Tumor microenvironment Flashcards
Which of the following statements concerning tumor hypoxia is TRUE?
A. Hypoxic regions in tumors may be detected using labeled bortezomib
B. As a tumor increases in size, the hypoxic fraction of cells decreases
C. Regions of chronic hypoxia may develop in tumors due to the intermittent opening and closing of blood vessels
D. In the absence of reoxygenation it is unlikely that all hypoxic cells would be eliminated from a tumor following a typical course of radiotherapy
E. Acutely hypoxic tumor cells usually exhibit slow reoxygenation while chronically hypoxic tumor cells reoxygenate rapidly
D
In the absence of reoxygenation it is unlikely that all hypoxic cells would be eliminated following a typical course of radiotherapy from a tumor possessing even a small percentage of hypoxic cells because hypoxic cells demonstrate approximately 3-fold greater radioresistance compared with aerated cells.
Hypoxic regions in tumors can be detected using a labeled nitroimidazole compound. Bortezomib (Velcade) is a proteasome inhibitor and would not label hypoxic regions within tumors (Answer Choice A).
Although not without exceptions, as tumors increase in size the hypoxic fraction generally also increases. This is because the typically abnormal tumor vasculature is insufficient to maintain oxygen demand (Answer Choice B).
Regions of acute or transient hypoxia may develop in tumors due to intermittent blood flow via the intermittent closing down of blood vessels. Chronic hypoxia, on the other hand, is defined as diffusion-limited hypoxia due to the inability of oxygen to diffuse farther than 100 um from a blood vessel (Answer Choice C).
Acutely hypoxic cells tend to exhibit rapid reoxygenation, whereas chronically hypoxic cells generally reoxygenate more slowly (Answer Choice E).
Bevacizumab (Avastin) is a monoclonal antibody that targets:
A. Basic fibroblast growth factor (bFGF; FGF2)
B. Hypoxia-inducible factor-1a (HIF-1a; HIF1A)
C. Von Hippel-Lindau (VHL) protein
D. Ras
E. Vascular endothelial growth factor (VEGF; VEGFA)
E
Bevacizumab (Avastin) binds to and neutralizes vascular endothelial growth factor (VEGF)-A ligand, thereby preventing its interaction with cell surface receptors, including the VEGF receptor (VEGFR).
The fibroblast growth factors (FGFs) are a family of pluripotent growth factors that stimulate proliferation of mesodermal or neuroectodermal cells and can play a role in angiogenesis (Answer Choice A). FGFs have yet to be successfully targeted pharmaceutically.
Hypoxia-inducible factor (HIF)-1a is a transcription factor that detects hypoxia and enhances angiogenesis (Answer Choice B).
The Von Hippel-Lindau (VHL) protein belongs to a complex that is involved in the ubiquitination and degradation of HIF (Answer Choice C).
The Ras proteins are a family of small GTPases involved in the activation of signaling cascades following activation of receptors. Ras is frequently mutated in human cancers but is difficult to target pharmacologically (Answer Choice D).
Which of the following responses is LEAST likely to be observed?
A. Exposure to hypoxia increases the expression of angiogenesis-promoting genes
B. Anti-angiogenic therapy improves tumor oxygenation
C. A chronically hypoxic environment increases the metastatic potential of tumor cells
D. Hypoxia inhibits apoptosis in tumor cells
E. Exposure to hypoxia inhibits cell proliferation
D
An increased apoptotic index is often observed in hypoxic regions of tumors.
The gene for vascular endothelial growth factor (VEGF/VEGFA) is one of the major genes under the control of the hypoxia responsive promoter, HRE, which binds the transcription factor, hypoxia-inducible-factor (HIF)-1 (Answer Choice A).
Studies in animal models have indicated that treatment with anti-angiogenics can cause “normalization” of tumor blood vessels and result in a transient improvement in tumor oxygenation before vessels start to deteriorate (Answer Choice B).
Pre-clinical studies with animal models as well as clinical studies have linked increased hypoxia in tumors to increased tumor aggressiveness and metastatic potential (Answer Choice C).
Exposure to severe hypoxia halts progression of cells through the cell cycle and therefore inhibits proliferation.
Which of the following statements concerning chronically hypoxic cells in tumors is TRUE? Chronically hypoxic cells:
A. Can be selectively targeted for killing with certain bioreductive drugs
B. Are resistent to hyperthermia
C. Are located within 10 um of capillaries
D. Exist in a high pH microenvironment
E. Are a consequence of intermittent blood flow
A
Chronically hypoxic regions in a tumor can be targeted for elimination by administering certain bioreductive drugs that preferentially kill hypoxic, but not aerobic, cells.
Chronically hypoxic cells tend to be sensitive to hyperthermia. This is because they exist in an acidic (low pH) microenvironment (Answer Choices B and D).
Acutely hypoxic cells are a consequence of intermittent blood flow in tumors (Answer Choice E).
It has been shown via model calculations of oxygen consumption rates in respiring tissues and through the use of hypoxia markers that chronically hypoxic cells rarely appear closer than about 70 um from capillaries (Answer Choice C).
Which of the following statements concerning tumor angiogenesis is TRUE?
A. Even without angiogenesis, tumors can grow up to 2 cm in diameter
B. For most tumor types a high microvessel density has been negatively correlated with metastatic spread
C. Vascular endothelial growth factor (VEGF) is induced under hypoxic conditions
D. Angiostatin and endostatin are stimulators of angiogenesis
E. Basic fibroblast growth factor (bFGF) is a negative regulator of
angiogenesis
C
Expression of vascular endothelial growth factor (VEGF) and downstream angiogenesis is induced under hypoxic conditions via hypoxia-inducible transcription factors that bind to the VEGF promoter to stimulate its transcription.
In the absence of angiogenesis, tumors would only be expected to reach a diameter of about 2 mm, not 2 cm (Answer Choice A).
Microvessel density, a measure of angiogenesis, has been correlated positively with metastatic spread for most tumor types (Answer Choice B).
Angiostatin and endostatin are inhibitors of angiogenesis while basic fibroblast growth factor (bFGF) is a positive regulator of angiogenesis (Answer Choice D and E).
The regulation of hypoxia-inducible factor-1a (HIF-1a; HIF1A) by oxygen concentration is best described by which of the following statements?
A. Under hypoxic conditions, HIF-1a transcription and translation are upregulated as well as translocation of HIF-1a from the cytosol to the nucleus
B. Under aerobic conditions, the HIF-1a heme moiety becomes oxygenated. This drives a conformational change in the protein that limits DNA binding and prevents upregulation of target genes
C. Under hypoxic conditions, HIF-1a is activated by bioreduction, thereby promoting the up-regulation of target genes
D. Under hypoxic conditions, the HIF-1a heme moiety becomes deoxygenated. This induces a conformational change in the protein that leads to enhancing DNA binding and subsequent upregulation of target genes
E. Under aerobic conditions, HIF-1a is hydroxylated by HIF prolyl hydroxylases that target the protein for ubiquitination and subsequent proteosomal degradation, thereby preventing the up-regulation of target genes
E
Hypoxia-inducible factor-1 (HIF-1) is a heterodimer that acts as a key regulator of several oxygen-responsive proteins, including erythropoietin and vascular endothelial growth factor (VEGF). HIF-1 was first identified as a DNA-binding protein that mediated the up-regulation of the erythropoietin gene under hypoxic stress. Subsequent studies have
implicated HIF-1 in the regulation of a broad range of oxygen responsive genes including VEGF, VEGF receptors, angiopoietins, nitric oxide synthase, fibroblast growth factors and platelet-derived growth factor (PDGF). Under aerobic conditions, HIF-1a is hydroxylated by HIF prolyl hydroxylases. Hydroxylation at two prolyl residues targets HIF-1a to the von Hippel-Lindau (VHL) E3 ubiquitin ligase and results in HIF-1a ubiquitination and subsequent proteosomal degradation, thereby limiting upregulation of target genes. Because the hydroxylation catalyzed by prolyl hydroxylases requires molecular oxygen, HIF-1 escapes inactivation under hypoxic conditions.
Which of the following statements best describes the “normalization hypothesis” proposed to explain the survival benefit associated with combining anti-angiogenics with traditional chemotherapy agents?
A. Anti-angiogenic therapy stimulates the formation of leaky blood vessels thereby enhancing access of chemotherapy agents to the tumor parenchyma
B. Anti-angiogenic therapy transiently reduces pericyte coverage of tumor blood vessels, which would otherwise form a significant mechanical and biochemical barrier to the delivery of chemotherapy to the tumor
C. Tumor cell-derived pro-angiogenic factors render endothelial cells resistant to chemotherapy-induced apoptosis. Anti-angiogenic therapy eliminates this protection and restores endothelial cell sensitivity to chemotherapeutic agents
D. Anti-angiogenic therapy reduces the secretion of anti-apoptotic factors by vascular endothelial cells that would otherwise render nearby cancer cells relatively resistant to chemotherapeutic agents
E. Anti-angiogenic therapy transiently restores the normal balance of pro- and anti-angiogenic factors in tumor tissue thereby reducing tumor vessel leakiness, dilation, and tortuosity as well as increasing pericyte coverage
E
When administered as a single agent, several anti-angiogenic drugs have not yielded a long-term survival benefit. In contrast, delivery of antiangiogenic agents with chemotherapy has produced a significant survival benefit in colon cancer and previously untreated lung and breast cancers. If the anti-angiogenic agent were destroying tumor vasculature in combination regimens, one would expect decreased tumor blood flow and compromised delivery of chemotherapy to the tumor. The survival benefits produced by the addition of an anti-angiogenic drug to traditional chemotherapeutic regimens therefore appears paradoxical. One possible explanation for this has been termed the “normalization hypothesis.” Under the pressure of pro-angiogenic factors, tumor vasculature is structurally and functionally abnormal. Anti-angiogenic therapy (transiently) restores the balance of pro- and anti-angiogenic factors. Consequently, immature and leaky blood vessels are pruned, pericyte coverage increases, and the basement membrane becomes more homogenous and normalized. As a result, the resultant vascular bed achieves greater organization by being less leaky, dilated, and tortuous. These morphological changes also result in functional changes, including decreased interstitial fluid pressure, increased tumor oxygenation, and improved penetration of drugs into the tumor parenchyma. Due to improved drug delivery, chemotherapy is more efficacious. Sustained or high-dose anti-angiogenic therapy, however, may drive an imbalance favoring anti-angiogenic factors leading to inadequate tumor blood supply and compromise of the efficacy of systemic therapies.
At a distance of 150 µm from the nearest tumor blood vessel, one might expect all of the following microenvironmental conditions, EXCEPT:
A. Increased hypoxia
B. Decreased pH
C. Decreased interstitial fluid pressure
D. Decreased glucose
E. Increased metabolic acids
C
Solid tumors develop regions of increased hypoxia (decreased pO2), decreased pH, decreased glucose, and increased (not decreased) interstitial fluid pressure. Low oxygen levels shift glucose metabolism from oxidative phosphorylation to glycolysis, which increases the production of metabolic acids. Oxygen can diffuse about 70 µm from the arterial end of a capillary.
Paclitaxel appears to be effective in radiosensitizing tumors in vivo for all the following mechanisms, EXCEPT:
A. Induction of apoptosis
B. Upregulation of HIF-1
C. Oxygenation of radioresistant hypoxic cells
D. Arrest of cells in the radiosensitive G2/M phase
E. Decrease of interstitial fluid pressure
B
Paclitaxel stabilizes microtubule polymers and protects them from disassembly. As a result, mitosis is consequently blocked and apoptosis is activated.
Paclitaxel has been shown to increase the radiation sensitivity of tumors by inducing apoptosis, increasing oxygenation of hypoxic cells in tumors, arresting cells in the radiosensitive G2/M phase of the cell cycle, and decreasing interstitial fluid pressure. Some of these studies have been conducted in animal models while others have been performed in clinical trials of human breast cancer patients. No studies have demonstrated upregulation of HIF-1 following treatment with paclitaxel; indeed, one might expect HIF-1 to be degraded more rapidly following reoxygenation.
Which of the following statements regarding the tumor microenvironment is FALSE?
A. Blood vessel supply is heterogeneous and irregular
B. Blood flow through micro-vessels may be sluggish
C. There tends to be an increase in vessel density compared to normal tissue
D. There are a greater number of hypoxic regions within the microenvironment of a tumor compared to that seen in normal tissue
E. Nutritional support to the tumor microenvironment is adequate and homogeneous
E
Tumor masses exhibit abnormal blood vessel networks that fail to provide adequate and homogeneous nutritional support.
Which of the following statements regarding angiogenesis is FALSE?
A. For a multi-cellular organism to grow, it must have the capacity to recruit new blood vessels via angiogenesis
B. Angiogenesis is normally regulated by pro-angiogenic, but not antiangiogenic, molecules
C. Angiogenesis is dysregulated in a neoplastic environment
D. Without a nearby blood vessel or effective angiogenesis, a tumor cannot grow beyond a critical size or metastasize to other organs
E. Tumor cell viability decreases beyond 70 um of blood vessels due to the diffusion limits of oxygen
B
Since oxygen is unable to diffuse more than approximately 70 µm from the arterial end of a capillary, tumors require the ability to develop new blood vessels in order to grow (Answer Choice A). This process is normally regulated by a balance of both pro-(including VEGF, FGF, PDGF) and anti-(thrombospondin, angiostatin, endostatin) angiogenic molecules.
Which of the following statements regarding anti-angiogenic therapy strategies is FALSE?
A. Anti-angiogenic therapies interfere with activators of angiogenesis
B. Anti-angiogenic therapies target receptor tyrosine kinases and related signal transductions
C. Anti-angiogenic therapies seek to amplify endogenous suppressors of angiogenesis.
D. Anti-angiogenic therapies use colchicine as an anti-angiogenic agent.
E. Anti-angiogenic therapies ultimately target VEGFR-1 in order to achieve inhibition of angiogenesis
D
Colchicine is an anti-inflammatory agent that binds tubulin. Colchicine itself induces vascular damage but only at doses that are limited by toxicity and therefore not used in the clinical setting for this purpose.
Which of the following statements concerning vasculogenesis is TRUE?
A. The process of vasculogenesis is specific to the developing embryo
B. Vasculogenesis refers to a subset of angiogenesis, in that it describes the formation of only venous vessels as tumors grow beyond 1-2 mm3
C. Tumors use vasculogenesis or angiogenesis in a mutually exclusive fashion
D. Vasculogenesis is critical for a tumor to achieve local tumor recurrence following radiotherapy
E. Vasculogenesis utilizes pre-existing blood vessels during the early stages of tumor development to facilitate tumor growth
D
Whereas angiogenesis is the sprouting of endothelial cells from nearby blood vessels, vasculogenesis is the formation of blood vessels from circulating endothelial progenitor cells (i.e. the bone marrow). Vasculogenesis is of particular importance following treatment with antiangiogenic agents as well as following irradiation. During early tumor development, both vasculogenesis and angiogenesis are likely utilized. Because tumor irradiation abrogates local angiogenesis, the tumor must rely on the vasculogenesis pathway for re-growth following irradiation. Tumor irradiation produces a marked influx of CD11b+ myeloid cells into tumors and are criticial in order for a tumor to achieve formation of blood vessels after irradiation as well as for tumor recurrence.
