Lipolytic breakdown of endogenous lipid pools in beta cells contributes to glucose stimulated insulin secretion (GSIS), and is mediated by acute activation of neutral lipases in the amplification pathway. Recently, it has been shown in other cell types, that endogenous lipid can be metabolised by autophagy, where macromolecules are delivered to the lysosome for degradation. This lipophagy is catalysed by lysosomal acid lipase (LAL). Our current aim was to investigate LAL in pancreatic beta cells, and its role in lipophagy and GSIS. 

RT-PCR and Western blot analysis showed abundant gene and protein expression of LAL in MIN6 cells, mouse and human islets. Using insulin RIA to measure insulin secretion, we saw GSIS upregulated 1.8 fold in MIN6 cells pretreated for 24hr with lalistat (LAL inhibitor) (n=3; P<0.001), while an acute 2hr treatment showed no difference. Manipulating LAL expression using siRNA, also caused an upregulation in GSIS (1.5 fold n=3; P=0.09). As expected, lalistat treatment of MIN6 cells increased neutral lipid pools, as measured by lipid GC-MS (n=4): cholesterol ester 6.7 fold (P<0.001), triacylglycerol 1.7 fold (P<0.05) and diacylglycerol 2 fold (P<0.05). Additionally, nile red staining of MIN6 cells and confocal microscopy, showed the lipid droplet number of MIN6 cells was greater with lalistat treatment than in untreated cells. Inhibition of autophagy with atg7 siRNA in MIN6 cells, resulted in a 1.7 fold increase in GSIS (n=4; P<0.05), and mouse islets treated with 5mmol/L 3-methyladenine for 24hr, to inhibit autophagy, showed a doubling in GSIS (n=4; p<0.05).

Our data suggests a novel pathway of lysosomal lipid degradation, utilising LAL and potentially lipophagy, is a constitutive negative regulator of GSIS. This process could be mediated by depletion of substrate for the neutral lipases that are activated by glucose.