Figure 8.

Summary: cholesterol homeostasis pathways in astrocytes and glioma cells. (A) At high plating density, normal astrocytes lower cholesterol levels compared to sparsely plated cells by both increasing ABCA1 (Figure S1) and by decreasing de novo synthesis through the mevalonate pathway¹⁵. The factors that activate ABCA1 in dense normal astrocytes are not defined, but might involve inhibition of FAK⁴⁹. (B) Glioma tumor initiating cells at high density keep cholesterol levels equivalent to sparse through the net effects of multiple feedback loops. Unlike normal astrocytes, they generate new cholesterol by keeping the mevalonate pathway on¹⁵. This cholesterol in turn is oxidized to oxysterols (Fig. 2), which act as activating ligands for LXR. Activation of LXR then in turn keeps cholesterol levels in check (Fig. 3) by increasing cholesterol efflux via upregulating ABCA1 RNA and protein (Figs 1 and 2) and through inhibition of the mevalonate pathway (Fig. 5). This enables the cells to survive and proliferate under conditions in which normal astrocytes cells are contact inhibited (Fig. 3 and ref.¹⁵). (C) In the absence of LXR, intracellular cholesterol levels are high (Fig. 3), likely due to low ABCA1 levels (Fig. 2) and increased activity in the mevalonate pathway (Fig. 5). This leads to decreased cell proliferation and viability, possibly due to toxic levels of cholesterol and/or oxysterols (Fig. 3). (D) In the absence of ABCA1, net cholesterol levels are equivalent in sparse and dense cells and the glioma cells continue to proliferate and are viable at high density (Fig. 4). While decreased ABCA1 should result in an accumulation of cholesterol due to decreased efflux, cholesterol is maintained at levels compatible with cell viability through feedback inhibition of the mevalonate pathway (Fig. 5), possibly through increased LXR activation by accumulated oxysterols, or though feedback inhibition of transcription factors such as SREBP⁴¹.