Role of microRNA-33 family in lipid metabolism in mammals and in Drosophila melanogaster

Regulation of synthesis, degradation and distribution of lipids is crucial for the homeostasis of organisms and cells. Sterol regulatory element-binding protein (SREBP) transcription factors are known to be master regulators of lipid metabolism. They are activated in situations of low lipid levels and downstream of certain signalling pathways. In turn, they activate the transcription of numerous genes involved in cholesterol, fatty acid and phospholipid metabolism. In the first part of my thesis, we provided evidence that the primary transcript of SREBP2 also contains a highly conserved intronic miRNA (miR-33a) that cooperates with SREBP2 to maintain normal cellular lipid levels. Specifically, we demonstrated that miR-33a reduces cellular cholesterol export by targeting the cholesterol-exporting transporter ABCA1. Thus in mammals, the genetic locus encoding SREBP2 and miR- 33a contains, on the one hand, a protein that increases cholesterol synthesis and, on the other hand, a miRNA that prevents export of newly synthesized cholesterol. Moreover, we demonstrated that miR-33a is also involved in fatty acid metabolism. Specifically, we showed that miR-33a inhibits translation of several transcripts encoding proteins involved in fatty acid β-oxidation including CPT1A, HADHB, and CROT, thereby reducing fatty acid degradation in mammals. In the second part of my thesis, we analysed the effect of miR-33 overexpression in the fat body of Drosophila melanogaster in different developmental stages. During larval stages, we observed that miR-33 overexpression partially prevented the decrease in cellular triacylglycerol (TAG) levels usually observed upon starvation. In parallel, transcript levels of cpt1, the rate-limiting step of β-oxidation, were decreased suggesting that decreased fatty acid degradation might have caused the observed phenotype. This indicates that the regulation of fatty acid β-oxidation by miR-33 is conserved between Drosophila and human and might be of functional importance. Surprisingly, overexpression of miR-33 in the fat body of adult flies led to the opposite phenotype with a marked decrease of TAG levels. We identified several transcripts coding for proteins involved in TAG synthesis as targets of miR-33, and demonstrate that they are downregulated in miR-33 overexpressing transgenic flies. Thus, miR-33 targets transcripts involved in either fatty acid synthesis or degradation depending on the context. Lipid synthesis and degradation are mutually exclusive processes within a cell. Our observations therefore indicate that miR-33 might play a role in preventing extreme fluctuations in cellular TAG levels in flies. Altogether, our results contribute to understand the collaboration between miR-33 family members and SREBPs in lipid metabolism and this has implications for the promising clinical use of miR-33 inhibitors.