Key Role for a Viral Lytic Gene in Kaposi's Sarcoma
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《新英格兰医药杂志》
Several types of cancer, including the majority of those typically associated with AIDS, are caused by gammaherpesviruses. Herpesviruses undergo both latent and lytic infection; most research on herpesvirus-induced tumors has focused on genes that mediate latency. However, a recent study by Grisotto et al.1 shows that a lytic gene of human herpesvirus 8 (HHV-8), also known as Kaposi's sarcoma–associated herpesvirus, may play a critical role in the development of Kaposi's sarcoma through direct and indirect mechanisms. This study highlights the role of lytic genes in viral oncogenesis.
The induction of tumors by viruses is best viewed as a biologic accident resulting from strategies that viruses have developed to perpetuate themselves. When gammaherpesviruses such as HHV-8 or Epstein–Barr virus establish latent infections, few viral genes are expressed. The proteins encoded by these genes help maintain the viral DNA, immortalize the cell, and contribute to resistance from immunologic attack. However, these functions can also promote the development of tumors. Herpesviruses spread by lytic replication (in which the virus-producing cell usually dies) and have developed strategies to facilitate this process. For example, the expression of antiapoptotic genes prolongs the survival of the virus-producing cell, thus increasing the number of viruses released into the extracellular milieu. As a consequence of these strategies, lytic genes can also promote tumor formation. However, because they are usually expressed only in cells that are destined to die, lytic genes have been viewed as less important than latent genes in cancer development.
Kaposi's sarcoma is a multicentric tumor that involves the hyperproliferation of endothelial-derived spindle cells. Most spindle cells are latently infected with HHV-8; few are lytically infected. Grisotto et al. used an animal model to study the role of a lytic HHV-8 gene in the pathogenesis of Kaposi's sarcoma. The gene encodes the viral G-protein–coupled receptor (vGPCR) and is thought to be a "pirated" human chemokine receptor — it was probably wrested from the human genome during viral evolution. Unlike most cellular chemokine receptors, however, the vGPCR is constitutively active. vGPCR signaling can result in cell proliferation, activation of the growth-inducing signaling molecule Akt, and the production of angiogenic factors (Figure 1). Its expression in mouse endothelial cells can lead to the formation of Kaposi's sarcoma–like lesions.2,3,4,5 Grisotto et al. developed a transgenic mouse model in which the administration of doxycycline induced the expression of vGPCR and a visible marker that was "turned on" in the cells in which vGPCR was activated. Both the gene for the vGPCR and the marker gene were controlled by a segment of DNA that normally controls the CD2 gene. When these mice were fed doxycycline for up to 40 days, vascularized angiogenic lesions developed in their ears, paws, and tails; these lesions contained a large number of endothelial-like cells expressing vGPCR (Figure 2A and 2B). This finding provides evidence that vGPCR directly triggers angioproliferation in endothelial cells by activating intracellular pathways, autocrine factors, or both. CD2 is predominantly expressed in thymocytes and bone marrow cells. The expression of vGPCR and the preferential growth of endothelial cells in the mouse model suggest that the CD2 promoter is active in these cells or their precursors and that these cells are then unusually sensitive to direct or autocrine activation by vGPCR signaling. Moreover, transplantation experiments suggest that these angiogenic cells do not come from the bone marrow, but rather from peripheral endothelial precursor cells.
Figure 1. Activities of the vGPCR Protein in HHV-8.
The constitutively active vGPCR of HHV-8 may promote the development of Kaposi's sarcoma by means of a variety of mechanisms.1,2,3,4,5 Signaling by vGPCR activates Akt, which directly induces cell transformation. The vGPCR also results in the production of a variety of other factors, including the nuclear factor (NF)-B–dependent factors interleukin-8, growth-related protein (GRO-), interleukin-6, interleukin-1, and tumor necrosis factor (TNF-); AP-1–dependent basic fibroblast growth factor (bFGF); platelet-derived growth factor B (PDGF-B); and placental growth factor (PlGF). Some, but not all, studies have found that vGPCR induces secretion of vascular endothelial growth factor (VEGF), and there is evidence that this secretion may be mediated by hypoxia-inducible factor (HIF). Also, vGPCR up-regulates the expression of the VEGF receptors 1 and 2 (VEGFR-1 and VEGFR-2, respectively). PlGF, VEGF, and other factors can act in an autocrine or paracrine fashion to promote Kaposi's sarcoma.
Figure 2. Tumor Induced by vGPCR in a Mouse Model.
In the transgenic mouse model developed by Grisotto et al.,1 exposure to doxycycline resulted in the endothelial expression of both vGPCR — a protein encoded by HHV-8 — and the marker LacZ; the latter is evident in the transfected cells (blue stain). After 5 days of continuous vGPCR expression (induced by doxycycline), LacZ-expressing endothelial cells lined some blood vessels (Panel A); 15 days after the induction of vGPCR expression, many more such cells and angioproliferation were observed (Panel B). (Panels A and B, X-galactosidase and biotinylated lycopersicon esculentum lectin stain.) After treatment with doxycycline for 120 days, Kaposi's sarcoma–like tumors developed in the tails of the mice (Panel C; X-galactosidase and nuclear fast red stain). As compared with cells at the periphery of the tumor, few cells within the tumor expressed LacZ (and hence vGPCR). (Panels A, B, and C courtesy of S. Lira and M. Grisotto.)
Longer treatment with doxycycline in these mice resulted in the development of tumor nodules resembling Kaposi's sarcoma. Few tumor cells expressed vGPCR (Figure 2C). However, the protein was expressed in a high percentage of the cells surrounding the tumor, especially in association with blood vessels. Thus, it appears that the cells that express vGPCR induce tumor development in vGPCR-negative cells, presumably by means of a paracrine or cell-to-cell contact mechanism.
These mice represent an important tool with which to dissect the role of the vGPCR but a somewhat artificial model of disease. In HHV-8 infection, vGPCR is generally expressed during lytic replication that can be triggered by hypoxia or other stimuli, and it generally leads to cell death. One puzzling question has been how a lytic gene could be so important in oncogenesis. Evidence that vGPCR can be expressed in the absence of full lytic replication4 perhaps helps explain this apparent paradox. Additional studies will be needed to determine the extent to which the transgenic mouse described by Grisotto et al. models the molecular events that lead to Kaposi's sarcoma. Moreover, it will be important to clarify the relative contributions of other HHV-8 genes in the development of Kaposi's sarcoma.
This and previous studies of vGPCR underscore the role of lytic genes in the viral oncogenesis of Kaposi's sarcoma and other tumors, and they suggest that such genes may provide new tumor targets. Lytic genes may be more amenable to attack than latent genes, if only because there are more of them. The article by Grisotto et al. and related work by others4,5 suggest that selectively removing the cells that are undergoing lytic replication or that express vGPCR would cause regression of Kaposi's sarcoma. During lytic replication, HHV-8–encoded kinases may render the cell susceptible to killing by certain nucleoside analogues, and this susceptibility could be exploited therapeutically. Also, if vGPCR is expressed in the absence of full lytic replication, it would be an attractive target, and inhibitors of vGPCR signaling could be developed for testing. Although the results of the study by Grisotto et al. will not immediately lead to a change in therapy for Kaposi's sarcoma, they provide helpful groundwork for future advances.
Dr. Yarchoan reports, as a full-time U.S. government employee, holding patents on interleukin-12 as a treatment for Kaposi's sarcoma. No other potential conflict of interest relevant to this article was reported.
Source Information
From the HIV and AIDS Malignancy Branch, Center for Cancer Research, National Cancer Institute, Bethesda, MD.
References
Grisotto MG, Garin A, Martin AP, et al. The human herpesvirus 8 chemokine receptor vGPCR triggers autonomous proliferation of endothelial cells. J Clin Invest 2006;116:1264-1273.
Bais C, Santomasso B, Coso O, et al. G-protein-coupled receptor of Kaposi's sarcoma-associated herpesvirus is a viral oncogene and angiogenesis activator. Nature 1998;391:86-89.
Schwarz M, Murphy PM. Kaposi's sarcoma-associated herpesvirus G protein-coupled receptor constitutively activates NF-kappa B and induces proinflammatory cytokine and chemokine production via a C-terminal signaling determinant. J Immunol 2001;167:505-513.
Sodhi A, Montaner S, Gutkind JS. Does dysregulated expression of a deregulated viral GPCR trigger Kaposi's sarcomagenesis? FASEB J 2004;18:422-427.
Montaner S, Sodhi A, Ramsdell AK, et al. The Kaposi's sarcoma-associated herpesvirus G protein-coupled receptor as a therapeutic target for the treatment of Kaposi's sarcoma. Cancer Res 2006;66:168-174.(Robert Yarchoan, M.D.)
The induction of tumors by viruses is best viewed as a biologic accident resulting from strategies that viruses have developed to perpetuate themselves. When gammaherpesviruses such as HHV-8 or Epstein–Barr virus establish latent infections, few viral genes are expressed. The proteins encoded by these genes help maintain the viral DNA, immortalize the cell, and contribute to resistance from immunologic attack. However, these functions can also promote the development of tumors. Herpesviruses spread by lytic replication (in which the virus-producing cell usually dies) and have developed strategies to facilitate this process. For example, the expression of antiapoptotic genes prolongs the survival of the virus-producing cell, thus increasing the number of viruses released into the extracellular milieu. As a consequence of these strategies, lytic genes can also promote tumor formation. However, because they are usually expressed only in cells that are destined to die, lytic genes have been viewed as less important than latent genes in cancer development.
Kaposi's sarcoma is a multicentric tumor that involves the hyperproliferation of endothelial-derived spindle cells. Most spindle cells are latently infected with HHV-8; few are lytically infected. Grisotto et al. used an animal model to study the role of a lytic HHV-8 gene in the pathogenesis of Kaposi's sarcoma. The gene encodes the viral G-protein–coupled receptor (vGPCR) and is thought to be a "pirated" human chemokine receptor — it was probably wrested from the human genome during viral evolution. Unlike most cellular chemokine receptors, however, the vGPCR is constitutively active. vGPCR signaling can result in cell proliferation, activation of the growth-inducing signaling molecule Akt, and the production of angiogenic factors (Figure 1). Its expression in mouse endothelial cells can lead to the formation of Kaposi's sarcoma–like lesions.2,3,4,5 Grisotto et al. developed a transgenic mouse model in which the administration of doxycycline induced the expression of vGPCR and a visible marker that was "turned on" in the cells in which vGPCR was activated. Both the gene for the vGPCR and the marker gene were controlled by a segment of DNA that normally controls the CD2 gene. When these mice were fed doxycycline for up to 40 days, vascularized angiogenic lesions developed in their ears, paws, and tails; these lesions contained a large number of endothelial-like cells expressing vGPCR (Figure 2A and 2B). This finding provides evidence that vGPCR directly triggers angioproliferation in endothelial cells by activating intracellular pathways, autocrine factors, or both. CD2 is predominantly expressed in thymocytes and bone marrow cells. The expression of vGPCR and the preferential growth of endothelial cells in the mouse model suggest that the CD2 promoter is active in these cells or their precursors and that these cells are then unusually sensitive to direct or autocrine activation by vGPCR signaling. Moreover, transplantation experiments suggest that these angiogenic cells do not come from the bone marrow, but rather from peripheral endothelial precursor cells.
Figure 1. Activities of the vGPCR Protein in HHV-8.
The constitutively active vGPCR of HHV-8 may promote the development of Kaposi's sarcoma by means of a variety of mechanisms.1,2,3,4,5 Signaling by vGPCR activates Akt, which directly induces cell transformation. The vGPCR also results in the production of a variety of other factors, including the nuclear factor (NF)-B–dependent factors interleukin-8, growth-related protein (GRO-), interleukin-6, interleukin-1, and tumor necrosis factor (TNF-); AP-1–dependent basic fibroblast growth factor (bFGF); platelet-derived growth factor B (PDGF-B); and placental growth factor (PlGF). Some, but not all, studies have found that vGPCR induces secretion of vascular endothelial growth factor (VEGF), and there is evidence that this secretion may be mediated by hypoxia-inducible factor (HIF). Also, vGPCR up-regulates the expression of the VEGF receptors 1 and 2 (VEGFR-1 and VEGFR-2, respectively). PlGF, VEGF, and other factors can act in an autocrine or paracrine fashion to promote Kaposi's sarcoma.
Figure 2. Tumor Induced by vGPCR in a Mouse Model.
In the transgenic mouse model developed by Grisotto et al.,1 exposure to doxycycline resulted in the endothelial expression of both vGPCR — a protein encoded by HHV-8 — and the marker LacZ; the latter is evident in the transfected cells (blue stain). After 5 days of continuous vGPCR expression (induced by doxycycline), LacZ-expressing endothelial cells lined some blood vessels (Panel A); 15 days after the induction of vGPCR expression, many more such cells and angioproliferation were observed (Panel B). (Panels A and B, X-galactosidase and biotinylated lycopersicon esculentum lectin stain.) After treatment with doxycycline for 120 days, Kaposi's sarcoma–like tumors developed in the tails of the mice (Panel C; X-galactosidase and nuclear fast red stain). As compared with cells at the periphery of the tumor, few cells within the tumor expressed LacZ (and hence vGPCR). (Panels A, B, and C courtesy of S. Lira and M. Grisotto.)
Longer treatment with doxycycline in these mice resulted in the development of tumor nodules resembling Kaposi's sarcoma. Few tumor cells expressed vGPCR (Figure 2C). However, the protein was expressed in a high percentage of the cells surrounding the tumor, especially in association with blood vessels. Thus, it appears that the cells that express vGPCR induce tumor development in vGPCR-negative cells, presumably by means of a paracrine or cell-to-cell contact mechanism.
These mice represent an important tool with which to dissect the role of the vGPCR but a somewhat artificial model of disease. In HHV-8 infection, vGPCR is generally expressed during lytic replication that can be triggered by hypoxia or other stimuli, and it generally leads to cell death. One puzzling question has been how a lytic gene could be so important in oncogenesis. Evidence that vGPCR can be expressed in the absence of full lytic replication4 perhaps helps explain this apparent paradox. Additional studies will be needed to determine the extent to which the transgenic mouse described by Grisotto et al. models the molecular events that lead to Kaposi's sarcoma. Moreover, it will be important to clarify the relative contributions of other HHV-8 genes in the development of Kaposi's sarcoma.
This and previous studies of vGPCR underscore the role of lytic genes in the viral oncogenesis of Kaposi's sarcoma and other tumors, and they suggest that such genes may provide new tumor targets. Lytic genes may be more amenable to attack than latent genes, if only because there are more of them. The article by Grisotto et al. and related work by others4,5 suggest that selectively removing the cells that are undergoing lytic replication or that express vGPCR would cause regression of Kaposi's sarcoma. During lytic replication, HHV-8–encoded kinases may render the cell susceptible to killing by certain nucleoside analogues, and this susceptibility could be exploited therapeutically. Also, if vGPCR is expressed in the absence of full lytic replication, it would be an attractive target, and inhibitors of vGPCR signaling could be developed for testing. Although the results of the study by Grisotto et al. will not immediately lead to a change in therapy for Kaposi's sarcoma, they provide helpful groundwork for future advances.
Dr. Yarchoan reports, as a full-time U.S. government employee, holding patents on interleukin-12 as a treatment for Kaposi's sarcoma. No other potential conflict of interest relevant to this article was reported.
Source Information
From the HIV and AIDS Malignancy Branch, Center for Cancer Research, National Cancer Institute, Bethesda, MD.
References
Grisotto MG, Garin A, Martin AP, et al. The human herpesvirus 8 chemokine receptor vGPCR triggers autonomous proliferation of endothelial cells. J Clin Invest 2006;116:1264-1273.
Bais C, Santomasso B, Coso O, et al. G-protein-coupled receptor of Kaposi's sarcoma-associated herpesvirus is a viral oncogene and angiogenesis activator. Nature 1998;391:86-89.
Schwarz M, Murphy PM. Kaposi's sarcoma-associated herpesvirus G protein-coupled receptor constitutively activates NF-kappa B and induces proinflammatory cytokine and chemokine production via a C-terminal signaling determinant. J Immunol 2001;167:505-513.
Sodhi A, Montaner S, Gutkind JS. Does dysregulated expression of a deregulated viral GPCR trigger Kaposi's sarcomagenesis? FASEB J 2004;18:422-427.
Montaner S, Sodhi A, Ramsdell AK, et al. The Kaposi's sarcoma-associated herpesvirus G protein-coupled receptor as a therapeutic target for the treatment of Kaposi's sarcoma. Cancer Res 2006;66:168-174.(Robert Yarchoan, M.D.)