Characterization of the role of HIF prolyl 4-hydroxylases in metabolism and adipocyte differentiation and transmembrane prolyl 4-hydroxylase in aged tissues

Hypoxia response is a physiological response to a decreased oxygen concentration. When the oxygen concentration inside the cell decreases, HIF-transcription factors are activated, and cause an altered gene expression ensuring cell survival. One of the central pathways HIFs regulate includes energy metabolism: In normal oxygen concentration maximum amount of energy can be extracted by oxidizing glucose, while in low oxygen environments alternative energy metabolism, that consume more glucose, are favored. Hypoxia response can be activated by decreasing oxygen concentration in the atmosphere, or by using inhibitors specific for HIF-controlling enzymes (HIF-P4Hs), or by creating HIF-P4H deficient mouse lines. Mice hypomorphic for Hif-p4h-2 have been shown to be lighter than their wild-type counterparts and have less adipose tissue and adipose tissue inflammation. Manipulation of the non-oxidative energy pathways conveyed by hypoxia response could potentially be utilized in the treatment of obesity and metabolic syndrome.

In this thesis we investigated the roles of each HIF-isoenzyme in metabolism by using pharmacological pan-HIF-P4H inhibitors and HIF-P4H deficient mice. Our studies indicate that inhibition of HIF-P4H-2 is metabolically most favorable as measured by increased glucose sensitivity and resistance to weight gain. Inhibition of HIF-P4H-1 was also deemed neutral, or positive in some cases, while inhibition of HIF-P4H-3 was deemed detrimental. We also studied hypoxia responses effect in adipocyte cell differentiation using pharmacological HIF-P4H-inhibitors and mice hypomorphic for HIF-P4H-2. Our results show that complete HIF-P4H inhibition is detrimental to adipocytes resulting in less mature adipocytes: Most likely HIF-P4H-2 inhibition contributes to positive changes seen in adult mice, while the inhibition of all HIF-P4Hs creates unintended effects. Finally, we studied P4H-TM enzyme, whose mutation causes severe intellectual disability, eye abnormalities and muscle weakness in human patients. We found mitochondrial changes in tracheal epithelial cells in in P4H-TM deficient mouse possibly explaining the consecutive pneumonies seen in human patients. We also found a novel localization site for P4H-TM in spinal cord in addition to brain: We suspect that mitochondrial defects seen in tracheal cells could be present also in selective spinal cord cell populations possibly contributing to patient phenotypes.