Role of Raf Kinase Inhibitor Protein in Helicobacter pylori-Mediated Signaling in Gastric Cancer

Abstract

Helicobacter pylori is a helical bacterium that colonizes the stomach in over half of the world’s population. Infection with this bacterium has been linked to peptic ulcer disease and gastric cancer. The bacterium has been shown to affect regulatory pathways in its host cells through specific virulence factors that control gene expression. Infection with H. pylori increases levels of phosphorylation of Raf kinase inhibitor protein (pRKIP) in gastric adenocarcinoma (AGS) cells in vitro and in vivo. We investigated the role of H. pylori in the phosphorylation of RKIP as a possible mechanism to downregulate pro-survival signals in gastric adenocarcinoma. pRKIP induces RKIP transcriptional activity, which serves to induce apoptosis of damaged cells to prevent further tumorigenesis. Infection of wild type and RKIP knockout mice with H. pylori for 2 months further confirmed roles of RKIP and pRKIP in the prevention of gastric cancer progression. Loss of RKIP in AGS cells results in increased expression of the Cag A virulence factor after H. pylori infection and RKIP overexpression inhibits H. pylori-mediated STAT3 phosphorylation and STAT3 and NF-κB transcriptional activity. We examined the role of mTOR (mammalian target of rapamycin) after H. pylori infection on the phosphorylation of RKIP. Cells treated with rapamycin, an inhibitor of mTOR, displayed less expression of pRKIP after H. pylori infection. Microarray antibody analysis was conducted on wild-type and RKIP-knockdown AGS cells and showed that in the absence of RKIP, there was increased expression of pro-tumorigenic proteins such as EGFR, Raf-1, and MAPKs. Although further work is needed to confirm the interaction of RKIP and mTOR in AGS cells as a result of H. pylori infection, we hypothesize that H. pylori-mediated induction of pro-survival signaling in gastric epithelial cells induces a feedback response through the activation of RKIP. The phosphorylated, or active, form of RKIP is important in protecting gastric epithelial cells from tumorigenesis after H. pylori infection.

An estimation of the incidence of tuberous sclerosis complex in a nationwide retrospective cohort study (1997–2010)

“…We do not know the reason why peptic ulcer is negatively associated with TSC. Increased expression of phosphorylated Raf kinase inhibitor protein by gastric cells infected with H. pylori is abrogated by rapamycin, an mTOR inhibitor. Thus, mTOR might modulate the signalling pathways in gastric cells infected with H. pylori , contributing to the pathogenesis of peptic ulcers.…”

RKIP: A Key Regulator in Tumor Metastasis Initiation and Resistance to Apoptosis: Therapeutic Targeting and Impact

“…At a post-translational level, a phosphorylation of RKIP at Serine 153 (pSer153-RKIP) by protein kinase C ζ (PKCζ) has been found in several cancer types to account for loss of RKIP activity as a Raf-1 proto-oncogene, serine/threonine kinase/mitogen-activated protein kinase kinase/extracellular signal-regulated kinase (Raf/MEK/ERK) and GPCR inhibitor, through inhibition of G protein-coupled receptor kinase 2(GRK2) [6,30,74,75,76,77,78]. In some cancers, the nuclear pSer153-RKIP levels significantly correlated with poor response to therapy and overall prognosis [75,79].…”

“…Contrarily, the inhibition of Raf-mediated STAT3 serine phosphorylation is attained through RKIP-induced suppression of Raf-1 activity [97]. Accordingly, RKIP expression has been found to be inversely correlated with phosphorylated STAT3 (pSTAT3) levels in different tumor types [75,76,98,99,100]. …”

Pathogenesis of Helicobacter pylori Infection

“…Furthermore, Du et al suggested that cholesterol glucosylation by H. pylori modulates the lipid-raft trafficking endocytic pathway that requires PI3K activation, thereby favoring prolonged bacterial survival inside macrophages. Finally, Nisimova et al investigated the role of H. pylori infection in the phosphorylation of pro-apoptotic factor Raf kinase inhibitor protein (RKIP) [48]. They concluded that H. pylori in vitro and in vivo infection can induce phosphorylation and subsequent activation of RKIP which in turn can downregulate protumorigenic factors such as EGFR, Raf-1, and MAPKs and therefore effectively attenuating pro-survival signals in gastric tumorigenesis.…”