Resistance to chemotherapy is a major obstacle for curative treatment of human gastric cancer (GC). However, the underlying molecular mechanisms are largely unknown. Wingless-type MMTV integration site family members (WNTs) are secreted glycoproteins involved in embryogenesis and, on inappropriate expression in the adult, in cancer. Here, we show expression of WNT6 in GC patient specimens, human GC cell lines and in a mouse model of GC. In human GC cells, WNT6 expression was enhanced by caveolin-1 (Cav1), a scaffold protein of plasma membrane caveolae. WNT6 knock-down and overexpression experiments demonstrated that WNT6 increased the resistance to apoptotic cell death induced by the anthracycline chemotherapeutics epirubicin (Epi) and doxorubicin (Dox). Epi increased the activity of the human WNT6 promoter through Cav1-dependent binding of β-catenin to the proximal WNT6 promoter. Epi increased both WNT6/Wnt6 and Cav1 expression in human GC cells and within the tumor area of a murine model of GC (CEA424-SV40 TAg). In GC patients, WNT6 expression was positively associated with the tumor stage and the nodal status, and inversely correlated with the response to ECF (Epi, cisplatin, 5-fluorouracil) chemotherapy. These results showed that WNT6 and Cav1 are upregulated by chemotherapeutics and enhance the resistance of GC cells to anthracycline drugs. Understanding the molecular mechanisms driving WNT6/Cav1-induced drug resistance will provide benefits in developing new therapies for GC.

Peroxisome proliferator-activated receptor ␥ (PPAR␥) is a transcription factor that promotes differentiation and cell survival in the stomach. PPAR␥ upregulates and interacts with caveolin-1 (Cav1), a scaffold protein of Ras/mitogen-activated protein kinases (MAPKs). The cytoplasmic-to-nuclear localization of PPAR␥ is altered in gastric cancer (GC) patients, suggesting a so-far-unknown role for Cav1 in spatial regulation of PPAR␥ signaling. We show here that loss of Cav1 accelerated proliferation of normal stomach and GC cells in vitro and in vivo. Downregulation of Cav1 increased Ras/MAPK-dependent phosphorylation of serine 84 in PPAR␥ and enhanced nuclear translocation and ligand-independent transcription of PPAR␥ target genes. In contrast, Cav1 overexpression sequestered PPAR␥ in the cytosol through interaction of the Cav1 scaffolding domain (CSD) with a conserved hydrophobic motif in helix 7 of PPAR␥'s ligand-binding domain. Cav1 cooperated with the endogenous Ras/MAPK inhibitor docking protein 1 (Dok1) to promote the liganddependent transcriptional activity of PPAR␥ and to inhibit cell proliferation. Ligand-activated PPAR␥ also reduced tumor growth and upregulated the Ras/MAPK inhibitors Cav1 and Dok1 in a murine model of GC. These results suggest a novel mechanism of PPAR␥ regulation by which Ras/MAPK inhibitors act as scaffold proteins that sequester and sensitize PPAR␥ to ligands, limiting proliferation of gastric epithelial cells.Peroxisome proliferator-activated receptor ␥ (PPAR␥) belongs to the nuclear receptor (NR) superfamily (31). Infection by Helicobacter pylori is a major risk factor for gastric cancer (GC) in humans (68). H. pylori increases the expression of PPAR␥, cytokines, and eicosanoids, while PPAR␥ protects the gastric epithelium by inhibiting apoptosis of host cells (19) and inflammation (42). PPAR␥ ligands (glitazones; 15-deoxy-prostaglandin J 2 ) have been shown to inhibit proliferation and induce growth arrest or apoptosis in human GC cell lines (32,52,53). PPAR␥ knockout (KO) mice are susceptible to chemically induced gastric carcinogenesis (36). In humans, the common "partial loss of function" gene polymorphism (Pro12Ala) is correlated with an increased risk of GC, suggesting a role for PPAR␥ as a tumor suppressor in the stomach (67).PPAR␥ inhibits cell proliferation by several mechanisms, including inhibition of cyclin D1 expression, promotion of its proteasomal degradation, and upregulation of cyclin-dependent kinase (CDK) inhibitors (20,55,65). Members of the Ras/mitogen-activated protein kinase (MAPK) cascade, such as extracellular signal-regulated kinases 1/2 (ERK1/2), counteract this effect by inducing cyclin D1 expression and reducing PPAR␥ activity by phosphorylation on serine 84 (serine 82 in mouse) in its N-terminal activation function (AF1) (7). Cav1, a scaffold protein of plasma membrane caveolae (46), attenuates ERK1/2 activation and cell growth by sequestration of upstream MAPK cascade components, including growth factor receptors, Ras, Raf, and MEK1. In contrast, Cav1...

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