Intracellular Signaling
Summary of Key Points
• Ligand binding and activation of cell surface and internal receptors triggers the activation and/or suppression of signaling cascades that regulate diverse cellular processes, including cell growth, proliferation, survival, and invasion, among others.
• Multiple nodes within these intracellular signaling networks are genetically and epigenetically altered in human cancers, leading to constitutive pathway activation or suppression.
• A subset of cancers are dependent on genomic alterations in oncogenes or tumor suppressor genes for their growth and survival, a phenomenon known as “oncogene addiction.”
• Drugs that selectively inhibit altered proteins that are critical for the maintenance of the transformed phenotype have shown unprecedented clinical activity in genetically defined subsets of cancers.
• Precision medicine is the use of genetic and epigenetic information to develop treatment regimens that target the driver oncogenes and tumor suppressors responsible for tumor progression in individual patients with cancer. Potential challenges to the application of this approach include the current inability to directly inhibit some oncogenic proteins (i.e., KRAS), the development of drug resistance, technical hurdles posed by limited tissue availability for genomic studies, and intratumoral and lesion-to-lesion genomic heterogeneity.
1. After ligand stimulation, cell surface receptors can transmit signals by:
A Dimerizing and autophosphorylating intracellular tyrosine residues, which recruit adaptor proteins and intracellular effectors to transduce signals to the nucleus
B Undergoing a conformational change that facilitates guanosine diphosphate to guanosine triphosphate exchange on coupled G proteins
C Clustering as an aggregate of two or three receptors chains, thereby facilitating activation of constitutively bound intracellular tyrosine kinases and phosphorylation of intracellular receptor tails
D Undergoing sequential proteolytic cleavage to generate an active moiety that acts as a direct transcriptional regulator
2. The Ras/mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3 kinase (PI3)/ mammalian target of rapamycin (mTOR) signaling cascades are parallel but interconnected pathways that are frequently mutated in human cancer. Which of the following therapeutic approaches has NOT been approved by the U.S. Food and Drug Administration as an anticancer strategy?
B Small molecule inhibitors of mTOR
C Monoclonal antibodies that bind to epidermal growth factor receptor (EGFR)
3. Tumor suppressors are altered in cancer by all the following mechanisms, except:
4. Which of the following are direct kinase inhibitors?
5. Intracellular signaling can be regulated on many levels, including which of the following?
A Ligand-mediated receptor internalization and/or desensitization
B Relocalization of key proteins to specific compartments (e.g., membrane and nucleus)
C (De-)phosphorylation of effectors by phosphatases and kinases to initiate or inhibit signaling
D Upregulation of negative feedback loops that control pathway output
1. Answer: E. Each choice above represents the general method of signal transduction downstream of receptor tyrosine kinases (A), G-protein coupled receptors (B), interferon and interleukin cytokine receptors (C), and notch receptors (D).
2. Answer: D. Vemurafenib, an adenosine triphosphate (ATP)–competitive inhibitor of BRAF, is approved for the treatment of patients with melanoma whose tumors harbor a mutation in the BRAF gene (August 2011). Everolimus, an inhibitor of the mTOR complex 1 (mTORC1), is used in the treatment of kidney cancers (March 2009; as well as temsirolimus, May 2007), pancreatic neuroendocrine tumors (May 2011), renal angiomyolipomas (April 2012), and estrogen receptor (ER) positive/human epidermal growth factor receptor 2 (HER2)–negative breast cancers (July 2012). Cetuximab (cEGFR + colorectal cancer, February 2004; head and neck squamous cell carcinoma, November 2011) and panitumumab (EGFR + colorectal cancer, September 2006) are monoclonal antibody therapies that bind to the extracellular domain of EGFR. Erlotinib (November 2004) and gefitinib (May 2003) are ATP-competitive inhibitors of EGFR. These kinase inhibitors are used in the treatment of non–small cell lung cancers and pancreatic cancers (November 2005). No direct inhibitor of Ras is available for clinical use. Farnesyltransferase inhibitors were tested but were found to be inactive in tumors in which Ras mutations are common, likely because of the ability of geranyl geranyl modifications to substitute for farnesylation in effectively targeting Ras to the membrane.
3. Answer: C. Tumor suppressors are commonly altered by mutations (which can cause protein inactivation or truncation) and by epigenetic silencing resulting from promoter methylation. Copy number alterations such as homozygous gene deletion (loss of both alleles) and loss of one allele (LOH) are also common. LOH is often accompanied by loss or mutation of the other allele. In cancer, tumor suppressor function is lost, thereby relieving the repression of the pathways involved. Thus gene amplification, which would in theory upregulate the expression of a tumor suppressor, is not a mechanism of tumorigenesis.
4. Answer: A and D. Erlotinib is an ATP-competitive, EGFR tyrosine kinase inhibitor that is approved for use in patients with non–small cell lung cancer. Sunitinib is a multitargeted, ATP-competitive kinase inhibitor used in the treatment of patients with renal cancer, gastrointestinal stromal tumors, and pancreatic neuroendocrine tumors. Rapamycin is a small molecule inhibitor that binds to FKBP12, and this complex binds to and inhibits mTORC1. Rapalogues are used for the treatment of kidney cancer, pancreatic neuroendocrine tumors, renal angiomyolipomas, and breast cancers. Bevacizumab is a humanized monoclonal antibody that binds to the vascular endothelial growth factor–antagonist ligand and is approved for use in colorectal and kidney cancers. Bevacizumab can thus indirectly inhibit vascular endothelial growth factor receptor activation by decreasing ligand availability, but it is not a direct inhibitor of the kinase.
5. Answer: E. A, B, C, and D are four methods, among many others, that tightly control cellular signaling, including ligand-receptor specificity, selective adapter recruitment, protein sequestration and degradation, proteolytic cleavage, other posttranslational modifications, and transcriptional co-activators/co-repressors, among others.
