Prevalent diseases such as obesity and type II diabetes are linked to aberrant lipid homeostasis. The C. elegans SREBP/SBP-1 transcription factor is, like its mammalian orthologs, crucial for fatty acid and lipid homeostasis. Depletion of
sbp-1 causes clear, sterile, lethal and slow growth defects and a reduction in oleic acid levels (OA). OA is a precursor for triacylglycerides (TAGs) and poly-unsaturated fatty acids and is produced by the SBP-1 regulated Stearoyl-CoA desaturases
fat-6 and
fat-7. Dietary supplementation with OA improves all
sbp-1 phenotypes, suggesting a central role for
fat-6/fat-7 expression and OA production in SBP-1-regulated lipid homeostasis. To identify additional regulators of lipid homeostasis we undertook two screens. First, in an auxotrophy screen, we tested the ability of dietary OA to rescue ""
sbp-1""-like phenotypes caused by 1311 RNAi library clones. Second, as
fat-6/fat-7 and OA play a key role in lipid homeostasis, we performed a genome-wide RNAi screen for modifiers of expression of a
fat-7 GFP promoter fusion. One gene,
tkt-1, an ortholog of human transketolase (TKT), was identified in both screens.
tkt-1 is necessary for normal lipid homeostasis as
tkt-1-depleted animals contain less TAGs and have reduced Sudan Black staining. Interestingly, TKT heterozygous mice are also lean, suggesting a conserved role for
tkt-1/TKT in lipid homeostasis. TKT acts in the Pentose Phosphate Pathway (PPP) and is necessary for the production of NADPH, a cofactor for the synthesis of lipids. Mammalian SREBP activates transcription of NADPH generating enzymes, suggesting that
tkt-1/TKT could also be regulated directly by SBP-1/SREBP. Indeed, we found that 3 NADPH-producing enzymes (
gspd-1,
idh-1 and T25B9.9) as well as
tkt-1 are down regulated in
sbp-1(RNAi) animals. In cultured mammalian cells, TKT is activated by SREBP, possibly by direct binding to the TKT promoter. We suggest that TKT may regulate lipid homeostasis through the requirement for NADPH and under (direct) SREBP activation. However, as
tkt-1 depletion also reduced the expression of
fat-7 and other putative SBP-1 targets,
tkt-1 might also act upstream of SBP-1 or in a feed forward loop. Indeed, preliminary experiments in cell culture using TKT siRNAs and compounds that modify the PPP do alter SREBP levels. This two-way regulation might allow animals to couple fatty acid production with PPP activity to ensure correct lipid levels which is critical in lipid homeostasis and obesity.