European Review for Medical and Pharmacological Sciences 2017; 21: 1335-1340 Regenerative in cardiovascular diseases – an update Y. CAO1, P.-Y. ZHANG2

1Department of , Xuzhou Hospital of Traditional Chinese Medicine, Xuzhou, Jiangsu, P.R. China 2Department of Cardiology, Xuzhou Central Hospital, The Affiliated Xuzhou Hospital of Medical College of Southeast University, Xuzhou, Jiangsu, P.R. China

Abstract. – The unique ability of regener- od vessels in the affected tissue. Although early ative medicine to differentiate into any cell of results from these trials were promising, and the the three germ layers (endoderm, ectoderm, treatment using these factors were found to be and mesoderm) is of immense clinical impor- safe but there were no lasting clinical affects3. tance. They have an unique capacity for unlim- This might be due to insufficient doses or a very ited self-renewal. Furthermore, pluripotent stem large amount of protein might be needed to have cells (PSC), including human embryonic (hESC) a significant effect, something that is unrealistic and induced pluripotent stem cells (hiPSCs), hold great potential as an unlimited source of in a clinical setting. functional as well as transplantable cells appli- cations. Specifically, in the context of cardio- vascular and ischemic diseases, it is believed Gene that hESC-derived endothelial cells (hESC- ECs) has a unique ability to admi- could be used to stimulate angiogenesis or vas- nister some different angiogenic factors. Ade- culogenesis in ischemic tissues, thereby restor- noviral vectors are used on a large scale during ing blood supply to the affected area. The pres- ent review article is focused on the current as- gene therapy studies; however, some high profile pects of the regenerative medicine during cardi- serious adverse events during clinical trials have 4 ac disorders. led to major questions about the safety . Another strategy, which has been investigated in the set- Key Words ting of PAD and CLI, is the administration of Regenerative medicine, Cardiac disorders, Stem naked plasmid DNA encoding angiogenic factors. cells. These primarily aimed to increase the duration of transgene expression. Despite this, expression of the transgene remained low, as plasmids have low transfection efficiency5. Introduction The first case of administration of naked pla- smid DNA, encoding for VEGF, was reported Regenerative medicine incorporates all strate- almost 20 years ago6, but without any definitive gies for the repair, replacement or regrowth of da- conclusions. Since then, numerous clinical and maged/destroyed tissues and organs. It involves pre-clinical studies have been performed, using the use of biomaterials, human genes, proteins a variety of VEGF family members and viral or cells. In peripheral arterial disease (PAD) and vectors. A phase I clinical trial confirmed the critical limb ischemia (CLI), early regenerative safety of adenoviral-mediated delivery of VEGF medicine strategies focused on the administration cDNA to patients with PAD, although no conclu- of angiogenesis-inducing growth factors, such as sions could be drawn about efficacy7. In 2012, a vascular endothelial growth factor (VEGF) or 10-year safety follow-up on patients treated with fibroblast growth factor (FGF)1,2. Angiogenesis an injection of either an adenoviral vector con- is defined as the migration and proliferation of taining a VEGF-encoding plasmid, or a plasmid/ differentiated endothelial cells (ECs) to allow liposome combination, directly into an ischemic sprouting of new capillary branches from existing lower limb, confirmed the long-term safety of this vessels to stimulate the development of new blo- approach8. Currently, there are 22 known FGF

Corresponding Author: Peiying Zhang, MD; e-mail: [email protected] 1335 Y. Cao, P.-Y. Zhang ligands involved in angiogenesis9, and cardiova- have also been reported, for example, adipose scular gene therapy trials have been performed tissue and skeletal muscle13. These cells are de- using some these isoforms. Trials with Sendai scribed as multipotent, non-hematopoietic and virus expressing human FGF-2, showed a signi- fibroblast-like, possessing the ability to diffe- ficant improvements in limb function10. Some rentiate into some cell types, including bone, gene therapy clinical trials, using other angioge- fat, and cartilage14. Although their cell surface nic factors, (such as hypoxia-inducible factor-1α marker profile has been debated15, their broad (HIF-1α), hepatocyte growth factor (HGF) and potential in the field of regenerative medicine is developmentally regulated endothelial locus-1), widely accepted16. In preclinical models of PAD have been performed in patients suffering from and CLI, treatment of animals with ECs from IC and CLI11,12. Although, no significant safety MSCs derived from numerous sources, perfu- issues were reported in any of these trials, and sion rates of ischemic limbs were significantly the results were mixed. Overall, strategies using higher in in treated animals as compared to angiogenic growth factors to treat PAD and CLI control animals17. However, in clinical trials, the have been largely unsuccessful. results have been varied, and many trials have shown no significant improvement in secondary outcomes18. Cell Therapies Currently, there are many clinical trials invol- ving therapies in the treatment of CLI Pluripotent Stem Cells but the results have been varied. The majority These cells have unique abilities as they could of these studies involved the transplantation or be differentiated into any of the three viz. endo- injection of autologous cell populations broadly derm, ectoderm, and mesoderm. Further, they called as adult stem cells, including mesenchymal have the capacity for unlimited self- renewal. stem cells (MSC), bone marrow mononuclear HPSCs hold great potential for some diverse cells (BM-MNC) and endothelial progenitor cells scientific and clinical applications, including (EPC). Autologous cell types have an advantage pharmacology, toxicology, cellular-based thera- of the decline in the risk of immune rejection. pies and regenerative medicine. Currently, there MSCs originate in the stromal compartment are 2 different types of pluripotent stem cells, of the bone marrow, where they make up only a which have been described – embryonic stem small fraction of the total nucleated cells. Other cells (ESCs) and induced pluripotent stem cells sources of cells with mesenchymal potential (iPSCs) (Table I).

Table I. Regenerative medicine prospects. Regenerative medicine Human Pluripotent Stem Cells (HPSCs) hold great potential for some diverse scientific and clinical applications, including regenerative medicine Types:

1. Human embryonic stem cells (HESCs) 2. Induced pluripotent stem cells (iPSCs)

Pros Pros • The cells were obtained from the • It involves the use of autologous programmed cells. inner cell mass (ICM) of the blastocyst stage embryo • It has potential to provide patient-and disease-specific • Regarded as the ‘gold standard’ autologous cells for transplantation • The method of iPSC derivation does not require the use of human embryos, so ethically more appropriate Cons Cons • Heterogeneity between these cell lines • The mechanisms governing pluripotency is complex • Ethical concerns, like fertilized embryos destruction and is poorly understood • The potential of teratoma formation is high • Highly efficient, 3D EB-based culture systems are difficult • Chances of rejection by the host are high to scale up to produce clinically relevant numbers of cells • Expensive

1336 Regenerative medicine in cardiovascular diseases – an update

Human Embryonic Stem Cells surface antigens (SSEA3, SSEA4, TRA1-60, and The human embryonic stem cells (hESCs) are TRA1-81) and gene expression (OCT4, NANOG, important tools in the fields of developmental bio- SOX2). Further, they were able to produce cells logy and regenerative medicine19. In 1998 Thom- from all 3-germ layers both in vitro and in vivo son20 used human embryos, produced by in vitro teratoma formation assays. Although hESCs are fertilization (IVF), to isolate and culture the first still considered the ‘gold standard’ regarding de- human-derived ESCs. The cells were obtained velopmental biology, these cells have the potential from the inner cell mass (ICM) of the blastocyst to provide patient-and disease-specific autologous stage embryo, isolated by immune- and cells for transplantation. Additionally, iPSCs could cultured on irradiated mouse embryonic fibro- be used as excellent in vitro models of disease27. blasts (MEF). Cells were grown as colonies and Moreover, as the method of iPSC derivation does individual undifferentiated colonies were manual- not require the use of human embryos, using these ly selected and dissociated into clumps until the cells circumvents the ethical issues faced with the cell line was established. These cells, generated use of hESCs. in 1998, are still used in research today and are widely regarded as the ‘gold standard’ hESC lines. Regulation of Pluripotency Moreover, efforts are made to advance the culture The mechanisms governing pluripotency in conditions to a clinically acceptable standard. In these cells are still relatively poorly understood. addition to the original 5 lines, a large number of Thus far, research has identified a group of key other hESC lines have now been generated. Howe- transcription factors, playing essential roles in ver, there is considerable heterogeneity between maintenance and control of pluripotency – OCT4, these cell lines21. Further, there are numerous SOX2, and NANOG. Indeed, these factors are caveats that must be addressed before such strate- used in the reprogramming of somatic cells to a gies could be considered for routine use clinically. pluripotent state. Much of the original work was Firstly, there are some ethical concerns, like fertili- performed in mESCs, before the derivation of zed embryos destruction22. Secondly, the presence hESCs in 1998, with many of the mechanisms of undifferentiated cells increases the potential of conserved between the two systems. teratoma formation. Another caveat is their immu- OCT4 has a key role in the regulation as well nogenicity. In a similar way to tissue and organs, as in the establishment of ICM pluripotency28. transplantation of hESCs is an allogeneic process, In hESCs in vitro, knockdown of OCT4 resul- and rejection by the host is a very real prospect. ts in rapid changes in morphology, a marked Several studies23 have shown varying degrees of reduction in growth rate and cell surface mar- immune response elicited by these cells, with some ker expression, including down-regulation of reports even suggesting that hESCs are immu- SSEA3, SSEA4, and TRA1-6029. Cells deficient ne-privileged. Circumvention of rejection might in OCT4 also showed a clear up-regulation of be possible via the generation of hESC banks con- differentiation-associated markers, particularly taining immune-phenotyped lines, although this genes associated with differentiation to trophec- requires a large time and economic investment24. toderm, endoderm, and mesoderm30. Up-regu- A recently published trial, evaluated hESC-deri- lation of OCT4 showed association with the ved retinal pigment epithelium in the treatment changes in genes associated with mesodermal of patients with age-related and endodermal differentiation31. Additionally, and Stargardt’s macular dystrophy25. This study RNAi-induced silencing of OCT4 induced a provided evidence of their safety, graft survival change in >1000 genes, with both positive (e.g. and possible biological activity. pluripotency-associated TFs) and negative (e.g. mesoderm, endoderm and ectoderm-associated Induced Pluripotent Stem Cells genes) regulation of different gene sets32. OCT4 One way to overcome the issue of hESC immu- also interacts with SOX2, a member of the ne-rejection is via the use of autologous cells. In SRY-related HMG-box (Sox) family, for the pur- 2007 Takahashi et al26 reprogrammed both mouse pose of regulation of pluripotency33. Further, it and human adult fibroblasts to pluripotency by was also shown that NANOG was expressed in transduction of four defined factors: OCT4, SOX2, the ICM of blastocyst stage pre-implantation hu- KLF4, and c-MYC. The pluripotent cells generated, man embryos, but not in some of the earlier-sta- known as induced pluripotent stem cells (iPSCs) ge embryos, demonstrating a role for NANOG are similar to hESCs in morphology, proliferation, in the maintenance of pluripotency.

1337 Y. Cao, P.-Y. Zhang

OCT4, SOX2, and NANOG were found to 2D Monolayer Culture Systems co-occupy the promoter region of 353 different This protocol resulted in up-regulation of en- genes, with binding sites occurring nearby. The dothelial-associated CD31 and down-regulation three factors were found to regulate pluripotency of pluripotency-associated genes such as OCT4. by binding and transcriptionally activating genes. These cells expressed high levels of endothe- Moreover, they bind and transcriptionally inacti- lial-associated genes, and performed functionally vate genes that promote development, such as in both in vivo and in vitro models42. Recently, a HOXB1and PAX634. Indeed, targeted down-regu- method for simultaneous derivation of ECs and lation of any one of these three factors results in a pericytes from hiPSCs has been published43. Fur- decrease in the expression of the other two. Thus, thermore, Patsch et al44 also described a highly a synergy exists between these three factors, efficient monolayer-based system for the deriva- forming an auto-regulatory loop, and working tion of ECs from hPSCs. to regulate a large number of differentiation and pluripotency associated genes. Conclusions Endothelial Differentiation of Pluripotent Stem Cells It could be concluded from above discussion, Regarding vascular and stimu- that regenerative medicine holds a strong poten- lation of angiogenesis, hPSC-derived endothelial tial for revascularization and angiogenesis. These cells (hPSC-ECs) are thought to have the greatest data are promising for the potential translation of potential, although methods of derivation remain these technologies into the clinic, as a possible suboptimal. Thus far, there have been a large therapeutic approach. number of publications describing protocols for the derivation of ECs from hPSCs35. Further, there are two main approaches, which have been Conflict of Interests taken when generating hESC-ECs; 3D embryoid The Authors declare that they have no conflict of interests. body (EB)-based culture systems and 2D mono- layer culture systems. Endothelial-associated genes, including Pe- cam-1 (CD31), VE-Cadherin (CD144) and CD34, References are often elevated during spontaneous EB-based 36 1) Papalamprou A, Chang CW, Vapniarsky N, Clark A, differentiation of hESCs . However, the efficien- Walker N, Griffiths LG. 37 Xenogeneic cardiac extra- cy of these differentiations is low. Other studies cellular matrix scaffolds with or without seeded showed that addition of VEGF into the system mesenchymal stem cells exhibit distinct in vivo could increase the numbers of cells expressing immunosuppressive and regenerative properties. CD31 and CD144. Further, these cells could be Acta Biomater 2016; 45: 155-168. 2) Henry TD, Annex BH, Mckendall GR, Azrin MA, Lopez isolated and cultured to obtain higher percentages JJ, Giordano FJ, Shah PK, Willerson JT, Benza RL, Ber- 38 of CD31+ cells . man DS, Gibson CM, Bajamonde A, Rundle AC, Fine J, Mccluskey ER, Investigators V. The VIVA trial: vascular endothelial growth factor in ischemia for vascular 3D Embryoid Body (EB)-based angiogenesis. Circulation 2003; 107: 1359-1365. Culture Systems 3) Lederman RJ, Mendelsohn FO, Anderson RD, Sau- cedo JF, Tenaglia AN, Hermiller JB, Hillegass WB, 3D EB-based direct differentiation protocols Rocha-Singh K, Moon TE, Whitehouse MJ, Annex are efficient in generating hESC-EC or hiPSC- BH, Investigators T. Therapeutic angiogenesis ECs39. Rufaihah et al40 used hiPSCs to generate with recombinant fibroblast growth factor-2 for cells, which were positive for CD31, CD144, intermittent claudication (the TRAFFIC study): a endothelial nitric oxide synthase (eNOS) and randomized trial. Lancet 2002; 359: 2053-2058. 4) Gansbacher B, European Society Of Gene Therapy. von Willibrand Factor (vWF). Moreover, these Report of a second serious adverse event in a cells demonstrated a high degree of transcriptio- clinical trial of gene therapy for X- linked seve- nal similarity between hESC-ECs and post-natal re combined immune deficiency (X-SCID). Po- ECs41. Although highly efficient, 3D EB-based sition of the European Society of Gene Therapy culture systems are difficult to scale up to pro- (ESGT). J Gene Med 2003; 5: 261-262. 5) Raval Z, Losordo DW. Cell therapy of peripheral duce clinically relevant numbers of cells due to arterial disease: from experimental findings to methods of EB formation. clinical trials. Circ Res 2013; 112: 1288-1302.

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