Chemotherapy is one of the principal modes of treatment for cancer patients. Clinically, many tumors present a satisfactory response when they are first exposed to the chemotherapeutic drugs. However, drug resistance occurs sooner or later in these tumors, and the majority of the patients develop progressive disease. The mechanisms of treatment failure of chemotherapeutic drugs have been well studied. Via a unique protection system, i.e. multidrug resistance (MDR), the cancer cells can escape the toxic effect of most commonly used cancer drugs in spite of their different chemical structures and different mechanisms of intracellular activity. There are two classes of transporter proteins at the cellular surface which are responsible for MDR in tumors. One is the adenosine triphosphate-binding cassette transporter superfamily, which is an energy-requiring efflux pump with the function of extruding toxic chemotherapeutic drugs from the cancer cells. The other is the solute carrier transporter superfamily, which mediates the cellular uptake of anticancer drugs, and drug resistance may result from decreased activity of these transporters. Although transporters of MDR are responsible for the tumor resistance to many chemotherapeutic drugs currently used in cancer therapy, the mechanisms of resistance to platinum-based antitumor agents are through different pathways. In this article, the mechanisms of MDR transporters mediating resistance to the commonly used chemotherapeutic drugs and to platinum-based agents are reviewed. Finally, with the finding of cancer stem cells in more and more solid tumors, it is recognized that the cancer stem cell is spared along with its normal tissue stem cell counterparts with very subtle differences. One characteristic of the normal tissue stem cell is the self-protection ability through innate MDR transporters. Therefore, the essential self-protection property is also present in the cancer stem cells. The quiescent tumor stem cell with constitutive MDR is the main barrier to therapy. Successful cancer therapy will depend on the ability to discern the subtle differences between the tumor and normal stem cells so that approaches can be developed to eliminate the tumor stem cells without excessive toxicity to normal stem cells.

In 1983, Bokhman proposed a dualistic model of endometrial tumorigenesis based on the clinical observations and clinicopathologic correlations. The majority of endometrial cancers (approximately 70-80%), designated as type I carcinomas, follow the estrogen-related pathway. Histologically, most of the type I tumors seem to arise in the background of hyperplastic endometrium, show an endometrioid differentiation, and are of low grade. Clinically, they are overall characterized by a favorable behavior. Another 10-20% of endometrial cancers, designated as type II carcinomas, follow the estrogen-unrelated pathway and arise in the background of atrophic endometrium. Type II tumors usually occur at an older age, approximately 5-10 years later than type I tumors. They are typically high-grade carcinomas of nonendometrioid differentiation, most frequently serous, less frequently clear cell. Type II carcinomas behave as an aggressive clinical course and poor prognosis. This dualistic model was subsequently supported by the molecular studies, approximately a decade later. At present, endometrioid and serous carcinoma, which represent the major phenotypes of types I and II endometrial carcinomas, respectively, are characterized by distinctive types of genetic instability and molecular alterations. In endometrioid (type I) carcinoma, four major genetic changes are responsible for the tumorigenesis, i.e. silencing of PTEN tumor suppressor gene, presence of microsatellite instability due to alterations of the mismatch repair genes, mutation of K-ras protooncogene, and alteration of beta-catenin gene. On the other hand, p53 mutation and overexpression of Her2/neu oncogene are two major genetic alterations in serous and clear cell (type II) carcinomas. However, like in any model, there is evidence for exceptions. Many endometrial carcinomas are in the gray zone with overlapping clinical, morphologic, immunohistochemical, and molecular features of types I and II endometrial cancers. Finally, a small group of endometrial carcinoma is noted to be hereditary. It is known as the most common extracolonic malignancy in hereditary nonpolyposis colorectal cancer (Lynch syndrome), an autosomal dominantly inherited disorder of cancer susceptibility. Inactivation of the mismatch repair genes MSH2 and MSH6 seems to play a central role in the tumorigenesis.

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II-3.