Bacteria and cancer : from toxin delivery to carcinogenesis

Abstract

Epidemiological evidence link certain chronic bacterial infections to a higher risk of cancer development. Induction of an inflammatory circuit and the accumulation of genomic instability are considered mechanisms by which bacteria contribute to malignant transformation. Whether production of toxins, that directly induce DNA damage, enhances the tumor promoting effects of chronic inflammation is still unknown. This thesis investigates the role of the cytolethal distending toxin (CDT), the first bacterial genotoxin identified, in carcinogenesis. We have studied the cellular responses to acute and chronic CDT intoxication, as well as the toxin production and secretion during bacterial infection.

Acute CDT intoxication triggers the activation of the DNA damage response and induction of survival signals in the target cells, which may favor cancer growth. Through a screening of a Saccharomyces cerevisiaelibrary, we identified 78 genes whose deletion confers hypersensitivity to CDT exposure (paper I). Bioinformatics analysis revealed that DNA repair and endocytosis were the two most represented signaling pathways among the genes identified in the screening. We further demonstrated that in response to DNA damage, the flap-endonuclease 1 (FEN1) regulated the RHOAdependent activation of the actin cytoskeleton and cell survival via the ROCK and MAPK p38 kinases, respectively, revealing a complex and previously unrecognized crosstalk between DNA damage, cell survival and cytoskeleton dynamics.

As chronic exposure to DNA damaging agents is a well-characterized risk for cancer development, we assessed the effects of chronic CDT exposure in vitro(paper II). Cells grown for more than six months in the presence of sub-lethal toxin doses showed an altered DNA damage response, genomic instability, and acquisition of several hallmarks of tumor progression, such as enhanced oxidative stress and capacity of anchorage independent growth. Cell survival of the chronically intoxicated cells was dependent on sustained activation of the MAPK p38 pathway. To dissect the role of CDT in tumor development in vivo, we produced aSalmonella typhimuriumstrain that encode for the Salmonella typhiCDT-like toxin, known as typhoid toxin (TT). As control, we used an isogenic strain carrying an inactive toxin. Both strains successfully infected the immunocompetent sv129 mice for more than 2 months, however only the bacteria expressing the active genotoxin caused an enhanced inflammation in liver and spleen.

To understand how this potential bacterial carcinogen is delivered to the target cells, we studied the secretion of the Salmonella TT (paper III). We demonstrated that TT is secreted from the bacterium via outer membrane vesicles (OMVs). These vesicles are further released into the extracellular environment via an exocytosis-like process. The paracrine internalization of TT-loaded OMVs by bystander cells was dependent on dynamin-1-mediated endocytosis.

Taken together, our studies contribute to elucidate the survival strategy of cancer cells in response to CDT, its role in cancer progression and its secreting mechanisms.