Molecular Mechanism Of PGC-1β-Regulated Mitochondrial Biogenesis In Myotubes And Hepatic Heme Biosynthesis In Rat Hepatocytes

The peroxisome proliferator-activated receptor-gamma (PPAR-y) coactivator-1β(PGC-1β) is a well-established regulator of the (3-oxidation of fatty acids and the oxidative phosphorylation in mitochondria; however, the underlying mechanism of its action remains elusive. In the present study, we show that PGC-1βis highly induced during myogenic differentiation and knockdown of endogenous PGC-1βby siRNA leads to the decrease of expression of several mitochondria-realted genes. PGC-1βstimulates the expression of its target genes including cytochrome c, ATP synthase P and ALAS-1 by its interaction with at least two transcriptional factors, NRF-1 and ERRα. The deletion or mutation of NRF-1 and/or ERRa binding sites in target gene promoters attenuates their activation by PGC-1β. Furthermore, we verified the physical interaction between PGC-1βand NRF-1 in vivo and in vitro. Moreover, inhibition of endogenous NRF-1 or ERRa by siRNA ablated the aforementioned function of PGC-1βand compromised the oxidative phosphorylation and mitochondrial biogenesis. Taken together, these results provide evidence for the physical interaction of NRF-1 and ERRa with PGC-1βin mediating the PGC-1β-dependent regulation of mitochondrial biogenesis and energy homeostasis network. Heme is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. It involved in basic cellular functions such as oxygen sensing, cell respiration and metabolism. ALAS1 (5-aminolevulinate synthase 1) is a mitochondrial enzyme that catalyzes the first step of the heme biosynthetic pathway. In the present study, we first described hepatic ALAS1 is regulated by PGC1β(peroxisome proliferators-activated receptorγ1β) through interacting and coactivating with NRF1. Elevation of PGC1βin rat primary hepatocytes via adenoviral vectors induces the expression ALAS1, which was abolished by siRNA of NRF1. The deletion or mutation of NRF1 binding sites in ALAS1 promoter attenuates its activation by PGC1β. PGC1βtreatment also up-regulated the mRNA levels of the genes ALAD (5-aminolevulinate dehydratase), HMBS (hydroxymethylbilane synthase), UROD (uroporphyrinogen decarboxylase), CPOX (coproporphyrinogen oxidase), PPOX (protoporphyrinogen oxidase) and FECH (ferrochelatase) encoding for enzymes controlling further steps in heme biosynthesis, which was also abolished by inhibition of endogenous NRF1. In conclusion, these results strongly support a role of PGC1βin the regulation of rat ALAS1 and five additional genes of the heme pathway, consequently leading to increased heme synthesis.