Mitochondrial biogenesis fails in secondary biliary cirrhosis in rats leadingto mitochondrial DNA depletion and deletions

Abstract

Chronic cholestasis is characterizedby mitochondrial dysfunction, associated with loss of mitochondrialmembrane potential, decreased activities of respiratory chaincomplexes, and ATP production. Our aim was to determine themolecular mechanisms that link long-term cholestasis to mitochondrialdysfunction. We studied a model of chronic cholestasis inducedby bile duct ligation in rats. Key sensors and regulators of theenergetic state and mitochondrial biogenesis, mitochondrial DNA(mtDNA)-to-nuclear DNA (nDNA) ratio (mtDNA/nDNA) relativecopy number, mtDNA deletions, and indexes of apoptosis (BAX,BCL-2, and cleaved caspase 3) and cell proliferation (PCNA) wereevaluated. Our results show that long-term cholestasis is associatedwith absence of activation of key sensors of the energetic state,evidenced by decreased SIRT1 and pyruvate dehydrogenase kinaselevels and lack of AMPK activation. Key mitochondrial biogenesisregulators (PGC-1α and GABP-α) decreased and NRF-1 was nottranscriptionally active. Mitochondrial transcription factor A (TFAM)protein levels increased transiently in liver mitochondria at 2 wk afterbile duct ligation, but they dramatically decreased at 4 wk. ReducedTFAM levels at this stage were mirrored by a marked decrease (65%)in mtDNA/nDNA relative copy number. The blockade of mitochondrialbiogenesis should not be ascribed to activation of apoptosis orinhibition of cell proliferation. Impaired mitochondrial turnover andloss of the DNA stabilizing effect of TFAM are likely the causativeevent involved in the genetic instability evidenced by accumulation ofmtDNA deletions. In conclusion, the lack of stimulation of mitochondrialbiogenesis leads to mtDNA severe depletion and deletions inlong-term cholestasis. Hence, long-term cholestasis should be considereda secondary mitochondrial hepatopathy.