Regulation of survival and proliferation of human intestinal epithelial cells by leukotriene D4

Sammanfattning: Popular Abstract in Swedish Inflammation is a protective response against injury or infection (the invading microbes or their toxins) with the aim being the removal of causal agent followed by the repair process. Without inflammation, infections would go unchecked and wounds would not heal. However, inflammation is like a double-edged sword. Though it is mainly a protective response, some times, in cases of hypersensitivity or autoimmunity, it can be life threatening. Normally, leukocytes (white blood cells) are present in the circulating blood stream. A critical function of inflammation is the delivery of leukocytes to the site of injury where they ingest offending agents, kill bacteria and other microbes. This is facilitated through increase in the vascular permeability, that permit plasma proteins and leukocytes to leave the blood circulation, followed by their journey to the injured area. One class of inflammatory mediators that bring about these changes, are leukotrienes. Inflammatory bowel diseases (IBD), comprising of ulcerative colitis and Crohn disease, are chronic, relapsing inflammatory disorders. They are caused due to the activation of inflammatory cells whose products cause non-specific tissue injury. There is a strong correlation between inflammation in the gut and occurence of colon cancer. Patients suffering from IBDs have increased levels of leukotrienes in their body and interestingly, these patients have high risk of developing colon cancer. So, in the present study, I studied the effects of inflammatory mediator leukotriene D4 (LTD4), on intestinal epithelial cells and examined if this treatment would lead to any changes that might be essential for the development of cancer. Three major hall marks of colon cancer cell as compared to normal cell are: 1) Enhanced cell survival: Normal intestinal epithelial cells when they reach the top of the intestinal crypt undergo programmed cell death (apoptosis). In contrast, the signals that drive the cells to apoptosis are deregulated in the cancer cell that makes it survive better compared to normal cells and grow constantly. 2) Increased cell proliferation: Cell proliferation/growth is strictly controlled in normal cells. However, this mechanism is totally de-regulated in cancer cells leading to unlimited divisions of cells resulting in the formation of tumour/ adenoma. 3) Migration/ metastasis: This is another characteristic feature of cancer cell. After the formation of tumour, in some cases, the tumour cells acquire the ability to detach from the parental tumour, migrate and enter the blood and lymphatic vessels leading to the spread of the cancer to other organs. We tried to understand the phenomenon of inflammation induced cancer progression using LTD4 and intestinal epithelial cells as a model system. Normal intestinal epithelial cell line, Int 407 was treated with the inflammatory mediator LTD4 and its effects on cell survival and growth was examined in the papers I and II. When cells are stimulated with LTD4, the signal is received through its receptors present on the cell membrane and is relayed through activation of series of proteins. This process is called signal transduction. Signals are finally transduced to the transcription factors in the cell nucleus that are responsible to turn on the machinary leading to enhanced growth processes. Treatment with LTD4 enhanced both cell survival and proliferation through activation of several signaling pathways. We also found that these cells can produce LTD4. In the III paper, I used a compound (antagonist) that can block this LTD4 receptor signaling and examined the changes in cell survival and growth. To our surprise, this treatment led to dramatic cell death. This suggests that endogenous LTD4 signaling is essential for the cells to survive. Further, I examined if treatment with LTD4 can increase the movement/migration of cells in paper IV. LTD4 treatment enhanced intestinal epithelial cell migration through activation of another signaling pathway. In conclusion, these results suggest that inflammatory mediators such as LTD4 has a role in increasing cell survival, proliferation and migration of cells that may further facilitate the progression of cancer. Targetting the inhibition of this mediator might offer a novel therapeutic strategy in the treatment of inflammation-induced cancer progression.