DLC1 in inflammation-driven gastric carcinogenesis
Hematology, Oncology
Final Report Abstract
Chronic infection of the stomach by the bacterium Helicobacter pylori (HP) is a risk factor for gastric cancer (GC) in humans. “Deleted in liver cancer-1” (DLC1) is a caveolae-associated protein and tumor suppressor which inhibits RhoGTPases that act as major oncogenic driver mutations in human diffuse-type GC. The DLC1 gene is (epi)genetically silenced in many tumor entities including GC. However, the role of DLC1 in HP-related gastric disease was hitherto unknown. We previously identified DLC1 in a complex with caveolin-1 (Cav1) in gastric epithelial cells to be protective against HP-induced cytoskeletal changes and cell damage. Now we show that, in human GC cells, an N-terminally truncated protein variant (DLC1v4) was retained, in contrast to the full-length (FL) protein (DLC1v1) which was detectable only in non-neoplastic cells and tissues. Re-expression of both DLC1 isoforms in human GC cells inhibited migration but promoted cell adhesion. DLC1 counteracted the cell elongating (“humming bird”) effect of CagA, the major oncogenic virulence factor of HP, by promoting cell spreading via focal adhesions. DLC1 lowered CagA-induced activation of RHOA and stress-related MAP-kinases and altered transcription from mitogenic and hypoxia-sensitive reporter genes. Mechanistically, CagA and DLC1 interacted via their N- and C-terminal protein domains, indicating that the protective action of DLC1 against HP could be based on the neutralization of its most important virulence factor CagA. Live HP bacteria but also CagA alone downregulated the activity of the FL DLC1v1 and the stress-sensitive DLC1v4 promoters in cells. Chronic infection with HP also diminished gastric Dlc1 mRNA in C57BL/6 mice in vivo, and transfection of ectopic CagA in cells reduced DLC1v4 mRNA in vitro. Thus, DLC1 was transcriptionally downregulated by HP-CagA, and the two proteins fulfilled opposite functions by protein complex formation. This data argued for a functional antagonistic cross-talk between DLC1 and CagA. In the healthy human and mouse stomach, DLC1v1 protein was present in single (enterochromaffin-like) cells adjacent to acid-secreting parietal cells and interspersed between the glands at the base of the stomach mucosa. DLC1+ cells were reduced in areas of HP-related gastritis and gastric atrophy. DLC1 protein expression negatively correlated with tumor type (Lauren), stage (UICC) and grade (G) in GC patients, indicating that loss of DLC1 is an early molecular event in the transformation of normal to neoplastic tissue, which proposes it as a possible future target or marker for cancer prevention and/or patient’s risk monitoring. DLC1 down-regulation was confirmed in mouse models of gastritis (HK-IL1btg), murine GC (CEA-SV40 TAg) and DLC1-deficiency (Dlc1gt/+) which all suffered from gastric inflammatory infiltration and imbalanced cytokine and hormone profiles. In vivo therapy of a preclinical GC model (CEA-SV40 TAg) with an inhibitor of RHO-signaling attenuated tumor growth. In sum, our data suggest a protective role of DLC1 in initial steps of gastric disease by antagonizing CagA-mediated oncogenic signaling. Due to the involvement of CagA and DLC1 in regulation of RHOA, HP-related gastric disease may be assigned to the diffuse-type of GC. This association may provide a new stratification for Helicobacter pylori-infected GC patients. Unexpectantly, mice (CID) with simultaneous KO of the two cooperating tumor suppressors CAV1 and DLC1 and concomitant transgenic overexpression of human IL1b, under the control of the murine H+K+ATPase promoter specific for gastric acid secreting cells, spontaneously developed a more severe gastritis phenotype and even progressed to the adenoma stage than any previously in our hands HP-infected WT and CAV1-KO mice. We therefore concluded that genetic hits are dominant over HP infection in this mouse model(s), and did not further continue on experimental HP infection in these mice since it seems to be dispensable for the observed phenotype. Nevertheless, in future studies we plan to assess whether additional infection with CagA-proficient HP strains is able to further exacerbate CID-driven gastritis and adenoma formation.
Publications
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