Figure 4. dHNF4 regulates all levels of the β-oxidation pathway.

(A) Second instar larvae that were transheterozygous for precise excisions of the EP2449 and KG08976 P-elements (control), or dHNF4^Δ33/dHNF4^Δ17 mutants (dHNF4⁻), were fed or starved for 24 hours, after which RNA was isolated and analyzed by northern blot hybridization. dHNF4 mRNA is induced upon starvation, with no transcript seen in mutant animals. CG5321, CG2107, fatp, yip2, CG9577, CG6178, and Acox57D-d, which function in β-oxidation, are significantly down-regulated in the dHNF4 mutant, consistent with the microarray results. CG3523, which encodes a predicted fatty acid synthetase, desat1, which encodes a predicted stearoyl-CoA desaturase, and CG11198, which encodes a predicted acetyl-CoA carboxylase, are all down-regulated upon starvation in both control larvae and dHNF4 mutants, and their overall level of expression is reduced in the mutant. (B) A schematic representation of the mitochondrial β-oxidation pathway is depicted. At the top, stored lipid in the form of triglycerides are hydrolyzed into free fatty acids by lipases. Acyl-CoA synthetases, which reside in the outer mitochondrial membrane, convert fatty acids into acyl-CoA for entry into the β-oxidation pathway. The acyl group is transported through the outer and inner mitochondrial membranes via a carnitine intermediate, and then processed through four enzymatic steps, as depicted. Each cycle generates one FADH₂ and one NADH, which donate their high energy electrons to the electron transport chain for ATP production. Each cycle of β-oxidation results in an acyl-CoA that is shortened by two carbons. This acyl-CoA can be processed through successive cycles to produce more FADH₂ and NADH. The names of genes that are down-regulated in dHNF4 mutants are listed next to their predicted enzymatic or transport functions.