CTGF is a therapeutic target for metastatic melanoma

Metastatic melanoma remains a devastating disease with a 5-year survival rate of less than five percent. Despite recent advances in targeted therapies for melanoma, only a small percentage of melanoma patients experience durable remissions. Therefore, it is critical to identify new therapies for the treatment of advanced melanoma. Here, we define connective tissue growth factor (CTGF) as a therapeutic target for metastatic melanoma. Clinically, CTGF expression correlates with tumor progression and is strongly induced by hypoxia through HIF-1 and HIF-2-dependent mechanisms. Genetic inhibition of CTGF in human melanoma cells is sufficient to significantly reduce orthotopic tumor growth, as well as metastatic tumor growth in the lung of severe combined immunodeficient (SCID) mice. Mechanistically, inhibition of CTGF decreased invasion and migration associated with reduced matrix metalloproteinase-9 expression. Most importantly, the anti-CTGF antibody, FG-3019, had a profound inhibitory effect on the progression of established metastatic melanoma. These results offer the first preclinical validation of anti-CTGF therapy for the treatment of advanced melanoma and underscore the importance of tumor hypoxia in melanoma progression.

The role of hypoxia-inducible factors in tumorigenesis

Hypoxia-inducible factors (HIFs) are essential mediators of the cellular oxygen-signaling pathway. They are heterodimeric transcription factors consisting of an oxygen-sensitive alpha subunit (HIF-alpha) and a constitutive beta subunit (HIF-beta) that facilitate both oxygen delivery and adaptation to oxygen deprivation by regulating the expression of genes that control glucose uptake, metabolism, angiogenesis, erythropoiesis, cell proliferation, and apoptosis. In most experimental models, the HIF pathway is a positive regulator of tumor growth as its inhibition often results in tumor suppression. In clinical samples, HIF is found elevated and correlates with poor patient prognosis in a variety of cancers. In summary, HIF regulates multiple aspects of tumorigenesis, including angiogenesis, proliferation, metabolism, metastasis, differentiation, and response to radiation therapy, making it a critical regulator of the malignant phenotype.

The Use of Plasma Surface-Enhanced Laser Desorption/Ionization Time-of-Flight Mass Spectrometry Proteomic Patterns for Detection of Head and Neck Squamous Cell Cancers

PURPOSE

Our study was undertaken to determine the utility of plasma proteomic profiling using surface-enhanced laser desorption/ionization time-of-flight (SELDI-TOF) mass spectrometry for the detection of head and neck squamous cell carcinomas (HNSCCs).

EXPERIMENTAL DESIGN

Pretreatment plasma samples from HNSCC patients or controls without known neoplastic disease were analyzed on the Protein Biology System IIc SELDI-TOF mass spectrometer (Ciphergen Biosystems, Fremont, CA). Proteomic spectra of mass:charge ratio (m/z) were generated by the application of plasma to immobilized metal-affinity-capture (IMAC) ProteinChip arrays activated with copper. A total of 37356 data points were generated for each sample. A training set of spectra from 56 cancer patients and 52 controls were applied to the "Lasso" technique to identify protein profiles that can distinguish cancer from noncancer, and cross-validation was used to determine test errors in this training set. The discovery pattern was then used to classify a separate masked test set of 57 cancer and 52 controls. In total, we analyzed the proteomic spectra of 113 cancer patients and 104 controls.

RESULTS

The Lasso approach identified 65 significant data points for the discrimination of normal from cancer profiles. The discriminatory pattern correctly identified 39 of 57 HNSCC patients and 40 of 52 noncancer controls in the masked test set. These results yielded a sensitivity of 68% and specificity of 73%. Subgroup analyses in the test set of four different demographic factors (age, gender, and cigarette and alcohol use) that can potentially confound the interpretation of the results suggest that this model tended to overpredict cancer in control smokers.

CONCLUSIONS

Plasma proteomic profiling with SELDI-TOF mass spectrometry provides moderate sensitivity and specificity in discriminating HNSCC. Further improvement and validation of this approach is needed to determine its usefulness in screening for this disease.

Genetic determinants that influence hypoxia-induced apoptosis

The p53 tumour suppressor gene is capable of activating both death receptor and mitochondrial-signalled forms of apoptotic cell death in response to diverse stimuli. Studies have suggested that impairment of the mitochondrial-signalled Apaf/caspase 9 pathway and not the death receptor Fas pathway results in almost complete resistance to apoptotic cell death induced by a low oxygen environment. However, it is unclear how p53 signals the activation of this pathway and whether it is through already identified p53 effector genes such as the pro-apoptotic gene bax, or through novel effectors such as BNIP-3/BNIP-3L. Comparison of cell lines genetically matched at the bax, cytochrome c, apaf, caspase 9 and caspase 3 loci indicated that except for bax, all of these genes were essential for hypoxia induced apoptosis both in cell culture and in transplanted tumours. These data imply that cytochrome c plays a pivotal role in signalling cell death by apoptosis under hypoxic conditions, and that the release of cytochrome c is independent of both Bax and p53. In contrast to cytochrome c, p53 modulates the magnitude of apoptosis under hypoxic conditions, but in itself is not required for the activation of the caspase cascade.

Anticancer efficacy of systemically delivered anaerobic bacteria as gene therapy vectors targeting tumor hypoxia/necrosis

A major obstacle in cancer gene therapy is selective tumor delivery. Previous studies have suggested that genetically engineered anaerobes of the genus Clostridium might be gene therapy vectors because of their ability to proliferate selectively in the hypoxic/necrotic regions common to solid tumors. However, the tumor colonization efficiency of the strain previously used was insufficient to produce any antitumor effect. Here we describe for the first time the successful transformation of C. sporogenes, a clostridial strain with the highest reported tumor colonization efficiency, with the E. coli cytosine deaminase (CD) gene and show that systemically injected spores of these bacteria express CD only in the tumor. This enzyme can convert the nontoxic prodrug 5-fluorocytosine (5-FC) to the anticancer drug 5-fluorouracil (5-FU). Furthermore, systemic delivery of 5-FC into mice previously injected with CD-transformed spores of C. sporogenes produced greater antitumor effect than maximally tolerated doses of 5-FU. Since most human solid tumors have hypoxic and necrotic areas this vector system has considerable promise for tumor-selective gene therapy.

Hypoxia regulates insulin-like growth factor-binding protein 1 in human fetal hepatocytes in primary culture: suggestive molecular mechanisms for in utero fetal growth restriction caused by uteroplacental insufficiency

Intrauterine growth restriction (IUGR) can be a consequence of decreased uterine blood flow (uteroplacental insufficiency) and maternal and fetal hypoxia. Insulin-like growth factors (IGFs) and their binding proteins (IGFBPs) are key elements in fetal growth. IGF-I is a major growth promoter in utero. IGFBP-1 is primarily made in the liver, and it mostly inhibits IGF actions at the cellular level. IGFBP-1 is elevated in the fetal circulation of human and animal pregnancies complicated by IUGR caused by placental insufficiency and in utero hypoxia and is believed to restrict fetal growth by sequestering IGFs. In this study, we developed a protocol to establish highly pure primary cultures of human fetal hepatocytes in vitro and investigated their expression of IGFBP-1 messenger RNA (mRNA) and protein and the effects of hypoxia on their expression of IGFBP-1 mRNA and protein. Hepatocytes were isolated from second-trimester human fetal livers (n = 7) and purified by Percoll gradient centrifugation. Hepatocyte cultures were characterized by immunocytochemistry and were compared with hepatocytes in situ in human fetal liver tissue, by immunohistochemistry, using specific antibodies and indirect immunofluorescence. Cultures consisted primarily (>90%) of cells positive for cytokeratin 18, fibrinogen, and IGFBP-1, with less than 2% vascular cells and less than 8% macrophages. Identification of isolated hepatocytes was further confirmed by morphology. Hepatocytes were cultured in defined medium, and Northern analysis revealed expression of a 1.5-kb IGFBP-1 mRNA transcript in hepatocytes cultured under normoxic conditions, for 24 h, that did not increase in steady-state levels after 48 h in culture. Under hypoxic conditions (2% O(2)), IGFBP-1 mRNA expression increased 3- to 4-fold, compared with normoxic controls. Cells cultured under 10% O(2) did not demonstrate an increase in IGFBP-1 mRNA levels. IGFBP-1 protein in conditioned medium (CM) was measured by immunoradiometric assay and increased 3- to 4-fold under hypoxic (2% O(2)), compared with normoxic, conditions. Western ligand blot analysis of CM revealed the presence of IGFBP-1, IGFBP-2, IGFBP-3, and IGFBP-4. IGFBP-1 was the most abundant IGFBP in CM, and densitometric analysis revealed a 2.5-fold increase in IGFBP-1 under hypoxic, compared with normoxic, conditions, supporting the immunoradiometric assay results. A 3-fold increase in IGFBP-3 mRNA, but not other IGFBPs, was noted under hypoxic, compared with normoxic, conditions. This study demonstrates that human fetal hepatocytes can be cultured in defined medium, as primary cultures with high purity, and that they express IGFBP-1 mRNA and secrete IGFBP-1 protein in vitro. In addition, the data demonstrate that hypoxia up-regulates fetal hepatocyte IGFBP-1 mRNA steady-state levels and protein, with this being the major IGFBP derived from the fetal hepatocyte. The data support a role for the fetal liver as a source of elevated circulating levels of IGFBP-1 in fetuses with in utero hypoxia and IUGR.

Hypoxia activates a platelet-derived growth factor receptor/phosphatidylinositol 3-kinase/Akt pathway that results in glycogen synthase kinase-3 inactivation

Hypoxia initiates numerous intracellular signaling pathways important in regulating cell proliferation, differentiation, and death. In this study, we investigated the pathway that hypoxia uses to activate Akt and inactivate glycogen synthase kinase-3 (GSK-3), two proteins the functions of which are important in cell survival and energy metabolism. Severe hypoxia (0.01% oxygen) initiated a signaling cascade by inducing the tyrosine phosphorylation of the platelet-derived growth factor (PDGF) receptor within 1 h of treatment and increasing receptor association with the p85 subunit of phosphatidylinositol 3-kinase (PI 3-K). Hypoxia-induced signaling also resulted in PI 3-K-dependent phosphorylation of Akt on Ser-473, a modification of Akt that is important for its activation. This activation of Akt by hypoxia was substantially diminished in cells that possessed mutations in their PDGF receptor-PI 3-K interaction domain. In addition, Akt activation by hypoxia was resistant to treatment with the growth factor receptor poison suramin but was sensitive to treatment with the PI 3-K inhibitor wortmannin. Activation of Akt by hypoxia resulted in the phosphorylation of GSK-3alpha and GSK-3beta at Ser-9 and Ser-21, two well-documented Akt phosphorylation sites, respectively, that are inactivating modifications of each GSK-3 isoform. In support of the phosphorylation data, GSK-3 activity was significantly reduced under hypoxia. In conclusion, we propose that hypoxia activates a growth factor receptor/PI 3-K/Akt cascade that leads to GSK-3 inactivation, a pathway that can impact cell survival, proliferation, and metabolism.

p21(Cip1) and p27(Kip1) regulate cell cycle reentry after hypoxic stress but are not necessary for hypoxia-induced arrest

We investigated the role of the cyclin-dependent kinase inhibitors p21(Cip1) and p27(Kip1) in cell cycle regulation during hypoxia and reoxygenation. While moderate hypoxia (1 or 0.1% oxygen) does not significantly impair bromodeoxyuridine incorporation, at very low oxygen tensions (0.01% oxygen) DNA replication is rapidly shut down in immortalized mouse embryo fibroblasts. This S-phase arrest is intact in fibroblasts lacking the cyclin kinase inhibitors p21(Cip1) and p27(Kip1), indicating that these molecules are not essential elements of the arrest pathway. Hypoxia-induced arrest is accompanied by dephosphorylation of pRb and inhibition of cyclin-dependent kinase 2, which results in part from inhibitory phosphorylation. Interestingly, cells lacking the retinoblastoma tumor suppressor protein also display arrest under hypoxia, suggesting that pRb is not an essential mediator of this response. Upon reoxygenation, DNA synthesis resumes by 3.5 h and reaches aerobic levels by 6 h. Cells lacking p21, however, resume DNA synthesis more rapidly upon reoxygenation than wild-type cells, suggesting that this inhibitor may play a role in preventing premature reentry into the cell cycle upon cessation of the hypoxic stress. While p27 null cells did not exhibit rapid reentry into the cell cycle, cells lacking both p21 and p27 entered S phase even more aggressively than those lacking p21 alone, revealing a possible secondary role for p27 in this response. Cdk2 activity is also restored more rapidly in the double-knockout cells when returned to normoxia. These studies reveal that restoration of DNA synthesis after hypoxic stress, but not the S phase arrest itself, is regulated by p21 and p27.

Regulation of insulin-like growth factor-binding protein 1 by hypoxia and 3',5'-cyclic adenosine monophosphate is additive in HepG2 cells

Insulin-like growth factor-binding protein 1 (IGFBP-1) is important in regulating minute-to-minute IGF bioavailability in the circulation and is primarily an inhibitor of IGF action systemically and in most cellular systems. Understanding regulation of IGFBP-1 is, thus, important in understanding regulation of IGF actions. The IGFBP-1 promoter contains a cAMP response element, and cAMP stimulates IGFBP-1 gene expression at the transcriptional level. Recently, we have found three consensus sequences for the hypoxia response element in intron 1 of the IGFBP-1 gene. Herein, we have investigated the effects of hypoxia and a cAMP analog, 8-bromoadenosine-3',5'-cyclic monophosphate (8-Br-cAMP), on IGFBP-1 expression in HepG2 cells, a model system for IGFBP-1 gene regulation. HepG2 cells were exposed to normoxia (20% pO2) or hypoxia (2% pO2) for 24 h in the absence or presence of 8-Br-cAMP (0.1, 0.5, and 1 mM). Western ligand blotting revealed IGFBP-1 as the predominant IGFBP in HepG2-conditioned media, which increased in a dose-dependent manner after incubation with 8-Br-cAMP in normoxia and hypoxia (3-fold and 7-fold at 1 mM, respectively). Under hypoxic, compared with normoxic, conditions, IGFBP-1 protein and messenger RNA (mRNA) levels increased approximately 10-fold and 20-fold, respectively. In normoxia, 8-Br-cAMP stimulated IGFBP-1 protein and mRNA levels in a dose-dependent manner (7-fold and 10-fold at 1 mM). Hypoxia and 8-Br-cAMP showed additive stimulatory effects on IGFBP-1 protein and mRNA levels (35-fold and 50-fold at 1 mM) that were time and dose dependent. Primary transcripts of IGFBP-1 mRNA were increased concordantly with IGFBP-1 mRNA. The half-life of the IGFBP-1 mRNA was markedly increased (approximately 6-fold) by hypoxia, and cAMP minimally enhanced this effect. These results demonstrate that hypoxia and compounds that increase intracellular cAMP additively regulate IGFBP-1 gene expression by transcriptional and posttranscriptional mechanisms. Regulation of IGFBP-1 mRNA and protein by cAMP and hypoxia may be important for understanding the physiologic and pathophysiologic roles of IGFBP-1.

p53 promotes selection for Fas-mediated apoptotic resistance

Although p53 inactivation is implicated as a mechanism to explain diminished apoptotic response, it is clear that tumor cells that possess transcriptionally functional p53 can also be resistant to diverse apoptotic stimuli. We hypothesize that oncogenic activation and DNA damage are sufficient stimuli to increase the p53-dependent transcription of Fas and thereby establish a situation in which cell to cell contact could be a selective pressure to either lose p53 function or inactivate components of the Fas death pathway. Examination of genetically matched tumor cell lines that possessed either wild-type or null p53 loci indicated that cells possessing functional p53 increased their surface levels of Fas and Fas ligand (FasL) in response to DNA damage. In contrast, stress induced by changes in the tumor microenvironment such as decreased oxygen did not up-regulate Fas or FasL. Cells with wild-type p53 underwent Fas-mediated killing in the presence of either FasL-expressing killer cells or activating Fas antibodies, whereas cells in which p53 was deleted or inactivated were protected from such killing. Furthermore, Fas and FasL expression and induction became transcriptionally repressed in transformed cells with wild-type p53 with increasing passage, whereas other p53 downstream targets and functions, such as p21 inducibility and cell cycle arrest, remained intact. Repression of the Fas locus could be reverted by treatment with the histone deacetylase inhibitor trichostatin A. These results support a model of tumor progression in which oncogenic transformation drives tumor cells to lose either p53 or their Fas sensitivity as a means of promoting their survival and evade immune surveillance.

Regulation of insulin-like growth factor-binding protein-1 by nitric oxide under hypoxic conditions

Nitric oxide (NO) is believed to play an important, but as yet undefined, role in regulating hypoxia inducible gene expression. Recently, we have reported evidence suggesting that the human insulin-like growth factor-binding protein-1 (IGFBP-1) gene is directly regulated by hypoxia through the hypoxia-inducible factor-1 pathway. The goal of the current study was to investigate NO regulation of hypoxic induction of IGFBP-1 gene expression using HepG2 cells, a model system of hepatic gene expression. We report that a NO generator, sodium nitroprusside, significantly diminishes hypoxic activation of IGFBP-1 protein and messenger ribonucleic acid expression. Furthermore, these effects are independent of guanylate cyclase/ cGMP signaling, as two different inhibitors, LY 83583, a specific inhibitor of guanylate cyclase, and KT 5823, a protein kinase G inhibitor, had no effect on IGFBP-1 induction by hypoxia. Hypoxic induction of a reporter gene containing four tandemly ligated hypoxia response elements was completely blocked by sodium nitroprusside, but not by 8-bromo-cGMP, an analog ofcGMP. These results suggest that NO blocks hypoxic induction of IGFBP-1 by a guanylate cyclase/ cGMP-independent pathway, possibly at the level of oxygen sensing. The impaired hypoxia regulation of IGFBP-1 by nitric oxide may play a key role in the hyperinduction of IGFBP-1 observed in pathophysiological conditions such as fetal hypoxia and preeclampsia where dysregulation of NO has been observed.

p53 checkpoint-defective cells are sensitive to X rays, but not hypoxia

X-ray-induced damage leads to cell-cycle "checkpoint" arrest by p53-dependent induction of the cyclin-dependent kinase inhibitor p21 (Waf1/Cip1/Sdi1). Human tumor cells that lack this response fail to arrest after exposure to DNA-damaging agents, undergo multiple rounds of endoreduplicative DNA synthesis, and eventually commit to an apoptotic cell death. Since low oxygen tension can also induce p53 protein accumulation, and can lead to cell-cycle arrest or apoptosis, we examined the expression of p21 in tumor cells under normoxic and hypoxic conditions. In a survey of cells, mRNA for the p21 gene was induced two- to threefold in response to hypoxia in a seemingly p53-independent manner. We therefore examined genetically matched cells that differ in their p21 and p53 status for response to ionizing radiation and hypoxia. We found that both p21-deficient and p53-deficient cells exhibit an increase in chromosome instability, an increased level of apoptosis, and a failure to arrest after exposure to ionizing radiation. However, cells that lack either p21 or p53 exhibit no increase in chromosome instability or elevated apoptosis and still arrest in response to hypoxia. Thus, the mechanism responsible for the differential response to either hypoxia or X rays presumably lies in the control of cell-cycle progression in response to stress and its dependence on p21. Since the loss of a DNA-damage-dependent checkpoint does not sensitize cells to killing by stresses that elicit a DNA-damage-independent checkpoint, targeting the function of p21 pharmacologically will not kill tumor cells in situ in the absence of a DNA damage signal.

Opposing effects of hypoxia on expression of the angiogenic inhibitor thrombospondin 1 and the angiogenic inducer vascular endothelial growth factor

Tumor angiogenesis, the development of new blood vessels during malignant progression, is a regulated process that has both genetic and physiological controls. Physiologically, angiogenesis is stimulated by decreases in tissue oxygenation (i.e., hypoxia). We investigated the effect of hypoxia on the expression of two angiogenic factors reported to be genetically regulated by the p53 tumor suppressor gene: (a) the angiogenic inhibitor thrombospondin 1 (TSP-1); and (b) the angiogenic inducer vascular endothelial growth factor (VEGF). Analysis of rodent cells that differ in their p53 genotype (p53+/+ or p53-/-) indicated that in vitro exposure to hypoxia simultaneously suppressed TSP-1 and induced VEGF expression, regardless of the p53 genotype. On transformation of these cells with E1A and oncogenic H-ras, the basal level of TSP-1 expression was strongly diminished, whereas that of VEGF could still be induced by hypoxia. Consistent with these in vitro findings, sections of tumors derived from the transformed p53+/+ and p53-/- cells showed that VEGF protein overlapped with regions of hypoxia, whereas TSP-1 protein was below the limits of detection in tumor tissue. Using a panel of normal/immortalized and transformed human cells, it was found that the ability of hypoxia to inhibit TSP-1 expression depends on the cell type and/or the degree of transformation. In contrast, VEGF expression was induced by hypoxia in all of the human cell types examined. Together, these findings suggest that hypoxic and oncogenic signals could interact in the tumor microenvironment to inhibit TSP-1 and induce VEGF expression, promoting the switch to the angiogenic phenotype.

Development of a hypoxia-responsive vector for tumor-specific gene therapy

We are developing new gene therapy vectors whose expression is selectively activated by hypoxia, a unique feature of human solid tumors. As vascular endothelial growth factor (VEGF) is upregulated by hypoxia, such regulatory mechanisms would enable us to achieve hypoxia-inducible expression of therapeutic genes. Constructs with five copies of hypoxia-responsive elements (HREs) derived from the 5'-untranslated region (UTR) of the human VEGF showed excellent transcriptional activation at low oxygen tension relevant to tumor hypoxia. In an attempt to achieve higher responsiveness, various combinations of HREs and promoters were examined. In addition, we also investigated whether the 3' UTR of the VEGF gene would confer increased post-transcriptional mRNA stability under hypoxic conditions. However, despite increases in the hypoxic/aerobic ratio of luciferase activity, gene expression with 3' UTR was lower due to mRNA destabilization by AU-rich elements (AREs). Thus, we found no benefit from the inclusion of the 3' UTR in our vectors. Of all the vectors tested, the combination of 5HRE and a CMV minimal promoter exhibited hypoxia responsiveness (over 500-fold) to the similar level to the intact CMV promoter. We propose that this vector would be useful for tumor selective gene therapy.

Loss of PTEN facilitates HIF-1-mediated gene expression

In glioblastoma-derived cell lines, PTEN does not significantly alter apoptotic sensitivity or cause complete inhibition of DNA synthesis. However, in these cell lines PTEN regulates hypoxia- and IGF-1-induced angiogenic gene expression by regulating Akt activation of HIF-1 activity. Restoration of wild-type PTEN to glioblastoma cell lines lacking functional PTEN ablates hypoxia and IGF-1 induction of HIF-1-regulated genes. In addition, Akt activation leads to HIF-1alpha stabilization, whereas PTEN attenuates hypoxia-mediated HIF-1alpha stabilization. We propose that loss of PTEN during malignant progression contributes to tumor expansion through the deregulation of Akt activity and HIF-1-regulated gene expression.

Candidate genes for the hypoxic tumor phenotype

In this study, we have analyzed changes induced by hypoxia at the transcriptional level of genes that could be responsible for a more aggressive phenotype. Using a series of DNA array membranes, we identified a group of hypoxia-induced genes that included plasminogen activator inhibitor-1 (PAI-1), insulin-like growth factor-binding protein 3 (IGFBP-3), endothelin-2, low-density lipoprotein receptor-related protein (LRP), BCL2-interacting killer (BIK), migration-inhibitory factor (MIF), matrix metalloproteinase-13 (MMP-13), fibroblast growth factor-3 (FGF-3), GADD45, and vascular endothelial growth factor (VEGF). The induction of each gene was confirmed by Northern blot analysis in two different squamous cell carcinoma-derived cell lines. We also analyzed the kinetics of PAI-1 induction by hypoxia in more detail because it is a secreted protein that may serve as a useful molecular marker of hypoxia. On exposure to hypoxia, there was a gradual increase in PAI-1 mRNA between 2 and 24 h of hypoxia followed by a rapid decay after 2 h of reoxygenation. PAI-1 levels were also measured in the serum of a small group of head and neck cancer patients and were found to correlate with the degree of tumor hypoxia found in these patients.

Hypoxia induces p53 accumulation through MDM2 down-regulation and inhibition of E6-mediated degradation

Hypoxia, a result of DNA-damaging agents such as ionizing radiation, induces the nuclear accumulation of the p53 tumor suppressor protein. However, unlike the effect in ionizing radiation, hypoxia readily induces the nuclear accumulation of p53 in HPV E6-infected cells. In HPV-infected cells, a key regulator of p53 protein levels is the E6 oncoprotein. In association with the endogenous cellular protein E6-associated protein (E6AP), E6 can accelerate the degradation of p53 under aerobic conditions. To better define the mechanism of p53 induction in E6-infected cells by hypoxia, we studied the expression and association of E6 and E6AP with p53 in vivo. We found that hypoxia did not alter the protein levels of E6 or E6AP as compared with those found under aerobic growth conditions, indicating that protein inhibition of E6 or E6AP alone is not sufficient to explain the increased accumulation of p53 under hypoxic conditions. However, p53 did fail to coprecipitate with E6AP under hypoxia, indicating that hypoxia uncouples the interaction of p53 with E6 and E6AP. We also present evidence to indicate that hypoxia decreases the expression of the endogenous cellular regulator of p53 protein, the human MDM2 protein, resulting in an inhibition of p53 export from the nucleus to the cytoplasm for degradation. Taken together, these results suggest that the hypoxic induction of p53 is attributable to the down-regulation of MDM2 protein levels and uncoupling of p53 from its interaction with the E6/E6AP complex.

A p53 and apoptotic independent role for p21waf1 in tumour response to radiation therapy

Loss of p21 in human cancer cells results in checkpoint failure, induction of polyploidy and subsequent apoptosis following DNA damage. Tumours in immunodeficient mice derived from cells lacking p21 are also more sensitive to ionizing radiation than their wild-type counterparts. Abrogation of p53 in the p21+/+ parental cells results in an in vitro phenotype that is indistinguishable from that of the p21 knockout cells. Thus, the in vitro phenotype resulting from loss of p21 is consistent with its well-established role in the p53/p21 damage response pathway. However, despite the similar in vitro phenotype, p21+/+ cells with abrogated p53 show no evidence of the sensitivity observed in the p21-/- cells when grown as tumours in immunodeficient mice. The increased radio-sensitization stabilization of p21-/- tumours is also unrelated to the increase in apoptosis observed in these tumours following radiation treatment. Apoptosis in the p21-/- tumours was significantly reduced by expression of bcl-2 without any corresponding change in the overall response of the tumour. Similarly, abrogation of p53 in the p21+/+ tumours substantially increased radiation-induced apoptosis within the tumours without increasing their radiation sensitivity. Dissociation of these in vivo and in vitro phenotypes indicates that p21 participates in a novel in vivo specific damage response pathway that is distinct from its role in the p53 pathway, and therefore that it may be an effective therapeutic target for cancer therapy.

Hypoxic microenvironment within an embryo induces apoptosis and is essential for proper morphological development

Recent studies have suggested the importance of hypoxia-inducible transcription factors in development, yet the questions of whether hypoxia actually exists in a developing embryo in vivo and, if so, what role it plays in development remain unanswered. In this study, we directly demonstrate that regions of hypoxia, most prominently the hindbrain, otic vesicle, and first branchial arch, exist in a gestational day (GD) 11 rat embryo grown in utero. We also show that varying the oxygen environment of an embryo affects its morphological development. Rat embryos which were grown at 45% oxygen from GD 9-11 showed gross morphological abnormalities, including defective cranial neural tube closure, incomplete otic vesicle invagination, and abnormal somite formation and embryo turning. These embryos, in addition, exhibited reduced cell death. On the other hand, embryos which were grown at 5% oxygen during the same period were stunted in overall growth, yet morphologically normal, and displayed prominent areas of apoptosis. In this study, we propose that embryonic development, like tumor development, requires two different but interactive sets of signals. One set exists in the genetic program for development; the other set arises from changes in the microenvironment of the embryo. Therefore, it is the interplay between these two sets of cues that drives normal embryonic development. The requirement for hypoxia to activate apoptotic cell death is but one example of such interactions.

Hypoxia and cAMP stimulate vascular endothelial growth factor (VEGF) in human endometrial stromal cells: potential relevance to menstruation and endometrial regeneration

The human female reproductive tract shows unique cycle-specific changes in vascularization. Vascular endothelial growth factor (VEGF) is a specific vascular endothelial mitogen which is produced by human endometrium and is known to be regulated by steroid hormones. Vasoconstriction during menstruation leads to endometrial hypoxia, a possible stimulus for angiogenesis. In the current study we tested the hypothesis that hypoxia and cAMP, a known stimulus for endometrial decidualization, can induce VEGF in human endometrial stromal cells. Decidualized as well as non decidualized stromal cells from 6 patients were exposed to normoxia (20% oxygen) and hypoxia (2% oxygen) for up to 72h. VEGF levels were assessed by Northern analysis using a 605 bp BamHI fragment of the human VEGF cDNA, and hybridization signals were normalized to levels of 18S RNA. VEGF protein was determined by ELISA. Hypoxia stimulated VEGF mRNA in decidualized stromal cells by 10.2 fold at 48h compared to normoxic controls. VEGF protein increased 10 fold by 48h and increased further to 13 fold at 72h. In the presence of 2% oxygen VEGF mRNA in nondecidualized endometrial stromal cells was increased 1.2-8 fold between 2 and 72h of treatment. VEGF protein also increased 1.2-9 fold in this time period. cAMP regulated both VEGF mRNA and protein in non decidualized stromal cells. VEGF mRNA increased 2-4 fold in 2-72h and protein production showed a 2-6 fold increase. VEGF was seen to be regulated by both cAMP and hypoxia in an additive manner. These results demonstrate that both non-decidualized and decidualized endometrial stromal cells respond to hypoxia with increasing levels of VEGF. 8Br-cAMP, which is shown to increase VEGF levels in endometrial cells per se, has an additive effect on VEGF production under hypoxic conditions. This effect may have physiologic and pathophysiologic relevance during the process of menstruation and in post menstrual endometrial repair and angiogenesis.

Mitogen-activated protein kinase phosphatase-1 (MKP-1) expression is induced by low oxygen conditions found in solid tumor microenvironments. A candidate MKP for the inactivation of hypoxia-inducible stress-activated protein kinase/c-Jun N-terminal protein kinase activity

Pathophysiological hypoxia is an important modulator of gene expression in solid tumors and other pathologic conditions. We observed that transcriptional activation of the c-jun proto-oncogene in hypoxic tumor cells correlates with phosphorylation of the ATF2 transcription factor. This finding suggested that hypoxic signals transmitted to c-jun involve protein kinases that target AP-1 complexes (c-Jun and ATF2) that bind to its promoter region. Stress-inducible protein kinases capable of activating c-jun expression include stress-activated protein kinase/c-Jun N-terminal protein kinase (SAPK/JNK) and p38 members of the mitogen-activated protein kinase (MAPK) superfamily of signaling molecules. To investigate the potential role of MAPKs in the regulation of c-jun by tumor hypoxia, we focused on the activation SAPK/JNKs in SiHa human squamous carcinoma cells. Here, we describe the transient activation of SAPK/JNKs by tumor-like hypoxia, and the concurrent transcriptional activation of MKP-1, a stress-inducible member of the MAPK phosphatase (MKP) family of dual specificity protein-tyrosine phosphatases. MKP-1 antagonizes SAPK/JNK activation in response to diverse environmental stresses. Together, these findings identify MKP-1 as a hypoxia-responsive gene and suggest a critical role in the regulation of SAPK/JNK activity in the tumor microenvironment.

Apaf-1 and caspase-9 in p53-dependent apoptosis and tumor inhibition

The ability of p53 to promote apoptosis in response to mitogenic oncogenes appears to be critical for its tumor suppressor function. Caspase-9 and its cofactor Apaf-1 were found to be essential downstream components of p53 in Myc-induced apoptosis. Like p53 null cells, mouse embryo fibroblast cells deficient in Apaf-1 and caspase-9, and expressing c-Myc, were resistant to apoptotic stimuli that mimic conditions in developing tumors. Inactivation of Apaf-1 or caspase-9 substituted for p53 loss in promoting the oncogenic transformation of Myc-expressing cells. These results imply a role for Apaf-1 and caspase-9 in controlling tumor development.

Highly conserved RNA sequences that are sensors of environmental stress

The putative function of highly conserved regions (HCRs) within 3' untranslated regions (3'UTRs) as regulatory RNA sequences was efficiently and quantitatively assessed by using modular retroviral vectors. This strategy led to the identification of HCRs that alter gene expression in response to oxidative or mitogenic stress. Databases were screened for UTR sequences of >100 nucleotides that had retained 70% identity over more than 300 million years of evolution. The effects of 10 such HCRs on a standard reporter mRNA or protein were studied. To this end, we developed a modular retroviral vector that can allow for a direct comparison of the effects of different HCRs on gene expression independent of their gene-intrinsic 5'UTR, promoter, protein coding region, or poly(A) sequence. Five of the HCRs tested decreased mRNA steady-state levels 2- to 10-fold relative to controls, presumably by altering mRNA stability. One HCR increased translation, and one decreased translation. Elevated mitogen levels caused four HCRs to increase protein levels twofold. One HCR increased protein levels fourfold in response to hypoxia. Although nonconserved UTR sequences may also have a role, these results provide evidence that sequences that are highly conserved during evolution are good candidates for RNA motifs with posttranscriptional regulatory functions in gene expression.

Hypoxia stimulates insulin-like growth factor binding protein 1 (IGFBP-1) gene expression in HepG2 cells: a possible model for IGFBP-1 expression in fetal hypoxia

IGFBP-1 is elevated in fetuses with long-term, chronic hypoxia and intrauterine growth restriction. We investigated the hypothesis that hypoxia regulates IGFBP-1 in the human fetus in vivo and IGFBP-1 gene expression and protein in vitro. Umbilical artery IGFBP-1 levels (mean +/- SEM) from term babies with respiratory acidosis (acute hypoxia), normal babies, and those with mixed respiratory/metabolic acidosis (more profound and prolonged hypoxia) were measured using an immunoradiometric assay. IGFBP-1 levels were similar in normal (n = 12) and acutely hypoxic (n = 6) babies (189.1 +/- 71.8 vs. 175.8 +/- 45.9 ng /ml, respectively, P = 0.789). However, with more profound and prolonged hypoxia (n = 19), IGFBP-1 levels were markedly elevated (470.6 +/- 80.0 ng /ml, P = 0.044). To investigate IGFBP-1 regulation by hypoxia in vitro, HepG2 cells were incubated under hypoxia (pO2 = 2%) and normoxia (pO2 = 20%). IGFBP-1 protein and mRNA increased 8- and 12-fold, respectively, under hypoxic conditions. Hypoxia did not affect protein or mRNA levels of IGFBP-2 or -4. IGFBP-5 and -6 mRNAs, undetectable in control cells, were not induced by hypoxia, whereas minimally expressed IGFBP-3 mRNA increased twofold. Investigation into IGFBP-1 gene structure revealed three potential consensus sequences for the hypoxia response element (HRE) in the first intron. To investigate functionality, a 372-bp fragment of IGFBP-1 intron 1, containing putative HREs, was placed 5' to a heterologous hsp70 promoter in a plasmid using luciferase as a reporter gene. Under hypoxia, reporter gene activity increased up to 30-fold. Mutations in the middle HRE abolished reporter activity in response to hypoxia, suggesting that this HRE is functional in the IGFBP-1 hypoxia response. Cotransfection of HRE reporter genes with a constitutively expressing hypoxia-inducible factor 1 plasmid in HepG2 cells resulted in a fourfold induction of reporter activity, suggesting a role for hypoxia-inducible factor 1 in hypoxia induction of IGFBP-1 gene expression. These data support the hypothesis that hypoxia regulation of IGFBP-1 may be a mechanism operating in the human fetus to restrict insulin-like growth factor-mediated growth in utero under conditions of chronic hypoxia and limited substrate availability.

Heat shock protein 72 modulates pathways of stress-induced apoptosis

The resistance to stress-induced apoptosis conferred by the thermotolerant state or by exogenous expression of HSP72 was measured in mouse embryo fibroblasts. The induction of thermotolerance protects cells from heat, tumor necrosis factor alpha (TNFalpha), and ceramide-induced apoptosis but not from ionizing radiation. Because the development of thermotolerance is associated with increased levels of heat shock proteins, we determined whether constitutive expression of one of the major inducible heat shock proteins, HSP72, could also protect cells from stress-induced apoptosis. Cells expressing constitutive HSP72 were shown to have significantly reduced levels of apoptosis after heat, TNFalpha, and ceramide but not after ionizing radiation. Activation of stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) was found to be strongly inhibited in thermotolerant cells after heat shock but not after other stresses. Cells that constitutively express HSP72 did not demonstrate decreased SAPK/JNK activation after any of these stresses. Thus, factors other than HSP72 that are induced in the thermotolerant state are able to reduce activation of SAPK/JNK after heat stress. Notably, the level of activation of SAPK/JNK did not correlate with the amount of apoptosis detected after different stresses. Constitutive HSP72 expression inhibited poly(ADP-ribose) polymerase cleavage in cells after heat shock and TNFalpha but not after ceramide or ionizing radiation. The results suggest either that SAPK/JNK activation is not required for apoptosis in mouse embryo fibroblasts or that HSP72 acts downstream of SAPK/JNK. Furthermore, the data support the concept that caspase activity, which can be down-regulated by HSP72, is a crucial step in stress-induced apoptosis. Based on data presented here and elsewhere, we propose that the heat shock protein family can be classified as a class of anti-apoptotic genes, in addition to the Bcl-2 and inhibitor of apoptosis protein families of genes.

The E6 and E7 genes of human papilloma virus-type 16 protect primary astrocyte cultures from injury

Many oncogenes are implicated in the regulation of apoptosis as well as in control of the cell cycle, and several have been shown to protect cells from injury. We tested whether the expression of human papillomavirus type 16 genes E6 and E7 could protect primary astrocytes from injury. Retroviral vectors were used to express E6, E7, or E6E7 in primary murine astrocyte cultures. Astrocytes expressing E6E7 suffered less than half the injury seen in controls after exposure to 400 microM H2O2. When we compared astrocytes expressing only E6 or E7, cells expressing E7 alone were protected to a greater extent and from more severe injury than those expressing E6. E6E7, E6 and E7 all provided protection from 30 h glucose deprivation, but again E7 provided the best protection, reducing injury to less than a third of that seen in controls. Expression of E7 alone decreases vulnerability to both hydrogen peroxide and glucose deprivation injury while E6 alone primarily decreases glucose deprivation injury. These results demonstrate that expression of human papillomavirus oncogenes can reduce the vulnerability of astrocytes to oxidative stress and nutrient deprivation.

Direct evidence that apoptosis enhances tumor responses to fractionated radiotherapy

Currently, the contribution of cellular apoptotic sensitivity to tumor response after radiation therapy remains controversial. To address this issue, the survival of Rat-1 fibroblasts containing a 4-hydroxytamoxifen-regulated c-Myc allele, c-MycER (T. D. Littlewood et al., Nucleic Acids Res., 23: 1686-1690, 1995), after single and fractionated doses of radiation was investigated. This model system allows pharmacological regulation of apoptosis sensitivity in the same cells in vitro and as xenograft tumors derived from these cells in vivo (G. I. Evan et al., Cell, 69: 119-128, 1992; R. M. Alarcon et al., Cancer Res., 56: 4315-4319, 1996). Activating c-MycER in vitro resulted in marked sensitization of Rat-1 fibroblasts to the effects of both single-dose and fractionated irradiation as measured by the induction of apoptosis and clonogenic survival. Overexpression of the antiapoptosis protein Bcl-2 suppressed the induction of apoptosis and increased clonogenic survival in cells with activated c-Myc after single-dose and fractionated radiation. Systemic time-release implant delivery of 4-hydroxytamoxifen to severe combined immunodeficient mice bearing Rat-1-MycER tumors over the course of either single-dose (10 Gy) or fractionated (five fractions of 2 Gy) radiotherapy resulted in prolonged tumor growth delay relative to identical tumors from mice that received placebo implants. Furthermore, tumors derived from Rat-1-MycER cells that overexpressed Bcl-2 exhibited shorter tumor growth delays relative to similarly treated Rat-1-MycER tumors. The length of tumor growth delay after single-dose or fractionated radiotherapy strongly correlated with the extent of radiation-induced apoptosis in the xenograft tumors as measured by terminal deoxynucleotidyl transferase-mediated nick end labeling. These in vivo results provide direct evidence that increasing the sensitivity of tumor cells to die by apoptosis increases the efficacy of fractionated radiotherapy by reducing tumor cell clonogenic survival.

The unique physiology of solid tumors: opportunities (and problems) for cancer therapy

The physiology of solid tumors differs from that of normal tissues in a number of important aspects, the majority of which stem from differences between the two vasculatures. Compared with the regular, ordered vasculature of normal tissues, blood vessels in tumors are often highly abnormal, distended capillaries with leaky walls and sluggish flow. Tumor growth also requires continuous new vessel growth, or angiogenesis. These physiological differences can be problems for cancer treatment; for example, hypoxia in solid tumors leads to resistance to radiotherapy and to some anticancer drugs. However, these differences can also be exploited for selective cancer treatment. Here we review four such areas that are under active investigation: (a) hypoxia-selective cytotoxins take advantage of the unique low oxygen tension in the majority of human solid tumors. Tirapazamine, a drug in the final stages of clinical trials, is one of the more promising of these agents; (b) leaky tumor blood vessels can be exploited using liposomes that have been sterically stabilized to have a long intravascular half-life, allowing them to selectively accumulate in solid tumors; (c) the tumor microenvironment is a stimulus to angiogenenesis, and inhibition of angiogenesis can be a powerful anticancer therapy not susceptible to acquired drug resistance; and (d) we discuss attempts to use gene therapy activated either by the low oxygen environment or by necrotic regions of tumors.

Acidosis reduces neuronal apoptosis

Acidosis is a well established concomitant of tissue ischemia. Acidosis in the pH range 6.0-7.0 is seen in cerebral ischemia and within solid tumors. Extracellular acidosis of pH 6.0 and 6.4 provided essentially complete protection from 48 h serum deprivation induced apoptotic death of cultured primary murine neurons. We tested the effect of p53 using transformed mouse embryo fibroblasts of either p53+/+ or p53-/- genotype. Both were markedly protected from serum deprivation by acidity. Hypoxia induced fibroblast injury was also reduced at pH 6.8. Lower pH resulted in a shift from apoptotic to necrotic morphology after 42 h hypoxia. Acidosis reduces apoptosis of both normal and transformed cells, irrespective of p53 status.

p53 mediates apoptosis induced by c-Myc activation in hypoxic or gamma irradiated fibroblasts

Deregulated c-Myc expression leads to a cellular state where proliferation and apoptosis are equally favored depending on the cellular microenvironment. Since the apoptotic sensitivity of many cells is influenced by the status of the p53 tumor suppressor gene, we investigated whether the induction of apoptosis by DNA damage or non-genotoxic stress are also influenced by the p53 status of cells with altered c-Myc activity. Rat-1 fibroblasts expressing a conditional c-Myc allele (c-MycER), were transfected to express an antisense RNA complimentary to p53 mRNA. Expression of antisense p53 RNA decreased p53 protein levels and delayed p53 accumulation following c-Myc activation. Under hypoxic or low serum conditions, cells expressing antisense p53 were substantially more resistant to c-Myc-induced apoptosis than were control cells. c-Myc activation also sensitized Rat-1 cells to radiation-induced apoptosis. Rat-1 cells expressing antisense p53 RNA were more resistant to apoptosis induced by the combined effects of c-Myc activation and gamma irradiation. In a similar manner, apoptosis induced by c-Myc in serum starved, hypoxic or gamma irradiated fibroblasts was also inhibited by Bcl-2. These data indicate that p53 is involved in c-Myc-mediated apoptosis under a variety of stresses which may influence tumor growth, evolution and response to therapy.

Loss of p21Waf1/Cip1 sensitizes tumors to radiation by an apoptosis-independent mechanism

Cellular checkpoints are important mediators of the response of normal cells following genotoxic damage, and interruption of these checkpoints is a common feature of many solid tumors. Although the effects of loss in checkpoint function in tumor cells are well understood in terms of cell cycle control, there is little information on their role in determining treatment efficacy in vivo. We have examined both the in vitro and in vivo responses of isogenic lines differing only in the p53-transactivated checkpoint gene, p21Waf1/Cip1. When assayed in vitro, loss of p21 in human colon tumor cells results in a selective induction of apoptosis [Waldman, T., et al., Nature (Lond.), 381: 713-716, 1996.] but no difference in the clonogenic survival. However, when grown as xenografts and irradiated in situ, p21-deficient tumors were significantly more sensitive to radiation as assessed both by clonogenic survival and by regrowth of the tumors following treatment. These data indicate that loss of p21 results in increased sensitivity to killing by ionizing radiation that is independent of the induction of apoptosis and cell cycle arrest but that is specific to cells when they are grown as a solid tumor. These results have important implications for assessing both the genetic determinants of sensitivity to anticancer agents and efficacy of anticancer agents.

Induction of vascular endothelial growth factor by hypoxia is modulated by a phosphatidylinositol 3-kinase/Akt signaling pathway in Ha-ras-transformed cells through a hypoxia inducible factor-1 transcriptional element

Tumor angiogenesis, the development of new blood vessels, is a highly regulated process that is controlled genetically by alterations in oncogene and tumor suppressor gene expression and physiologically by the tumor microenvironment. Previous studies indicate that the angiogenic switch in Ras-transformed cells may be physiologically promoted by the tumor microenvironment through the induction of the angiogenic mitogen, vascular endothelial growth factor (VEGF). In this report, we show Ras-transformed cells do not use the downstream effectors c-Raf-1 or mitogen activated protein kinases (MAPK) in signaling VEGF induction by hypoxia as overexpression of kinase-defective alleles of these genes does not inhibit VEGF induction under low oxygen conditions. In contrast to the c-Raf-1/MAP kinase pathway, hypoxia increases phosphatidylinositol 3-kinase (PI 3-kinase) activity in a Ras-dependent manner, and inhibition of PI 3-kinase activity genetically and pharmacologically results in inhibition of VEGF induction. We propose that hypoxia modulates VEGF induction in Ras-transformed cells through the activation of a stress inducible PI 3-kinase/Akt pathway and the hypoxia inducible factor-1 (HIF-1) transcriptional response element.

Selection of human cervical epithelial cells that possess reduced apoptotic potential to low-oxygen conditions

Since human papillomavirus (HPV) infection is strongly associated with cervical neoplasia and tumor hypoxia has prognostic significance in human cervical carcinomas, we examined the relationship between hypoxia and apoptosis in human cervical epithelial cells expressing high-risk HPV type 16 oncoproteins. In vitro, hypoxia stimulated both p53 induction and apoptosis in primary cervical epithelial cells infected with the HPV E6 and E7 genes but not in cervical fibroblasts infected with E6 and E7. Furthermore, cell lines derived from HPV-associated human cervical squamous cell carcinomas were substantially less sensitive to apoptosis induced by hypoxia, indicating that these cell lines have acquired additional genetic alterations that reduced their apoptotic sensitivity. Although the process of long-term cell culturing resulted in selection for subpopulations of HPV oncoprotein-expressing cervical epithelial cells with diminished apoptotic potential, the exposure of cells to hypoxia greatly accelerated the selection process. These results provide evidence for the role of hypoxia-mediated selection of cells with diminished apoptotic potential in the progression of human tumors and can in part explain why cervical tumors that possess low pO2 values are more aggressive.

Anaerobic bacteria as a gene delivery system that is controlled by the tumor microenvironment

A fundamental obstacle in gene therapy for cancer treatment is the specific delivery of an anticancer gene product to a solid tumor. Although several strategies exist to control gene expression once a vector is directly introduced into a tumor, as yet no systemic delivery system exists that specifically targets solid tumors. Nonpathogenic, obligate anaerobic bacteria of the genus Clostridium have been used experimentally as anticancer agents because of their selective growth in the hypoxic regions of solid tumors after systemic application. In this report we further describe a novel approach to cancer gene therapy in which genetically engineered clostridia are used as tumor-specific vectors for the delivery of antitumor genes. We have introduced into a strain of C. beijerinckii the gene for an E. coli nitroreductase known to activate the nontoxic prodrug CB 1954 to a toxic anticancer drug. Nitroreductase produced by these clostridia enhanced the killing of tumor cells in vitro by CB 1954, by a factor of 22. To demonstrate the specificity of this approach for tumor targeting, we intravenously injected the inactive spore form of C. beijerinckii, which upon transition to a reproductive state will express the E. coli nitroreductase gene. Nitroreductase activity was detectable in 10 of 10 tumors during the first 5 days after intravenous injection of inactive clostridial spores, indicating a rapid transition from spore to reproductive state. Tumors harboring clostridial spores which did not possess the E. coli nitroreductase gene were devoid of nitroreductase activity. Most importantly, E. coli nitroreductase protein was not found in a large survey of normal mouse tissues following intravenous injection of nitroreductase containing clostridia, strongly suggesting that obligate anaerobic bacteria such as clostridia can be utilized as highly specific gene delivery vectors for cancer therapy.

The DNA damage response in DNA-dependent protein kinase-deficient SCID mouse cells: replication protein A hyperphosphorylation and p53 induction

Severe combined immunodeficient (SCID) mice display an increased sensitivity to ionizing radiation compared with the parental, C.B-17, strain due to a deficiency in DNA double-strand break repair. The catalytic subunit of DNA-dependent protein kinase (DNA-PKCS) has previously been identified as a strong candidate for the SCID gene. DNA-PK phosphorylates many proteins in vitro, including p53 and replication protein A (RPA), two proteins involved in the response of cells of DNA damage. To determine whether p53 and RPA are also substrates of DNA-PK in vivo following DNA damage, we compared the response of SCID and MO59J (human DNA-PKcs-deficient glioblastoma) cells with their respective wild-type parents following ionizing radiation. Our findings indicate that (i) p53 levels are increased in SCID cells following ionizing radiation, and (ii) RPA p34 is hyperphosphorylated in both SCID cells and MO59J cells following ionizing radiation. The hyperphosphorylation of RPA p34 in vivo is concordant with a decrease in the binding of RPA to single-stranded DNA in crude extracts derived from both C.B-17 and SCID cells. These results suggest that DNA-PK is not the only kinase capable of phosphorylating RPA. We conclude that the DNA damage response involving p53 and RPA is not associated with the defect in DNA repair in SCID cells and that the physiological substrate(s) for DNA-PK essential for DNA repair has not yet been identified.

Modulation of c-Myc activity and apoptosis in vivo

We have developed an animal tumor model system to study the effects of c-Myc activation on apoptosis induction in vivo. Tumors were generated in SCID mice from Rat-1 fibroblasts that constitutively express an inactive c-Myc-estrogen receptor fusion protein (T.D. Littlewood et al, Nucleic Acids Res., 23: 1686 -1690, 1995), which is activated in vivo by the administration of 4-hydroxytamoxifen in time release pellets. We demonstrate that activation of c-Myc results in a substantial increase in the number of apoptotic tumor cells and that this apoptosis is predominant in regions of tumor hypoxia. c-Myc-induced apoptosis of hypoxic cells is inhibited in tumors that overexpress the human Bcl-2 protein. Bcl-2, however, does not prevent p53 protein accumulation or the down-regulation of the cyclin-cdk inhibitor p27 protein following c-Myc activation by 4-hydroxytamoxifen. This result suggests that Bcl-2 does not affect c-Myc function directly but acts downstream of c-Myc to inhibit apoptosis. We propose that the ability of activated c-Myc to enhance cellular proliferation might contribute to the genesis of early neoplasms that are held in check by the alternate ability of c-Myc to induce apoptosis of cells that have outgrown their supply of oxygen or other factors associated with hypoxic regions of solid tumors. Secondary genetic lesions downstream of c-Myc that suppress the apoptotic potential of tumor cells, such as Bcl-2 overexpression, might play an important role in the malignant progression of these tumors because they would disrupt the balance between apoptosis and proliferation initiated by c-Myc deregulation.

Oncogenic transformation and hypoxia synergistically act to modulate vascular endothelial growth factor expression

Hypoxia can select for cells that have lost their apoptotic potential, thereby making them resistant to adverse conditions. However, long-term survival of transformed cells which have diminished apoptotic sensitivity when exposed to low oxygen conditions would require the activation of their angiogenic program to compensate for an insufficient oxygen supply. In this report, we show that the activity (of oncogenic Ha-ras, either constitutively or transiently, enhances the induction of the angiogenic mitogen, vascular endothelial growth factor (VEGF), by hypoxia. Analysis of the 5' flanking region of the VEGF promoter indicates that a HIF-1-like sequence is to promote a 15-fold increase in reporter gene activity in Ha-ras-transformed cells when exposed to hypoxia, whereas mutations in the same site totally inhibited VEGF induction. Under low oxygen conditions, VEGF induction is inhibited in cells expressing a mutant inhibitory allele of Ha-ras (RasN17), indicating a direct role for Ras in modulating VEGF activity. We propose that the angiogenic switch in Ras-transformed cells may be physiologically promoted by the tumor microenvironment through VEGF induction.

Anaerobic bacteria as a delivery system for cancer gene therapy: in vitro activation of 5-fluorocytosine by genetically engineered clostridia

Certain species of anaerobic bacteria have been shown to localise and germinate specifically in the hypoxic regions of tumours, resulting in tumour lysis. We propose an innovative approach to cancer gene therapy in which genetically engineered anaerobic bacteria of the genus Clostridium are used to achieve tumour-specific gene delivery. Our strategy involves enzyme/prodrug therapy, in which the Escherichia coli enzyme cytosine deaminase is used to convert the non-toxic prodrug 5-fluorocytosine to the active chemotherapeutic agent 5-fluorouracil. The E. coli gene encoding cytosine deaminase has been cloned into a clostridial expression vector and transformed into Clostridium beijerinckii, resulting in constitutive expression of cytosine deaminase and significant levels of active enzyme in the bacterial medium. When added to an in vitro clonogenic survival assay, supernatant from clostridia expressing cytosine deaminase increased the sensitivity of murine EMT6 carcinoma cells to 5-fluorocytosine approximately 500-fold. This high level of prodrug activation, combined with the specificity of clostridia for hypoxic regions of tumours, indicates a potential use in cancer gene therapy.

Mechanism of heat shock protein 72 induction in primary cultured astrocytes after oxygen-glucose deprivation

Induction of stress proteins is thought to be important in the protection of cells from a variety of environmental insults including heat, hypoxia and ischemia. The aim of this study was to compare the mechanism of induction of heat shock protein 72 (HSP72) in primary cultures of murine cortical astrocytes by heat and combined oxygen-glucose deprivation (OGD), a model of in vitro ischemia. 35S-methionine labeling and immunoblotting showed increased HSP72 synthesis and accumulation lasting for up to 24 h following heat or OGD. Heat induced a markedly greater amount of HSP72 mRNA and protein than did OGD. We then sought evidence of heat shock transcription factor-1 (HSF-1) activation. An increase in apparent molecular weight of nuclear HSF-1 after heat or OGD was observed, consistent with increased phosphorylation. To seek an explanation of the difference between heat and OGD as inducers of HSP72 we examined the binding activity of HSP72 + 73 to other proteins. More cellular protein was found to co-immunoprecipitate with HSP72 + 73, and more HSP72 + 73 was found in the pellet fraction after heat shock compared to OGD. These results suggest that HSP72 induction is regulated in astrocytes at least in part at the level of HSF activation, by both heat and OGD. Reduced availability of free HSP72 + 73 in heated cells could be responsible for the greater magnitude of HSP72 induction after heat compared to OGD.

Hypoxia-mediated selection of cells with diminished apoptotic potential in solid tumours

Apoptosis is a genetically encoded programme of cell death that can be activated under physiological conditions and may be an important safeguard against tumour development. Regions of low oxygen (hypoxia) and necrosis are common features of solid tumours. Here we report that hypoxia induces apoptosis in oncogenically transformed cells and that further genetic alterations, such as loss of the p53 tumour-suppressor gene or overexpression of the apoptosis-inhibitor protein Bcl-2, substantially reduce hypoxia-induced cell death. Hypoxia also selects for cells with defects in apoptosis, because small numbers of transformed cells lacking p53 overtake similar cells expressing wild-type p53 when treated with hypoxia. Furthermore, highly apoptotic regions strongly correlate with hypoxic regions in transplanted tumours expressing wild-type p53, whereas little apoptosis occurs in hypoxic regions of p53-deficient tumours. We propose that hypoxia provides a physiological selective pressure in tumours for the expansion of variants that have lost their apoptotic potential, and in particular for cells acquiring p53 mutations.

New directions for radiation biology research in cancer of the uterine cervix

A simplified model for tumorigenesis, locoregional growth, and metastases is proposed for carcinoma of the cervix. With the use of this model, four potential areas for future directions for radiobiologic-clinical research are identified. The first area concerns the influence of human papillomavirus infection and p53 mutations on tumor biology, with particular reference to radiosensitivity and metastatic potential. Research in this area should be most fruitful. The second area focuses on the influence of hypoxia on clinical outcome in carcinoma of the cervix. The use of selective hypoxic cell toxins (e.g., tirapazamine) for phase II testing in hypoxic tumors is recommended. The third area concerns the development and clinical confirmation of assays for the prediction of intrinsic tumor radiosensitivity (e.g., surviving fraction after 2 Gy) and normal tissue radiosensitivity. The need exists for more rapid assays so that their results can be available prior to institution of therapy. The influence of the intrinsic radiosensitivity of normal tissues (especially in patients who are heterozygotes for ataxia-telangiectasia and patients with autoimmune disease) may permit identification of those at increased risk for complications so that alternative, less toxic treatment can be allocated. The fourth area for additional study concerns the influence of both intrinsic (c-myc amplification, matrix metalloproteinase levels) and extrinsic factors (fever, immunosuppression) on the development of distant metastases. Such investigations will permit identification of patients at high risk of developing distant metastases so that adjuvant treatments (e.g., chemotherapy or metalloproteinase inhibitors) can be explored. It is believed that future clarification of our proposed model will lead to other worthwhile areas for therapeutic intervention.

Mitogen-activated protein kinase acts as a negative regulator of the heat shock response in NIH3T3 cells

Exposure of NIH3T3 cells to elevated temperatures induces the phosphorylation and activation of mitogen-activated protein (MAP) kinases [or extracellular signal-regulated kinases (ERKs)]. To investigate the significance of MAP kinase activation by heat shock, we examined the effect of inhibiting the activity of MAP kinase on heat shock protein 70 (hsp 70) expression. Overexpression of a dominant inhibitory mutant of ERK1, but not ERK2, in heat-shocked cells increased hsp70 reporter gene activity, suggesting that ERK1 acts as a repressor of hsp70 gene expression. Increases in ERK1 activity through treatment of cells with sodium vanadate (SV), an inhibitor of the dual-specificity MAP kinase phosphatase 1 (PAC1), resulted in increased phosphorylation of the heat shock transcription factor-1 (HSF-1) in unheated cells, delayed the activation of HSF-1 by heat shock, and inhibited the induction of hsp 70 by heat shock. Furthermore, the induction of thermotolerance was reduced significantly in cells that increased ERK1 activity by SV pretreatment. Immune complex kinase assays of heat shocked or SV-pretreated cells indicated that HSF-1 is a potential in vivo substrate for ERK1 phosphorylation. Taken together, these results suggest that agents that modulate MAP kinase act as negative regulators of the heat shock response in mammalian cells by modulating HSF-1 activity and hsp 70 expression.