Biochemistry - Cell Signaling

Molecular mechanisms of cardiotoxicity of tyrosine kinase inhibition.

Force, T et al., 2007. Nat. Rev. Cancer 7(5):332-344.

Abstract

Cancer therapy has progressed remarkably in recent years. In no area has this been more apparent than in the development of "targeted therapies", particularly those using drugs that inhibit the activity of certain tyrosine kinases, activating mutations or amplifications of which are causal, or strongly contributory, to tumorigenesis. However, some of these therapies have been associated with toxicity to the heart. Here we summarize what is known about the cardiotoxicity of cancer drugs that target tyrosine kinases. We focus on basic mechanisms through which interruption of specific signalling pathways leads to cardiomyocyte dysfunction and/or death, and contrast this with therapeutic responses in cancer cells.

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EGFR mutation and resistance of non-small-cell lung cancer to gefitinib.

Kobayashi, S et al., 2005. N. Engl. J. Med. 352(8):786-792.

Abstract

Mutations of the epidermal growth factor receptor ( EGFR ) gene have been identified in specimens from patients with non-small-cell lung cancer who have a response to anilinoquinazoline EGFR inhibitors. Despite the dramatic responses to such inhibitors, most patients ultimately have a relapse. The mechanism of the drug resistance is unknown. Here we report the case of a patient with EGFR -mutant, gefitinib-responsive, advanced non-small-cell lung cancer who had a relapse after two years of complete remission during treatment with gefitinib. The DNA sequence of the EGFR gene in his tumor biopsy specimen at relapse revealed the presence of a second point mutation, resulting in threonine-to-methionine amino acid change at position 790 of EGFR. Structural modeling and biochemical studies showed that this second mutation led to gefitinib resistance.

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Tyrosine kinases as targets for cancer therapy.

Krause, DS & Van Etten, RA, 2005.  N. Engl. J. Med. 353(2):172-187. A review.

Abstract

Protein tyrosine kinases (TKs) are enzymes that catalyze the transfer of phosphate from ATP to tyrosine residues in polypeptides. The human genome contains about 90 TK and 43 TK-like genes, the products of which regulate cellular proliferation, survival, differentiation, function, and motility. More than 25 years ago, TKs were implicated as oncogenes in animal tumors induced by retroviruses. However, they were largely ignored in drug development because of a paucity of evidence for a causative role in human cancer and concerns about drug specificity and toxicity. The landscape was changed radically by the success of imatinib mesylate, an inhibitor of the BCR-ABL TK in chronic myeloid leukemia (CML) - a result heralded as a proof-of-principle and a triumph of targeted cancer therapy. TKs are now regarded as excellent targets for cancer chemotherapy, but reality lies somewhere between the extremes of triumph and tribulation. In this article we will review mechanisms of aberrant TK signaling and strategies to inhibit TKs in cancer, summarize the status of TK-directed cancer therapies, and discuss challenges and prospects for the future.

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Activating mutations in the epidermal growth factor receptor underlying responsiveness of non-small-cell lung cancer to gefitinib.

Lynch, TJ et al., 2004. N. Engl. J. Med. 350(21):2129-2139.

Abstract

BACKGROUND: Most patients with non-small-cell lung cancer have no response to the tyrosine kinase inhibitor gefitinib, which targets the epidermal growth factor receptor (EGFR). However, about 10 percent of patients have a rapid and often dramatic clinical response. The molecular mechanisms underlying sensitivity to gefitinib are unknown. METHODS: We searched for mutations in the EGFR gene in primary tumors from patients with non-small-cell lung cancer who had a response to gefitinib, those who did not have a response, and those who had not been exposed to gefitinib. The functional consequences of identified mutations were evaluated after the mutant proteins were expressed in cultured cells. RESULTS: Somatic mutations were identified in the tyrosine kinase domain of the EGFR gene in eight of nine patients with gefitinib-responsive lung cancer, as compared with none of the seven patients with no response (P<0.001). Mutations were either small, in-frame deletions or amino acid substitutions clustered around the ATP-binding pocket of the tyrosine kinase domain. Similar mutations were detected in tumors from 2 of 25 patients with primary non-small-cell lung cancer who had not been exposed to gefitinib (8 percent). All mutations were heterozygous, and identical mutations were observed in multiple patients, suggesting an additive specific gain of function. In vitro, EGFR mutants demonstrated enhanced tyrosine kinase activity in response to epidermal growth factor and increased sensitivity to inhibition by gefitinib. CONCLUSIONS: A subgroup of patients with non-small-cell lung cancer have specific mutations in the EGFR gene, which correlate with clinical responsiveness to the tyrosine kinase inhibitor gefitinib. These mutations lead to increased growth factor signaling and confer susceptibility to the inhibitor. Screening for such mutations in lung cancers may identify patients who will have a response to gefitinib.

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Molecular determinants of the response of glioblastomas to EGFR kinase inhibitors.

Mellinghoff, IK et al., 2005. N. Engl. J. Med. 353(19):2012-2024.

Abstract

Background The epidermal growth factor receptor (EGFR) is frequently amplified, overexpressed, or mutated in glioblastomas, but only 10 to 20 percent of patients have a response to EGFR kinase inhibitors. The mechanism of responsiveness of glioblastomas to these inhibitors is unknown. Methods We sequenced kinase domains in the EGFR and human EGFR type 2 (Her2/neu) genes and analyzed the expression of EGFR, EGFR deletion mutant variant III (EGFRvIII), and the tumor-suppressor protein PTEN in recurrent malignant gliomas from patients who had received EGFR kinase inhibitors. We determined the molecular correlates of clinical response, validated them in an independent data set, and identified effects of the molecular abnormalities in vitro. Results Of 49 patients with recurrent malignant glioma who were treated with EGFR kinase inhibitors, 9 had tumor shrinkage of at least 25 percent. Pretreatment tissue was available for molecular analysis from 26 patients, 7 of whom had had a response and 19 of whom had rapid progression during therapy. No mutations in EGFR or Her2/neu kinase domains were detected in the tumors. Coexpression of EGFRvIII and PTEN was significantly associated with a clinical response (P<0.001; odds ratio, 51; 95 percent confidence interval, 4 to 669). These findings were validated in 33 patients who received similar treatment for glioblastoma at a different institution (P=0.001; odds ratio, 40; 95 percent confidence interval, 3 to 468). In vitro, coexpression of EGFRvIII and PTEN sensitized glioblastoma cells to erlotinib. Conclusions Coexpression of EGFRvIII and PTEN by glioblastoma cells is associated with responsiveness to EGFR kinase inhibitors

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Tamoxifen resistance by a conformational arrest of the estrogen receptor a after PKA activation in breast cancer.

Michalides, R et al., 2004. Cancer Cell 5(6):597-605.

Abstract

Using a novel approach that detects changes in the conformation of ERa, we studied the efficacy of anti-estrogens to inactivate ERa under different experimental conditions. We show that phosphorylation of serine-305 in the hinge region of ERa by protein kinase A (PKA) induced resistance to tamoxifen . Tamoxifen bound but then failed to induce the inactive conformation, invoking ERa-dependent transactivation instead. PKA activity thus induces a switch from antagonistic to agonistic effects of tamoxifen on ERa. In clinical samples, we found that downregulation of a negative regulator of PKA, PKA-RIa, was associated with tamoxifen resistance prior to treatment. Activation of PKA by downregulation of PKA-RIa converts tamoxifen from an ERa inhibitor into a growth stimulator, without any effect on ICI 182780 (Fulvestrant). Estrogen receptor-positive breast cancer patients are commonly treated with the anti-estrogen tamoxifen , which gives a 50% reduction in recurrence. Here, we describe that inactivation of the estrogen receptor by anti-estrogens leads to rapid conformational changes in the receptor, which can be followed by FRET (fluorescence resonance energy transfer). Using this method, we can determine the efficacy of two different anti-estrogens, tamoxifen and Fulvestrant. Breast cancers resistant to tamoxifen are often still sensitive to Fulvestrant. We demonstrate that resistance to tamoxifen is mediated by a modification of the estrogen receptor by protein kinase A, not only in experimental setting, but in breast cancer patients as well. This modification converts the antagonist tamoxifen into an agonist, reversing its effect on tumor cell growth.

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