論文の公開元へEnhanced G1 arrest and apoptosis via MDM4/MDM2 double knockdown and MEK inhibition in wild-type TP53 colon and gastric cancer cells with aberrant KRAS signaling
概要
1.1 Relationship of TP53, MDM2 and MDM4, and reactivation of p53
The tumor protein (TP) 53, a tumor suppressor gene, has been reported to be inactivated mainly by missense mutation in approximately 50% of various advanced human cancers (1, 2). Even in cancers carrying wild-type (wt) TP53, p53 is often suppressed by upregulated murine double minute homolog 2 (MDM2) and MDM4 (3). MDM2 inhibits the p53 transcriptional activity and ubiquitinates p53 leading to its degradation (4). MDM4 forms a complex with MDM2 that participates in p53 degradation. Furthermore, MDM4 can block p53-mediated transcription through directly binding to the transactivation domain of p53. MDM4 is also a target of MDM2-mediated ubiquitination and subsequent degradation (5, 6). In normal cells, an autoregulatory feedback loop formed with MDM2, MDM4, and p53 ensures a dynamic equilibrium between these molecules (4, 7). In cancer cells, the regulatory relationship between these three proteins is disrupted by TP53 mutation or MDM2 and MDM4 upregulation, contributing to carcinogenesis and tumor progression.
1.2 siRNA therapies for cancers carrying wild-type TP53
It has been expected that synthetic small interfering RNAs (siRNAs) are promising therapeutics to be applied to cancer therapy. They can be designed to specifically target cancer-driver genes in a sequence-specific manner, enabling more precise and personalized treatments (8). We previously reported that the MDM2 inhibitor nutlin-3 or siRNAs with DNA-substituted seed arms targeting MDM2 (chiMDM2) inhibited tumor cell growth and viability by inducing G1 arrest and apoptosis in colon and gastric cancer cells carrying wt TP53 (9-11). Furthermore, we revealed that MDM4 knockdown using chiMDM4 could greatly enhance the antitumor effects of nutlin-3 and chiMDM2 in those cancer cells via augmented p53 activation (10).
1.3 Present status of targeted therapies aiming at EGFR and downstream signaling pathway in colon and gastric cancer
Epidermal growth factor receptor (EGFR) and human epidermal growth factor receptor 2 (HER2), and their downstream signaling-molecules, i.e., of RAS-RAF- mitogen-activated protein kinase kinase (MEK)- extracellular signal-regulated kinase (ERK) cascade, play pivotal roles in the development and disease progression of various cancers (12). Most tumor cells of patients with advanced colorectal cancer overexpress EGFR, and half of the patients harbor RAS or RAF mutations (13). RAS and BRAF (V600E) mutations are associated with aggressive diseases, poorer survival and resistance to therapies employing anti-EGFR antibodies (14). HER2 amplifications are detected in 13% of patients with advanced gastric cancer (15). Combination chemotherapies with anti-HER2 monoclonal antibody trastuzumab benefit patients with HER2-positive gastric cancer. Despite of recent significant advances in chemotherapy for advanced gastric cancer, the prognosis remains unsatisfactory.
Trametinib is a MEK inhibitor firstly approved for clinical use in 2013 that specifically binds to MEK1 and MEK 2, and inhibits signaling of growth factor receptors including EGFR families (16). Trametinib is effective in the treatment of metastatic malignant melanoma and colon cancer with BRAF V600E or V600K mutations used as a single agent or as combination with BRAF inhibitors (17). However, trametinib has failed to show demonstrable clinical activity in treatment of colon cancer with mutant- type (mt) KRAS when used as a single agent (18), possibly due to mutual activation of KRAS and PI3K signaling pathways.
1.4 Crosstalk between p53 pathway and MAPK signaling pathway
The interaction between the p53 pathway and the RAS-RAF- MEK- ERK cascade has been previously reported (19, 20). ERK1/2 upregulates phosphorylation of MDM2 at Ser-166 and promotes MDM2-mediated p53 degradation. Dual targeting of the p53 pathway and the RAS-RAF-MEK-ERK cascade may be a rational therapeutic strategy for wt TP53 expressing colorectal and gastric cancers with activated EGFR pathways. Further, this strategy might be particularly important for tumors carrying mt RAS and mt RAF, exhibiting resistance to antibodies targeted against these receptors. Recently, the synergism of MDM2 and MEK inhibitors was demonstrated in colorectal and non-small cell lung cancer cells harboring mt KRAS (21). However, the precise mechanism of action remains unclear.
