Vitamin D A Negative Regulator of Triggering Receptor Expressed on Myeloid Cell-1 (TREM-1) in Hepatocellular Carcinoma Cells

Abstract

Hepatocellular carcinoma (HCC) accounting for more than 90% of cases of primary liver cancer, is the third most common cause of cancer-related death worldwide. Exposure to toxic substance and exposure to infectious agents like HBV (hepatitis B virus) and HCV (hepatitis C virus) are the main risk factors for development of HCC. Chronic inflammation precedes the development of cirrhosis and HCC. TREM-1(triggering receptor expressed on myeloid cell-1) is an inflammatory marker and amplifier of inflammation. HMGB-1 secreted from necrotizing cells activates TREM-1 at cell surface. It further signals through PI3K and ERK1/2 to activate NF-κB transcription factor resulting in the production of pro-inflammatory cytokines namely IL-6 and TNF-α. These pro-inflammatory cytokines further enhance the inflammation to change the microenvironment and predisposing the liver to carcinogenesis. Low levels of vitamin D have been associated with liver diseases such as alcoholic liver disease, non-alcoholic steatohepatitis, and HBV and HCV infection. Low level of vitamin D is a poor prognostic factor and supplementation of vitamin D with antiviral therapy in HCV has proven beneficial. Vitamin D is an anti-inflammatory agent and acts through vitamin D receptor in the cells to suppress inflammation in various diseases. Vitamin D also decreases proliferation in various tumors and hence it can be used to decrease chronic inflammation in liver diseases. I found significantly increased expression of TREM-1, HMGB-1 and DAP-12 in HepG-2 cells (hepatocellular carcinoma cell line) compared to THLE-2 (epithelial cells transformed with Sv40 large T antigen) cells. Pro-inflammatory cytokines, IL-6 and TNF-α further significantly increased the TREM-1 expression in HepG-2 and THLE-2 cells. Treatment of HepG-2 cells with calcitriol attenuated the TREM-1 expression as well as the stimulatory effect of pro-inflammatory cytokines IL-6 and TNF-α on TREM-1 expression. Further, calcitriol also decreased the proliferation, invasion and migration of HepG-2 cells, suggesting that vitamin D supplementation can be used to attenuate the ongoing chronic inflammation in hepatitis and cirrhosis, and the progression to hepatocellular carcinoma can be delayed. This study provides molecular and biochemical evidence that suppression of TREM-1 by vitamin D supplementation together with conventional treatment of hepatitis, fibrosis and chronic hepatitis may be a novel strategy and promising target for delaying the progression of liver disease and hepatocellular carcinoma.