Joo-Won Jeong

Hypoxia-inducible factor (HIF-1)|[alpha]|: its protein stability and biological functions

Hypoxia-inducible factor (HIF-1) is an oxygen-dependent transcriptional activator, which plays crucial roles in the angiogenesis of tumors and mammalian development. HIF-1 consists of a constitutively expressed HIF-1β subunit and one of three subunits (HIF-1α, HIF-2α or HIF-3α). The stability and activity of HIF-1α are regulated by various post-translational modifications, hydroxylation, acetylation, and phosphorylation. Therefore, HIF-1α interacts with several protein factors including PHD, pVHL, ARD-1, and p300/CBP. Under normoxia, the HIF-1α subunit is rapidly degraded via the von Hippel-Lindau tumor suppressor gene product (pVHL)- mediated ubiquitin-proteasome pathway. The association of pVHL and HIF-1α under normoxic conditions is triggered by the hydroxylation of prolines and the acetylation of lysine within a polypeptide segment known as the oxygen-dependent degradation (ODD) domain. On the contrary, in the hypoxia condition, HIF-1α subunit becomes stable and interacts with coactivators such as p300/CBP to modulate its transcriptional activity. Eventually, HIF-1 acts as a master regulator of numerous hypoxia-inducible genes under hypoxic conditions. The target genes of HIF-1 are especially related to angiogenesis, cell proliferation/survival, and glucose/iron metabolism. Moreover, it was reported that the activation of HIF-1α is closely associated with a variety of tumors and oncogenic pathways. Hence, the blocking of HIF-1a itself or HIF-1α interacting proteins inhibit tumor growth. Based on these findings, HIF-1 can be a prime target for anticancer therapies. This review summarizes the molecular mechanism of HIF-1a stability, the biological functions of HIF-1 and its potential applications of cancer therapies.

Hepatitis B virus X protein induces angiogenesis by stabilizing hypoxia-inducible factor-1α

Hepatitis B virus X protein (HBx) is closely involved in the development of hepatocellular carcinoma, a highly vascularized solid tumor. Here we show that HBx increases the transcriptional activity and protein level of hypoxia‐inducible factor‐1α (HIF‐1α) under both normoxic and hypoxic conditions, and it also stimulates angiogenesis. HBx directly interacted with the bHLH/PAS domain of HIF‐1α but not with the von Hippel‐Lindau protein (pVHL). HBx decreased the binding of pVHL to HIF‐1α and prevented ubiquitin‐dependent degradation of HIF‐1α. In HBx‐transgenic mice, HIF‐1α and vascular endothelial growth factor were strongly detected in the dysplastic lesion, where HBx was also more highly expressed than in the non‐neoplastic region of the liver. An immunohistochemical study showed that microvessels are more abundant in the dysplastic lesion than in the non‐neoplastic region. Our data suggest that HBx stabilizes HIF‐1α and leads to angiogenesis during hepatocarcinogenesis.

Melatonin suppresses tumor angiogenesis by inhibiting HIF-1α stabilization under hypoxia

Abstract:  Angiogenesis is an important mediator of tumor progression. As tumors expand, diffusion distances from the existing vascular supply increases, resulting in hypoxia in the cancer cells. Sustained expansion of a tumor mass requires new blood vessel formation to provide rapidly proliferating tumor cells with an adequate supply of oxygen and nutrients. The key regulator of hypoxia‐induced angiogenesis is the transcription factor known as hypoxia‐inducible factor (HIF)‐1. HIF‐1α is stabilized by hypoxia‐induced reactive oxygen species (ROS) and enhances the expression of several types of hypoxic genes, including that of the angiogenic activator known as vascular endothelial cell growth factor (VEGF). In this study, we found that melatonin, a small lipophilic molecule secreted primarily by the pineal gland, destabilizes hypoxia‐induced HIF‐1α protein levels in the HCT116 human colon cancer cell line. This destabilization of HIF‐1α resulted from the antioxidant activity of melatonin against ROS induced by hypoxia. Moreover, under hypoxia, melatonin suppressed HIF‐1 transcriptional activity, leading to a decrease in VEGF expression. Melatonin also blocked in vitro tube formation and invasion and migration of human umbilical vein endothelial cells induced by hypoxia‐stimulated conditioned media of HCT116 cells. These findings suggest that melatonin could play a pivotal role in tumor suppression via inhibition of HIF‐1‐mediated angiogenesis.

AMP-activated protein kinase is activated in Parkinson’s disease models mediated by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

The selective loss of dopaminergic neurons in the substantia nigra pars compacta is a feature of Parkinsons disease (PD). 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced neurotoxicity is the most common experimental model used to investigate the pathogenesis of PD. Administration of MPTP in mice produces neuropathological defects as observed in PD and 1-methyl-4-pyridinium (MPP(+)) induces cell death when neuronal cell cultures are used. AMP-activated protein kinase (AMPK) is a key regulator of energy homeostasis. In the present study, we demonstrated that AMPK is activated by MPTP in mice and MPP(+) in SH-SY5Y cells. The inhibition of AMPK by compound C resulted in an increase in MPP(+)-induced cell death. We further showed that overexpression of AMPK increased cell viability after exposure to MPP(+) in SH-SY5Y cells. Based on these results, we suggest that activation of AMPK might prevent neuronal cell death and play a role as a survival factor in PD.

Melatonin suppresses tumor progression by reducing angiogenesis stimulated by HIF-1 in a mouse tumor model

The sustained expansion of a tumor mass requires new blood vessel formation to provide rapidly proliferating tumor cells with an adequate supply of oxygen and nutrients. Hypoxia‐inducible factor‐1 (HIF‐1) plays an essential role in tumor angiogenesis and growth by regulating the transcription of genes in response to hypoxic stress. This study was designed to investigate the effects of melatonin on tumor growth and angiogenesis, as well as the mechanism underlying the antitumor activities of melatonin. In this study, we show that the administration of melatonin inhibits tumor growth and blocks tumor angiogenesis in mice. Moreover, melatonin diminished the expression of the HIF‐1α protein within the tumor mass during tumorigenesis. Our findings suggest that melatonin is a promising anti‐angiogenic therapeutic agent targeting HIF‐1α in cancer. Considering that HIF‐1α is overexpressed in a majority of human cancers, melatonin could offer a potent therapeutic agent for cancer.

Antiangiogenic Effects of P-Coumaric Acid in Human Endothelial Cells

p‐Coumaric acid, a hydroxy derivative of cinnamic acid, has been known to possess antioxidant and anticancer activities. Despite its potential contribution to chemopreventive effects, the mechanism by which p‐coumaric acid exerts its antiangiogenic actions remains elusive. In this study, we revealed that p‐coumaric acid inhibited the sprouting of endothelial cells in rat aortic rings and inhibited the tube formation and migration of endothelial cells. We observed that p‐coumaric acid could downregulate mRNA expression levels of the key angiogenic factors vascular endothelial growth factor and basic fibroblast growth factor. Also, we demonstrated that p‐coumaric acid inhibited both the AKT and ERK signaling pathways, which are known to be crucial for angiogenesis. Using a mouse model, we also showed that p‐coumaric acid effectively suppressed tumor growth in vivo by lowering hemoglobin contents. Collectively, these findings indicate that p‐coumaric acid possesses potent anticancer properties due to the inhibition of angiogenesis in vivo. Copyright

The Anti-Angiogenic Activities of Glycyrrhizic Acid in Tumor Progression

Glycyrrhizic acid (GA) is the bioactive compound of licorice and has been used as a herbal medicine because of its anti‐viral, anti‐cancer, and anti‐inflammatory properties. This study was designed to investigate the effects of GA on tumor growth, angiogenesis, and the mechanisms underlying the anti‐angiogenic activities of GA. We observed that GA inhibited tumor growth and angiogenesis in mice. GA decreased angiogenic activities, such as the migration, invasion, and tube formation of endothelial cells. We also demonstrated that GA reduced the production of reactive oxygen species and activation of ERK in endothelial cells. Our findings suggest that GA is a promising anti‐angiogenic therapeutic agent that targets the ERK pathway. Considering that angiogenesis is highly stimulated in the majority of cancers, GA could offer a potent therapeutic agent for cancer. Copyright

Ninjurin1 is expressed in myeloid cells and mediates endothelium adhesion in the brains of EAE rats

Ninjurin1 (nerve injury-induced protein, Ninj1) is an adhesion molecule that is essential for cell-to-cell interactions. However, little is known about the function of Ninj1 in the central nervous system (CNS). To address its role in the CNS, we analyzed the expression pattern of Ninj1 in normal rats and in an experimental autoimmune encephalomyelitis (EAE) model. Ninj1 was expressed in three major compartments of brains, meninges, the choroid plexus, and parenchymal perivascular spaces. In the EAE brains, Ninj1 was strongly expressed in myeloid cells (macrophages/monocytes and neutrophils) and partially expressed in endothelial cells (ECs). Furthermore, Ninj1 enhanced adhesion between BV2 cells (murine monocyte lineage microglia) and HBMECs (human brain microvascular endothelial cells). Collectively, our findings suggest that Ninj1 may mediate the entry of myeloid cells into the CNS in normal and EAE brains, and it is a potential therapeutic target for regulating myeloid cell trafficking across the blood-brain barrier (BBB) in CNS immune processes.

Tid-1 Interacts with the von Hippel-Lindau Protein and Modulates Angiogenesis by Destabilization of HIF-1α

The von Hippel-Lindau protein (pVHL) is a major tumor suppressor protein and also associated with the inhibition of angiogenesis via HIF-1alpha ubiquitination and proteasomal degradation. To further elucidate the biological activity of pVHL in angiogenesis, pVHL-interacting proteins were screened using the yeast two-hybrid system. We found that a mouse homologue of the long form of Drosophila tumor suppressor l(2)tid, Tid-1(L), directly interacts with pVHL in vitro and in vivo. Furthermore, Tid-1(L) protein; enhanced the interaction between HIF-1alpha and pVHL, leading to the destabilization of HIF-1alpha protein; therefore, Tid-1(L) protein decreased vascular endothelial growth factor expression and inhibited angiogenesis in vivo and in vitro. These findings propose that Tid-1(L) may play a critical role in pVHL-mediated tumor suppression by modulating the pVHL-dependent HIF-1alpha stability.

Protein kinase C-δ regulates the stability of hypoxia-inducible factor-1α under hypoxia

Hypoxia is a state of deficiency of available oxygen in the blood and tissues, and it occurs during several pathophysiological processes, including tumorigenesis. Under hypoxia, hypoxia‐inducible factor‐1 (HIF‐1) plays an essential role in cellular oxygen homeostasis. In the present article protein kinase C‐δ (PKC‐δ) is activated by hypoxia, increases the protein stability and transcriptional activity of HIF‐1α in human cervical adenocarcinoma cells. Moreover, the knockdown of PKC‐δ inhibited vascular endothelial growth factor expression and angiogenic activity under hypoxia. These effects were completely reversed by PKC‐δ overexpression following the knockdown of PKC‐δ. Collectively, these findings demonstrate the role of PKC‐δ as a new regulator of hypoxia‐induced angiogenesis. (Cancer Sci 2007; 98: 1476–1481)