How an organism responds to oxygen availability in order to modulate its own metabolism balance remains an unresolved question in the field of developmental biology. In our study, we use Drosophila larvae as a model to study how hypoxia disrupts metabolism homeostasis. We have previously shown that hypoxic conditions cause growth restriction and lipid aggregation in larger droplets in the larval fat body, the main site of fat metabolism. However, the mechanism of lipid aggregation remains unknown. Here we demonstrate that Gbb signaling decreases in larval fat body tissue under hypoxia conditions. Overexpression of the ligand, Gbb does not rescue these lipid metabolism defects under hypoxia conditions. However, overexpressing an activated form of Mad protein, the transcription factor downstream of GBB signaling, rescues hypoxia-induced lipid metabolism defects in larval fat body. Therefore, a decrease in Gbb signaling is responsible for the larval lipid metabolism defects observed under hypoxia conditions. Interestingly, down-regulation of CDK8 in the larval fat body restores both lipid metabolism defects and Mad activity under hypoxic conditions. Therefore, an increase in CDK8 activity decreases GBB signaling and plays an important role in hypoxia-induced lipid metabolism defects.
We have previously demonstrated that the larval growth defects observed under hypoxic conditions are due to a block in insulin signaling. Drosophila insulin-like peptides (DILPs) are produced by insulin-producing cells (IPC) located in the larval brain. After production, they are secreted to promote protein synthesis and cell growth. Our laboratory has previously shown that hypoxia disrupts the insulin signaling pathway by causing retention of Dilp2 in the IPCs. Fat body specific over-expression of InR under hypoxia conditions does not rescue lipid metabolism defects under hypoxic conditions. However, knockdown of the downstream negative regulator of InR signaling, PTEN rescues hypoxia-induced fat body lipid loss. Interestingly, fat body specific knockdown of PTEN under hypoxic condition also increases Mad activity under hypoxia conditions. Therefore, inhibition of GBB and insulin signaling pathways regulates fat body lipid storage under hypoxic conditions.
In conclusion, a reduction in insulin signaling and enhanced activity of CDK8 in the fat body causes a decline in Gbb/BMP signaling that is required for the maintenance of proper lipid metabolism during times of limited oxygen availability.