Novel MEK inhibitor trametinib and other retinoblastoma gene (RB)‐reactivating agents enhance efficacy of 5‐fluorouracil on human colon cancer cells

Abstract

Chemotherapy for colorectal cancer has become more complicated and diversified with the appearance of molecular-targeting agents. 5-Fluorouracil (5-FU) has been a mainstay of chemotherapy for colorectal cancer, but it is still unknown whether the combining of 5-FU with novel molecular-targeting agents is effective. Thymidylate synthase (TS) is a direct target of 5-FU, and the low TS level has been generally supposed to sensitize 5-FU's efficacy. We therefore hypothesized that RB-reactivating agents could enhance the efficacy of 5-FU, because the RB-reactivating agents could suppress the function of transcription factor E2F of TS gene promoter. We used three RB-reactivating agents, trametinib ⁄ GSK1120212 (MEK inhibitor), fenofibrate (PPARa agonist), and LY294002 (PI3K inhibitor), with 5-FU against human colon cancer HT-29 and HCT15 cells. Trametinib induced p15 and p27 expression and reduced cyclin D1 levels in HT-29 cells. Fenofibrate also dephosphorlated ERK1 ⁄ 2 and reduced cyclin D1 levels in HT-29 cells. LY294002 induced p27 expression in HCT15 cells. All three agents caused dephosphorylation of RB protein and G1-phase arrest with a reduction of TS expression. As a consequence, the combination of 5-FU with each of the agents resulted in a significant decrease of colony numbers in HT-29 or HCT15 cells. These results suggest "RB-reactivation therapy" using molecular-targeting agents to be a new strategy for 5-FU-based chemotherapy. In particular, we strongly expect trametinib, which was discovered in Japan and was recently submitted to FDA for approval, to be used together with established regimens for colorectal cancer. (Cancer Sci 2013; 104: 687-693) 5-F luorouracil (5-FU) has been widely used for the treatment of advanced colorectal cancers. In recent years, combined treatments based on 5-FU have improved the outcome of patients with advanced colorectal cancers.(1,2) However, 5-FU has problems with chemosensitivity or toxicity, including considerable bone marrow suppression in some cases.( 3) 5-Fluorouracil inhibits thymidylate synthase (TS), which plays a central role in DNA biosynthesis. Thymidylate synthase expression is crucial to the efficacy of 5-FU. (4,5) Actually, the level of TS expression is inversely correlated with the clinical response to 5-FU-based chemotherapy for colorectal and other cancers. (6) . Therefore, the downregulation of TS expression might be a promising way to enhance the efficacy of 5-FU.(7) On the other hand, TS is a target of E2F1, whose transcriptional ability is repressed by direct binding to hypophosphorylated RB protein. (8) We then hypothesized that clinically used molecular-targeting agents that reactivate RB might be useful to improve the efficacy of 5-FU.In the present study, we used three RB-reactivating agents; the MEK inhibitor trametinib ⁄ GSK1120212 (also called JTP-74057), (9) PPARa agonist fenofibrate, and PI3K inhibitor LY294002. Trametinib is a novel allosteric MEK1 ⁄ 2 inhibitor, originally identified as a p15 inductive compound by us,shown to have broad ...

Coadministration of Trametinib and Palbociclib Radiosensitizes KRAS-Mutant Non–Small Cell Lung Cancers In Vitro and In Vivo

“…While many treatments have been developed for NSCLCs that have activating mutations in signal transduction pathways, KRASmutant NSCLCs have continually led down a path of therapeutic resistance (12)(13). Recent findings highlight that KRAS mutations act as a negative predictive marker for tumor response in patients with NSCLCs treated with adjuvant chemotherapy or anti-EGFR therapies (7,27,34,35).…”

“…This mutation leads to decreased efficacy and resistance to chemotherapeutics and radiotherapy (11)(12)(13)(14). Previous efforts have shown the use of MEK inhibitor (MEKi) in cancer cells harboring KRAS mutations (13,15,16).…”

“…Recent studies in colon cancers have shown the retinoblastoma (RB)-reactivating activity of trametinib (GSK1120212), which is an allosteric inhibitor of MEK1/2 (13,26). CDK4/RB pathway involvement in MEKi sensitivity led us to investigate this cell-cycle regulatory pathway.…”

Molecular Pathways: The Basis for Rational Combination Using MEK Inhibitors in KRAS-Mutant Cancers

“…Because phosphorylation of Rb results in G 1 -S cell-cycle progression, deregulation of the RAS-ERK pathway leads to sustained cell-cycle progression. MEK inhibitors have been shown to reduce cyclin D1 levels and induce the p27 KIP1 expression as well as cause dephosphorylation of Rb, arresting human cancer cells in the G 1 phase (23,24).…”

The Significant Role of Cyclin D1 in the Synergistic Growth-inhibitory Effect of Combined Therapy of Vandetanib with 5-Fluorouracil for Gastric Cancer

“…by a decrease in TYMS (10). Thus, the development of a new therapeutic strategy with 5-FU to reduce TYMS expression would be of interest.…”