Thymidylate synthase (TS) is a folate-dependent enzyme that catalyzes the reductive methylation of 2'-deoxyuridine-5'-monophosphate to 2'-deoxythymidine-5'-monophosphate. This pathway provides the sole intracellular de novo source of 2'-deoxythymidine-5'-triphosphate; therefore, TS represents a critical target in cancer chemotherapy. 5-Fluorouracil (5-FU) was synthesized in 1957 and represents the first class of antineoplastic agents to be developed as inhibitors of TS. While 5-FU has been widely used to treat various human malignancies, its overall clinical efficacy is limited. Therefore, significant efforts have focused on the design of novel, more potent inhibitor compounds of TS. These agents fall into two main categories: folate analogs and nucleotide analogs. Five antifolate analogs are currently being evaluated in the clinic: raltitrexed, pemetrexed, nolatrexed, ZD9331, and GS7904L. Our laboratory has identified a novel mechanism of resistance that develops to TS inhibitor compounds, namely drug-mediated acute induction of new TS synthesis; this mechanism is directly controlled at the translational level. The ability of cancer cells to acutely induce the expression of TS may represent a novel mechanism for the development of cellular drug resistance. The future success of TS inhibitor compounds in the clinic may depend on novel strategies to selectively inhibit TS and on novel combination therapies to overcome cellular drug resistance.

The vacuolar-ATPase (v-ATPase) is a proton transporter found on many intra-cellular organelles and the plasma membrane (PM). The v-ATPase on PMs of cancer cells may contribute to their invasive properties in vitro. Its relevance to human cancer tissues remains unclear. We investigated whether the expression and cellular localization of v-ATPase corresponded to the stage of human pancreatic cancer, and its effect on matrix metalloproteinase (MMP) activation in vitro. The intensity of v-ATPase staining increased significantly across the range of pancreatic histology from normal ducts to pancreatic intra-epithelial neoplasms (PanIN) and finally pancreatic ductal adenocarcinoma (PDAC). Low-grade PanIN lesions displayed polarized staining confined to the basal aspect of the cell in the majority (86%) of fields examined. High-grade PanIN lesions and PDAC demonstrated intense and diffuse v-ATPase localization. In pancreatic cancer cells, PM-associated v-ATPase co-localized with cortactin, a component of the leading edge that helps direct MMP release. Blockade of the v-ATPase with concanamycin or shRNA targeting the V1E subunit reduced MMP-9 activity; this effect was greatest in cells with prominent PM-associated v-ATPase. In cells with detectable MMP-2 activities, however, treatment with concanamycin markedly increased MMP-2’s most activated forms. V-ATPase blockade inhibited functional migration and invasion in those cells with predominantly MMP-9 activity. These results indicate that human PDAC specimens demonstrate loss of v-ATPase polarity and increased expression that correlates with increasing invasive potential. Thus, v-ATPase selectively modulates specific MMPs that may be linked to an invasive cancer phenotype.

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Studies from our laboratory have shown that the folate-dependent enzyme, thymidylate synthase (TS), functions as an RNA binding protein. There is evidence that TS, in addition to interacting with its own TS mRNA, forms a ribonucleoprotein complex with a number of other cellular mRNAs, including those corresponding to the p53 tumor suppressor gene and the myc family of transcription factors. Using both in vitro and in vivo model systems, we have demonstrated that the functional consequence of binding of TS protein to its own cognate mRNA, as well as binding of TS to the p53 mRNA, is translational repression. Herein, we review current work on the translational autoregulatory control of TS expression and discuss the molecular elements that are required for the TS protein-TS mRNA interaction. TS may play a critical role in regulating the cell cycle and the process of apoptosis through its regulatory effects on expression of p53 and perhaps other cell cycle related proteins. Finally, the ability of TS to function as a translational regulator may have important consequences with regard to the development of cellular resistance to various anticancer drugs.

Protein kinase D (PKD) signaling plays a critical role in the regulation of DNA synthesis, proliferation, cell survival, adhesion, invasion/migration, motility, and angiogenesis. To date, relatively little is known about the potential role of PKD in the development and/or progression of human colorectal cancer (CRC). We evaluated the expression of different PKD isoforms in CRC and investigated the antitumor activity of PKD inhibitors against human CRC. PKD2 was the dominant isoform expressed in human colon cancer cells. PKD3 expression was also observed but PKD1 expression, at both the RNA and protein levels, was not detected. Suppression of PKD using the small molecule inhibitors, CRT0066101 and kb-NB142-70, resulted in low micromolar in vitro antiproliferative activity against multiple human CRC cell lines. Drug treatment was associated with dose-dependent suppression of PKD2 activation. Incubation with CRT0066101 resulted in G2/M phase arrest and induction of apoptosis in human CRC cells. Further studies showed that CRT0066101 treatment gave rise to a dose-dependent increase in expression of cleaved PARP and activated caspase-3, in addition to inhibition of AKT and ERK signaling, and suppression of NF-κB activity. Transfection of PKD2-targeted siRNAs resulted in similar effects on downstream pathways as observed with small molecule inhibitors. Daily administration of CRT0066101 resulted in significant inhibition of tumor growth in HCT116 xenograft nude mice. Taken together, our studies show that PKD plays a significant role in mediating growth signaling in CRC and may represent a novel chemotherapeutic target for the treatment of CRC.

Cloves (Syzygium aromaticum) have been used as a traditional Chinese medicinal herb for thousands of years. Cloves possess antiseptic, antibacterial, antifungal, and antiviral properties, but their potential anticancer activity remains unknown. In this study, we investigated the in vitro and in vivo antitumor effects and biological mechanisms of ethyl acetate extract of cloves (EAEC) and the potential bioactive components responsible for its antitumor activity. The effects of EAEC on cell growth, cell cycle distribution, and apoptosis were investigated using human cancer cell lines. The molecular changes associated with the effects of EAEC were analyzed by Western blot and (qRT)-PCR analysis. The in vivo effect of EAEC and its bioactive component was investigated using the HT-29 tumor xenograft model. We identified oleanolic acid (OA) as one of the components of EAEC responsible for its antitumor activity. Both EAEC and OA display cytotoxicity against several human cancer cell lines. Interestingly, EAEC was superior to OA and the chemotherapeutic agent 5-fluorouracil at suppressing growth of colon tumor xenografts. EAEC promoted G0/G1 cell cycle arrest and induced apoptosis in a dose-dependent manner. Treatment with EAEC and OA selectively increased protein expression of p21WAF1/Cip1 and γ-H2AX and downregulated expression of cell cycle-regulated proteins. Moreover, many of these changes were at the mRNA level, suggesting transcriptional regulation by EAEC treatment. Our results demonstrate that clove extract may represent a novel therapeutic herb for the treatment of colorectal cancer, and OA appears to be one of the bioactive components.

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RNA interference is a post-transcriptional mechanism by which double-stranded RNA specifically silence expression of a corresponding gene. Small interfering double-stranded RNA (siRNA) of 21-23 nucleotides can induce the process of RNA interference. Studies from our laboratory have shown that translation of thymidylate synthase (TS) mRNA is controlled by its own protein end-product TS in a negative autoregulatory manner. Disruption of this process gives rise to increased synthesis of TS and leads to the development of cellular drug resistance to TS-targeted compounds. As a strategy to inhibit TS expression at the mRNA level, siRNAs were designed to target nucleotides 1058 -1077 on human TS mRNA. Transfection of TS1058 siRNA into human colon cancer RKO cells resulted in a dose-dependent inhibition of TS expression with an IC 50 value of 10 pM but had no effect on the expression of ␣-tubulin or topoisomerase I.