Current Cancer Drug Targets - Volume 3, Issue 2, 2003

Transport mechanisms for the exclusion of toxic xenobiotics and their metabolites from cellular environment are crucial for living organisms. Accumulation of these toxins may affect a number of regulatory and other functions, ultimately leading to cell death. This trafficking of toxins and their metabolites is an energy dependent, primary active process, involving the hydrolysis of nucleotide triphosphates (ATP or GTP), while transferring substrate molecules across the cell membrane, against a concentration gradient of the substrate. Therefore, specific membrane associated proteins, known as efflux pumps, are required to remove these undesirable molecules from the cellular environment. These transport proteins have diverse structural characteristics with molecular weights ranging from 28 kDa to 190 kDa and a broad substrate specificity ranging from anionic to weakly cationic compounds. While these transport mechanisms constitute an important part of the cellular defense machinery, they also pose a formidable threat to the efficacy of chemotherapy against pathogenic bacteria and cancer cells. In cancer cells, the over expression of these proteins may confer a multidrug resistance (MDR) phenotype. This problem of MDR in cancer cells has so far been attributed to the two major families of efflux pumps, P-glycoprotein (Pgp) and multidrug resistance associated proteins (MRP), which belong to the ATP-binding cassette (ABC) super family. However, the existence of these pumps has not been able to explain all types of acquired MDR. Therefore, the importance of transport mechanisms other than these ABC-transporters cannot be ruled out. One such transporter is DNP-SG ATPase, whose identity has recently been established with RLIP76, a Ral binding GTPase activating protein known to be involved in the Ras-Rho-Ral mediated signaling mechanism. In the present article, we review the comparative functional, structural, and molecular characteristics of some transporters and discuss their role in xenobiotic transport and multidrug resistance.

Oncogenic Ras proteins have been seen as an important target for novel anticancer drugs. Due to the functional role of Ras farnesylation, fanesyltransferase (FTase) inhibition was thought to be a strategy for interfering with Ras-dependent transformation. When farnesylation is blocked, the function of Ras protein is severely impaired because of the inability of the nonfarnesylated protein to anchor to the membrane. Although it has been clearly demonstrated that FTase inhibitors (FTIs) inhibit Ras farnesylation, it is uncertain whether the antiproliferative effects of these compounds result exclusively from the effects on Ras. Moreover, no consensus has been reached as to the relevant targets(s) of FTIs that can explain their mosaic pharmacology. In searching for downstream targets for FTIs effects, CENP-E and CENP-F / mitosin were identified. Different studies showed that the inhibition of farnesylation interferes with CENP-E-microtubule association. In the presence of FTIs, chromosome alignment to the metaphase plate is delayed, suggesting that farnesylated proteins are involved in a step critical to bipolar spindle formation and chromosome alignment. An important question is whether these biological effects might contribute to the chemotherapeutic effects of the FTIs. However, FTIs, triggering the spindle checkpoint, might elevate the rate of cellular missegregation to levels that are incompatible with cell viability, as well as have a reduced (but still significant?) effect on checkpointproficient normal cells. As an example, RPR-115135 induced micronuclei (MN) increase in cancer cells displaying high chromosome instability (CIN) levels, whereas in normal cells it is devoid of activity. Cancer cells showing high CIN level might represent an ideal target for the activity of some FTIs.

Techniques for the construction of phage display libraries of combinatorial proteins have dramatically improved. This has allowed researchers to expand the applications to the field of cancer biology. The most direct use of protein phage-displayed libraries is the selection of ligands for individual proteins. This includes identification of peptide ligands for receptor signaling molecules: integrins, cytokine and growth factor receptors. Selected peptides may be used as competitors for natural ligands and for the mapping of binding epitopes. This approach has been exploited for delineation of intracellular signal transduction pathways and for the selection of enzyme substrates and inhibitors. Recently, more complicated biological systems were used as targets for biopanning. This includes combination of soluble proteins, cellular surfaces and even the vasculature of whole organs. cDNA expression libraries in phage-based vectors have been recently introduced. The use of phage as a vector for targeted gene therapy is also considered. These and other applications of phage display for cancer research will be reviewed.

Survivin, an inhibitor of apoptosis (IAP) containing one baculovirus IAP repeat (BIR) domain, has been reported to be capable of regulating both cellular proliferation and apoptotic cell death. Survivin splice variants, survivin-ΔEx3 and survivin-2B, have apparently retained and lost anti-apoptotic potential, respectively. As survivin was first discovered due to its high homology with effector cell protease receptor (EPR-1), a protein involved in blood coagulation, it has been suggested (but not proven) that EPR-1 may act as a natural anti-sense to survivin in cells. Survivin homologs have been found in non-human species. Survivin expression has been described during embryonic development and in adult cancerous tissues, with greatly reduced expression in adult normal differentiated tissues, particularly if their proliferation index is low. Survivin has been defined as a universal tumor antigen and as the fourth most significant transcriptosome expressed in human tumors. Although survivin is usually located in the cell cytoplasmic region and associated with poor prognosis in cancer, nuclear localisation, indicative of favorable prognosis, has also been reported. Survivin expression has also been reported in a number of proliferating normal adult tissues.Extensive research has been conducted, aimed at increasing our understanding of survivin, by determining its sub-cellular structure and location, mechanism(s) of action and control of expression. While much important information on this molecule has been accumulated, there are still many areas of controversy or limited information. Further research may enable exploitation of survivin overexpression in cancer compared to normal tissues, making survivin a potentially attractive target for cancer therapeutics.

Replicative senescence is a programmed cellular response in normal cells, the induction of which depends on the accumulated number of cell divisions. Unlike cells undergoing apoptosis, senescent cells have a large and flat morphology, express acidic β-galactosidase (β-gal) and show a permanent cell cycle G1 phase arrest. Recently, senescent-like growth arrest has been observed in many types of tumor cell lines after exposure to certain chemotherapeutic drugs. These senescent-like cancer cells show similar morphology, growth arrest and β-gal expression to normal cells undergoing replicative senescence. However, unlike replicative senescence during the aging process, the chemodrug-induced senescent-like growth arrest is independent of cell cycle distribution, telomere length or cell cycle inhibitors. These observations suggest that induction of senescentlike response may provide a novel target leading to permanent growth arrest in cancer cells. So far, cell lines derived from more than 14 types of cancers have shown senescent-like growth arrest by either introduction of tumor suppressor genes or treatment with chemotherapeutic drugs. In addition, the drug-induced β-gal expression has been correlated with cancer cells undergoing terminal senescent-like growth arrest, which provides a possible marker for this process. This review will describe the evidence on senescent-like growth arrest in human cancer cells and the molecular changes that differ between chemodrug-induced senescent-like growth arrest and apoptosis. In addition, the possible factors and mechanisms involved in this process are also discussed. Finally, the implications on how senescent-like growth arrest might be exploited as a possible new target for anti-cancer drugs are addressed.