Gene set enrichment analysis reveals several globally affected pathways due to SKI-1/S1P inhibition in HepG2 cells.

Sterol regulatory element-binding proteins (SREBPs) are transcription factors governing transcription of genes related to cholesterol and fatty acid metabolism. To become active, SREBPs must undergo a proteolytic cleavage to allow an active NH(2)-terminal segment to translocate into the nucleus. SKI-1/S1P is the first protease in the proteolytic activation cascade of SREBPs. SREBP inhibition may be useful, for example, in the treatment of liver steatosis caused by homocysteine-induced lipid synthesis. Accordingly, we overexpressed inhibitory prodomains (proSKI) of SKI-1/S1P in HepG2 cells to block SREBP activation to evaluate the potential of SKI-1/S1P in controlling cellular cholesterol synthesis. SKI-1/S1P inhibition resulted in reduced cholesterol synthesis and mRNA levels of the rate-limiting enzymes, HMG-CoA reductase and squalene epoxidase, in the cholesterol synthetic pathway. The inhibitory effect was maintained in the presence of homocysteine-induced endoplasmic reticulum stress. A gene set enrichment analysis was performed to elucidate other metabolic effects caused by SKI-1/S1P inhibition. SKI-1/S1P inhibition was observed to affect a number of other metabolic pathways, including glycolysis and citric acid cycle. These results demonstrate that inhibition of SREBPs decreases cholesterol synthesis in HepG2 cells both in the absence and presence of homocysteine. SKI-1/S1P inhibition may cause widespread changes in other key metabolic pathways.

Secreted apolipoprotein E reduces macrophage-mediated LDL oxidation in an isoform-dependent way.

As an inflammatory cell, the macrophage produces various oxidizing agents, such as free radical species. These can modify LDL as a secondary effect and doing so may favor atherogenic processes. Any molecule able to counteract these reactions would be of much benefit, especially if secreted by the macrophage itself at the lesion site. Such is the case for apolipoprotein E (apoE), which has been shown to exert antioxidant properties in some studies, mostly in relation to Alzheimer's disease. In this study, we assessed the antioxidant potential of the various isoforms of apoE (E2, E3, and E4) using a metal-induced LDL oxidation system with exogenous recombinant apoE and an in vitro model of macrophage-mediated LDL oxidation. We found that all three isoforms had an antioxidant capacity. However, whereas apoE2 was the most protective isoform in the cell-free system, the opposite was observed in apoE-transfected J774 macrophages. In the latter model, cellular cholesterol efflux was found to be more important with apoE2, possibly explaining the larger quantity of oxidative indices observed in the medium. It is proposed that the antioxidant property of apoE results from a balance between direct apoE antioxidant capacities, such as the ability to trap free radicals, and potentially pro-oxidative indirect events associated with cholesterol efflux from cells. Our observations add to the therapeutic potential of apoE. However, they also suggest the need for more experiments in order to achieve careful selection of the apoE isoform to be targeted, especially in the perspective of apoE transgene use.