Different dose-dependent effects of palmitate and oleate on mitochondrial dynamic behaviour and early endoplasmic reticulum stress onset in HEPG2-cells

Mitochondrial dysfunction and disruption of mitochondrial dynamics towards fission processes, as well as endoplasmic reticulum (ER) stress have been suggested to play a key role in insulin resistance development. Mitochondria are functionally and structurally linked to ER, which undergoes stress in conditions of lipids oversupply, activating the unfolded protein response, which in turn activates the principal inflammatory pathways that impair insulin action. Among the nutrients, dietary fats are believed to play key roles in insulin resistance onset. However, not all dietary fats exert the same effects on cellular energy metabolism. The aim of the present work was to evaluate the dose-dependent effects of saturated and monounsaturated fatty acids on mitochondrial dynamics behaviour and ER stress in vitro. To this end, HEPG-2 cells were treated for 24 hours with 10 μM, 50 μM, 100 μM, 250 μM and 500 μM of sodium palmitate or oleate. Western blot analysis was performed to evaluate the protein content of: a) mitofusin 2 (Mfn2), marker of mitochondrial fusion process; b) dynamin-related protein 1 (DRP1), marker of mitochondrial fission process; c) 78-kDa glucose-regulated protein (GRP78), marker of early ER stress onset. The preliminary results showed that palmitate induced an increase in DRP1 content mainly at the highest concentration, with no significant changes in Mfn2 content. Grp78 content increased at low concentrations of palmitate and returned at the baseline level at the highest concentration. On the other hand, oleate induced an increase in Mfn2 content mainly starting from 50 μM concentration, whereas DRP1 content did not significantly changed among the different oleate concentrations. GRP78 content increased at high concentrations of oleate. In conclusion, palmitate and oleate showed different dose-dependent effects on dynamic mitochondrial behaviour and ER stress induction. Palmitate induced an increase in mitochondrial fission at high concentrations, whereas the increase in GRP78 content at the lowest concentration suggested an induction of early ER stress even at low palmitate doses. Conversely, oleate induced mitochondrial fusion starting at low concentrations, whereas the induction of early ER stress could be observed only at the high concentrations. Given that both mitochondrial fission and ER stress have been associated to insulin resistance onset, the present data could confirm the induction of cellular mechanisms involved in insulin resistance development in presence of palmitate, whereas they showed that oleate may counteract early ER stress at low concentrations probably by inducing mitochondrial fusion processes. With the limitation that further studies are needed to analyse insulin resistance development, the present preliminary data suggested the involvement of mitochondrial dynamics behaviour in the protective effects of oleate towards insulin resistance onset.