DEAD-Box Protein RNA-Helicase DDX6 Regulates the Expression of HER2 and FGFR2 at the Post-Transcriptional Step in Gastric Cancer Cells

Abstract

The human DEAD/H-box RNA helicase DDX6 (RCK/p54) is a protein encoded by the fusion gene from the t(11;14)(q23;q32) chromosomal translocation observed in human B-cell lymphoma cell line RC-K8. DDX6 has a variety of functions such as translation initiation, pre-mRNA splicing, and ribosome assembly. However, details of the regulatory mechanism governing DDX6 and the functions of DDX6 are largely unknown. Previously, we reported that DDX6 is overexpressed in most malignant cell lines and clinical colorectal tumor samples and that DDX6 positively contributes to the pathogenesis of various cancers. In the current study, we aimed at revealing the function of DDX6 in HER2 and FGFR2 related human gastric cancer (GC) by using clinical samples and GC cell lines. DDX6 protein was overexpressed in about 60% of the clinical samples; HER2, in 35%; and FGFR2, in 30%, (n = 20). Interestingly, the DDX6 protein was overexpressed in all HER2-positive samples (n = 7), and in 83% (5 of 6) of the FGFR2-positive samples, which could reflect the contribution of DDX6 to the expression of HER2 and FGFR2. In the GC cell line MKN7, which has HER2 amplification, the knockdown of DDX6 by siR-DDX6 led to the decreased expression of the HER2 protein. On the other hand, the knockdown of HER2 did not influence the DDX6 expression. Similar results were also obtained for the KATO-III and HSC39 cell lines having amplified FGFR2 expression. The increased expression of DDX6 induced a significantly increased expression of the HER2 protein without increasing the mRNA expression. The results of an RNP Immunoprecipitation (RIP)-assay using GC cells indicated that the DDX6 protein acted as an RNA-binding protein for HER2 and FGFR2 mRNAs and positively regulated their post-transcriptional processes. These findings demonstrated that DDX6 was an upstream molecule that positively regulated the expression of HER2 and FGFR2 at the post-transcriptional step in GC cells.

Synthetic miR-143 Inhibits Growth of HER2-Positive Gastric Cancer Cells by Suppressing KRAS Networks Including DDX6 RNA Helicase

“…Since HER2 was not a direct target of miR-143, we sought to elucidate the mechanism underlying the down-regulation of HER2. As we had previously reported that DDX6 post-transcriptionally regulates HER2 expression [16], we next examined the expression level of DDX6. As expected, the expression level of DDX6 was significantly decreased in MKN-7 and KATO-III cells treated with syn-miR-143 (Figure 4B).…”

“…In our previous studies, we detected the overexpression of DDX6 in most of the tumor samples and cell lines examined, and found that DDX6 positively regulates c-Myc expression at the translational step in various cancers [22,23,24]. Furthermore, DDX6 also positively regulates HER2 expression at the translation step in GC cells [16]. In the present study, we demonstrated DDX6 to be a direct target of syn-miR-143 and reconfirmed the regulatory function of DDX6 in HER2 expression (Figure 4A–D, Supplementary Figure S2).…”

“…Previously, we reported the anti-tumor effect of a novel synthetic miR-143 (syn-miR-143) on colorectal cancer, which miR acts on by suppressing the KRAS signaling networks in colorectal cancer cells [15]. In addition, we recently found the regulatory function of DEAD-box RNA helicase 6 (DDX6) in the expression of HER2 at the post-transcriptional level in GC cells [16]. In the current study, we clarified the inhibitory effect of syn-miR-143 on the KRAS signaling networks in HER2-positive GC by targeting KRAS and its effector molecules, AKT and ERK.…”

Morphine may act via DDX49 to inhibit hepatocellular carcinoma cell growth

“…We focused on the DEAD box protein DDX49 as one of the genes whose expression was most altered by morphine. Other members of the DEAD box family have already been implicated in cancer [ 20 ]. For example, overexpression of DDX5 (also called p68) can transform healthy cells, and it is up-regulated in colorectal cancer [ 21 – 24 ].…”

RNA Helicases as Shadow Modulators of Cell Cycle Progression

“…Taken together, these findings suggest that DDX6 plays an important evolutionarily conserved role in cell cycle progression and proliferation, probably through the regulation of translation of specific key mRNAs. However, another group showed that overexpression of DDX6 leads to inhibition of cell growth and a decrease in anchorage-independent growth, a hallmark of malignant transformation [ 67 ]. Since DDX6 may become a valuable target for cancer therapy [ 68 , 69 ], the significant role of this helicase in the regulation of the cell cycle should be taken into account in order to avoid secondary side effects.…”