Published online before print November 1, 2007, doi: 10.1161/ATVBAHA.107.156786
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© 2008 American Heart Association, Inc.
Editorials |
Twin Layers of Lightning
A New Role for the Chaperone Hsp90 in Angiogenesis
From the Carolina Cardiovascular Biology Center (X.P., C.P.) and the Division of Cardiology (C.P.), University of North Carolina, Chapel Hill.
Correspondence to Cam Patterson, MD, Director, Division of Cardiology, and Carolina Cardiovascular Biology Center, University of North Carolina at Chapel Hill, 8200 Medical Biomolecular Research Building, Chapel Hill, NC 27599-7126. E-mail cpatters{at}med.unc.edu
Key Words: Hsp90 • VEGF • migration
Blood vessel formation occurs through two sequential mechanisms: the de novo formation of blood vessels during embryonic development (vasculogenesis) and the formation of new capillaries from preexisting vessels (angiogenesis). The principle mechanism of vessel formation in adults is angiogenesis and malfunction of this process leads to a wide range of diseases including tumors, inflammatory diseases, psoriasis, rheumatoid arthritis, and diabetic retinopathy, presciently and collectively referred to as "angiogenesis-dependent diseases."1 Most embryonic vessels and proliferating endothelial cells during angiogenesis are under control of a key molecule, vascular endothelial growth factor (VEGF). More than 10 new drugs targeting VEGF have been approved since 2004 by the Food and Drug Administration in the United States for the treatment of cancer and age-related macular degeneration, but it remains to be determined what the ultimate therapeutic success of those angiogenic inhibitors will be. It is generally accepted that VEGF-dependent endothelial cell migration is required for angiogenic responses; however, specific roles for VEGF-enhanced cell motility have been incompletely characterized.
See page 105
To fill this gap, a report published in this issue of Arteriosclerosis, Thrombosis, and Vascular Biology2 illuminates a critical role for the 90-kDa heat-shock protein (Hsp90) in VEGF-stimulated endothelial migration. Specifically, Miao and colleagues designed a dominant-negative Hsp90 construct (D88N-Hsp90β), cleverly based on a bioinformatic comparison of ATP-binding domain sequences of Hsp82 in yeast with Hsp90
and β isoforms in mammalian cells. They demonstrate that D88N-Hsp90β potently inhibits VEGF-stimulated Akt and eNOS activation, which are critical modulators of VEGF-induced cell migration and angiogenesis. This report dissects the mechanism by which Hsp90 regulates endothelial migration using a novel genetic tool, supporting a scaffold role for Hsp90 in the complex formation of PDK1, Akt, and eNOS in an ATP-dependent manner and their serial activation cascades (Figure). D88N-Hsp90β destabilizes the complex of Hsp90, PDK1, Akt, and eNOS, consequently blocking nitric oxide release, Rac1 activation, and stress fiber formation through its effects on the availability of the proximal protein phosphatase for Akt, PP2A. One remarkable observation in this report is that D88N-Hsp90β inhibits Rac1 activity, which has broad cellular effects. However, it is unclear from the present studies whether Rac1 activation and stress fiber formation are modulated entirely by the Akt-eNOS-NO pathway in these studies, or whether Rac1 may also be a new client of Hsp90.
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Hsp90 belongs to the chaperone systems that assist protein folding and assembly; Hsp90 family members are highly conserved and are essential for viability in yeast.3 Given that Hsp90 comprises as much as 1% to 2% of total cellular protein content, a long list of Hsp90 targets and diverse cellular functions have been identified, including effects on cell cycle, cytokinesis, and other processes that were recently identified using a remarkable genome-wide chemical–genetic screen in yeast.4 Hsp90 appears to be unique among all the chaperones in that it possesses not only substrate-specific folding activity but also mediates the conformational regulation of tyrosine kinases and steroid hormone receptors (reviewed by Buchner,5 Picard,6 and Young7). Over the past decade, several small-molecule drugs targeting Hsp90 have been identified as potential anticancer agents. Akt is one major client for Hsp90 in cancer cell lines, and the destabilization of Akt by Hsp90 inhibitors potently inhibits tumor growth.8–10 This is relevant to the present studies because, in angiogenesis, Akt is a major modulator of VEGF-mediated vessel formation in vivo and in vitro, and one is led to believe through the impressive studies of Miao and colleagues that it may also be a major target of Hsp90 in the endothelium. Previous reports have indicated that Akt is regulated by Hsp90 because the Hsp90 inhibitor 17-aminoallyl geldanamycin (17-AAG) markedly inhibits Akt protein expression in human umbilical endothelial cells.11 However, D88N-Hsp90β inhibited the formation of the ternary complex of Hsp90, eNOS, and Akt and decreased Akt phosphorylation without affecting its protein levels in this report. It would be of potential therapeutic relevance given these studies to determine whether PDK activity or protein levels are also directly regulated by D88N-Hsp90β. Although the confusing effects of 17-AAG and D88N-Hsp90β need further investigation, the importance of Hsp90 in the regulation of Akt is emphasized and may represent another important target for antiangiogenic therapy. The involvement of Hsp90 in angiogenesis may partially explain the efficiency of Hsp90 inhibitors as anticancer drugs.
Our understanding of the role of Akt in angiogenesis has grown remarkably in the last several years. Akt plays a key role in maintaining the survival of a wide range of cell types.12 Several lines of evidence suggest a link between Akt and neovascularization. Akt directly phosphorylates eNOS, which points to a central role of endothelial nitric oxide for postnatal neovascularization.13–15 Indeed, eNOS knockout animals are characterized by impaired angiogenesis in response to VEGF or ischemia,16 convincingly indicating nitric oxide is a key modulator of angiogenesis even though the downstream effector pathways are still not clear. For example, other downstream substrates of Akt such as Bad or caspase-9 responding to VEGF may block endothelial cell apoptosis and thus also participate in this critical pathophysiologic pathway. The present studies provide critical evidence that Akt-dependent nitric oxide synthesis contributes to endothelial cell migration and highlight the Akt–eNOS pathway as another important therapeutic target for angiogenesis-dependent diseases.
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Acknowledgments |
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Sources of Funding
Work in the author’s laboratory is supported by NIH grants GM61728, HL65619, AG02482, and HL61656 (to C.P.) and a postdoctoral fellowship from the American Heart Association (to X.P.). C.P. is an Established Investigator of the American Heart Association and a Burroughs Wellcome Fund Clinical Scientist in Translational Research.
Disclosures
None.
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Arterioscler Thromb Vasc Biol 2008 28: 105-111.
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