Abstract Body: Background: Microsomal triglyceride transfer protein (MTP) is a key regulator of lipid metabolism, facilitating lipid transfer and lipoprotein assembly in enterocytes and hepatocytes. We have shown that MTP interacts with adipose triglyceride lipase (ATGL) in adipocytes and modulates lipolysis. Molecular basis and physiological significance of these interactions remain unclear.
Aim: To examine physiological importance and nature of MTP-ATGL interactions.
Methods: Physiological MTP-ATGL interactions were studied in human adipocytes during adipogenesis and fasting. We used HDOC software to predict motifs and amino acids in MTP-ATGL interactions and site directed mutagenesis to test their importance. MTP expression and activity were determined with western blot and TG transfer assay. MTP mutants with robust activity and expression were used to study interactions with purified human ATGL using ELISA and co-immunoprecipitations.
Results: Immunoprecipitation experiments and activity assays showed that MTP expression and activity remain constant during adipogenesis and fasting in human adipocytes. In contrast, ATGL expression was detectable on the fifth day during adipogenesis. Fasting had no effect on ATGL protein expression but enhanced activity. MTP-ATGL interactions coincided with the expression of ATGL. However, ATGL binding to MTP decreased with fasting. MTP-ATGL interactions were unaffected by high salt concentrations but decreased with increasing detergent concentrations. In silico modeling predicted that the N-terminal region of MTP interacts with nonpolar residues in the hydrophobic domain of ATGL. Mutagenesis studies revealed that Y138E MTP mutant retained TG transfer activity but showed significantly less inhibition of ATGL binding and activity, highlighting the importance of this residue in protein-protein interactions and in regulating lipolysis.
Conclusion: Our data show that MTP–ATGL interactions occur as soon as ATGL protein is expressed during adipogenesis. However, these interactions are reduced with fasting likely increasing ATGL activity in adipocytes under nutrient deprivation. These studies suggest MTP-ATGL interactions are hydrophobic and dynamic in nature that they are affected by protein expression and fasting. Targeting MTP-ATGL interactions in adipocytes could modulate lipid metabolism and help prevent obesity and hepatic steatosis.