers are highly dependent upon glucose for proliferation and survival. Therapeutic modalities that suppress glucose, insulin and IGF-1 have promise as adjuvant therapies to suppress breast cancer progression, metastasis and recurrence. Suppression of glucose availability can trigger the activation of the AMP-dependent kinase (AMPK) in breast cancer cells which may suppress growth. However, AMPK activation can also lead to activation of survival pathways that promote cell survival and expression of VEGF. Previous investigations indicated that estrogen receptor alpha negative tumor cells respond to the AMPK activator, metformin, by repressing proliferation in vitro and conversely inducing tumor growth in vivo (Phoenix et al., Breast Cancer Res Treat, 2008). This study evaluated the efficacy of two metabolic interventions on the 66cl4 murine breast tumor growth and metastasis model.
 Methods: The 66cl4 murine breast tumor was inoculated into the mammary fat pad of 4-6 week old female Balb/c mice (10 mice/group, 1x106cells/animal). Tumors were allowed to progress to palpable size of 150mm3 volume. Treatment with caloric restriction (70% ab libitum) and metformin at three doses (41, 412 and 825mg/kg/d) were continued for 5 weeks. Tumor volumes were measured twice weekly, blood glucose and animal weight weekly and tumor weight at harvest. Tumors and lungs were analyzed for tumor vascularization, proliferation, apoptosis and cytokine expression and serum evaluated for IGF-1.
 Results: Caloric restriction significantly suppressed weight gain in Balb/c tumor bearing animals and significantly suppressed glucose compared to ab libitum controls (74.7 +/-2.6 vs. 95.1 +/-2.5, p<0.001). Tumor growth was significantly suppressed over the time course of 6 weeks in the caloric restriction group. Distal metastasis to the lung was also nearly completely inhibited by caloric restriction compared to normal controls. Metabolic intervention with metformin showed a trend of lower serum glucose with increasing doses, however this was not statistically significant. IGF-1 levels at harvest were lower in control mice than those at the highest dose of metformin. Interestingly, high dose metformin resulted in slightly larger tumors and higher tumor weights at harvest. However, histological analysis revealed less viable area and increased apoptosis, although microvascular density and Ki67 proliferation were not affected. High dose metformin reduced distal metastatic lung lesions, consistent with the loss of viable cells in the primary tumors.Conclusions: These results indicate that metabolic interventions may be effective at stabilizing aggressive breast cancers. It is likely that glucose and insulin/IGF-1 can significantly contribute to both primary tumor growth and metastasis. Metformin, the AMPK activator, may be useful in reducing insulin/IGF-1 and glucose in those patients with metabolic syndrome or diabetes which could reduce risk of recurrence where glucose contributes to tumor expansion and survival.

Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 6024.

Hypoxia stimulates breast carcinoma cell invasion through MT1-MMP and MMP-2 activation

The process of cancer cell invasion involves degradation of the extracellular matrix (ECM) by proteases, integrin adhesion and cell motility. The role of ECM degrading proteases on the hypoxia-induced invasion of breast carcinoma cells was investigated. Hypoxia markedly increased the invasion capacity of MDA-MB-231 and MDA-MB-435 breast carcinoma cell lines. Matrix metalloproteinase (MMP) inhibitors blocked the hypoxia-induced invasion, whereas other protease inhibitors had no effect. Antibodies or siRNAs blocking either membrane type-1 MMP (MT1-MMP) or MMP-2 were effective in reducing the hypoxia-induced invasion. Serum-free reconstitution experiments confirmed the involvement of the MT1-MMP/MMP-2/tissue inhibitor of metalloproteinase-2 complex in this hypoxia-induced response. Overexpression of MT1-MMP in a poorly invasive breast cancer cell line, T47-D, promoted hypoxia-induced invasion and MMP-2 activation. Cell surface accumulation and activation of MT1-MMP without apparent regulation at the mRNA or protein levels indicated a post-translational adaptive response to hypoxia. Inhibition of the small GTPase RhoA eliminated the hypoxia-induced invasion and blocked the localization of MT1-MMP to the plasma membrane. Zymographic and molecular analysis of human breast tumors showed a strong correlation between hypoxic microenvironments and MMP-2 activation without changes in MT1-MMP expression. Our studies suggest that hypoxic tumor microenvironments promote breast cancer invasion through an MT1-MMP-dependent mechanism.

Induction of tumor lymphangiogenesis by VEGF-C promotes breast cancer metastasis

Metastasis of breast cancer occurs primarily through the lymphatic system, and the extent of lymph node involvement is a key prognostic factor for the disease. Whereas the significance of angiogenesis for tumor progression has been well documented, the ability of tumor cells to induce the growth of lymphatic vessels (lymphangiogenesis) and the presence of intratumoral lymphatic vessels have been controversial. Using a novel marker for lymphatic endothelium, LYVE-1, we demonstrate here the occurrence of intratumoral lymphangiogenesis within human breast cancers after orthotopic transplantation onto nude mice. Vascular endothelial growth factor (VEGF)-C overexpression in breast cancer cells potently increased intratumoral lymphangiogenesis, resulting in significantly enhanced metastasis to regional lymph nodes and to lungs. The degree of tumor lymphangiogenesis was highly correlated with the extent of lymph node and lung metastases. These results establish the occurrence and biological significance of intratumoral lymphangiogenesis in breast cancer and identify VEGF-C as a molecular link between tumor lymphangiogenesis and metastasis.

Role of AP-1 and HIF-1 transcription factors in TGF-beta activation of VEGF expression

Aberrant expression of vascular endothelial growth factor (VEGF) has been demonstrated to be associated with most human solid tumors. Here we report that TGF-beta potently induces VEGF expression in human HT-1080 fibrosarcomas primarily through transcriptional activation with no significant changes in mRNA turnover. The tyrosine kinase inhibitor genistein and AP-1 inhibitor curcumin significantly blocked TGF-beta induction of VEGF expression while SP-1 and MKK1 inhibitors did not. TGF-beta enhanced both AP-1 and HIF-1 DNA binding activities whereas SP-1, AP-2 and NF-1 did not show major changes. Transcriptional reporter assays provided further evidence that TGF-beta augmented both AP-1 and HIF-1 activities. Moreover, TGF-beta-treated HT-1080 cells contained higher levels of HIF-1alpha and c-jun proteins in nuclear extracts. TGF-beta and hypoxia synergistically induced VEGF mRNA expression. Given the fact that most tumors respond to hypoxic stress with increased VEGF expression via HIF-1-dependent transcription, this study identifies for the first time that TGF-beta also increases VEGF mRNA in an AP-l/HIF-1-dependent mechanism and may potentiate the hypoxic response.

Fibroblast growth factor 2 activation of stromal cell vascular endothelial growth factor expression and angiogenesis

Angiogenesis is a key component of human cancer progression and metastasis. In an effort to recapitulate early events in tumor-induced angiogenesis, we have employed a subcutaneous Matrigel implant model using immunodeficient mice as hosts. Matrigel-containing fibroblast growth factor 2 (FGF-2; 1.2 microg/ml) induced stromal cell infiltration into the Matrigel/skin interface within 4 days and maximal neovascularization at 7 days. Cells staining positive for the endothelial cell marker, platelet-endothelial cell adhesion molecule 1 (PECAM-1), were present in neovessels and in isolated cells within the Matrigel matrix. Immunohistochemical analysis revealed high levels of vascular endothelial growth factor (VEGF) deposited in the stromal interface present only in the FGF-2-containing but not in control Matrigel implants. VEGF expression was confirmed with in situ hybridization. High VEGF mRNA levels were observed in the infiltrating stromal cells but not in endothelial or endothelial precursors as defined by PECAM-1 staining. In vitro analysis of FGF-2-treated embryonic fibroblasts, Balb/c 3T3 cells, showed an induction of VEGF transcription, mRNA synthesis, and protein secretion as defined by transcriptional reporter, Northern blot, and ELISA assays. The FGF-2-induced VEGF expression was not dependent on select matrix adherence or signaling components because VEGF mRNA expression induced by FGF-2 was equally activated on serum, basement membrane, and fibronectin matrix substrates. Systemic application of anti-VEGF antibodies significantly repressed FGF-2-induced angiogenesis over control antibody by 88% (p < 0.001). These data support an FGF-2 angiogenic model that is dependent on endothelial cell activation, stromal cell infiltration, and VEGF expression by the infiltrating stromal cell population.

Two-chain high molecular weight kininogen induces endothelial cell apoptosis and inhibits angiogenesis: partial activity within domain 5

We previously reported that the binding of two-chain high molecular weight kininogen (HKa) to endothelial cells may occur through interactions with endothelial urokinase receptors. Since the binding of urokinase to urokinase receptors activates signaling responses and may stimulate mitogenesis, we assessed the effect of HKa binding on endothelial cell proliferation. Unexpectedly, HKa inhibited proliferation in response to several growth factors, with 50% inhibition caused by approximately 10 nM HKa. This activity was Zn(2+) dependent and not shared by either single-chain high molecular weight kininogen (HK) or low molecular weight kininogen. HKa selectively inhibited the proliferation of human umbilical vein and dermal microvascular endothelial cells, but did not affect that of umbilical vein or human aortic smooth muscle cells, trophoblasts, fibroblasts, or carcinoma cells. Inhibition of endothelial proliferation by HKa was associated with endothelial cell apoptosis and unaffected by antibodies that block the binding of HK or HKa to any of their known endothelial receptors. Recombinant HK domain 5 displayed activity similar to that of HKa. In vivo, HKa inhibited neovascularization of subcutaneously implanted Matrigel plugs, as well as rat corneal angiogenesis. These results demonstrate that HKa is a novel inhibitor of angiogenesis, whose activity is dependent on the unique conformation of the two-chain molecule.

Upregulation of vascular endothelial growth factor by cobalt chloride-simulated hypoxia is mediated by persistent induction of cyclooxygenase-2 in a metastatic human prostate cancer cell line

Upregulation of vascular endothelial growth factor (VEGF) expression induced by hypoxia is crucial event leading to neovascularization. Cyclooxygenase-2, an inducible enzyme that catalyzes the formation of prostaglandins (PGs) from arachidonic acid, has been demonstrated to be induced by hypoxia and play role in angiogenesis and metastasis. To investigate the potential effect of COX-2 on hypoxia-induced VEGF expression in prostate cancer. We examined the relationship between COX-2 expression and VEGF induction in response to cobalt chloride (CoCl2)-simulated hypoxia in three human prostate cancer cell lines with differing biological phenotypes. Northern blotting and ELISA revealed that all three tested cell lines constitutively expressed VEGF mRNA, and secreted VEGF protein to different degrees (LNCaP > PC-3 > PC3ML). However, these cell lines differed in the ability to produce VEGF in the presence of CoCl2-simulated hypoxia. CoCl2 treatment resulted in 40% and 75% increases in VEGF mRNA, and 50% and 95% in protein secretion by LNCaP and PC-3 cell lines, respectively. In contrast, PC-3ML cell line, a PC-3 subline with highly invasive, metastatic phenotype, exhibits a dramatic upregulation of VEGF, 5.6-fold in mRNA and 6.3-fold in protein secretion after treatment with CoCl2. The upregulation of VEGF in PC-3ML cells is accompanied by a persistent induction of COX-2 mRNA (6.5-fold) and protein (5-fold). Whereas COX-2 expression is only transiently induced in PC-3 cells and not affected by CoCl2 in LNCaP cells. Moreover, the increases in VEGF mRNA and protein secretion induced by CoCl2 in PC-3ML cells were significantly suppressed following exposure to NS398, a selective COX-2 inhibitor. Finally, the effect of COX-2 inhibition on CoCl2-induced VEGF production was reversed by the treatment with exogenous PGE2. Our data demonstrate that VEGF induction by cobalt chloride-simulated hypoxia is maintained by a concomitant, persistent induction of COX-2 expression and sustained elevation of PGE2 synthesis in a human metastatic prostate cancer cell line, and suggest that COX-2 activity, reflected by PGE2 production, is involved in hypoxia-induced VEGF expression, and thus, modulates prostatic tumor angiogenesis.

Expression of vascular endothelial growth factor induces an invasive phenotype in human squamous cell carcinomas

Inhibition of the vascular endothelial growth factor (VEGF) receptor Flk-1 has been shown to prevent invasion of experimental squamous cell carcinomas (SCC). To directly investigate the role of VEGF in tumor invasion, we stably transfected human SCC-13 cells, which are characterized by a noninvasive phenotype in vivo, with expression vectors containing murine VEGF(164) in sense (SCC/VEGF+) or antisense (SCC/VEGF-) orientation or with vector alone (SCC/vec). SCC/vec cells formed slowly growing, well-differentiated tumors with well-defined borders between tumor and stroma, after intradermal or subcutaneous injection. In contrast, SCC/VEGF+ tumors were characterized by rapid tumor growth, with small cell groups and single cells invading into the surrounding tissue, and by admixture of blood vessels and tumor cells in areas of tumor invasion. We detected an increase in tumor vessel density and size in VEGF-overexpressing tumors, resulting in a more than fourfold increase in total vascular areas. In contrast, SCC/VEGF- clones formed noninvasive, sharply circumscribed tumors with reduced vascular density. These findings demonstrate that selective VEGF overexpression was sufficient to induce tumor invasiveness, and they provide further evidence for an active role of the tumor stroma in cancer progression.

Vascular endothelial cell adherens junction assembly and morphogenesis induced by sphingosine-1-phosphate

Vascular endothelial cells undergo morphogenesis into capillary networks in response to angiogenic factors. We show here that sphingosine-1-phosphate (SPP), a platelet-derived bioactive lipid, activates the EDG-1 and -3 subtypes of G protein-coupled receptors on endothelial cells to regulate angiogenesis. SPP induces the Gi/mitogen-activated protein kinase/cell survival pathway and the small GTPase Rho- and Raccoupled adherens junction assembly. Both EDG-1-and EDG-3-regulated signaling pathways are required for endothelial cell morphogenesis into capillary-like networks. Indeed, SPP synergized with polypeptide angiogenic growth factors in the formation of mature neovessels in vivo. These data define SPP as a novel regulator of angiogenesis.

Gangliosides influence angiogenesis in an experimental mouse brain tumor

Gangliosides are sialated glycosphingolipids present on the plasma membranes of all vertebrate cells. Tumors shed gangliosides into the extracellular microenvironment, which may influence tumor-host cell interactions. We have investigated the role of gangliosides on the growth and angiogenesis of the EPEN experimental mouse brain tumor. EPEN cells express only ganglioside G(M3), and the solid tumors formed in vivo are sparsely vascularized with extensive necrosis. We stably transfected the EPEN cells with the cDNA for N-acetylgalactosaminyl transferase, a key enzyme for the synthesis of complex gangliosides. In addition to G(M3), the transfected cell line (EPEN-GNT) expressed complex gangliosides G(M2), G(M1), and G(D1a). The EPEN-GNT tumor was more densely vascularized with less necrosis and grew more rapidly than the nontransfected EPEN or mock-transfected (EPEN-V) control tumors. Also, VEGF gene expression was higher in the EPEN-GNT tumor than in the control tumors. The synthesis of complex gangliosides in the EPEN-GNT tumor cells also stimulated vascularization in an in vivo Matrigel assay for angiogenesis. These results indicate that the ratio of G(M3) to complex gangliosides can influence the growth and angiogenic properties of the EPEN experimental brain tumor and are consistent with previous findings in other systems. We conclude that gangliosides may be important modulators of brain tumor angiogenesis.

Role of protein kinase C isoforms in phorbol ester-induced vascular endothelial growth factor expression in human glioblastoma cells

Aberrant expression of the potent angiogenic cytokine, vascular endothelial growth factor (VEGF), has been demonstrated to be associated with most human solid tumors. Both transcriptional and post-transcriptional mechanisms have been shown to modulate VEGF expression in a multitude of cell types. Here we report that when protein kinase C (PKC) pathways were activated in human glioblastoma U373 cells by phorbol 12-myristate 13-acetate (PMA), VEGF mRNA expression was up-regulated via a post-transcriptional mRNA stabilization mechanism. PMA treatment exhibited no increase in VEGF-specific transcriptional activation as determined by run-off transcription assays and VEGF promoter-luciferase reporter assays. However, PMA increased VEGF mRNA half-life from 0.8 to 3.6 h which was blocked by PKC inhibitors but not by protein kinase A or cyclic nucleotide-dependent protein kinase inhibitors. When U373 cells were transfected with antisense oligonucleotide sequences to the translation start sites of PKC-alpha, -beta, -gamma, -delta, -epsilon, or -zeta isoforms, both PKC-alpha and -zeta antisense oligonucleotides showed substantial inhibition of PMA-induced VEGF mRNA. In addition, overexpression of PKC-zeta resulted in a strong constitutive up-regulation of VEGF mRNA expression. This study demonstrates for the first time that specific PKC isoforms regulate VEGF mRNA expression through post-transcriptional mechanisms.

TNF-alpha impairs insulin signaling and insulin stimulation of glucose uptake in C2C12 muscle cells

Physiological stressors such as sepsis and tissue damage initiate an acute immune response and cause transient systemic insulin resistance. This study was conducted to determine whether tumor necrosis factor-alpha (TNF-alpha), a cytokine produced by immune cells during skeletal muscle damage, decreases insulin responsiveness at the cellular level. To examine the molecular mechanisms associated with TNF-alpha and insulin action, we measured insulin receptor substrate (IRS)-1- and IRS-2-mediated phosphatidylinositol 3-kinase (PI 3-kinase) activation, IRS-1-PI 3-kinase binding, IRS-1 tyrosine phosphorylation, and the phosphorylation of two mitogen-activated protein kinases (MAPK, known as p42(MAPK) and p44(MAPK)) in cultured C2C12 myotubes. Furthermore, we determined the effects of TNF-alpha on insulin-stimulated 2-deoxyglucose (2-DG) uptake. We observed that TNF-alpha impaired insulin stimulation of IRS-1- and IRS-2-mediated PI 3-kinase activation by 54 and 55% (P < 0.05), respectively. In addition, TNF-alpha decreased insulin-stimulated IRS-1 tyrosine phosphorylation by 40% (P < 0.05). Furthermore, TNF-alpha repressed insulin-induced p42(MAPK) and p44(MAPK) tyrosine phosphorylation by 81% (P < 0.01). TNF-alpha impairment of insulin signaling activation was accompanied by a decrease (P < 0.05) in 2-DG uptake in the muscle cells (60 +/- 4 vs. 44 +/- 6 pmol. min-1. mg-1). These data suggest that increases in TNF-alpha may cause insulin resistance in skeletal muscle by inhibiting IRS-1- and IRS-2-mediated PI 3-kinase activation as well as p42(MAPK) and p44(MAPK) tyrosine phosphorylation, leading to impaired insulin-stimulated glucose uptake.

Regulation of human vascular endothelial growth factor mRNA stability in hypoxia by heterogeneous nuclear ribonucleoprotein L

A 126-base region of human vascular endothelial growth factor (VEGF) 3'-untranslated region, which we identified as the hypoxia stability region, forms seven hypoxia-inducible RNA-protein complexes with apparent molecular masses ranging from 40 to 90 kDa in RNA-UV-cross-linking assays. In this study, we show that proteins that form the 60-kDa RNA-protein complex with the hypoxia stability region were present in both cytoplasmic and nuclear compartments. We purified the protein associated in the 60-kDa complex and identified it as heterogeneous nuclear ribonucleoprotein L (hnRNP L) by protein sequencing. Removal of hnRNP L by immunoprecipitation specifically abolished formation of the 60-kDa complex. Synthetic deoxyribonucleotide competition studies defined the RNA-binding site of hnRNP L as a 21-base-long sequence, 5'-CACCCACCCACAUACAUACAU-3'. Immunoprecipitation of hnRNP L followed by reverse transcription-polymerase chain reaction showed that hnRNP L specifically interacts with VEGF mRNA in hypoxic cells in vivo. Furthermore, when M21 cells transfected with antisense oligodeoxyribonucleotide to the hnRNP L RNA-binding site, the VEGF mRNA half-life was significantly reduced under hypoxic conditions. Thus, we propose that specific association of hnRNP L with VEGF mRNA under hypoxia may play an important role in hypoxia-induced post-transcriptional regulation of VEGF mRNA expression.

Hypoxia-mediated regulation of gene expression in mammalian cells

The molecular mechanism underlying oxygen sensing in mammalian cells has been extensively investigated in the areas of glucose transport, glycolysis, erythropoiesis, angiogenesis and catecholamine metabolism. Expression of functionally operative representative proteins in these specific areas, such as the glucose transporter 1, glycolytic enzymes, erythropoietin, vascular endothelial growth factor and tyrosine hydroxylase are all induced by hypoxia. Recent studies demonstrated that both transcriptional activation and post-transcriptional mechanisms are important to the hypoxia-mediated regulation of gene expression. In this article, the cis-acting elements and trans-acting factors involved in the transcriptional activation of gene expression will be reviewed. In addition, the mechanisms of post-transcriptional mRNA stabilization will also be addressed. We will discuss whether these two processes of regulation of hypoxia-responsive genes are mechanistically linked and co-operative in nature.

Activation of Sp1-mediated vascular permeability factor/vascular endothelial growth factor transcription requires specific interaction with protein kinase C zeta

The transcription factor Sp1 is ubiquitously expressed and plays a significant role in the constitutive and induced expression of a variety of mammalian genes and may even contribute to tumorigenesis. Here, we describe a novel pathway whereby Sp1 promotes the transcription of vascular permeability factor/vascular endothelial growth factor (VPF/VEGF), a potent angiogenic factor, by interacting directly and specifically with protein kinase C zeta (PKC zeta) isoform in renal cell carcinoma. PKC zeta binds and phosphorylates the zinc finger region of Sp1. Moreover, in the presence of the wild type von Hippel-Lindau gene product, the interaction of Sp1 with PKC zeta is inhibited, and in this manner steady state levels of Sp1 phosphorylation are decreased significantly. Co-transfection of renal cell carcinoma cells and human fibrosarcoma cells with a plasmid overexpressing PKC zeta and VPF/VEGF promoter luciferase constructs results in activation of Sp1-mediated transcription, whereas expression of a dominant-negative mutant of PKC zeta repressed this activation. Taken together, our results suggest a new pathway of cell signaling through PKC zeta and provide an insight into PKC zeta and Sp1-dependent transcriptional regulation of VPF/VEGF expression and thus tumor angiogenesis.

Mast cells can secrete vascular permeability factor/ vascular endothelial cell growth factor and exhibit enhanced release after immunoglobulin E-dependent upregulation of fc epsilon receptor I expression

Vascular permeability factor/vascular endothelial cell growth factor (VPF/VEGF) can both potently enhance vascular permeability and induce proliferation of vascular endothelial cells. We report here that mouse or human mast cells can produce and secrete VPF/VEGF. Mouse mast cells release VPF/VEGF upon stimulation through Fcepsilon receptor I (FcepsilonRI) or c-kit, or after challenge with the protein kinase C activator, phorbol myristate acetate, or the calcium ionophore, A23187; such mast cells can rapidly release VPF/VEGF, apparently from a preformed pool, and can then sustain release by secreting newly synthesized protein. Notably, the Fc epsilonRI-dependent secretion of VPF/VEGF by either mouse or human mast cells can be significantly increased in cells which have undergone upregulation of Fc epsilonRI surface expression by a 4-d preincubation with immunoglobulin E. These findings establish that at least one cell type, the mast cell, can be stimulated to secrete VPF/VEGF upon immunologically specific activation via a member of the multichain immune recognition receptor family. Our observations also identify a new mechanism by which mast cells can contribute to enhanced vascular permeability and/or angiogenesis, in both allergic diseases and other settings.

Tumor growth modulation by sense and antisense vascular endothelial growth factor gene expression: effects on angiogenesis, vascular permeability, blood volume, blood flow, fluorodeoxyglucose uptake, and proliferation of human melanoma intracerebral xenografts

Vascular endothelial growth factor (VEGF), also known as vascular permeability factor, has been investigated as a potent mediator of brain tumor angiogenesis and tumor growth. We evaluated the effect of VEGF expression on the pathophysiology of tumor growth in the brain. Human SK-MEL-2 melanoma cells, with minimal VEGF expression, were stably transfected with either sense or antisense mouse VEGF cDNA and used to produce intracerebral xenografts. Vascular permeability, blood volume, blood flow, and tumor fluorodeoxyglucose metabolism were assessed using tissue sampling and quantitative autoradiography. Tumor proliferation was assessed by measuring bromodeoxyuridine labeling indices. Tumor vascular density and morphological status of the blood-brain barrier were evaluated by immunohistochemistry. SK-MEL-2 cells transfected with sense VEGF (V+) expressed large amounts of mouse and human VEGF protein; V+ cells formed well-vascularized, rapidly growing tumors with minimal tumor necrosis. V+ tumors had substantial and significant increases in blood volume, blood flow, vascular permeability, and fluorodeoxyglucose metabolism compared to wild-type and/or V- (antisense VEGF) tumors. VEGF antisense transfected V- expressed no detectable VEGF protein and formed minimally vascularized tumors. V- tumors had a very low initial growth rate with central necrosis; blood volume, blood flow, vascular permeability, and glucose metabolism levels were low compared to wild-type and V+ tumors. A substantial inhibition of intracerebral tumor growth, as well as a decrease in tumor vascularity, blood flow, and vascular permeability may be achieved by down-regulation of endogenous VEGF expression in tumor tissue. VEGF-targeted antiangiogenic gene therapy could be an effective component of a combined strategy to treat VEGF-producing brain tumors.

Increased microvascular density and enhanced leukocyte rolling and adhesion in the skin of VEGF transgenic mice

Vascular endothelial growth factor (VEGF) has been implicated in the pathologic angiogenesis observed in psoriasis and other chronic inflammatory skin diseases that are characterized by enhanced expression of VEGF by epidermal keratinocytes and of VEGF receptors by tortuous microvessels in the upper dermis. To investigate the functional importance of chronic VEGF overexpression in vivo, we used a keratin 14 promoter expression cassette containing the gene for murine VEGF164 to selectively target VEGF expression to basal epidermal keratinocytes in transgenic mice. These mice demonstrated an increased density of tortuous cutaneous blood capillaries with elevated expression levels of the high affinity VEGF receptors, VEGFR-1 and VEGFR-2, most prominently during the neonatal period. In contrast, no abnormalities of lymphatic vessels were detected. In addition, the number of mast cells in the upper dermis was significantly increased in transgenic skin. Intravital fluorescence microscopy revealed highly increased leukocyte rolling and adhesion in postcapillary skin venules that were both inhibited after injection of blocking antibodies against E- and P-selectin. Combined blocking antibodies against intercellular adhesion molecule-1 and lymphocyte function-associated antigen-1 were without effect, whereas an anti-vascular cell adhesion molecule-1/VLA-4 antibody combination almost completely normalized the enhanced leukocyte adhesion in transgenic mice. This study reveals VEGF as a growth factor specific for blood vessels, but not lymphatic vessels, and demonstrates that chronic orthotopic overexpression of VEGF in the epidermis is sufficient to induce cardinal features of chronic skin inflammation, providing a molecular link between angiogenesis, mast cell accumulation, and leukocyte recruitment to sites of inflammation.

Identification of a human VPF/VEGF 3' untranslated region mediating hypoxia-induced mRNA stability

Hypoxia is a prominent feature of malignant tumors that are characterized by angiogenesis and vascular hyperpermeability. Vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) has been shown to be up-regulated in the vicinity of necrotic tumor areas, and hypoxia potently induces VPF/VEGF expression in several tumor cell lines in vitro. Here we report that hypoxia-induced VPF/VEGF expression is mediated by increased transcription and mRNA stability in human M21 melanoma cells. RNA-binding/electrophoretic mobility shift assays identified a single 125-bp AU-rich element in the 3' untranslated region that formed hypoxia-inducible RNA-protein complexes. Hypoxia-induced expression of chimeric luciferase reporter constructs containing this 125-bp AU-rich hypoxia stability region were significantly higher than constructs containing an adjacent 3' untranslated region element without RNA-binding activity. Using UV-cross-linking studies, we have identified a series of hypoxia-induced proteins of 90/88 kDa, 72 kDa, 60 kDa, 56 kDa, and 46 kDa that bound to the hypoxia stability region element. The 90/88-kDa and 60-kDa species were specifically competed by excess hypoxia stability region RNA. Thus, increased VPF/VEGF mRNA stability induced by hypoxia is mediated, at least in part, by specific interactions between a defined mRNA stability sequence in the 3' untranslated region and distinct mRNA-binding proteins in human tumor cells.

Angiogenesis promoted by vascular endothelial growth factor: regulation through alpha1beta1 and alpha2beta1 integrins

Vascular endothelial growth factor (VEGF), also known as vascular permeability factor, is a cytokine of central importance for the angiogenesis associated with cancers and other pathologies. Because angiogenesis often involves endothelial cell (EC) migration and proliferation within a collagen-rich extracellular matrix, we investigated the possibility that VEGF promotes neovascularization through regulation of collagen receptor expression. VEGF induced a 5- to 7-fold increase in dermal microvascular EC surface protein expression of two collagen receptors-the alpha1beta1 and alpha2beta1 integrins-through induction of mRNAs encoding the alpha1 and alpha2 subunits. In contrast, VEGF did not induce increased expression of the alpha3beta1 integrin, which also has been implicated in collagen binding. Integrin alpha1-blocking and alpha2-blocking antibodies (Ab) each partially inhibited attachment of microvascular EC to collagen I, and alpha1-blocking Ab also inhibited attachment to collagen IV and laminin-1. Induction of alpha1beta1 and alpha2beta1 expression by VEGF promoted cell spreading on collagen I gels which was abolished by a combination of alpha1-blocking and alpha2-blocking Abs. In vivo, a combination of alpha1-blocking and alpha2-blocking Abs markedly inhibited VEGF-driven angiogenesis; average cross-sectional area of individual new blood vessels was reduced 90% and average total new vascular area was reduced 82% without detectable effects on the pre-existing vasculature. These data indicate that induction of alpha1beta1 and alpha2beta1 expression by EC is an important mechanism by which VEGF promotes angiogenesis and that alpha1beta1 and alpha2beta1 antagonists may prove effective in inhibiting VEGF-driven angiogenesis in cancers and other important pathologies.

The von Hippel-Lindau gene product inhibits vascular permeability factor/vascular endothelial growth factor expression in renal cell carcinoma by blocking protein kinase C pathways

Mutation or loss of function of the von Hippel-Lindau (VHL) tumor suppressor gene is regularly found in sporadic renal cell carcinomas (RCC), well vascularized malignant tumors that characteristically overexpress vascular permeability factor/vascular endothelial growth factor (VPF/VEGF). The wild-type VHL (wt-VHL) gene product acts to suppress VPF/VEGF expression, which is overexpressed when wt-VHL is inactive. The present study investigated the pathways by which VHL regulates VPF/VEGF expression. We found that inhibition of protein kinase C (PKC) represses VPF/VEGF expression in RCC cells that regularly overexpress VPF/VEGF. The wt-VHL expressed by stably transfected RCC cells forms cytoplasmic complexes with two specific PKC isoforms, zeta and delta, and prevents their translocation to the cell membrane where they otherwise would engage in signaling steps that lead to VPF/VEGF overexpression. Other experiments implicated mitogen-activated protein kinase (MAPK) phosphorylation as a downstream step in PKC regulation of VPF/VEGF expression. Taken together, these data demonstrate that wt-VHL, by neutralizing PKC isoforms zeta and delta and thereby inhibiting MAPK activation, plays an important role in preventing aberrant VPF/VEGF overexpression and the angiogenesis that results from such overexpression.

Meningiomas: role of vascular endothelial growth factor/vascular permeability factor in angiogenesis and peritumoral edema

OBJECTIVE

Vascular endothelial growth factor (VEGF)/vascular permeability factor (VPF) is a potent angiogenic growth factor implicated in the tumor angiogenesis/metastasis of a number of human cancers. Activation of receptors for VEGF/VPF is specifically mitogenic to endothelial cells and increases their permeability. Although extensive literature exists regarding VEGF/VPF in human astrocytomas, little is known about its potential biological role(s) in meningiomas. Our interest in meningiomas was initiated by the observation that some meningiomas are extremely vascular and are occasionally associated with a considerable degree of peritumoral brain edema, both potentially related to the biological attributes of VEGF/VPF.

METHODS

As a first test of this hypothesis, we examined a cohort of 18 meningiomas for expression of VEGF/VPF at the messenger ribonucleic acid and protein levels and correlated expression with pathological characteristics, vascularity, and degree of peritumoral edema.

RESULTS

The majority of meningiomas expressed VEGF/VPF at both the messenger ribonucleic acid and protein levels. Corresponding serial sections were stained with an endothelial cell marker to obtain a microvascular density count, which positively correlated (P = 0.0005) with expression of VEGF/VPF. Furthermore, meningiomas with a large amount of peritumoral edema, as determined from the preoperative computed tomographic scans or magnetic resonance imaging scans, had elevated expression of VEGF/VPF (P = 0.05).

CONCLUSION

These data suggest that VEGF/VPF may play a role in both meningioma vascularity and peritumoral edema.

Hypoxia regulates the expression of vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) and its receptors in human skin

Tissue hypoxia is a characteristic feature of malignant tumors and healing wounds, conditions that are associated with angiogenesis and with increased expression of vascular permeability factor (VPF; also called vascular endothelial growth factor, VEGF), a selective endothelial cell mitogen inducing microvascular hyperpermeability in vivo. We investigated the regulation of VPF/VEGF and its receptors by tissue hypoxia in normal human skin explants and in cultured skin cells in vitro. VPF/VEGF mRNA expression was dramatically upregulated in epidermal keratinocytes, dermal fibroblasts, and dermal microvessels after 24 h of skin organ culture. Hypoxia also enhanced the expression of VPF/VEGF in cultured epidermal keratinocytes and dermal microvascular endothelial cells (predominantly VPF/VEGF121 and VPF/VEGF165) and in dermal fibroblasts (additional upregulation of VPF/VEGF189). The expression of the VPF/VEGF receptor Flt-1 was selectively induced on dermal microvessels in skin explant cultures and in dermal endothelial cell monolayer cultures under hypoxic conditions. In contrast, the KDR receptor was downregulated by hypoxia. These results suggest that hypoxia likely regulates cutaneous angiogenesis and microvascular permeability by two distinct mechanisms: (i) Induction of VPF/VEGF in epithelial and mesenchymal cells, including endothelial cells. (ii) Differential modulation of VPF/VEGF receptor expression by microvascular endothelial cells. These mechanisms may be of importance in the pathogenesis of healing wounds and some malignant tumors that are commonly characterized by hypoxia and overexpression of VPF/VEGF.

Vascular permeability factor/vascular endothelial growth factor: a multifunctional angiogenic cytokine

VPF/VEGF is a multifunctional cytokine that contributes to angiogenesis by both direct and indirect mechanisms. On the one hand, VPF/VEGF stimulates the endothelial cells lining nearby microvessels to proliferate, to migrate and to alter their pattern of gene expression. On the other hand, VPF/VEGF renders these same microvascular endothelial cells hyperpermeable so that they spill plasma proteins into the extravascular space, leading to profound alterations in the extracellular matrix that favor angiogenesis. These same principles apply in tumors, in several examples of non-neoplastic pathology, and in physiological processes that involve angiogenesis and new stroma generation. In all of these examples, microvascular hyperpermeability and the introduction of a provisional, plasma-derived matrix precede and accompany the onset of endothelial cell division and new blood vessel formation. It would seem, therefore, that tumors have made use of fundamental pathways that developed in multicellular organisms for purposes of tissue defense, renewal and repair. VPF/VEGF, therefore, has taught us something new about angiogenesis; namely, that vascular hyperpermeability and consequent plasma protein extravasation are important--perhaps essential--elements in its generation. However, this finding raises a paradox. While VPF/VEGF induces vascular hyperpermeability, other potent angiogenic factors apparently do not, at least in sub-toxic concentrations that are more than sufficient to induce angiogenesis (Connolly et al., 1989a). Nonetheless, wherever angiogenesis has been studied, the newly generated vessels have been found to be hyperpermeable. How, therefore, do angiogenic factors other than VPF/VEGF lead to the formation of new and leaky blood vessels? We do not as yet have a complete answer to this question. One possibility is that at least some angiogenic factors mediate their effect by inducing or stimulating VPF/VEGF expression. In fact, there are already clear example of this. A number of putative angiogenic factors including small molecules (e.g. prostaglandins, adenosine) as well as many cytokines (e.g. TGF-alpha, bFGF, TGF-beta, TNF-alpha, KGF, PDGF) have all been shown to upregulate VPF/VEGF expression. Further studies that elucidate the crosstalk among various angiogenic factors are likely to contribute significantly to a better understanding of the mechanisms by which new blood vessels are formed in health and in disease.

Stimulation of endothelial cell migration by vascular permeability factor/vascular endothelial growth factor through cooperative mechanisms involving the alphavbeta3 integrin, osteopontin, and thrombin

We have identified several mechanisms by which the angiogenic cytokine vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) likely regulates endothelial cells (EC) migration. VPF/VEGF induced dermal microvascular EC expression of mRNAs encoding the alphav and beta3 integrin subunits resulting in increased levels of the alphavbeta3 heterodimer at the cell surface, and VPF/VEGF also induced mRNA encoding osteopontin (OPN), an alphavbeta3 ligand. OPN promoted EC migration in vitro; and VPF/VEGF induction of alphavbeta3 was accompanied by increased EC migration toward OPN. Because thrombin cleavage of OPN results in substantial enhancement of OPN's adhesive properties, and because VPF/VEGF promotes increased microvascular permeability leading to activation of the extrinsic coagulation pathway, we also investigated whether VPF/VEGF facilitates thrombin cleavage of OPN in vivo. Consistent with this hypothesis, co-injection of VPF/VEGF together with OPN resulted in rapid cleavage of OPN by endogenous thrombin. Furthermore, in comparison with native OPN, thrombin-cleaved OPN stimulated a greater rate of EC migration in vitro, which was additive to the increased migration associated with induction of alpha v beta 3. Thus, these data demonstrate cooperative mechanisms for VPF/VEGF regulation of EC migration involving the alphavbeta3 integrin, the alphavbeta3 ligand OPN, and thrombin cleavage of OPN. These findings also illustrate an operational link between VPF/VEGF induction of EC gene expression and VPF/VEGF enhancement of microvascular permeability, suggesting that these distinct biological activities may act accordingly to stimulate EC migration during angiogenesis.

Regulation of VEGF/VPF expression in tumor cells: consequences for tumor growth and metastasis

Vascular endothelial growth factor (VEGF), also known as vascular permeability factor (VPF) is a multifunctional cytokine which potently stimulates angiogenesis in vivo. VEGF/VPF expression is elevated in pathological conditions including cancer, proliferative retinopathy, psoriasis and rheumatoid arthritis. The angiogenesis associated with human tumors is likely a central component in promoting tumor growth and metastatic potential. The regulation of VEGF/VPF expression during tumor progression may involve diverse mechanisms including activated oncogenes, mutant or deleted tumor suppressor genes, cytokine activation, hormonal modulators, and a particularly effective activator, hypoxia. Understanding the diverse mechanisms by which tumor cells overexpress VEGF/VPF, and which mechanisms are operating in specific tumor types is important for the design of effective anti-cancer therapies.

Expression of vascular permeability factor/vascular endothelial growth factor by melanoma cells increases tumor growth, angiogenesis, and experimental metastasis

Vascular permeability factor (VPF)/vascular endothelial growth factor (VEGF) is an angiogenic cytokine expressed by many human and animal tumors. Hypoxia often up-regulates VPF/VEGF expression further. To better define the role of VPF/VEGF in tumor biology, we screened tumorigenic lines for those expressing minimal constitutive and hypoxia-inducible VPF/VEGF. Human melanoma SK-MEL-2 cells best fit these criteria and formed small, poorly vascularized tumors in immunodeficient mice. We transfected SK-MEL-2 cells stably with sense or antisense mouse VPF/VEGF cDNA or with vector alone. Cells transfected with sense VPF/VEGF (V+) expressed and secreted large amounts of mouse VPF/VEGF and formed well-vascularized tumors with hyperpermeable blood vessels and minimal necrosis in nude/SCID mice. Antisense-transfected VPF/VEGF (V-) cells expressed reduced constitutive VPF/VEGF and no detectable mouse VPF/VEGF, and formed small, minimally vascularized tumors exhibiting extensive necrosis. Vector-alone transfectants (N1 cells) behaved like parental cells. V+ cells formed numerous lung tumor colonies in SCID mice, approximately 50-fold more than N1 cells, whereas V- cells formed few or none. These experiments demonstrate that VPF/VEGF promotes melanoma growth by stimulating angiogenesis and that constitutive VPF/VEGF expression dramatically promotes tumor colonization in the lung.

Participation of glomerular endothelial cells in the capillary repair of glomerulonephritis

In many glomerular diseases severe injury to the mesangium may occur, leading to matrix dissolution and damage to the glomerular capillaries. Although the destruction of glomerular architecture may lead to permanent injury, in some cases spontaneous recovery occurs. The mechanisms that mediate this recovery are unknown. In this study we provide evidence for glomerular capillary repair (angiogenesis) in the adult injured glomerulus. Injection of anti-Thy 1 antibody into rats results in severe mesangiolysis with capillary ballooning, microaneurysm formation, and loss of endothelial cells in addition to mesangial cells. Although mesangial proliferation is a major response to injury, proliferation of endothelial cells also can be documented from days 2 to 14 in association with repair of the capillaries. The endothelial cell proliferation peaks on days 2 and 7, when it is seven- to ninefold greater than normal. Many of the endothelial cells display morphological features of angiogenesis. The initial wave of endothelial cell proliferation can be reduced by 40% with neutralizing anti-basic fibroblast growth factor antibodies (P < 0.001). The later glomerular endothelial cell proliferation is associated with upregulated expression of vascular permeability factor/endothelial cell growth factor (VPF/VEGF) and an increase of flk, a VPF/VEGF receptor. Although PDGF is expressed in this model, anti-PDGF antibody treatment did not affect the endothelial cell proliferative response. In summary, glomerular endothelial cells have an active role in the glomerular response to injury. Glomeruli are capable of healing microaneurysms, and the mechanism involves basic fibroblast growth factor- and VPF/VEGF-mediated endothelial proliferative responses.

Vascular permeability factor/vascular endothelial growth factor: an important mediator of angiogenesis in malignancy and inflammation

Vascular permeability factor (VPF), also known as vascular endothelial growth factor (VEGF), is a multifunctional cytokine that is overexpressed in many transplantable animal and autochtonous human cancers, in healing wounds, and in chronic inflammatory disorders such as psoriasis and rheumatoid arthritis. All of these entities are characterized by angiogenesis, altered extracellular matrix, and variable degrees of hypoxia. In addition, two VPF/VEGF receptors, flt-1 and kdr, are overexpressed by endothelial cells that line the microvessels that supply these tumors/inflammatory reactions. On the basis of these and other data, we have proposed a model of angiogenesis in which VPF/VEGF plays a central role: this model is applicable to tumors and also to the angiogenesis that occurs in non-neoplastic processes.

Structural requirements for dimerization, glycosylation, secretion, and biological function of VPF/VEGF

Vascular permeability factor (VPF) also known as vascular endothelial growth factor (VEGF), is a dimeric protein that affects endothelial cell (EC) and vascular functions including enhancement of microvascular permeability and stimulation of EC growth. To investigate the structural features of VPF/VEGF necessary for efficient dimerization, secretion, and biological activities, we employed site-directed mutagenesis with a Cos-1 cell expression system. Several cysteine residues essential for VPF dimerization were identified by mutation analysis of the Cys-25, Cys-56, and Cys-67 residues. Mutant VPF isoforms lacking either of these cysteines were secreted as monomers and were completely inactive in both vascular permeability and endothelial cell mitotic assays. VPF Cys-145 mutant protein was efficiently secreted as a glycosylated, dimeric polypeptide, but had a reduction in biological activities. The site of N-linked glycosylation was directly identified as Asn-74, which, when mutated produced an inefficiently secreted dimeric protein without post-translational glycosylation, yet maintained full vascular permeability activity. Finally, we found that one VPF mutant isoform Cys-101 was not secreted and this mutant functioned as a dominant-negative suppressor of wild-type VPF secretion as demonstrated by co-expression assays in Cos-1 cells.

Independent regulation of adipose tissue-specificity and obesity response of the adipsin promoter in transgenic mice

Transcription of the adipocyte-specific adipsin gene is dramatically reduced in the adipose tissue of a number of genetically and chemically-induced obese rodents. To map the region of the adipsin gene that confers this response to obesity, transgenic mice were made containing -114, -250, -400, -700, and -938 base pairs (bp) to +35 bp of the promoter linked to the bacterial chloramphenicol acetyltransferase gene. Transgenic mice containing as few as 114 bp of the adipsin promoter had high levels of chloramphenicol acetyltransferase activity in adipose tissue. However, only those mice with 938 bp of the adipsin upstream regulatory region showed suppression of expression in adipose tissue in mice that were induced to become obese with monosodium glutamate. Using gel retardation assays, we showed that a 56-bp fragment of DNA mapping between -687 and -743 bp upstream from the start of adipsin expression was bound by protein factors in nuclear extracts prepared from adipose tissue. There was much greater retardation of this fragment with nuclear extracts prepared from adipose tissue of lean versus obese mice. These results indicate that a tissue-specific transcription factor(s) that regulates adipsin expression is less active in the adipose tissue of obese animals.

Overexpression of vascular permeability factor/vascular endothelial growth factor and its receptors in psoriasis

Psoriatic skin is characterized by microvascular hyperpermeability and angioproliferation, but the mechanisms responsible are unknown. We report here that the hyperplastic epidermis of psoriatic skin expresses strikingly increased amounts of vascular permeability factor (VPF; vascular endothelial growth factor), a selective endothelial cell mitogen that enhances microvascular permeability. Moreover, two VPF receptors, kdr and flt-1, are overexpressed by papillary dermal microvascular endothelial cells. Transforming growth factor alpha (TGF-alpha), a cytokine that is also overexpressed in psoriatic epidermis, induced VPF gene expression by cultured epidermal keratinocytes. VPF secreted by TGF-alpha-stimulated keratinocytes was bioactive, as demonstrated by its mitogenic effect on dermal microvascular endothelial cells in vitro. Together, these findings suggest that TGF-alpha regulates VPF expression in psoriasis by an autocrine mechanism, leading to vascular hyperpermeability and angiogenesis. Similar mechanisms may operate in tumors and in healing skin wounds which also commonly express both VPF and TGF-alpha.

Vascular endothelial growth factor. Regulation by cell differentiation and activated second messenger pathways

Vascular endothelial growth factor (VEGF) is an angiogenic polypeptide that has been isolated from a variety of tumorigenic and nontransformed cell lines. Because of the importance of blood vessel growth to cell and tissue development, we have examined VEGF gene expression in a variety of mouse tissues and rodent models of cellular differentiation. Using a cloned murine VEGF cDNA we show that VEGF mRNA is expressed at relatively low levels in many adult mouse tissues examined. However, this message is dramatically induced in two models of cell differentiation: 3T3-adipose conversion and C2C12 myogenic differentiation. VEGF protein secretion is also induced in adipocyte differentiation. VEGF mRNA is markedly regulated in a pheochromocytoma (PC12) cell model of transformation and differentiation. The transformed undifferentiated cells express moderate levels of VEGF mRNA and this expression is virtually extinguished when cells differentiate into non-malignant neuron-like cells. Experiments employing phorbol esters and cAMP analogues indicate that VEGF mRNA expression is stimulated in preadipocytes by both protein kinase C and protein kinase A-mediated pathways. These results suggest that VEGF mRNA levels are closely linked to the process of cellular differentiation; they also clearly demonstrate that expression of this angiogenic factor is specifically regulated in a transformed cell line, possibly via aberrant activation of cellular second messenger pathways.

Control of the adipsin gene in adipocyte differentiation. Identification of distinct nuclear factors binding to single- and double-stranded DNA

The mouse adipsin gene encodes a serine protease with complement factor D activity that is expressed during adipocyte differentiation and is deficient in several animal models of obesity. We have investigated the regulation of adipsin expression by transfecting preadipocytes and adipocytes with plasmids containing the 5'-flanking region of the adipsin gene linked to a reporter gene. Constructions containing a -950 to +35 segment of the adipsin promoter were preferentially expressed in adipose cells. Deletion experiments identified a region from -114 to -38 which contains a large inverted repeat sequence and negatively regulated gene expression in preadipocytes and positively regulated expression in fat cells. Exonuclease III protection and gel retardation assays indicated that this region of duplex DNA had multiple binding sites for nuclear factors, several of which were preadipose specific. In addition, we also identified two distinct factors that bound symmetrically and sequence specifically to the inverted repeat sequences only when they were in single-stranded form; one of these factors was induced during adipocyte differentiation. These results suggest that the control of the adipsin promoter in differentiation may involve an interplay of multiple regulated DNA-binding proteins, including two that have preferential affinity for single-stranded DNA.

Properties and differential regulation of two fatty acid binding proteins in the rat kidney

A protein from rat kidney was characterized that had several properties common to a multigene family of fatty acid binding proteins identified in other tissues. The putative kidney fatty acid binding protein (FABP) was purified from the soluble fraction of kidney homogenates using gel filtration and ion exchange chromatography. It was relatively abundant, had an apparent molecular mass of 15.5 kDa as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, bound equimolar amounts of oleic acid, and could be distinguished from other FABPs on the basis of size, amino acid composition, and tissue distribution. Polyclonal antibodies to kidney FABP were obtained and used to show that only kidney contained the 15.5-kDa protein, although the antibodies also recognized a slightly larger and less abundant protein in kidney that also was present in bladder. Rat kidney also contained heart FABP, and the properties of both FABPs in rat kidney were compared. The distribution of both proteins within the kidney differed, with kidney FABP being localized almost exclusively within the cortex, whereas heart FABP was found both in cortex and medulla. Kidney FABP was expressed developmentally after the neonatal period, whereas heart FABP was present in both neonatal and adult kidney at comparable amounts. Hypertension induced by mineralocorticoids or infusion of angiotensin II caused a marked suppression of kidney FABP expression, whereas amounts of heart FABP in kidney were unchanged. The studies showed that rat kidney contains at least two FABPs, and that these proteins are differentially regulated, suggesting that functional differences between the proteins may exist.

Hypertension induces tissue-specific gene suppression of a fatty acid binding protein in rat aorta

The effect of hypertension on the expression of a fatty acid binding protein localized in the rat aorta was studied. The presence of rat heart fatty acid binding protein (hFABP) was documented in aortic tissue by using a cDNA probe and polyclonal antibodies. Hypertension was induced in groups of rats by implantation of deoxycorticosterone acetate in conjunction with 1% salt in the drinking water (deoxycorticosterone/salt). By the third week of this treatment a marked reduction (by a factor of 20) in the expression of hFABP mRNA in aorta was found, concomitant with a reduction in immunologically detectable protein, suggesting transcriptional regulation. This effect was tissue specific, since no change in the normal amounts of hFABP mRNA in heart, skeletal muscle, or kidney was found. This reduction in aortic hFABP mRNA was also found in mildly hypertensive uninephrectomized rats given salt but no deoxycorticosterone and in normotensive rats given deoxycorticosterone but no excess salt intake. A marked decrease in aortic hFABP mRNA also was observed in the Goldblatt two kidney-one clip hypertensive model, and administration of angiotensin II for 6 days by osmotic minipump also caused a reduction. These findings suggest that hFABP is under complex regulation in aortic tissue and is suppressed by arterial hypertension.

Cloning and tissue distribution of rat heart fatty acid binding protein mRNA: identical forms in heart and skeletal muscle

A fatty acid binding protein (FABP) has been identified and characterized in rat heart, but the function and regulation of this protein are unclear. In this study the cDNA for rat heart FABP was cloned from a lambda gt11 library. Sequencing of the cDNA showed an open reading frame coding for a protein with 133 amino acids and a calculated size of 14776 daltons. Several differences were found between the sequence determined from the cDNA and that reported previously by protein sequencing techniques. Northern blot analysis using rat heart FABP cDNA as a probe established the presence of an abundant mRNA in rat heart about 0.85 kilobases in length. This mRNA was detected, but was not abundant, in fetal heart tissue. Tissue distribution studies showed a similar mRNA species in red, but not white, skeletal muscle. In general, the mRNA tissue distribution was similar to that of the protein detected by Western immunoblot analysis, suggesting that heart FABP expression may be regulated at the transcriptional level. S1 nuclease mapping studies confirmed that the mRNA hybridized to rat heart FABP cDNA was identical in heart and red skeletal muscle throughout the entire open reading frame. The structural differences between heart FABP and other members of this multigene family may be related to the functional requirements of oxidative muscle for fatty acids as a fuel source.

Measurement of rat heart fatty acid binding protein by ELISA. Tissue distribution, developmental changes and subcellular distribution

A class of soluble, low molecular weight proteins collectively called fatty acid binding proteins (FABP) are thought to function in the intracellular movement of fatty acids. To understand more clearly the role of FABP in cardiac metabolism, we used ELISA and immunoblotting techniques to study the distribution of heart FABP in several rat tissues, compare male and female rat heart content, quantitate developmental changes, and determine its subcellular distribution. Immunoreactive protein was found in appreciable amounts in rat heart, red skeletal muscle and kidney. Adult rat heart contained about 1.5 mg FABP/g tissue wet weight with the atrial content being approximately 50% of the ventricular concentration. No significant difference was detected between the sexes. The amount of FABP increased progressively during development from fetal to adult animals, and measureable amounts were found in 17-day-old fetal tissue. Comparisons between myoglobin and FABP showed that FABP appeared earlier than myoglobin in development, but myoglobin was more abundant than FABP at birth. Using immunoblots it was determined that rat heart FABP was localized in the cytosol with no detectable intramitochondrial material.