HR3/RORa-mediated cholesterol sensing regulates TOR signaling

Cells and organisms adjust their growth based on the availability of cholesterol, which is essential for cellular functions. However, the mechanisms by which cells sense cholesterol levels and translate these into growth signals are not fully understood. We report that cholesterol rapidly activates the master growth-regulatory TOR pathway in Drosophila tissues. We identify the nuclear receptor HR3, an ortholog of mammalian RORa, as an essential factor in cholesterol-induced TOR activation. We demonstrate that HR3 binds cholesterol and promotes TOR pathway activation through a non-genomic mechanism acting upstream of the Rag GTPases. Similarly, we find that RORa is necessary for cholesterol-mediated TOR activation in human cells, suggesting that HR3/RORa represents a conserved mechanism for cholesterol sensing that couples cholesterol availability to TOR-pathway activity. These findings advance our understanding of how cholesterol influences cell growth, with implications for cholesterol-related diseases and cancer. Overall design: To determine the biological effects of dietary cholesterol replenishment, and to assess which effects require HR3 or TOR, we performed RNAseq on whole larvae expressing no RNAi, Hr3-RNAi, or Tor-RNAi throughout the body (using ubiquitous driver Tub-GAL80TS; Tub-GAL4), after the animals had been stimulated with dietary cholesterol or not. Thus there are three genotypes x two feeding conditions, with five replicates each. Each replicate contains five late-third-instar larvae.