'The metabolism of tumours': 70 years later.

Otto Warburg's classic treatise on the reprogramming of tumour metabolism from oxidative to glycolytic metabolism was published in London in 1930. Although the Warburg effect is one of the most universal characteristics of solid tumours, the molecular basis for this phenomenon has only recently been elucidated by studies indicating that increased expression of genes encoding glucose transporters and glycolytic enzymes in tumour cells is mediated by the transcription factors c-MYC and HIF-1. Whereas c-myc is a direct target for oncogenic mutations, expression of hypoxia-inducible factor 1 (HIF-1) is indirectly up-regulated via gain-of-function mutations in oncogenes and loss-of-function mutations in tumour suppressor genes that result increased HIF-1alpha protein expression and/or increased HIF-1 transcriptional activity in a cell-type-specific manner. As a result of genetic alterations and intratumoral hypoxia, HIF-1alpha is overexpressed in the majority of common human cancers relative to the surrounding normal tissue. In human breast cancer and brain tumours, HIF-1alpha overexpression is strongly correlated with tumour grade and vascularity.

HER2 (neu) signaling increases the rate of hypoxia-inducible factor 1alpha (HIF-1alpha) synthesis: novel mechanism for HIF-1-mediated vascular endothelial growth factor expression.

Hypoxia-inducible factor 1 (HIF-1) is a transcriptional activator composed of HIF-1alpha and HIF-1beta subunits. Several dozen HIF-1 targets are known, including the gene encoding vascular endothelial growth factor (VEGF). Under hypoxic conditions, HIF-1alpha expression increases as a result of decreased ubiquitination and degradation. The tumor suppressors VHL (von Hippel-Lindau protein) and p53 target HIF-1alpha for ubiquitination such that their inactivation in tumor cells increases the half-life of HIF-1alpha. Increased phosphatidylinositol 3-kinase (PI3K) and AKT or decreased PTEN activity in prostate cancer cells also increases HIF-1alpha expression by an undefined mechanism. In breast cancer, increased activity of the HER2 (also known as neu) receptor tyrosine kinase is associated with increased tumor grade, chemotherapy resistance, and decreased patient survival. HER2 has also been implicated as an inducer of VEGF expression. Here we demonstrate that HER2 signaling induced by overexpression in mouse 3T3 cells or heregulin stimulation of human MCF-7 breast cancer cells results in increased HIF-1alpha protein and VEGF mRNA expression that is dependent upon activity of PI3K, AKT (also known as protein kinase B), and the downstream kinase FRAP (FKBP-rapamycin-associated protein). In contrast to other inducers of HIF-1 expression, heregulin stimulation does not affect the half-life of HIF-1alpha but instead stimulates HIF-1alpha synthesis in a rapamycin-dependent manner. The 5'-untranslated region of HIF-1alpha mRNA directs heregulin-inducible expression of a heterologous protein. These data provide a molecular basis for VEGF induction and tumor angiogenesis by heregulin-HER2 signaling and establish a novel mechanism for the regulation of HIF-1alpha expression.

Expression of hypoxia-inducible factor 1alpha in brain tumors: association with angiogenesis, invasion, and progression.

BACKGROUND: Hypoxia inducible factor-1 (HIF-1) plays a critical role in angiogenesis during vascular development. The authors tested the hypothesis that HIF-1 expression correlates with progression and angiogenesis in brain tumors. METHODS: The authors investigated the expression of the HIF-1alpha and HIF-1beta subunits in human glioma cell lines and brain tumor tissues using Western blot analysis and immunohistochemistry. RESULTS: In glioblastomas multiforme (GBMs), HIF-1alpha primarily was localized in pseudopalisading cells around areas of necrosis and in tumor cells infiltrating the brain at the tumor margin. In contrast, HIF-1alpha was expressed in stromal cells throughout hemangioblastomas (HBs). Like HIF-1alpha, HIF-1beta was most highly expressed in high grade tumors but was expressed more widely than HIF-1alpha, including cells away from necrotic zones. In the brains of mice injected with Glioma 261 cells, a pattern of HIF-1alpha expression identical to that observed in human GBMs was noted. CONCLUSIONS: In GBMs, the heterogeneous pattern of HIF-1alpha expression appears to be determined at least in part by tissue oxygenation, whereas in HBs the homogeneous expression of HIF-1alpha may be driven by an oncogenic rather than a physiologic stimulus.

Hypoxia, HIF-1, and the pathophysiology of common human diseases.

Hypoxia plays a fundamental role in the pathophysiology of common causes of mortality, including ischemic heart disease, stroke, cancer, chronic lung disease, and congestive heart failure. In these disease states, hypoxia induces changes in gene expression in target organs that either fail to result in adequate adaptation or directly contribute to disease pathogenesis. Hypoxia-inducible factor 1 (HIF-1) is a transcriptional activator that is expressed in response to cellular hypoxia and mediates multiple cellular and systemic homeostatic responses to hypoxia. Recent studies have provided evidence that important pathophysiological responses to hypoxia in pulmonary hypertension, myocardial ischemia, and cancer are mediated by HIF-1. Pharmacologic and gene therapy strategies designed to modulate HIF-1 activity may represent a novel and effective therapeutic approach to these common disorders.

Hypoxia-inducible factor 1alpha protein expression is controlled by oxygen-regulated ubiquitination that is disrupted by deletions and missense mutations.

Hypoxia-inducible factor 1 (HIF-1) is a transcription factor that mediates cellular and systemic homeostatic responses to reduced O(2) availability in mammals, including angiogenesis, erythropoiesis, and glycolysis. HIF-1 activity is controlled by the O(2)-regulated expression of the HIF-1alpha subunit. Under nonhypoxic conditions, HIF-1alpha protein is subject to ubiquitination and proteasomal degradation. Here we report that missense mutations and/or deletions involving several different regions of HIF-1alpha result in constitutive expression and transcriptional activity in nonhypoxic cells. We demonstrate that hypoxia results in decreased ubiquitination of HIF-1alpha and that missense mutations increase HIF-1alpha expression under nonhypoxic conditions by blocking ubiquitination.

Modulation of hypoxia-inducible factor 1alpha expression by the epidermal growth factor/phosphatidylinositol 3-kinase/PTEN/AKT/FRAP pathway in human prostate cancer cells: implications for tumor angiogenesis and therapeutics.

Dysregulated signal transduction from receptor tyrosine kinases to phosphatidylinositol 3-kinase (PI3K), AKT (protein kinase B), and its effector FKBP-rapamycin-associated protein (FRAP) occurs via autocrine stimulation or inactivation of the tumor suppressor PTEN in many cancers. Here we demonstrate that in human prostate cancer cells, basal-, growth factor-, and mitogen-induced expression of hypoxia-inducible factor 1 (HIF-1) alpha, the regulated subunit of the transcription factor HIF-1, is blocked by LY294002 and rapamycin, inhibitors of PI3K and FRAP, respectively. HIF-1-dependent gene transcription is blocked by dominant-negative AKT or PI3K and by wild-type PTEN, whereas transcription is stimulated by constitutively active AKT or dominant-negative PTEN. LY294002 and rapamycin also inhibit growth factor- and mitogen-induced secretion of vascular endothelial growth factor, the product of a known HIF-1 target gene, thus linking the PI3K/PTEN/AKT/FRAP pathway, HIF-1, and tumor angiogenesis. These data indicate that pharmacological agents that target PI3K, AKT, or FRAP in tumor cells inhibit HIF-1alpha expression and that such inhibition may contribute to therapeutic efficacy.

Overexpression of hypoxia-inducible factor 1alpha in common human cancers and their metastases.

Neovascularization and increased glycolysis, two universal characteristics of solid tumors, represent adaptations to a hypoxic microenvironment that are correlated with tumor invasion, metastasis, and lethality. Hypoxia-inducible factor 1 (HIF-1) activates transcription of genes encoding glucose transporters, glycolytic enzymes, and vascular endothelial growth factor. HIF-1 transcriptional activity is determined by regulated expression of the HIF-1alpha subunit. In this study, HIF-1alpha expression was analyzed by immunohistochemistry in 179 tumor specimens. HIF-1alpha was overexpressed in 13 of 19 tumor types compared with the respective normal tissues, including colon, breast, gastric, lung, skin, ovarian, pancreatic, prostate, and renal carcinomas. HIF-1alpha expression was correlated with aberrant p53 accumulation and cell proliferation. Preneoplastic lesions in breast, colon, and prostate overexpressed HIF-1alpha, whereas benign tumors in breast and uterus did not. HIF-1alpha overexpression was detected in only 29% of primary breast cancers but in 69% of breast cancer metastases. In brain tumors, HIF-1alpha immunohistochemistry demarcated areas of angiogenesis. These results provide the first clinical data indicating that HIF-1alpha may play an important role in human cancer progression.

Regulation of cardiovascular development and physiology by hypoxia-inducible factor 1.

Hypoxia is an essential pathophysiologic component of ischemic cardiovascular disease. A better understanding of the molecular mechanisms underlying adaptive responses to hypoxia may lead to novel therapeutic strategies. Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric basic-helix-loop-helix-PAS domain transcription factor that mediates changes in gene expression in response to changes in O2 concentration. Genes that are transcriptionally activated by HIF-1 in hypoxic cells encode proteins that increase O2 delivery or allow metabolic adaptation to limited O2 availability. HIF-1 target genes include those encoding vascular endothelial growth factor (VEGF), erythropoietin, glucose transporters, and glycolytic enzymes. In anemic fetal sheep, increased myocardial vascularization was associated with concomitant increases in the expression of HIF-1 and VEGF. Expression of HIF-1 target genes was not induced by hypoxia in embryonic stem cells lacking expression of the O2-regulated HIF-1 alpha subunit. Mouse embryos lacking HIF-1 alpha expression arrested in their development by E9.0 and died by E10.5 with cardiovascular malformations and massive cell death throughout the embryo. These studies indicate that HIF-1 functions as a master regulator of O2 homeostasis that controls the establishment of essential physiologic systems during embryogenesis as well as their subsequent utilization during fetal and postnatal life.

Defective vascularization of HIF-1alpha-null embryos is not associated with VEGF deficiency but with mesenchymal cell death.

Hypoxia-inducible factor 1 (HIF-1) is a dimeric transcription factor composed of HIF-1alpha and HIF-1beta subunits that plays an essential role in mammalian O2 homeostasis. In Hif1a-/- knockout mice, complete deficiency of HIF-1alpha resulted in cardiac and vascular malformations and embryonic lethality at E10.5. Between E8. 75 and E9.25 striking vascular regression and abnormal remodeling occurred in the cephalic region concomitant with marked mesenchymal cell death. Similar vascular defects were observed in HIF-1alpha- and VEGF-deficient embryos and VEGF mRNA expression was not induced by hypoxia in Hif1a-/- embryonic stem cells. Surprisingly, Hif1a-/- embryos demonstrated increased VEGF mRNA expression compared to wild-type embryos. In tissue culture cells, VEGF mRNA expression was induced by glucose deprivation independent of HIF-1alpha, providing a mechanism for increased VEGF mRNA expression in Hif1a-/- embryos, in which absence of adequate tissue perfusion resulted in both O2 and glucose deprivation. Rather than being associated with VEGF deficiency, the vascular defects in Hif1a-/- embryos were spatially correlated with cell death, the onset of which preceded vascular regression.

Increased expression of hypoxia inducible factor-1alpha in rat and human prostate cancer.

Hypoxia-inducible factor 1 (HIF-1) is a transcription factor that regulates genes involved in adaptation to hypoxia. Expression of HIF-1alpha was evaluated in rat and human prostate cancer cell lines. Increased expression of HIF-1alpha mRNA in rat prostate cancer cell lines and hypoxia-induced expression of HIF-1alpha protein in human prostate cancer cell lines are associated with increased cell growth rates and metastatic potential. HIF-1alpha mRNA was undetectable in the normal rat ventral prostate by Northern blot hybridization. HIF-1alpha protein expression and HIF-1 DNA binding activity were detected in normoxic PC-3 cells. Human prostate cancer cells plated at low density manifested higher functional HIF-1alpha expression than cells plated at high density independent of O2 tension. HIF-1alpha may become dysregulated in prostate cancer and thus drive the transcription of hypoxia-adaptive genes involved in tumor progression. This is also the first evidence that human cancer cells can express functional HIF-1alpha protein under normoxic conditions.

Carbon monoxide and nitric oxide suppress the hypoxic induction of vascular endothelial growth factor gene via the 5' enhancer.

Vascular endothelial growth factor (VEGF) plays an important role in angiogenesis and blood vessel remodeling. Its expression is up-regulated in vascular smooth muscle cells by a number of conditions, including hypoxia. Hypoxia increases the transcriptional rate of VEGF via a 28-base pair enhancer located in the 5'-upstream region of the gene. The gas molecules nitric oxide (NO) and carbon monoxide (CO) are important vasodilating agents. We report here that these biological molecules can suppress the hypoxia-induced production of VEGF mRNA and protein in smooth muscle cells. In transient expression studies, both NO and CO inhibited the ability of the hypoxic enhancer we have previously identified to activate gene transcription. Furthermore, electrophoretic mobility shift assays indicated decreased binding of hypoxia-inducible factor 1 (HIF-1) to this enhancer by nuclear proteins isolated from CO-treated cells, although HIF-1 protein levels were unaffected by CO. Given that both CO and NO activate guanylyl cyclase to produce cGMP and that a cGMP analog (8-Br-cGMP) showed a similar suppressive effect on the hypoxic induction of the VEGF enhancer, we speculate that the suppression of VEGF by these two gas molecules occurs via a cyclic GMP-mediated pathway.

Cellular and developmental control of O2 homeostasis by hypoxia-inducible factor 1 alpha.

Hypoxia is an essential developmental and physiological stimulus that plays a key role in the pathophysiology of cancer, heart attack, stroke, and other major causes of mortality. Hypoxia-inducible factor 1 (HIF-1) is the only known mammalian transcription factor expressed uniquely in response to physiologically relevant levels of hypoxia. We now report that in Hif1a-/- embryonic stem cells that did not express the O2-regulated HIF-1alpha subunit, levels of mRNAs encoding glucose transporters and glycolytic enzymes were reduced, and cellular proliferation was impaired. Vascular endothelial growth factor mRNA expression was also markedly decreased in hypoxic Hif1a-/- embryonic stem cells and cystic embryoid bodies. Complete deficiency of HIF-1alpha resulted in developmental arrest and lethality by E11 of Hif1a-/- embryos that manifested neural tube defects, cardiovascular malformations, and marked cell death within the cephalic mesenchyme. In Hif1a+/+ embryos, HIF-1alpha expression increased between E8.5 and E9.5, coincident with the onset of developmental defects and cell death in Hif1a-/- embryos. These results demonstrate that HIF-1alpha is a master regulator of cellular and developmental O2 homeostasis.

Characterization of gene expression of a p53 homologue in the soft-shell clam (Mya arenaria).

Expression of a clam p53 homologue was examined in tissues of the soft-shell clam, Mya arenaria, from Beal's Island, Maine. Southern analysis reveals that p53, in this population, is a single copy gene. A 1.7 to 1.9-kb p53 mRNA was detected at very low levels in normal adult gonadal tissue. This transcript is similar in size to that of vertebrate p53 genes. RNAs were harvested from several tissues, including individual clam gonads during gametogenesis. These were hybridized in ribonuclease (RNase) protection assays to a p53 antisense probe designed from the clam p53 cDNA sequence. RNase protection profiles indicate that p53 mRNA is expressed in adductor muscle, gill, and gonads of both sexes. Although p53 mRNA is expressed throughout gametogenesis in mature male and female gonads, ovaries have significantly higher levels of expression. The significance of our findings to the study of normal clam gametogenesis and to etiology of gonadal tumors is discussed.

Perinatal lethality and multiple craniofacial malformations in MSX2 transgenic mice.

MSX2 is a homeodomain transcription factor that has been implicated in craniofacial morphogenesis on the basis of its expression pattern during mouse development and the finding of a missense mutation (P148H) in humans affected with Boston-type craniosynostosis. We have generated transgenic mice carrying a 34 kb DNA fragment encompassing a human MSX2 gene encoding either wild-type or mutant (P148H) MSX2. Inheritance of either transgene resulted in perinatal lethality and multiple craniofacial malformations of varying severity, including mandibular hypoplasia, cleft secondary palate, exencephaly, and median facial cleft, which are among the severe craniofacial malformations observed in humans. Transgenic mice also manifested aplasia of the interparietal bone and decreased ossification of the hyoid. Transgene-induced malformations involved cranial neural-crest derivatives, were characterized by a deficiency of tissue, and were similar to malformations associated with embryonic exposure to ethanol or retinoic acid, teratogens that cause increased cell death. Together with previous observations implicating MSX2 expression in developmentally-programmed cell death, these results suggest that wild-type levels of MSX2 activity may establish a balance between survival and apoptosis of neural crest-derived cells required for proper craniofacial morphogenesis.