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Inhibition of tumor angiogenesis by cell-permeable dominant negative SOX18 mutants
Medical Hypotheses (2008) 70, 880–882
http://intl.elsevierhealth.com/journals/mehy
Inhibition of tumor angiogenesis by cell-permeable dominant negative SOX18 mutants Min Luo a, Xiao-Tong Guo b, Wei Yang c, Liang-Qi Liu c, Li-Wen Li Xiao-Yan Xin a,*
c,*
,
a
Department of Obstetrics and Gynecology, Xijing Hospital, The Fourth Military Medical University, No.17 Changle Western Road, Xi’an 710032, PR China b Center of Clinical Oncology, The Fourth Military Medical University, No.17 Changle Western Road, Xi’an 710032, PR China c Department of Bioscience, College of Life Sciences, Northwest University, No.229 North Taibai Road, Xi’an 710069, PR China Received 10 July 2007; accepted 12 July 2007
Summary Angiogenesis play a key roles in tumor growth, invasion and metastasis, and has become an attractive target for anticancer drug development. Though a number of anti-angiogenic agents had entered clinical trials, few of them could reproduce the spectacular results in cancer patients as that had been seen in pre-clinical tumor models. Therefore, exploring novel anti-angiogenic agents is highly deserved. SOX18, a member of the Sry-related HMG box-containing family of transcription factors, is expressed transiently in endothelial cells during the development of blood vessels. And mutations resulting in expression of dominant negative SOX18 have been shown to severely impair the vascular development. Recent research demonstrated that SOX18 is expressed during the initial steps of tumor vascularization and involved in regulation of the expression of the VEGF receptor Flk-1 and the vascular cell adhesion molecule-1 (VCAM-1). Moreover, allograft tumor growth in mice heterozygous for RaOp (RaOp mice) which express a dominant negative mutant form of SOX18 (SOX18RaOp) that does not interact effectively with the endothelial partner protein MEF2C, was dramatically slower than that of wild-type mice. In this article, we postulate that recombinant cell-permeable dominant negative SOX18 mutants, prepared by fusion with protein transduction domains, would inhibit tumor angiogenesis with high efficiency by impairing endothelial tube formation. If the hypothesis was proved to be practical, the fusion proteins would show promise as single anti-angiogenic agents in cancer therapy. c 2007 Elsevier Ltd. All rights reserved.
Introduction * Corresponding authors. Tel./fax: +86 29 88302634(Li-Wen Li); tel./fax: +86 29 84775387(Xiao-Yan Xin). E-mail addresses: [email protected] (L.-W. Li), xinxy. [email protected] (X.-Y. Xin).
Angiogenesis induced by pro-angiogenic growth factors produced by cancer cells, including VEGF and bFGF, has been recognized as a critical factor in solid tumor growth, progression, development of
0306-9877/$ - see front matter c 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mehy.2007.07.024
Inhibition of tumor angiogenesis by cell-permeable dominant negative SOX18 mutants distant metastases and cancer-related death [1–5]. In the absence of tumor angiogenesis, the tumor enters a state of dormancy characterized by a balance between cell proliferation and apoptosis and the tumor mass stabilizes at a volume of a few cubic millimeters [6]. Anti-angiogenic therapy has been proved to be an efficient strategy to inducing tumor regression in pre-clinical tumor models. And a number of anti-angiogenic agents had entered clinical trials, but these trials generally failed to confirm in patients the spectacular results seen in mice. Till now, only tumor necrosis factor-a and Avastin have been approved as vascular disrupting and antiangiogenic agents, respectively, for the treatment of human cancers [7,8]. Therefore, exploring novel anti-angiogenic agents targeting the key steps of tumor angiogenesis is highly deserved.
881
and hair follicle anomalies that mimic the phenotype of human hypotrichosis-lymphedema -telangiectas syndrome [13]. Several researches demonstrated that SOX18 is involved in transcriptional up-regulation of Flk-1 and VCAM-1. SOX18 mRNA expression was detected in capillaries within the granulation tissue of fullthickness skin wounds and showed an identical pattern of distribution to Flk-1 mRNA expression in endothelial cells [12]. VCAM-1 is an important adhesion molecular that expressed on endothelial cells and has a critical role in endothelial activation. Both SOX18 and VCAM-1 have very similar spatiotemporal patterns of expression. Moreover, SOX18 has been shown to directly target and transactivate the expression of VCAM-1 [15]. These results strongly imply that SOX18 represents a transcription factor involved in the induction of angiogenesis.
Presentation of the hypothesis SOX18, which is expressed transiently in endothelial cells during the development of all blood vessels in the mouse embryo [9], is a member of the Sry-related HMG box-containing family of transcription factors [10,11]. SOX18 binds to the common SOX target sequence (A/T)ACAA(A/T)G and contains a discrete transactivation domain. And SOX18 participates in regulation of the expression of vascular cell adhesion molecule-1 (VCAM-1) and the VEGF receptor Flk1 through a mechanism of interacting with the muscle and endothelial transcription factor MEF2C [12]. Mutations resulting in expression of dominant negative SOX18 have been shown to severely impair the vascular development [13]. In this article, we postulate that cell-permeable dominant negative SOX18 mutants would inhibit tumor angiogenesis by impairing endothelial tube formation.
Theoretic fundamental of the hypothesis SOX18 mutations and vascular defects The importance of SOX18 for vascular development had been revealed by the vascular defects caused by SOX18 mutations both in humans and in mice. In humans, SOX18 mutations result in hypotrichosislymphedema-telangiectasia syndrome, characterized by localized vascular leakage and edema [14]. Also, vascular and coat anomalies in ragged (Ra) mutant mice result from mutations in SOX18. Among the four allelic forms of Ra, the most severe phenotype is conferred by ragged-opossum (RaOp). Mice heterozygous for this allele show severe vascular
Inhibition of tumor growth by dominant negative SOX18 mutants SOX7, SOX17 and SOX18 constitute group F of the SOX family of HMG box transcription factor genes. It has been demonstrated that the all the three members are co-expressed during the initial steps of tumor vascularization. And allograft tumor growth in mice heterozygous for RaOp, which express a dominant negative mutant form of SOX18 (SOX18RaOp) that does not interact effectively with the endothelial partner protein MEF2C, was dramatically slower than that of wild-type mice and had fewer intratumoral CD31- and NG2expressing vessels [16]. Meanwhile, human umbilical vein endothelial cells (HUVECs) transfected with SOX18RaOp showed a dramatic reduction in tube formation compared with that cells transfected with empty vector [16]. Recent research reveals that loss of function of SOX18 by gene targeting in mice does not result in a vascular phenotype [17], but SOX17(+/ )– SOX18( / ) double mutant mice show reduced neovascularization in the liver sinusoids and kidney outer medulla vasa recta [18]. And in vitro angiogenesis assays showed that the SOX17(+/ )–SOX18( / ) endothelial cells were defective in endothelial sprouting and remodeling of the vasculature [18]. In contrast, RaOp mice show an impaired phenotype of vascular development due to the failure of SOX18RaOp in activation of angiogenic target genes [13,19]. These findings therefore strongly suggest that SOX17 and SOX18 are genetic redundant in developing vasculature and the SOX18RaOp mutant protein could exert its dominant negative effect on both SOX17 and SOX18.
882
Intracellular delivery of dominant negative SOX18 mutants by fusion with protein transduction domains Belonging to the SOX family of HMG box transcription factors, SOX17 and SOX18 regulate the expression of angiogenic genes in cellular nucleus. And the inability of dominant negative SOX18 mutant proteins to across cellular membrane is the main obstacle to serve as inhibitors to antagonize the biological functions of SOX17 and SOX18. With the development and rapid progress of protein transduction technique [20–23], recombinant cell-permeable dominant negative SOX18 mutants could be easily prepared by fusion with protein transduction domains from HIV-1 TAT and HSV-1 VP22 or PTD4 [24,25]. In conclusion, we postulate that recombinant cell-permeable dominant negative SOX18 mutants, prepared by fusion with protein transduction domains, would achieve their dominant negative effects on both SOX17 and SOX18, thereby inducing tumor regression by inhibiting tumor angiogenesis with high efficiency through impairing endothelial tube formation.
Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 30400450 and No. 30600750).
References [1] Sun S, Schiller JH. Angiogenesis inhibitors in the treatment of lung cancer. Crit Rev Oncol Hematol 2007;62(2):93–104. [2] Carmeliet P. VEGF as a key mediator of angiogenesis in cancer. Oncology 2005;69(Suppl. 3):4–10. [3] Andreasson P, Carlsson R. Targeting angiogenesis with antibodies for the treatment of cancer. IDrugs 2005;8(9):730–3. [4] Tortora G, Melisi D, Ciardiello F. Angiogenesis: a target for cancer therapy. Curr Pharm Des 2004;10(1):11–26. [5] Bamias A, Dimopoulos MA. Angiogenesis in human cancer: implications in cancer therapy. Eur J Intern Med 2003;14(8):459–69. [6] Holmgren L, O’Reilly MS, Folkman J. Dormancy of micrometastases: balanced proliferation and apoptosis in the presence of angiogenesis suppression. Nat Med 1995;1(2): 149–53. [7] Ruegg C, Hasmim M, Lejeune FJ, Alghisi GC. Antiangiogenic peptides and proteins: from experimental tools to clinical drugs. Biochim Biophys Acta 2006;1765(2):155–77. [8] Vokes E, Herbst R, Sandler A. Angiogenesis inhibition in the treatment of lung cancer. Clin Adv Hematol Oncol 2006;4(11 Suppl. 23):1–12.
Luo et al. [9] Downes M, Koopman P. SOX18 and the transcriptional regulation of blood vessel development. Trends Cardiovasc Med 2001;11(8):318–24. [10] Dunn TL, Mynett-Johnson L, Wright EM, Hosking BM, Koopman PA, Muscat GE. Sequence and expression of Sox18 encoding a new HMG-box transcription factor. Gene 1995;161(2):223–5. [11] Azuma T, Seki N, Yoshikawa T, Saito T, Masuho Y, Muramatsu M. cDNA cloning, tissue expression, and chromosome mapping of human homolog of SOX18. J Hum Genet 2000;45(3):192–5. [12] Darby IA, Bisucci T, Raghoenath S, Olsson J, Muscat GE, Koopman P. Sox18 is transiently expressed during angiogenesis in granulation tissue of skin wounds with an identical expression pattern to Flk-1 mRNA. Lab Invest 2001;81(7):937–43. [13] Pennisi D, Gardner J, Chambers D, Hosking B, Peters J, Muscat G, et al. Mutations in Sox18 underlie cardiovascular and hair follicle defects in ragged mice. Nat Genet 2000;24(4):434–7. [14] Irrthum A, Devriendt K, Chitayat D, Matthijs G, Glade C, Steijlen PM, et al. Mutations in the transcription factor gene SOX18 underlie recessive and dominant forms of hypotrichosis-lymphedema-telangiectasia. Am J Hum Genet 2003;72(6):1470–8. [15] Hosking BM, Wang SC, Downes M, Koopman P, Muscat GE. The VCAM-1 gene that encodes the vascular cell adhesion molecule is a target of the Sry-related high mobility group box gene, Sox18. J Biol Chem 2004;279(7):5314–22. [16] Young N, Hahn CN, Poh A, Dong C, Wilhelm D, Olsson J, et al. Effect of disrupted SOX18 transcription factor function on tumor growth, vascularization, and endothelial development. J Natl Cancer Inst 2006;98(15):1060–7. [17] Pennisi D, Bowles J, Nagy A, Muscat G, Koopman P. Mice null for sox18 are viable and display a mild coat defect. Mol Cell Biol 2000;20(24):9331–6. [18] Matsui T, Kanai-Azuma M, Hara K, Matoba S, Hiramatsu R, Kawakami H, et al. Redundant roles of Sox17 and Sox18 in postnatal angiogenesis in mice. J Cell Sci 2006;119(Pt 17): 3513–26. [19] Hosking BM, Wang SC, Chen SL, Penning S, Koopman P, Muscat GE. SOX18 directly interacts with MEF2C in endothelial cells. Biochem Biophys Res Commun 2001;287(2): 493–500. [20] Shi M, Guo XT, Shu MG, Li LW. Enhancing tumor radiosensitivity by intracellular delivery of survivin antagonists. Med Hypotheses 2007;68(5):1056–8. [21] Shi M, Guo XT, Shu MG, Chen FL, Li LW. Cell-permeable hypoxia-inducible factor-1 (HIF-1) antagonists function as tumor radiosensitizers. Med Hypotheses 2007;69(1):33–5. [22] Shu MG, Guo XT, Zhen HN, Han Y, Chen FL, Li LW, et al. Enhancing skin flap survival by a cell-permeable wild-type survivin. Med Hypotheses 2007. doi:10.1016/j.mehy.2006. 11.055. [23] Guo XT, Shi M, Shu MG, Xue Y, Li LW, Liu WC. Ex vivo expanding hematopoietic stem cells by intracellular delivery of Cdx4 fusion proteins. Med Hypotheses 2007;68(6):1389–91. [24] Chauhan A, Tikoo A, Kapur AK, Singh M. The taming of the cell penetrating domain of the HIV Tat: myths and realities. J Control Release 2007;117(2):148–62. [25] Noguchi H, Matsumoto S. Protein transduction technology: a novel therapeutic perspective. Acta Med Okayama 2006;60(1):1–11.
http://intl.elsevierhealth.com/journals/mehy
Inhibition of tumor angiogenesis by cell-permeable dominant negative SOX18 mutants Min Luo a, Xiao-Tong Guo b, Wei Yang c, Liang-Qi Liu c, Li-Wen Li Xiao-Yan Xin a,*
c,*
,
a
Department of Obstetrics and Gynecology, Xijing Hospital, The Fourth Military Medical University, No.17 Changle Western Road, Xi’an 710032, PR China b Center of Clinical Oncology, The Fourth Military Medical University, No.17 Changle Western Road, Xi’an 710032, PR China c Department of Bioscience, College of Life Sciences, Northwest University, No.229 North Taibai Road, Xi’an 710069, PR China Received 10 July 2007; accepted 12 July 2007
Summary Angiogenesis play a key roles in tumor growth, invasion and metastasis, and has become an attractive target for anticancer drug development. Though a number of anti-angiogenic agents had entered clinical trials, few of them could reproduce the spectacular results in cancer patients as that had been seen in pre-clinical tumor models. Therefore, exploring novel anti-angiogenic agents is highly deserved. SOX18, a member of the Sry-related HMG box-containing family of transcription factors, is expressed transiently in endothelial cells during the development of blood vessels. And mutations resulting in expression of dominant negative SOX18 have been shown to severely impair the vascular development. Recent research demonstrated that SOX18 is expressed during the initial steps of tumor vascularization and involved in regulation of the expression of the VEGF receptor Flk-1 and the vascular cell adhesion molecule-1 (VCAM-1). Moreover, allograft tumor growth in mice heterozygous for RaOp (RaOp mice) which express a dominant negative mutant form of SOX18 (SOX18RaOp) that does not interact effectively with the endothelial partner protein MEF2C, was dramatically slower than that of wild-type mice. In this article, we postulate that recombinant cell-permeable dominant negative SOX18 mutants, prepared by fusion with protein transduction domains, would inhibit tumor angiogenesis with high efficiency by impairing endothelial tube formation. If the hypothesis was proved to be practical, the fusion proteins would show promise as single anti-angiogenic agents in cancer therapy. c 2007 Elsevier Ltd. All rights reserved.
Introduction * Corresponding authors. Tel./fax: +86 29 88302634(Li-Wen Li); tel./fax: +86 29 84775387(Xiao-Yan Xin). E-mail addresses: [email protected] (L.-W. Li), xinxy. [email protected] (X.-Y. Xin).
Angiogenesis induced by pro-angiogenic growth factors produced by cancer cells, including VEGF and bFGF, has been recognized as a critical factor in solid tumor growth, progression, development of
0306-9877/$ - see front matter c 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mehy.2007.07.024
Inhibition of tumor angiogenesis by cell-permeable dominant negative SOX18 mutants distant metastases and cancer-related death [1–5]. In the absence of tumor angiogenesis, the tumor enters a state of dormancy characterized by a balance between cell proliferation and apoptosis and the tumor mass stabilizes at a volume of a few cubic millimeters [6]. Anti-angiogenic therapy has been proved to be an efficient strategy to inducing tumor regression in pre-clinical tumor models. And a number of anti-angiogenic agents had entered clinical trials, but these trials generally failed to confirm in patients the spectacular results seen in mice. Till now, only tumor necrosis factor-a and Avastin have been approved as vascular disrupting and antiangiogenic agents, respectively, for the treatment of human cancers [7,8]. Therefore, exploring novel anti-angiogenic agents targeting the key steps of tumor angiogenesis is highly deserved.
881
and hair follicle anomalies that mimic the phenotype of human hypotrichosis-lymphedema -telangiectas syndrome [13]. Several researches demonstrated that SOX18 is involved in transcriptional up-regulation of Flk-1 and VCAM-1. SOX18 mRNA expression was detected in capillaries within the granulation tissue of fullthickness skin wounds and showed an identical pattern of distribution to Flk-1 mRNA expression in endothelial cells [12]. VCAM-1 is an important adhesion molecular that expressed on endothelial cells and has a critical role in endothelial activation. Both SOX18 and VCAM-1 have very similar spatiotemporal patterns of expression. Moreover, SOX18 has been shown to directly target and transactivate the expression of VCAM-1 [15]. These results strongly imply that SOX18 represents a transcription factor involved in the induction of angiogenesis.
Presentation of the hypothesis SOX18, which is expressed transiently in endothelial cells during the development of all blood vessels in the mouse embryo [9], is a member of the Sry-related HMG box-containing family of transcription factors [10,11]. SOX18 binds to the common SOX target sequence (A/T)ACAA(A/T)G and contains a discrete transactivation domain. And SOX18 participates in regulation of the expression of vascular cell adhesion molecule-1 (VCAM-1) and the VEGF receptor Flk1 through a mechanism of interacting with the muscle and endothelial transcription factor MEF2C [12]. Mutations resulting in expression of dominant negative SOX18 have been shown to severely impair the vascular development [13]. In this article, we postulate that cell-permeable dominant negative SOX18 mutants would inhibit tumor angiogenesis by impairing endothelial tube formation.
Theoretic fundamental of the hypothesis SOX18 mutations and vascular defects The importance of SOX18 for vascular development had been revealed by the vascular defects caused by SOX18 mutations both in humans and in mice. In humans, SOX18 mutations result in hypotrichosislymphedema-telangiectasia syndrome, characterized by localized vascular leakage and edema [14]. Also, vascular and coat anomalies in ragged (Ra) mutant mice result from mutations in SOX18. Among the four allelic forms of Ra, the most severe phenotype is conferred by ragged-opossum (RaOp). Mice heterozygous for this allele show severe vascular
Inhibition of tumor growth by dominant negative SOX18 mutants SOX7, SOX17 and SOX18 constitute group F of the SOX family of HMG box transcription factor genes. It has been demonstrated that the all the three members are co-expressed during the initial steps of tumor vascularization. And allograft tumor growth in mice heterozygous for RaOp, which express a dominant negative mutant form of SOX18 (SOX18RaOp) that does not interact effectively with the endothelial partner protein MEF2C, was dramatically slower than that of wild-type mice and had fewer intratumoral CD31- and NG2expressing vessels [16]. Meanwhile, human umbilical vein endothelial cells (HUVECs) transfected with SOX18RaOp showed a dramatic reduction in tube formation compared with that cells transfected with empty vector [16]. Recent research reveals that loss of function of SOX18 by gene targeting in mice does not result in a vascular phenotype [17], but SOX17(+/ )– SOX18( / ) double mutant mice show reduced neovascularization in the liver sinusoids and kidney outer medulla vasa recta [18]. And in vitro angiogenesis assays showed that the SOX17(+/ )–SOX18( / ) endothelial cells were defective in endothelial sprouting and remodeling of the vasculature [18]. In contrast, RaOp mice show an impaired phenotype of vascular development due to the failure of SOX18RaOp in activation of angiogenic target genes [13,19]. These findings therefore strongly suggest that SOX17 and SOX18 are genetic redundant in developing vasculature and the SOX18RaOp mutant protein could exert its dominant negative effect on both SOX17 and SOX18.
882
Intracellular delivery of dominant negative SOX18 mutants by fusion with protein transduction domains Belonging to the SOX family of HMG box transcription factors, SOX17 and SOX18 regulate the expression of angiogenic genes in cellular nucleus. And the inability of dominant negative SOX18 mutant proteins to across cellular membrane is the main obstacle to serve as inhibitors to antagonize the biological functions of SOX17 and SOX18. With the development and rapid progress of protein transduction technique [20–23], recombinant cell-permeable dominant negative SOX18 mutants could be easily prepared by fusion with protein transduction domains from HIV-1 TAT and HSV-1 VP22 or PTD4 [24,25]. In conclusion, we postulate that recombinant cell-permeable dominant negative SOX18 mutants, prepared by fusion with protein transduction domains, would achieve their dominant negative effects on both SOX17 and SOX18, thereby inducing tumor regression by inhibiting tumor angiogenesis with high efficiency through impairing endothelial tube formation.
Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 30400450 and No. 30600750).
References [1] Sun S, Schiller JH. Angiogenesis inhibitors in the treatment of lung cancer. Crit Rev Oncol Hematol 2007;62(2):93–104. [2] Carmeliet P. VEGF as a key mediator of angiogenesis in cancer. Oncology 2005;69(Suppl. 3):4–10. [3] Andreasson P, Carlsson R. Targeting angiogenesis with antibodies for the treatment of cancer. IDrugs 2005;8(9):730–3. [4] Tortora G, Melisi D, Ciardiello F. Angiogenesis: a target for cancer therapy. Curr Pharm Des 2004;10(1):11–26. [5] Bamias A, Dimopoulos MA. Angiogenesis in human cancer: implications in cancer therapy. Eur J Intern Med 2003;14(8):459–69. [6] Holmgren L, O’Reilly MS, Folkman J. Dormancy of micrometastases: balanced proliferation and apoptosis in the presence of angiogenesis suppression. Nat Med 1995;1(2): 149–53. [7] Ruegg C, Hasmim M, Lejeune FJ, Alghisi GC. Antiangiogenic peptides and proteins: from experimental tools to clinical drugs. Biochim Biophys Acta 2006;1765(2):155–77. [8] Vokes E, Herbst R, Sandler A. Angiogenesis inhibition in the treatment of lung cancer. Clin Adv Hematol Oncol 2006;4(11 Suppl. 23):1–12.
Luo et al. [9] Downes M, Koopman P. SOX18 and the transcriptional regulation of blood vessel development. Trends Cardiovasc Med 2001;11(8):318–24. [10] Dunn TL, Mynett-Johnson L, Wright EM, Hosking BM, Koopman PA, Muscat GE. Sequence and expression of Sox18 encoding a new HMG-box transcription factor. Gene 1995;161(2):223–5. [11] Azuma T, Seki N, Yoshikawa T, Saito T, Masuho Y, Muramatsu M. cDNA cloning, tissue expression, and chromosome mapping of human homolog of SOX18. J Hum Genet 2000;45(3):192–5. [12] Darby IA, Bisucci T, Raghoenath S, Olsson J, Muscat GE, Koopman P. Sox18 is transiently expressed during angiogenesis in granulation tissue of skin wounds with an identical expression pattern to Flk-1 mRNA. Lab Invest 2001;81(7):937–43. [13] Pennisi D, Gardner J, Chambers D, Hosking B, Peters J, Muscat G, et al. Mutations in Sox18 underlie cardiovascular and hair follicle defects in ragged mice. Nat Genet 2000;24(4):434–7. [14] Irrthum A, Devriendt K, Chitayat D, Matthijs G, Glade C, Steijlen PM, et al. Mutations in the transcription factor gene SOX18 underlie recessive and dominant forms of hypotrichosis-lymphedema-telangiectasia. Am J Hum Genet 2003;72(6):1470–8. [15] Hosking BM, Wang SC, Downes M, Koopman P, Muscat GE. The VCAM-1 gene that encodes the vascular cell adhesion molecule is a target of the Sry-related high mobility group box gene, Sox18. J Biol Chem 2004;279(7):5314–22. [16] Young N, Hahn CN, Poh A, Dong C, Wilhelm D, Olsson J, et al. Effect of disrupted SOX18 transcription factor function on tumor growth, vascularization, and endothelial development. J Natl Cancer Inst 2006;98(15):1060–7. [17] Pennisi D, Bowles J, Nagy A, Muscat G, Koopman P. Mice null for sox18 are viable and display a mild coat defect. Mol Cell Biol 2000;20(24):9331–6. [18] Matsui T, Kanai-Azuma M, Hara K, Matoba S, Hiramatsu R, Kawakami H, et al. Redundant roles of Sox17 and Sox18 in postnatal angiogenesis in mice. J Cell Sci 2006;119(Pt 17): 3513–26. [19] Hosking BM, Wang SC, Chen SL, Penning S, Koopman P, Muscat GE. SOX18 directly interacts with MEF2C in endothelial cells. Biochem Biophys Res Commun 2001;287(2): 493–500. [20] Shi M, Guo XT, Shu MG, Li LW. Enhancing tumor radiosensitivity by intracellular delivery of survivin antagonists. Med Hypotheses 2007;68(5):1056–8. [21] Shi M, Guo XT, Shu MG, Chen FL, Li LW. Cell-permeable hypoxia-inducible factor-1 (HIF-1) antagonists function as tumor radiosensitizers. Med Hypotheses 2007;69(1):33–5. [22] Shu MG, Guo XT, Zhen HN, Han Y, Chen FL, Li LW, et al. Enhancing skin flap survival by a cell-permeable wild-type survivin. Med Hypotheses 2007. doi:10.1016/j.mehy.2006. 11.055. [23] Guo XT, Shi M, Shu MG, Xue Y, Li LW, Liu WC. Ex vivo expanding hematopoietic stem cells by intracellular delivery of Cdx4 fusion proteins. Med Hypotheses 2007;68(6):1389–91. [24] Chauhan A, Tikoo A, Kapur AK, Singh M. The taming of the cell penetrating domain of the HIV Tat: myths and realities. J Control Release 2007;117(2):148–62. [25] Noguchi H, Matsumoto S. Protein transduction technology: a novel therapeutic perspective. Acta Med Okayama 2006;60(1):1–11.