Medium-chain fatty acid metabolism in hepatocytes and adipocytes

Abstract

Fatty acids (FAs), in free form or incorporated into complex lipids, constitute an inseparable part of the daily human diet. Different carbon atom chain length and saturation rate leads to a huge variety of FAs of which long-chain fatty acids (LCFAs) being the dominant type. The main focus of this work is medium-chain FAs (MCFAs), a group of saturated FAs, mainly found in coconut oil, palm kernel oil and mammalian milk. MCFAs have shorter carbon atom chain lengths and therefore different properties compared to longer FAs. The cellular metabolism of MCFAs is still insufficiently investigated particularly when compared to LCFAs. <br /> This thesis analyzes the metabolism of MCFAs with aim to better comprehend their effect on the human body, to examine their unique properties, and to contribute to the general understanding of MCFAs.<br /> The metabolism of MCFAs was investigated in different cell lines, in mouse organs, in differentiating 3T3-L1 cells, and in dedifferentiating hepatocytes. Alkyne-FAs and click labeling technique were used to analyze the metabolism of MCFAs. The triacylglycerol (TAG) synthesis from MCFAs was the main object of research. It was shown that different cell lines routinely used in cell culture experiments could not synthesize TAGs from MCFAs, however, primary hepatocytes, 3T3-L1 adipocytes and organ slices from liver and gut could. It was also found that differentiating 3T3-L1 cells gain the function of TAG synthesis from MCFAs, whereas primary hepatocytes lose this function after longer cultivation in 2D cell culture. mRNA sequencing data obtained for differentiating 3T3-L1 cells and dedifferentiating hepatocytes showed changes in various genes related to lipid metabolism.<br /> MCFA uptake into cells was also investigated. Cells that could not synthesize MCT (medium chain triacylglycerols) could still uptake MCFAs. Moreover, a very interesting effect of CPT1 inhibitor etomoxir on the MCFA metabolism was observed. Etomoxir reduced TAG synthesis from MCFAs and enhanced the formation of unknown substances secreted into the medium by C11 MCFA fed hepatocytes. <br /> Further, different enzymatic activities in the glycerol phosphate pathway were analyzed in Huh7 cells, primary hepatocytes and 3T3-L1 cells. It was shown that although Huh7 cells and dedifferentiated hepatocytes cannot synthesize TAGs from MCFAs, but their lysates possess different enzymatic activities for this synthesis.<br /> This thesis is concluded by introducing hypothetical models for MCFA metabolism in hepatocytes and 3T3-L1 cells, which suggest the involvement of mitochondria in MCFA-TAG synthesis.