Vascular gene transfer driven by endoglin and ICAM-2 endothelial-specific promoters

Applications of tissue-specific and cancer-selective gene promoters for cancer diagnosis and therapy

Current treatment of solid tumors with standard of care chemotherapies, radiation therapy and/or immunotherapies are often limited by severe adverse toxic effects, resulting in a narrow therapeutic index. Cancer gene therapy represents a targeted approach that in principle could significantly reduce undesirable side effects in normal tissues while significantly inhibiting tumor growth and progression. To be effective, this strategy requires a clear understanding of the molecular biology of cancer development and evolution and developing biological vectors that can serve as vehicles to target cancer cells. The advent and fine tuning of omics technologies that permit the collective and spatial recognition of genes (genomics), mRNAs (transcriptomics), proteins (proteomics), metabolites (metabolomics), epiomics (epigenomics, epitranscriptomics, and epiproteomics), and their interactomics in defined complex biological samples provide a roadmap for identifying crucial targets of relevance to the cancer paradigm. Combining these strategies with identified genetic elements that control target gene expression uncovers significant opportunities for developing guided gene-based therapeutics for cancer. The purpose of this review is to overview the current state and potential limitations in developing gene promoter-directed targeted expression of key genes and highlights their potential applications in cancer gene therapy.

ALDH2 (Aldehyde Dehydrogenase 2) Protects Against Hypoxia-Induced Pulmonary Hypertension

OBJECTIVE

Hypoxia-induced pulmonary hypertension (HPH) increases lipid peroxidation with generation of toxic aldehydes that are metabolized by detoxifying enzymes, including ALDH2 (aldehyde dehydrogenase 2). However, the role of lipid peroxidation and ALDH2 in HPH pathogenesis remain undefined. Approach and Results: To determine the role of lipid peroxidation and ALDH2 in HPH, C57BL/6 mice, ALDH2 transgenic mice, and ALDH2 knockout (ALDH2^-/-) mice were exposed to chronic hypoxia, and recombinant tissue-specific ALDH2 overexpression adeno-associated viruses were introduced into pulmonary arteries via tail vein injection for ALDH2 overexpression. Human pulmonary artery smooth muscle cells were used to elucidate underlying mechanisms in vitro. Chronic hypoxia promoted lipid peroxidation due to the excessive production of reactive oxygen species and increased expression of lipoxygenases in lung tissues. 4-hydroxynonenal but not malondialdehyde level was increased in hypoxic lung tissues which might reflect differences in detoxifying enzymes. ALDH2 overexpression attenuated the development of HPH, whereas ALDH2 knockout aggravated it. Specific overexpression of ALDH2 using AAV1 (adeno-associated virus)-ICAM (intercellular adhesion molecule) 2p-ALDH2 and AAV2-SM22αp (smooth muscle 22 alpha)-ALDH2 viral vectors in pulmonary artery smooth muscle cells, but not endothelial cells, prevented the development of HPH. Hypoxia or 4-hydroxynonenal increased stabilization of HIF (hypoxia-inducible factor)-1α, phosphorylation of Drp1 (dynamin-related protein 1) at serine 616, mitochondrial fission, and pulmonary artery smooth muscle cells proliferation, whereas ALDH2 activation suppressed the latter 3.

CONCLUSIONS

Increased 4-hydroxynonenal level plays a critical role in the development of HPH. ALDH2 attenuates the development of HPH by regulating mitochondrial fission and smooth muscle cell proliferation suggesting ALDH2 as a potential new therapeutic target for pulmonary hypertension.

Vascular and Neurogenic Rejuvenation in Aging Mice by Modulation of ASM

Although many reports have revealed dysfunction of endothelial cells in aging, resulting in blood-brain barrier (BBB) breakdown, the underlying mechanism or mechanisms remain to be explored. Here, we find that acid sphingomyelinase (ASM) is a critical factor for regulating brain endothelial barrier integrity. ASM is increased in brain endothelium and/or plasma of aged humans and aged mice, leading to BBB disruption by increasing caveolae-mediated transcytosis. Genetic inhibition and endothelial-specific knockdown of ASM in mice ameliorated BBB breakdown and neurocognitive impairment during aging. Using primary mouse brain endothelial cells, we found that ASM regulated the caveolae-cytoskeleton interaction through protein phosphatase 1-mediated ezrin/radixin/moesin (ERM) dephosphorylation and apoptosis. Moreover, mice with conditional ASM overexpression in brain endothelium accelerated significant BBB impairment and neurodegenerative change. Overall, these results reveal a novel role for ASM in the control of neurovascular function in aging, suggesting that ASM may represent a new therapeutic target for anti-aging.

Endoglin and alk1 as therapeutic targets for hereditary hemorrhagic telangiectasia

INTRODUCTION

Hereditary Haemorrhagic Telangiectasia (HHT) is as an autosomal dominant trait characterized by frequent nose bleeds, mucocutaneous telangiectases, arteriovenous malformations (AVMs) of the lung, liver and brain, and gastrointestinal bleedings due to telangiectases. HHT is originated by mutations in genes whose encoded proteins are involved in the transforming growth factor β (TGF-β) family signalling of vascular endothelial cells. In spite of the great advances in the diagnosis as well as in the molecular, cellular and animal models of HHT, the current treatments remain just at the palliative level. Areas covered: Pathogenic mutations in genes coding for the TGF-β receptors endoglin (ENG) (HHT1) or the activin receptor-like kinase-1 (ACVRL1 or ALK1) (HHT2), are responsible for more than 80% of patients with HHT. Therefore, ENG and ALK1 are the main potential therapeutic targets for HHT and the focus of this review. The current status of the preclinical and clinical studies, including the anti-angiogenic strategy, have been addressed. Expert opinion: Endoglin and ALK1 are attractive therapeutic targets in HHT. Because haploinsufficiency is the pathogenic mechanism in HHT, several therapeutic approaches able to enhance protein expression and/or function of endoglin and ALK1 are keys to find novel and efficient treatments for the disease.

Critical regulators of endothelial cell functions: for a change being alternative

SIGNIFICANCE

The endothelium regulates vessel dilation and constriction, balances hemostasis, and inhibits thrombosis. In addition, pro- and anti-angiogenic molecules orchestrate proliferation, survival, and migration of endothelial cells. Regulation of all these processes requires fine-tuning of signaling pathways, which can easily be tricked into running the opposite direction when exogenous or endogenous signals get out of hand. Surprisingly, some critical regulators of physiological endothelial functions can turn malicious by mere alternative splicing, leading to the expression of protein isoforms with opposite functions.

RECENT ADVANCES

While reviewing the evidence of alternative splicing on cellular physiology, it became evident that expression of splice factors and their activities are regulated by externally triggered signaling cascades. Furthermore, genome-wide identification of RNA-binding sites of splicing regulatory proteins now offer a glimpse into the splicing code responsible for alternative splicing of molecules regulating endothelial functions.

CRITICAL ISSUES

Due to the constantly growing number of transcript and protein isoforms, it will become more and more important to identify and characterize all transcripts and proteins regulating endothelial cell functions. One critical issue will be a non-ambiguous nomenclature to keep consistency throughout different laboratories.

FUTURE DIRECTIONS

RNA-deep sequencing focusing on exon-exon junction needs to more reliably identify alternative splicing events combined with functional analyses that will uncover more splice variants contributing to or inhibiting proper endothelial functions. In addition, understanding the signals mediating alternative splicing and its regulation might allow us to derive new strategies to preserve endothelial function by suppressing or upregulating specific protein isoforms. Antioxid. Redox Signal. 22, 1212-1229.

Endoglin for tumor imaging and targeted cancer therapy

INTRODUCTION

Although cancer treatment has evolved substantially in the past decades, cancer-related mortality rates are still increasing. Therapies targeting tumor angiogenesis, crucial for the growth of solid tumors, mainly target vascular endothelial growth factor (VEGF) and have been clinically applied during the last decade. However, these therapies have not met high expectations, which were based on therapeutic efficacy in animal models. This can partly be explained by the upregulation of alternative angiogenic pathways. Therefore, additional therapies targeting other pro-angiogenic pathways are needed.

AREAS COVERED

The transforming growth factor (TGF)-β signaling pathway plays an important role in (tumor) angiogenesis. Therefore, components of this pathway are interesting candidates for anti-angiogenic therapy. Endoglin, a co-receptor for various TGF-β family members, is specifically overexpressed in tumor vessels and endoglin expression is associated with metastasis and patient survival. Therefore, endoglin might be a good candidate for anti-angiogenic therapy. In this review, we discuss the potential of using endoglin to target the tumor vasculature for imaging and therapeutic purposes.

EXPERT OPINION

Considering the promising results from various in vitro studies, in vivo animal models and the first clinical trial targeting endoglin, we are convinced that endoglin is a valuable tool for the diagnosis, visualization and ultimately treatment of solid cancers.

Construction of a novel expression cassette for increasing transgene expression in vivo in endothelial cells of large blood vessels

The success of gene therapy hinges on achievement of adequate transgene expression. To ensure high transgene expression, many gene-therapy vectors include highly active virus-derived transcriptional elements. Other vectors include tissue-specific eukaryotic transcriptional elements, intended to limit transgene expression to specific cell types, avoid toxicity and prevent immune responses. Unfortunately, tissue specificity is often accompanied by lower transgene expression. Here, we use eukaryotic (murine) transcriptional elements and a virus-derived posttranscriptional element to build cassettes designed to express a potentially therapeutic gene (interleukin (IL)-10) in large-vessel endothelial cells (ECs) at levels as high as obtained with the cytomegalovirus (CMV) immediate early promoter, while retaining EC specificity. The cassettes were tested by incorporation into helper-dependent adenoviral vectors, and transduction into bovine aortic EC in vitro and rabbit carotid EC in vivo. The murine endothelin-1 promoter showed EC specificity, but expressed only 3% as much IL-10 mRNA as CMV. Inclusion of precisely four copies of an EC-specific enhancer and a posttranscriptional regulatory element increased IL-10 expression to a level at or above the CMV promoter in vivo, while retaining--and possibly enhancing--EC specificity, as measured in vitro. The cassette reported here will likely be useful for maximizing transgene expression in large-vessel EC, while minimizing systemic effects.

Physiological and tissue-specific vectors for treatment of inherited diseases

After more than 1500 gene therapy clinical trials in the past two decades, the overall conclusion is that for gene therapy (GT) to be successful, the vector systems must still be improved in terms of delivery, expression and safety. The recent development of more efficient and stable vector systems has created great expectations for the future of GT. Impressive results were obtained in three primary immunodeficiencies and other inherited diseases such as congenital blindness, adrenoleukodystrophy or junctional epidermolysis bullosa. However, the development of leukemia in five children included in the GT clinical trials for X-linked severe combined immunodeficiency and the silencing of the therapeutic gene in the chronic granulomatous disease clearly showed the importance of improving safety and efficiency. In this review, we focus on the main strategies available to achieve physiological or tissue-specific expression of therapeutic transgenes and discuss the importance of controlling transgene expression to improve safety. We propose that tissue-specific and/or physiological viral vectors offer the best balance between efficiency and safety and will be the tools of choice for future clinical trials in GT of inherited diseases.

The physiological role of endoglin in the cardiovascular system

Endoglin (CD105) is an integral membrane glycoprotein that serves as a coreceptor for members of the transforming growth factor-β superfamily of proteins. A major role for endoglin in regulating transforming growth factor-β-dependent vascular remodeling and angiogenesis has been postulated based on the following: 1) endoglin is the gene mutated in hereditary hemorrhagic telangiectasia type 1, a disease characterized by vascular malformations; 2) endoglin knockout mice die at midgestation because of defective angiogenesis; 3) endoglin is overexpressed in neoangiogenic vessels, during inflammation, and in solid tumors; and 4) endoglin regulates the expression and activity of endothelial nitric oxide synthase, which is involved in angiogenesis and vascular tone. Besides the predominant form of the endoglin receptor (long endoglin isoform), two additional forms of endoglin have been recently reported to play a role in the vascular pathology and homeostasis: the alternatively spliced short endoglin isoform and a soluble endoglin form that is proteolytically cleaved from membrane-bound endoglin. The purpose of this review is to underline the role that the different forms of endoglin play in regulating angiogenesis, vascular remodeling, and vascular tone, as well as to analyze the molecular and cellular mechanisms supporting these effects.

Targeting cancer vasculature via endoglin/CD105: a novel antibody-based diagnostic and therapeutic strategy in solid tumours

Endoglin/CD105 is well acknowledged as being the most reliable marker of proliferation of endothelial cells, and it is overexpressed on tumour neovasculature. Our current knowledge of its structure, physiological role, and tissue distribution suggests that targeting of endoglin/CD105 is a novel and powerful diagnostic and therapeutic strategy in human malignancies, through the imaging of tumour-associated angiogenesis and the inhibition of endothelial cell functions related to tumour angiogenesis. Among biotherapeutic agents, monoclonal antibodies have shown a major impact on the clinical course of human malignancies of different histotypes. Along this line, the potential efficacy of anti-endoglin/CD105 antibodies and their derivatives for clinical purposes in cancer is supported by a large body of available pre-clinical in vitro and in vivo data. In this review, the main findings supporting the translation of antibody-based endoglin/CD105 targeting from pre-clinical studies to clinical applications in human cancer are summarized and discussed.

Shear stress-induced transcriptional regulation via hybrid promoters as a potential tool for promoting angiogenesis

Among the key effects of fluid shear stress on vascular endothelial cells is modulation of gene expression. Promoter sequences termed shear stress response elements (SSREs) mediate the responsiveness of endothelial genes to shear stress. While previous studies showed that shear stress responsiveness is mediated by a single SSRE, these endogenous promoters often encode for multiple SSREs. Moreover, hybrid promoters encoding a single SSRE rarely respond to shear stress at the same magnitude as the endogenous promoter. Thus, to better understand the interplay between the various SSREs, and between SSREs and endothelial-specific sequences (ESS), we generated a series of constructs regulated by SSREs cassettes alone, or in combination with ESS, and tested their response to shear stress and endothelial specific expression. Among these constructs, the most responsive promoter (NR1/2) encoded a combination of two GAGACC/SSREs, the Sp1/Egr1 sequence, as well as a TPA response element (TRE). This construct was four- to five-fold more responsive to shear stress than a promoter encoding a single SSRE. The expression of constructs containing other SSRE combinations was unaffected or suppressed by shear stress. Addition of ESS derived from the Tie2 promoter, either 5' or 3' to NR1/2 resulted in shear stress transcriptional suppression, yet retained endothelial specific expression. Thus, the combination and localization order of the various SSREs in a single promoter is crucial in determining the pattern and degree of shear stress responsiveness. These shear stress responsive cassettes may prove beneficial in our attempt to time the expression of an endothelial transgene in the vasculature.

S-endoglin expression is induced in senescent endothelial cells and contributes to vascular pathology

Senescence of endothelial cells (ECs) may contribute to age-associated cardiovascular diseases, including atherosclerosis and hypertension. The functional and gene expression changes associated with cellular senescence are poorly understood. Here, we have analyzed the expression, during EC senescence, of 2 different isoforms (L, long; S, short) of endoglin, an auxiliary transforming growth factor (TGF)-beta receptor involved in vascular remodeling and angiogenesis. As evidenced by RT-PCR, the S/L ratio of endoglin isoforms was increased during senescence of human ECs in vitro, as well as during aging of mice in vascularized tissues. Next, the effect of S-endoglin protein on the TGF-beta receptor complex was studied. As revealed by coimmunoprecipitation assays, S-endoglin was able to interact with both TGF-beta type I receptors, ALK5 and ALK1, although the interaction with ALK5 was stronger than with ALK1. S-endoglin conferred a lower proliferation rate to ECs and behaved differently from L-endoglin in relation to TGF-beta-responsive reporters with ALK1 or ALK5 specificities, mimicking the behavior of the endothelial senescence markers Id1 and plasminogen activator inhibitor-1. In situ hybridization studies demonstrated the expression of S-endoglin in the endothelium from human arteries. Transgenic mice overexpressing S-endoglin in ECs showed hypertension, decreased hypertensive response to NO inhibition, decreased vasodilatory response to TGF-beta(1) administration, and decreased endothelial nitric oxide synthase expression in lungs and kidneys, supporting the involvement of S-endoglin in the NO-dependent vascular homeostasis. Taken together, these results suggest that S-endoglin is induced during endothelial senescence and may contribute to age-dependent vascular pathology.

Modification of kidney barrier function by the urokinase receptor

Podocyte dysfunction, represented by foot process effacement and proteinuria, is often the starting point for progressive kidney disease. Therapies aimed at the cellular level of the disease are currently not available. Here we show that induction of urokinase receptor (uPAR) signaling in podocytes leads to foot process effacement and urinary protein loss via a mechanism that includes lipid-dependent activation of alphavbeta3 integrin. Mice lacking uPAR (Plaur-/-) are protected from lipopolysaccharide (LPS)-mediated proteinuria but develop disease after expression of a constitutively active beta3 integrin. Gene transfer studies reveal a prerequisite for uPAR expression in podocytes, but not in endothelial cells, for the development of LPS-mediated proteinuria. Mechanistically, uPAR is required to activate alphavbeta3 integrin in podocytes, promoting cell motility and activation of the small GTPases Cdc42 and Rac1. Blockade of alphavbeta3 integrin reduces podocyte motility in vitro and lowers proteinuria in mice. Our findings show a physiological role for uPAR signaling in the regulation of kidney permeability.

New vectors and strategies for cardiovascular gene therapy

Cardiovascular diseases are the major cause of morbidity and mortality in both men and women in industrially developed countries. These disorders may result from impaired angiogenesis, particularly in response to hypoxia. Despite many limitations, gene therapy is still emerging as a potential alternative for patients who are not candidates for traditional revascularization procedures, like angioplasty or vein grafts. This review focuses on recent approaches in the development of new gene delivery vectors, with great respect to newly discovered AAV serotypes and their modified forms. Moreover, some new cardiovascular gene therapy strategies have been highlighted, such as combination of different angiogenic growth factors or simultaneous application of genes and progenitor cells in order to obtain stable and functional blood vessels in ischemic tissue.

Hereditary hemorrhagic telangiectasia, a vascular dysplasia affecting the TGF-beta signaling pathway

Hereditary hemorrhagic telangiectasia (HHT) is caused by mutations in endoglin (ENG; HHT1) or ACVRL1/ALK1 (HHT2) genes and is an autosomal dominant vascular dysplasia. Clinically, HHT is characterized by epistaxis, telangiectases and arteriovenous malformations in some internal organs such as the lung, brain or liver. Endoglin and ALK1 proteins are specific endothelial receptors of the transforming growth factor (TGF)-beta superfamily that are essential for vascular integrity. Genetic studies in mice and humans have revealed the pivotal role of TGF-beta signaling during angiogenesis. Through binding to the TGF-beta type II receptor, TGF-beta can activate two distinct type I receptors (ALK1 and ALK5) in endothelial cells, each one leading to opposite effects on endothelial cell proliferation and migration. The recent isolation and characterization of circulating endothelial cells from HHT patients has revealed a decreased endoglin expression, impaired ALK1- and ALK5-dependent TGF-beta signaling, disorganized cytoskeleton and the failure to form cord-like structures which may lead to the fragility of small vessels with bleeding characteristic of HHT vascular dysplasia or to disrupted and abnormal angiogenesis after injuries and may explain the clinical symptoms associated with this disease.

Characterization of murine S-endoglin isoform and its effects on tumor development

Endoglin is a transmembrane glycoprotein that acts as an auxiliary receptor for transforming growth factor-beta (TGF-beta) and modulates cellular responses to this pleiotropic cytokine. Endoglin is strongly expressed in endothelial cells, where it appears to exert a crucial role in vascular development and angiogenesis. Two endoglin isoforms (L and S), differing in their cytoplasmic domains, have been previously characterized in human tissues. We now demonstrate the existence of similar L- and S-endoglin variants in murine tissues with 47 and 35 amino acids, respectively, in their cytoplasmic tail. RT-PCR analysis showed that L is the predominant endoglin isoform expressed in mouse tissues, although S-endoglin mRNA is significantly expressed in liver and lung, as well as in endothelial cell lines. Furthermore, a protein of size equivalent to recombinant S-endoglin expressed in mammalian cells was detected in mouse endothelial cells by Western blot analysis. L- and S-endoglin isoforms can form disulfide-linked heterodimers, as demonstrated by cotransfection of L- and S-endoglin constructs. To address the role of S-endoglin in vivo, an S-Eng(+) transgenic mouse model that targets S-endoglin expression to the endothelium was generated. The lethal phenotype of endoglin-null (Eng(-/-)) mice was not rescued by breeding S-Eng(+) transgenic mice into the endoglin-null background. S-Eng(+) mice exhibited reduced tumor growth and neovascularization after transplantation of Lewis lung carcinoma cells. In addition, S-Eng(+) mice showed a drastic inhibition of benign papilloma formation when subjected to two-stage chemical skin carcinogenesis. These results point to S-endoglin as an antiangiogenic molecule, in contrast to L-endoglin which is proangiogenic. Oncogene (2005) 24, 4450-4461. doi:10.1038/sj.onc.1208644 Published online 4 April 2005.

The hydrodynamics-based procedure for controlling the pharmacokinetics of gene medicines at whole body, organ and cellular levels

Hydrodynamics-based gene delivery, involving a large-volume and high-speed intravenous injection of naked plasmid DNA (pDNA), gives a significantly high level of transgene expression in vivo. This has attracted a lot of attention and has been used very frequently as an efficient, simple and convenient transfection method for laboratory animals. Until recently, however, little information has been published on the pharmacokinetics of the injected DNA molecules and of the detailed mechanisms underlying the efficient gene transfer. We and other groups have very recently demonstrated that the mechanism for the hydrodynamics-based gene transfer would involve, in part, the direct cytosolic delivery of pDNA through the cell membrane due to transiently enhanced permeability. Along with the findings in our series of studies, this article reviews the cumulative reports and other intriguing information on the controlled pharmacokinetics of naked pDNA in the hydrodynamics-based gene delivery. In addition, we describe various applications reported so far, as well as the current attempts and proposals to develop novel gene medicines for future gene therapy using the concept of the hydrodynamics-based procedure. Furthermore, the issues associated with the clinical feasibility of its seemingly invasive nature, which is probably the most common concern about this hydrodynamics-based procedure, are discussed along with its future prospects and challenges.

Endothelium-Targeted Gene and Cell-Based Therapies for Cardiovascular Disease

Most common cardiovascular diseases are accompanied by endothelial dysfunction. Because of its predominant role in the pathogenesis of cardiovascular disease, the vascular endothelium is an attractive therapeutic target. The identification of promoter sequences capable of rendering endothelial-specific transgene expression together with the recent development of vectors with enhanced tropism for endothelium may offer opportunities for the design of new strategies for modulation of endothelial function. Such strategies may be useful in the treatment of chronic diseases such as hypertension, atherosclerosis, and ischemic artery disease, as well as in acute myocardial infarction and during open heart surgery for prevention of ischemia and reperfusion (I/R)-induced injury. The recent identification of putative endothelial progenitor cells in peripheral blood may allow the design of autologous cell-based strategies for neovascularization of ischemic tissues and for the repair of injured blood vessels and bioengineering of vascular prosthesis. "Proof-of-concept" for some of these strategies has been established in animal models of cardiovascular disease. However the successful translation of these novel strategies into clinical application will require further developments in vector and delivery technologies. Further characterization of the processes involved in mobilization, migration, homing, and incorporation of endothelial progenitor cells into the target tissues is necessary, and the optimal conditions for therapeutic application of these cells need to be defined and standardized.

Highlights on endoglin (CD105): from basic findings towards clinical applications in human cancer

Antibody targeting of tumor-associated vasculature is a promising therapeutic approach in human cancer; however, a specific cell membrane marker for endothelial cells of tumor vasculature has not been discovered yet. Endoglin (CD105) is a cell-surface glycoprotein most recently identified as an optimal indicator of proliferation of human endothelial cells. The finding that CD105 is over-expressed on vascular endothelium in angiogenetic tissues has prompted several pre-clinical studies designed to get a deeper understanding on the role of CD105 in angiogenesis, and to evaluate the most appropriate clinical setting(s) to utilize CD105 as a therapeutic target. In this review, the foreseeable clinical applications of CD105 in human cancer are discussed.

Perfusion of99Tcm-labeled CD105 Mab into kidneys from patients with renal carcinoma suggests that CD105 is a promising vascular target

There is strong published and unpublished evidence that our CD105 Mab E9, which is highly reactive with angiogenic endothelial cells, could be a useful reagent to target the vasculature of solid tumors in man. Since Mab E9 does not cross-react with animal tissues, we undertook here to evaluate its localization using human kidney as an ex vivo model. Perfusion was performed through the renal artery of 99Tcm-labeled purified CD105 Mab in freshly excised kidneys from 7 patients with renal carcinoma. In all 7 cases, immunoscintigraphs showed the presence of well-defined radioactive hot spots, which matched the positions of the tumors as identified by presurgery MRI scans and subsequent histopathologic examination. Importantly, in one instance, where a presurgery MRI scan had identified only one tumor, immunoscintigraphs showed 2 distinct hot spots of radioactivity. The pathology report confirmed that the additional hot spot corresponded to a small secondary well-vascularized tumor. The implication of this finding is that the radiolabeled Mab, E9, may be of use in the detection of metastatic disease. That the labeling of tumors was specific was confirmed when prior perfusion of unlabeled mab E9 in 2 kidneys completely blocked the localization of 99Tcm-conjugated Mab E9. Radioactivity in samples of tumor and normal tissue taken from 7 kidneys was counted in a gamma counter. In all cases, there was a greater uptake of radioactivity in tumors compared with the corresponding normal kidneys. The median values, adjusted per gram wet weight, for 99Tcm were 14.8 times (range, 4.8-113.0) greater in kidney tumors than in normal kidney tissue (p < 0.007). Immunofluorescent staining of cryostat sections of tumor tissues in each of the 7 cases showed strong and uniform localization of Mab E9 in tumor microvessels. Interestingly, chimeric staining of endothelial cells (ECs) was seen in an occasional microvessel segment. That is, while most of the ECs lining a microvessel were strongly stained, an occasional EC was negative. This was not an artifact of staining. Unstained ECs may be nonangiogenic or apoptotic since CD105 is a proliferation/activation-associated antigen. Further investigations are warranted to establish the pharmacokinetics of 99Tcm-labeled CD105 antibody in vivo. This would enable us to determine whether an apparently highly successful ex vivo study has the potential for tumor imaging/therapeutic vascular targeting in patients with cancer.

Endoglin (CD105): a powerful therapeutic target on tumor-associated angiogenetic blood vessels

Among surface molecules expressed on endothelial cells, endoglin (CD105) is emerging as a prime vascular target for antiangiogenetic cancer therapy. CD105 is a cell membrane glycoprotein mainly expressed on endothelial cells and overexpressed on tumor-associated vascular endothelium, which functions as an accessory component of the transforming growth factor -beta receptor complex and is involved in vascular development and remodelling. Quantification of intratumoral microvessel density by CD105 staining and of circulating soluble CD105 has been suggested to have prognostic significance in selected neoplasias. In addition, the potential usefulness of CD105 in tumor imaging and antiangiogenetic therapy has been well documented utilizing different animal models.

Expression of the TGF-beta coreceptor endoglin in epidermal keratinocytes and its dual role in multistage mouse skin carcinogenesis

Endoglin is an integral membrane glycoprotein primarily expressed in the vascular endothelium, but also found on macrophages and stromal cells. It binds several members of the transforming growth factor (TGF)-beta family of growth factors and modulates TGF-beta(1)-dependent cellular responses. However, it lacks cytoplasmic signaling motifs and is considered as an auxiliary receptor for TGF-beta. We show here that endoglin is expressed in mouse and human epidermis and in skin appendages, such as hair follicles and sweat glands, as determined by immunohistochemistry. In normal interfollicular epidermis, endoglin was restricted to basal keratinocytes and absent in differentiating cells of suprabasal layers. Follicular expression of endoglin was high in hair bulb keratinocytes, but decreased in parts distal from the bulb. To address the role of endoglin in skin carcinogenesis in vivo, Endoglin heterozygous mice were subjected to long-term chemical carcinogenesis treatment. Reduction in endoglin had a dual effect during multistage carcinogenesis, by inhibiting the early appearance of benign papillomas, but increasing malignant progression to highly undifferentiated carcinomas. Our results are strikingly similar to those previously reported for transgenic mice overexpressing TGF-beta(1) in the epidermis. These data suggest that endoglin might attenuate TGF-beta(1) signaling in normal epidermis and interfere with progression of skin carcinogenesis.

CD105 prevents apoptosis in hypoxic endothelial cells

CD105, a marker of endothelial cells, is abundantly expressed in tissues undergoing angiogenesis and is a receptor for transforming growth factorbeta. The pivotal role of CD105 in the vascular system was demonstrated by the severe vascular defects that occur in CD105-knockout mice, but the exact mechanisms for CD105 regulation of vascular development have not been fully elucidated. In light of the function of CD105 and the importance of hypoxia in neovascularisation, we speculated that CD105 is involved in hypoxia-initiated angiogenesis. Using tissue-cultured human microvascular endothelial cells, we have investigated the effects of hypoxic stress on CD105 gene expression. Hypoxia induced a significant increase in membrane-bound and secreted CD105 protein levels. CD105 mRNA and promoter activity were also markedly elevated, the latter returning to the basal level after 16 hours of hypoxic stress. Hypoxia induced cell cycle arrest at the G0/G1 phases and massive cell apoptosis after 24 hours through a reduction in the Bcl-2 to Bax ratio, downregulation of Bcl-XL and Mcl-1, and upregulation of caspase-3 and caspase-8. The consequence of CD105 upregulation was revealed using an antisense approach and a TUNEL assay. Suppression of CD105 increased cell apoptosis under hypoxic stress in the absence of TGFbeta1. Furthermore, hypoxia and TGFbeta1 synergistically induced apoptosis in the CD105-deficient cells but not in the control cells. We conclude that hypoxia is a potent stimulus for CD105 gene expression in vascular endothelial cells, which in turn attenuates cell apoptosis and thus contributes to angiogenesis.

Targeting gene expression to endothelium in transgenic animals: a comparison of the human ICAM-2, PECAM-1 and endoglin promoters

It is highly likely that successful pig-to-human xenotransplantation of vascularized organs will require genetic modification of the donor pig, and in particular of donor vascular endothelium. Promoters are generally tested in transgenic mice before generating transgenic pigs. Several promoters have been used to drive endothelial cell-specific expression in mice but none have yet been tested in pigs. We compared the promoters of three human genes that are predominantly expressed in vascular endothelium: intercellular adhesion molecule 2 (ICAM-2), platelet endothelial cell adhesion molecule 1 (PECAM-1) and endoglin. Expression of human complement regulatory proteins (hCRPs), directed by each of the promoters in mice, was largely restricted to vascular endothelium and leukocyte subpopulations. However, expression from the PECAM-1 promoter was weak in liver and non-uniform in the small vessels of heart, kidney, and lung. Conversely, expression from the endoglin promoter was consistently strong in the small vessels of these organs but was absent in larger vessels. The ICAM-2 promoter, which produced strong and uniform endothelial expression in all organs examined, was therefore used to generate hCRP transgenic pigs. Leukocytes from 57 pigs containing at least one intact transgene were tested for transgene expression by flow cytometry. Forty-seven of these transgenic pigs were further analyzed by immunohistochemical staining of liver biopsies, and 18 by staining of heart and kidney sections. Only two of the pigs showed expression, which appeared to be restricted to vascular endothelium in heart and kidney but was markedly weaker than in transgenic mice produced with the same batch of DNA. Thus, in this case, promoter performance in mice and pigs was not equivalent. The weak expression driven by the human ICAM-2 promoter in pigs relative to mice suggests the need for additional regulatory elements to achieve species-specific gene expression in pigs.

Transcriptional Targeting in Cancer Gene Therapy

Cancer gene therapy has been one of the most exciting areas of therapeutic research in the past decade. In this review, we discuss strategies to restrict transcription of transgenes to tumour cells. A range of promoters which are tissue-specific, tumour-specific, or inducible by exogenous agents are presented. Transcriptional targeting should prevent normal tissue toxicities associated with other cancer treatments, such as radiation and chemotherapy. In addition, the specificity of these strategies should provide improved targeting of metastatic tumours following systemic gene delivery. Rapid progress in the ability to specifically control transgenes will allow systemic gene delivery for cancer therapy to become a real possibility in the near future.

The genetics of cerebrovascular malformations

Certain rare familial or congenital syndromes include cerebrovascular malformations among their constellations of abnormalities. In addition, recognition of familial clustering in a subset of patients with cerebrovascular malformations has led to studies investigating the underlying genetic basis for these lesions. Genetic defects have been identified that cause familial cerebral cavernous malformations and hereditary hemorrhagic telangiectasia, a syndrome that features cerebral arteriovenous malformations. In addition to enhancing presymptomatic screening, identification of the responsible genes may result in a better understanding of the pathogenesis of these lesions, and ultimately, in novel treatments.