Several 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 homologs, critical for fatty acid and lipid homeostasis. Depletion of SBP-1 or its co-activator MDT-15 results in clear, sterile, lethal and slow growth phenotypes and a reduction in the fatty acid oleic acid, a precursor for triacylglycerides (TAGs) and poly-unsaturated fatty acids. Dietary supplementation with oleic acid markedly improved the phenotypes of
sbp-1 and
mdt-15 knockdown animals, suggesting a central role for oleic acid in SBP-1-function and lipid homeostasis. To identify novel conserved regulators of lipid homeostasis we performed an oleic acid auxotrophy screen. We screened the Ahringer RNAi library clones that cause clear, sterile, lethal, and slow growth phenotypes (1,300 genes) in search of clones that are, like
sbp-1 and
mdt-15, rescued upon dietary addition of oleic acid. We identified 6 genes in the screen. Three genes are required for mitochondrial function:
sco-1 (C01F1.2) and mitochondrial S9 and S17 ((F09G8.3; C05D11.10) confirming the role of mitochondria in lipid homeostasis (McKay et al, 2003). We also found
cul-1,
pyr-1 and
tkt-1(F01F10.2) in our screen. We focused on
tkt-1, the homolog of human transketolase (TKT) since TKT mouse knockouts are lean. C. elegans
tkt-1 is also necessary for normal lipid homeostasis since
tkt-1 depleted animals contain less TAGs by GC-MS and show reduced Sudan Black staining. TKT is an enzyme of the Pentose Phosphate Pathway and is necessary for the production of NADPH. A large number of SREBP/SBP-1 targets need NADPH as a cofactor and mammalian SREBP activates transcription of NADPH generating enzymes. This activation seems to be conserved in C. elegans since 3 NADPH producing enzymes (G6PDH, PGD and cytosolic IDH) were identified in our microarray expression studies of
sbp-1 depleted animals.
tkt-1 seems to be a new, direct target of SBP-1 since Ptkt-1::gfp expression is dependent on SBP-1 and
tkt-1 was identified in our microarray analysis of
sbp-1(RNAi) animals. We are currently testing if mammalian TKTs are also activated by SREBP in cultured cells. Our oleic acid auxotrophy screen has identified genes necessary for lipid homeostasis and found a new target of SBP-1. We suggest a molecular mechanism for lipid depletion in TKT depleted animals, possibly across species, through SBP-1 and the requirement for NADPH.