sitagliptin phosphate receptor Although our knowledge of how

Although our knowledge of how FAs affect GLP-1-producing L-cells is sparse and incomplete, increased intracellular ceramide [6], ER stress [12], and increased production of ROS [9] are indicated mediators of the lipotoxicity induced by SFA (16:0). However, the role for/interaction between these mediators remain elusive or unknown. When it comes to acute effects of FAs on GLP-1-secretion, these effects have been attributed to activation of free fatty sitagliptin phosphate receptor receptor 1 (FFAR1), as well as to intracellular metabolic handling of the FAs [5,6], but a potential role for FFAR1 in the metabolic fate of FAs and potential alterations following long term exposure to elevated FAs have not been extensively studied.
Material and methods
Results
Discussion We demonstrate that the induction of lipotoxicity in GLP-1 secreting cells is mediated by ROS generating intracellular metabolic handling of SFA (16:0) and independent of FFAR1 signaling. Specifically, an increase in intracellular ceramide contributes to the increase in ROS production previously shown to mediate lipotoxicity [9]. A ceramide dependent increase in early ROS production is supported by numerous studies indicating direct effects of ceramides on the electron transport chain and inhibition of complex I and III resulting in enhanced ROS production [31,32]. However, further studies are needed to investigate if the ceramide induced ROS increase indeed results from inhibition of the electron transport chain in GLP-1 secreting cells. Interestingly, what may be considered a contradictory increase in ROS following treatment with the ceramide synthesis inhibitor FB(1) alone, in the absence of elevated exogenous SFA (16:0), was observed. However, despite known elevating effects of ceramide on ROS production, FB(1) alone has previously been demonstrated to increase ROS in parallel with an increase in endogenous antioxidants [33]. It may also be hypothesized that FB(1) induction of ROS is secondary to low levels of de novo ceramide synthesis being essential for normal cell function, i.e. it is elevated ceramide levels that induce a cytotoxic ROS increase. Using molecular oxygen as the final electron acceptor in the respiratory chain, with the formation of H2O, inevitably leads to the formation of toxic by-products in the form of ROS [34]. An increase in FAO following supplementation with substrate inevitably then eventually becomes a significant contributor to increased ROS, as indeed indicated herein. However, FAO generated ROS is unlikely a significant contributor to the lipotoxicity observed herein, as inhibiting FAO did not induce a significant reduction in caspase-3 activity after 48h. Considering the relatively rapid turnover of cells in the intestinal epithelium, the early ceramide mediated apoptosis indicated is most likely of relevance for the reduced number of intestinal GLP-1-producing cells indicated following a diet rich in saturated fat. The ROS reducing capacity of MUFA (18:1) occurs in association with reduced expression of enzymes that control lipotoxic ceramide synthesis as well as reduce ceramide content. Further, increased expression of ceramide kinase (CerK) indicates increased ceramide phosphorylation following cosupplementation with MUFA (18:1). Thus, a lipotoxic increase in intracellular ceramide may be counterbalanced by phosphorylation and generation of C1P - which has been implicated in prosurvival mechanisms [35]. The different effects of SFA (16:0) and/or MUFA (18:1) on CerK mRNA vs protein expression, may stem from effects on translation. However, further studies are needed to delineate the mechanisms of the effects of FAs on CerK. The increase in cellular respiration observed in response to cosupplementation with MUFA (18:1) may indicate either or both enhanced viability due to lipoprotection, and/or enhanced mitochondrial function that plays a role in enhancing SFA oxidation - and thereby contributing to less toxic species [36] – as well as a reduction in ROS.