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MUSCLE CELL BIOLOGY AND CELL MOTILITY

Calcium-regulated changes in mitochondrial phenotype in skeletal muscle cells

¹Department of Biology and ²Department of Kinesiology and Health Science, York University, Toronto, Ontario, Canada M3J 1P3

Submitted 30 September 2003 ; accepted in final form 23 December 2003

Cytochrome c expression and mitochondrial biogenesis can be invoked by elevated intracellular Ca²⁺ in muscle cells. To characterize the potential role of Ca²⁺ as a messenger involved in mitochondrial biogenesis in muscle, we determined the effects of the Ca²⁺ ionophore A-23187 on the expression of nuclear- and mitochondrially encoded genes. Treatment of myotubes with 1 µM A-23187 for 48–96 h increased nuclear-encoded -subunit F₁ATPase and malate dehydrogenase (MDH) mRNA levels by 50–100% (P < 0.05) but decreased mRNA levels of glutamate dehydrogenase (GDH) by 19% (P < 0.05). mRNA levels of the cytochrome c oxidase (COX) nuclear-encoded subunits IV, Vb, and VIc were unchanged, whereas the mitochondrially encoded subunits COX II and COX III were decreased by 30 and 70%, respectively (P < 0.05). This was paralleled by a 20% decrease (P < 0.05) in COX activity. These data suggest that cytoplasmic Ca²⁺ differentially regulates the mRNA level of nuclear and mitochondrial genes. The decline in COX II and III mRNA may be mediated by Tfam, because A-23187 modestly reduced Tfam levels by 48 h. A-23187 induced time-dependent increases in Egr-1 mRNA, along with the activation of ERK1/2 and AMP-activated protein kinase. MEK inhibition with PD-98059 attenuated the increase in Egr-1 mRNA. A-23187 also increased Egr-1, serum response factor, and Sp1 protein expression, transcription factors implicated in mitochondrial biogenesis. Egr-1 overexpression increased nuclear-encoded cytochrome c transcriptional activation by 1.5-fold (P < 0.05) and reduced GDH mRNA by 37% (P < 0.05) but had no effect on MDH or -subunit F₁ATPase mRNA. These results indicate that changes in intracellular Ca²⁺ can modify mitochondrial phenotype, in part via the involvement of Egr-1.

mitochondrial biogenesis; malate dehydrogenase; cytochrome c oxidase mitochondrial transcription factor-A; early growth response gene-1; glutamate dehydrogenase

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