Dealing with energy demand: the AMP-activated protein kinase

Stimulation of mitochondrial gene expression and proliferation of mitochondria following impairment of cellular energy transfer by inhibition of the phosphocreatine circuit in rat hearts

Mitochondria proliferate when cellular energy demand increases. However, the pathways leading to enhanced expression of mitochondrial genes are largely unknown. We tested the hypothesis that an altered flux through energy metabolism is the key regulatory event by decreasing mitochondrial energy supply to rat heart cells by creatine depletion. Electron microscopy showed that the density of mitochondria increased by 75% in such hearts (p < 0.01). Levels of representative mRNAs encoded on mitochondrial DNA (mtDNA) or on nuclear chromosomes were elevated 1.5 to 2-fold (p < 0.05), while the mtDNA content was unchanged. The mRNA for the nuclear encoded mitochondrial transcription factor A (mtTFA) was increased after GPA feeding (p < 0.05). Thus, we have shown that an impairment of mitochondrial energy supply causes stimulation of gene expression resulting in mitochondrial proliferation, probably as a compensatory mechanism. The observed activation of the mtTFA gene corroborates the important function of this protein in nuclear-mitochondrial communication.

Do cytoskeletal components control fatty acid translocation into liver mitochondria?

For two decades it has been assumed that inhibition of carnitine palmitoyltransferase I (CPT-I) by malonyl-CoA represents the main regulatory mechanism of liver ketogenesis. However, recent evidence indicates that CPT-I activity is also controlled by interactions between mitochondria and cytoskeletal components. This newly recognized mechanism emphasizes the emerging role of the cytoskeleton in the regulation of metabolic pathways.

Stimulation of mitochondrial gene expression and proliferation of mitochondria following impairment of cellular energy transfer by inhibition of the phosphocreatine circuit in rat hearts

Mitochondria proliferate when cellular energy demand increases. However, the pathways leading to enhanced expression of mitochondrial genes are largely unknown. We tested the hypothesis that an altered flux through energy metabolism is the key regulatory event by decreasing mitochondrial energy supply to rat heart cells by creatine depletion. Electron microscopy showed that the density of mitochondria increased by 75% in such hearts (p < 0.01). Levels of representative mRNAs encoded on mitochondrial DNA (mtDNA) or on nuclear chromosomes were elevated 1.5 to 2-fold (p < 0.05), while the mtDNA content was unchanged. The mRNA for the nuclear encoded mitochondrial transcription factor A (mtTFA) was increased after GPA feeding (p < 0.05). Thus, we have shown that an impairment of mitochondrial energy supply causes stimulation of gene expression resulting in mitochondrial proliferation, probably as a compensatory mechanism. The observed activation of the mtTFA gene corroborates the important function of this protein in nuclear-mitochondrial communication.

Expression of the AMP-activated protein kinase beta1 and beta2 subunits in skeletal muscle

A heterotrimeric member of the AMP-activated protein kinase (AMPK) isoenzyme family was purified from rat skeletal muscle by immunoaffinity chromatography, consisting of an alpha2 catalytic and two non-catalytic subunits, beta2 and gamma1. The AMPK beta2 cDNA (271 amino acids (aa), molecular weight (MW)=30¿ omitted¿307, pI 6. 3) was cloned from skeletal muscle and found to share an overall identity of 70% with beta1 (270 aa, MW=30¿ omitted¿475, pI 6.0). In the liver AMPK beta1 subunit, Ser-182 is constitutively phosphorylated whereas in skeletal muscle beta2 isoform, we find that Ser-182 is only partially phosphorylated. In addition, the autophosphorylation sites Ser-24, Ser-25 found in the beta1 are replaced by Ala-Glu in the beta2 isoform. beta2 contains seven more Ser and one less Thr residues than beta1, raising the possibility of differential post-translational regulation. Immunoblot analysis further revealed that soleus muscle (slow twitch) contains exclusively beta1 associated with alpha2, whereas extensor digitorum longus muscle alpha2 (EDL, fast twitch) associates with beta2 as well as beta1. Sequence analysis revealed that glycogen synthase, a known AMPK substrate, co-immunoprecipitated with the AMPK alpha2beta2gamma1 complex.