Ras oncogene point mutations in bladder cancer resistant to cisplatin

The role of oncogenes in drug resistance

Increasing evidences indicate that oncogenes can directly or indirectly impact on cancer-cell drug resistance. This chapter provides a conceptual review regarding the role of oncogenes in drug resistance. The review is focused on drug resistance mediated by oncogenes encoding growth factor receptors, signaling molecules, transcription factors, cell-cycle regulators, and apoptosis regulators. It is my hope that better undertsnading on the role of oncogenes in drug resistance will invoke ideas on new approaches to enhance the cytotoxicity of the standard chemotherapeutic agents by functional perturbation of resistance-inducing oncogenes.

Molecular mechanisms of cisplatin resistance in bladder cancer

Metastatic disease is the most common mechanism of death in patients with advanced bladder cancer. As for most solid tumors, chemotherapy remains the only realistic option for palliating or curing metastatic disease. However, bladder cancer is characterized by chemoresistance. Only modest response rates are obtained using multiagent regimens including cisplatin. These low response rates and the toxicity of these regimens limit their use to patients at highest risk. Here, we review the molecular mechanisms of cisplatin resistance. These include methods to reduce cisplatin bioavailability within a cell, and defects in the machinery that produces cell death following cisplatin-induced DNA damage. While overcoming these mechanisms is a potential therapeutic approach that can increase response rates, in the short term this knowledge could be used to predict response in individual tumors.

Pancreatic cancer cell radiation survival and prenyltransferase inhibition: the role of K-Ras

Activating K-ras mutations are found in approximately 90% of pancreatic carcinomas and may contribute to the poor prognosis of these tumors. Because radiotherapy is frequently used in pancreatic cancer treatment, we assessed the contribution of oncogenic K-ras signaling to pancreatic cancer radiosensitivity. Seven human pancreatic carcinoma lines with activated K-ras and two cell lines with wild-type ras were used to examine clonogenic cell survival after Ras inhibition. Ras inhibition was accomplished by small interfering RNA (siRNA) knockdown of K-ras expression and by blocking Ras processing using a panel of prenyltransferase inhibitors of differing specificity for the two prenyltransferases that modify K-Ras. K-ras knockdown by siRNA or inhibition of prenyltransferase activity resulted in radiation sensitization in vitro and in vivo in tumors with oncogenic K-ras mutations. Inhibition of farnesyltransferase alone was sufficient to radiosensitize most K-ras mutant tumors, although K-Ras prenylation was not blocked. These results show that inhibition of activated K-Ras can promote radiation killing of pancreatic carcinoma in a superadditive manner. The finding that farnesyltransferase inhibition alone radiosensitizes tumors with K-ras mutations implies that a farnesyltransferase inhibitor-sensitive protein other than K-Ras may contribute to survival in the context of mutant K-ras. Farnesyltransferase inhibitors could therefore be of use as sensitizers for pancreatic carcinoma radiotherapy.

Mutation of cell cycle regulators and their impact on superficial bladder cancer

Early cytogenetic studies in bladder cancer identify regions of chromosomal gain or loss that can be candidate loci for oncogenes and tumor suppressor genes. Oncogenes with potential prognostic significance identified in bladder cancer the RAS family, epidermal growth factor receptor, ERBB-2, MDM2, and cyclin D1. The TP53 gene has been the most thoroughly characterized tumor suppressor gene in bladder cancer, with correlation of TP53 alterations with type of carcinogenic exposure, tumor stage and grade, as well as prognosis. Studies evaluating alterations of the retinoblastoma pathway have identified the retinoblastoma gene, RB, p161NK4A/CDKN2, and E2F-1 as tumor suppressor genes with potential prognostic significance in patients with bladder cancer. Better understanding of the genetic mechanisms underlying bladder tumor development and progression will allow better prevention, diagnosis, and treatment strategies.