Molecular signaling Flashcards
Following exposure of cells to 3 Gy from a 6 MV X-ray beam, the ATM protein is activated and phosphorylates multiple intracellular targets. Which of the following is NOT a target for ATM phosphorylation?
A. Histone H2AX
B. p53 (TP53)
C. VEGF (VEGFA)
D. BRCA1
E. Artemis
C
ATM is a kinase that is activated in response to the presence of DNA double-strand breaks, such as following exposure to ionizing radiation. Activated ATM phosphorylates multiple distinct target proteins, including histone H2AX, p53, BRCA1, and Artemis. Phosphorylation of H2AX (to y-H2AX) results in chromatin modification that facilitates the recruitment of factors needed for DNA repair (Answer Choice A). The tumor suppressors, p53 and BRCA1, activate cell cycle checkpoint and/or DNA repair processes in response to genotoxic stress (Answer Choices B and D). VEGF is a secreted factor that promotes angiogenesis and is not a direct target of ATM phosphorylation (Answer Choice C).
Which of the following pairs of molecular events and their functional consequences is INCORRECT?
A. VHL inactivation —- angiogenesis
B. cyclin D1 repression — inhibition of proliferation
C. cytochrome c release — apoptosis
D. ATM phosphorylation —- epistasis
E. miRNAs mis-expression — carcinogenesis
D
Epistasis is a form of gene interaction in which an allele for one trait (at one locus) influences the expression of an allele, at a different locus, for a separate and independent trait; this process is unrelated to ATM phosphorylation.
Inactivation of the VHL (von Hippel-Lindau tumor suppressor) gene results in overexpression of many environmental stress-inducible mRNAs, including those involved in energy metabolism, apoptosis, and angiogenesis via the activation of vascular endothelial growth factor (VEGF; Answer Choice A).
Cyclin D1 repression is associated with anti-proliferation effects. Its overexpression has been observed in human cancers, including pancreatic, lung, and esophageal (Answer Choice B).
Release of cytochrome c and apoptosis-inducing factor (AIF) from mitochondria into the cytoplasm is a primary mitochondrial apoptogenic activity (Answer Choice C).
MicroRNAs (miRNAs) are small non-protein-coding RNAs that function as negative regulators of gene expression under normal physiological conditions. Mis-expression of, or mutations in, miRNAs are associated with the development of a variety of human cancers, including B-cell chronic lymphocytic leukemia, colorectal cancer, and breast cancer.
Which of the following pairs of transcription factors and genes they directly regulate is INCORRECT?
A. HIF-1 and VEGF (VEGFA)
B. p53 (TP53) and p21(CDKN1A)
C. FOS and BRCA2
D. E2F and CDC25A
E. p53 and PUMA
C
FOS is a transcription factor that has been shown to modulate a variety of genes involved in stress responses but has not been shown to modulate BRCA2.
HIF-1 is a hypoxia inducible factor known to regulate the expression of the VEGF gene and thus the regulation of angiogenesis (Answer Choice A).
p53 is a transcription factor that induces expression of p21 (Answer Choice B).
E2F is known to regulate a large number of proteins involved in cell cycle progression, including CDC25A (Answer Choice D).
PUMA is the major mediator of p53-dependent apoptosis following ionizing radiation in most cell types. PUMA, a pro-apoptotic BH3-only member of BCL2 family protein promotes BAX/BAK and mitochondria-dependent apoptosis in various cell types (Answer Choice E).
Which of the following statements concerning cytokines is TRUE?
A. NF-kB is the critical cytokine responsible for the development of lung fibrosis following irradiation
B. A paracrine response is the result of a cytokine targeting the same cell that produced the cytokine
C. Most cytokines are tyrosine kinases
D. Cytokines are proteins released by irradiated cells that stimulate tissues to produce a biological response
E. An autocrine response is the result of a cytokine targeting cells adjacent to the cell that produced the cytokine
D
Cytokines are proteins released by cells, including irradiated cells, that stimulate tissues to mount a biological response.
NF-kB is a transcription factor (not a cytokine). TGF-B1 (TGFB1) is an important example of one of the cytokines that has been associated with the development of lung fibrosis following irradiation (Answer Choice A).
A paracrine response is the result of a cytokine that acts upon a cell, other than itself, within a tissue or organ (Answer Choice B). In contrast, an autocrine response is the result of a cytokine targeting the cell from which it was produced (Answer Choice E).
Cytokines generally do not have tyrosine kinase activity (Answer Choice C).
Which of the following statements concerning the response of NF-kB to ionizing radiation exposure is FALSE?
A. NF-kB is a transcription factor
B. The Inhibitor of Nuclear factor (NF)-kB, IkB, is phosphorylated by ATM and subsequently degraded, allowing NF-kB to move from the cytoplasm into the nucleus
C. NF-kB generally acts to stimulate apoptosis and enhance the radiosensitivity of cells
D. Both DNA double-strand breaks and reactive oxygen species generated by radiation exposure can activate NF-kB
E. NF-kB is sequestered as an inactive form in the cytoplasm by interaction with an inhibitory subunit of the IkB
C
Nuclear factor (NF)-kB generally exerts a pro-survival influence through interference with apoptotic signals. It accomplishes this via the TNF receptor signaling pathway which, upon activation by an apoptotic signal, is coupled via the FADD adaptor to a caspase cascade involving the initiator caspases-8 or -10. In some cell types, however, this may not occur, since it may be opposed through the parallel triggering by TNF of a signaling pathway that activates NF-kB via the TRADD and TRAF adaptors. Active NF-kB induces transcription of a set of genes that encode the anti-apoptotic IAPs (“inhibitors of apoptosis”). NF-kB can also exert an anti-apoptotic effect by inducing transcription of anti-apoptotic proteins, such as Bcl-xL (BCL2L1), which act to prevent cytochrome c release and the subsequent caspase-9 activation. IkB binds to NF-kB to prevent its translocation to the nucleus. Following formation of DNA double-strand breaks and reactive oxygen species in irradiated cells, kinases (including ATM) phosphorylate IkB, targeting it for ubiquitination and degradation, which allows NF-kB to translocate to the nucleus from the cytoplasm where it can act as a transcription factor. NF-kB can exist as hetero- or homodimers of five different subunits. Different heterodimers activate different sets of genes while p50 and p52 homodimers, lacking transactivation domains, can selectively repress expression of their target genes. Post-transcriptional modifications and cofactor binding also help shape the specificity of the NF-kB response. Competition between p53 (TP53) and NF-kB for CBP/p300 may play an important role in determining the balance between apoptosis and cell cycle arrest following irradiation.
Based on transcriptomic studies using microarray profiling, which one of the following statements best describes the transcriptional response of irradiated cells and tissues?
A. Many genes are up-regulated by radiation exposure, but down-regulation of genes is rarely observed
B. The transcriptional response to radiation is complex, but for a given cell line similar responses will be seen between 2- and 24-hours post-irradiation
C. The transcriptional response is dynamic and varies with time after irradiation, but overall is similar for most cell lines examined to date
D. Transcriptional responses depend on the time elapsed after irradiation and on the cell’s tissue of origin but do not vary significantly between cell types derived from the same tissue or between different individuals
E. Variability observed in transcriptional profiles between individuals may provide a basis for prediction of individual therapeutic responses in the future as a basis for individualized medicine
E
Many genes are both up- and down-regulated following irradiation in both a time and tissue-dependent manner. In addition, variation is also seen between cells derived from the same tissue and between tissue samples taken from different individuals. This inter-individual variation is seen both in the response to stressors such as ionizing radiation and in the normal basal gene expression patterns. One of the major driving factors in the science of microarray profiling is the hope that a better understanding of this variability in gene expression may lead to a more “personalized” diagnosis of disease, prognosis and prediction of the best therapeutic approach for cancer and other diseases.
Concerning the p21 (CDKN1A) protein, which of the following statements is TRUE?
A. Its transcription is transactivated by p53 (TP53) in response to ionizing radiation exposure.
B. It is required for entry into S phase of the cell cycle.
C. It is up-regulated only in cells exposed to radiation doses greater than 1 Gy.
D. Overexpression of p21 causes arrest in the G2 phase of the cell cycle.
E. It binds to Bcl-xL (BCL2L1) to promote apoptosis.
A
p21 is one of the most strongly p53-transactivated genes, and codes for the p21 protein. It responds robustly at both the mRNA and protein levels to ionizing and UV radiation as well as to most other stress-inducing agents. p21 is a CDK inhibitor and also binds to PCNA to prevent entry of cells into S phase. The predominant role of p21 appears to be in mediating G1 phase arrest, although it also plays roles in differentiation, senescence, and regulation of apoptosis.
The two most frequently activated signaling pathways in prostate cancer are driven by androgen receptor (AR) and PI(3)K-Akt. Inhibitors of the PI(3)K pathway are in early clinical trials, while androgen-deprivation therapy (ADT) via inhibition of the AR is able to confer a clinical response in most patients. Which of the following statements most CORRECTLY describes the relationship between these two pathways and explains mechanistically why single inhibition of AR or the PI(3)K-Akt pathways rarely induces tumor regression in preclinical models?
A. ADT represses an AR gene program governing DNA repair and inhibits repair of ionizing radiation–induced DNA damage
B. AR and PI(3)K pathways regulate each other by reciprocal negative feedback, such that inhibition of one activates the other
C. ADT represses the PI(3)K/Akt/target of rapamycin (TOR) pathway
D. ADT activates the unfolded protein response
E. All of the above
B
Prostate cancer is characterized by its dependence on androgen receptor (AR) signaling as well as frequent activation of PI(3)K signaling. AR transcriptional output is decreased in human and murine tumors with PTEN deletion. In addition, PI(3)K pathway inhibition activates AR signaling by relieving feedback inhibition of HER kinases. Similarly, AR inhibition activates Akt signaling by reducing levels of the Akt phosphatase, PHLPP. These two oncogenic pathways therefore cross-regulate each other by reciprocal feedback. Inhibition of one pathway leads to activation of the other thereby maintaining tumor cell survival. Combined pharmacologic inhibition of PI(3)K and AR signaling causes near complete prostate cancer regression in a PTEN-deficient murine prostate cancer model and in human prostate cancer xenografts, indicating that both pathways coordinately support survival.
The phenomenon of “oncogene addiction” is most clearly represented in which of the following clinical scenarios.
A. A Chronic Myeloid Luekemia (CML) patient treated with imatinib
B. An EGFR-mutant lung adenocarcinoma patient treated with bevacizumab
C. A BRAF-mutant melanoma patient treated with ipilimumab
D. An EML4-ALK positive lung adenocarcinoma patient treated with olaparib
E. A FLT3 mutated Acute Myeloid Leukemia (AML) patient treated with interferon.
A
“Oncogene addiction” was first coined by Bernard Weinstein and refers to the dependence of some tumors on a single dominant oncogene for continued growth and survival and that inhibition of this specific oncogene product is sufficient to halt the neoplastic phenotype. Answer Choice A is correct because imatinib is correctly paired with its target, BCR-ABL. The other answers are examples of oncogene-addicted cancers that are incorrectly paired with agents that do not target the dominant oncogene product.
EGFR-mutant lung adenocarcinoma patient can be treated with tyrosine kinase inhibitors such as gefitinib or erlotinib. A BRAF-mutant melanoma patient can be treated with a BRAF inhibitor such as venvurafinib or dabrafinib. An EML4-ALK positive lung adenocarcinoma patient treated with an ALK inhibitor such as crizotinib or alectinib. Lastly, a FLT3-mutated AML patient can be treated with FLT inhibitor, midostaurin, along with induction chemotherapy.
Which of the following pairs of genes or portions of genes and corresponding descriptors is CORRECT?
A. Tumor suppressor genes – activated in many human tumors
B. Exon – the non-coding region of a gene
C. Promoter – involved in regulating gene transcription
D. DNA repair gene – EGFR
E. Oncogene – activated through loss of heterozygosity
C
The promoter region is the regulatory portion of a gene that plays a critical role in directing whether a gene is transcribed or not. Tumor suppressor genes are generally inactivated in many cancers, typically resulting in a loss of control over cell proliferation. Exons are the expressed, or coding, regions of genes, whereas introns are the non-coding sequences. The protein encoded by the EGFR (epidermal growth factor receptor) gene is a cell surface tyrosine kinase receptor that is activated by epidermal growth factor (EGF) ligand, among others, and is important for cell proliferation. Loss of heterozygosity is a common mechanism by which tumor suppressor genes are inactivated. Oncogenes are generally activated by mechanisms including deletion/point mutation, chromosome rearrangement, retroviral integration, or gene amplification.
