Impact of the GTPase ARFRP1 on chylomicron maturation

Background: Abnormalities in lipid metabolism and lipoprotein and lipid droplet (LD) formation can lead to dyslipidaemia, fatty liver disease and type 2 diabetes. Lipoprotein and LD synthesis are initiated in the ER, further maturation occurs in the Golgi apparatus. We have recently shown that deletion of GTPase ARFRP1 in the intestinal epithelium results in impaired chylomicron (CM) formation. ARFRP1 recruits ARL1 and Golgins, that interact with several Rab GTPases to the trans-Golgi and is thereby involved in cellular trafficking events. The aim was to clarify the mechanisms involved in the lipidation and release of CMs.

Methods: Members of the ARFRP1 cascade were depleted in Caco-2 cells and lipid release and LD composition were analysed.

Results and Conclusion: Knockdown of Arfrp1, Arl1, Golgin-245 and Rab6 significantly reduced lipid release from Caco-2 cells. Western blot analysis of isolated LD fraction from Caco-2 cells that were depleted of Arfrp1 or Arl1 showed an increase of Rab6 and Perilipin-2 and simultaneously lower amounts of Perilipin-3/TIP47. We hypothesise that correct sorting of Rab6 to LDs is required for Perilipin-3/TIP47 assembly to LDs and the subsequent transfer of triglycerides from LDs to CMs. Actually, suppression of Perilipin-3/TIP47 inhibited the CM maturation in Caco-2 cells. As depletion of Arfrp1 in 3T3-L1 adipocytes also affected the association of Rab6 with LDs, we believe that ARFRP1 and its downstream effectors are regulating the sorting of Rab6 to LDs and thereby modulating the pattern of LD coating proteins and lipidation processes.