Cell-based GATA4 cardiac gene transfer using cell-penetrating peptide.

Congestive heart failure (CHF) is the only cardiovascular disease that is increasing in prevalence.1 Patients with CHF who have symptoms with mild activity or at rest (Class III and Class IV) have a poor long-term outcome, with 5-year age-adjusted mortality of 50%.2,3 Heart transplantation has an 80% 5-year survival rate, but only 3000 cardiac transplants are performed in the US each year, where the prevalence for severe CHF is estimated at 5 million patients.4 Angiotensin converting enzyme inhibitors, aldosterone antagonists, -adrenergic receptor antagonists, and implanted cardiac defibrillators have improved survival. However, even with optimal medical and device management, heart failure is an inexorable disease associated with unacceptably high morbidity and mortality.1 Because the prevalence of CHF is high and the outlook dismal, new therapeutic approaches are needed. In gene transfer experiments, one generally studies how increased expression of a single molecule influences signal transduction and cardiac function. These studies have identified a large number of potential therapeutic targets in cardiac myocytes, including cell membrane receptors and their ligands, calcium handling proteins, intracellular signaling molecules, transcription factors, and contractile proteins in cardiac myocytes.5–9 An abundance of therapeutic candidates is thwarted by the Achilles’ heel of cardiovascular gene transfer—the paucity of evidence for a vector and delivery method that will provide adequate cardiac expression safely. The exuberance engendered by cardiovascular gene therapy has subsided over the last 10 years, supplanted by similar hopes for cell-based therapy. Bone marrow–derived cells, endothelial progenitor cells, and embryonic or mesenchymal stem cells have been used successfully in animal models,10,11 although debate continues regarding cardiac myocyte regeneration per se.12 Clinical trials using bone marrow–derived cells in patients with acute myocardial infarction13 and CHF14 have been somewhat encouraging, but the mechanism for functional improvement may be attributable to angiogenesis rather than generation of cardiac myocytes.15 Thus, both gene transfer and cell-based treatments, although intellectually attractive and abounding with suitable therapeutic candidates, are hindered by challenging methodological problems. With this backdrop, Bian and colleagues, in this issue of Circulation Research16 describe a novel means to treat regional dysfunction associated with myocardial infarction in rats using GATA4 gene transfer. Although GATA4 is an essential transcription factor for cardiac myocyte hypertrophy,17–19 its inability to enter cells has impeded its use in cardiovascular gene therapy. Bian and colleagues resolve this problem by engineering cultured cardiac fibroblasts to express a GATA4:VP22 fusion protein. VP22, a cellpenetrating peptide, enables GATA4 to enter cells after secretion from implanted engineered cardiac fibroblasts, thereby exerting biological effects in the targeted area, the most impressive of which is cardiac myocyte hypertrophy. In essence, Bian et al have combined cell-based therapy and gene transfer while side-stepping the baggage associated with virus vectors and stem cells. Cell-penetrating peptides (CPP), also known as protein transduction domains, are basic and amphipathic peptides that transport proteins, peptides, plasmid DNA, siRNA, and liposomes across mammalian cell plasma membranes.17 The discovery of CPPs originated from the finding that the Tat transactivator of HIV virus type 1 translocates across cell membranes.18,19 It subsequently was found that Tat also could import fused protein into cells.20 Protein translocation was mediated by a 13–amino acid sequence in Tat, GRKKRRQRRRPPQ, and this basic and amphipathic peptide was both necessary and sufficient to carry fusion proteins across cell membranes of cultured cells.21 Other proteins possess translocation capability, including herpes simplex virus type 1 structural tegument protein VP22,22 and even synthetic peptides.17 Although the exact translocation mechanism is unknown, CPPs have also been used to transfer biological macromolecules, such as -galactosidase23 and Bcl-xL,24 into a variety of organs in vivo. Bian and colleagues16 tested whether delivery of the transcription factor GATA4, in the form of a GATA4:VP22 fusion protein, could reduce regional cardiac dysfunction associated with myocardial infarction in rats (Figure). To ensure long term expression, the authors used liposomal transfection of an expression vector encoding GATA4:VP22 into cultured rat cardiac fibroblasts. Four weeks after coronary artery ligation, GATA4:VP22 expressing fibroblasts were injected into the infarct border region. Improved cardiac function was found both 4 and 6 weeks after cell injection. This study is important because it tested the potential therapeutic effects of GATA4, an essential transcription factor for cardiac hypertrophy,25–27 using cell-based gene transfer in a clinically relevant animal model of cardiac dysfunction. The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Department of Medicine, University of California San Diego, and VA San Diego Healthcare System, San Diego, Calif. Correspondence to H. Kirk Hammond, MD, VA San Diego Healthcare System (111A), 3350 La Jolla Village Drive, San Diego, California 92161. E-mail [email protected] (Circ Res. 2007;100:1540-1542.) © 2007 American Heart Association, Inc.