Proximal tubular cytochrome c efflux: Determinant, and potential marker, of mitochondrial injury

Abstract

Background. Cytochrome c (cyt c) is released from mitochondria after tissue injury, but little is known of its subsequent fate. This study was undertaken to ascertain: (1) does cyt c readily gain access to the extracellular space; (2) if so, what are some determinants of this process; and (3) might cyt c release be a potentially useful marker of in vivo tissue damage. Methods. Isolated mouse proximal tubules (PT) were subjected to site 1 (rotenone; Rot), site 2 (antimycin A, AA), or site 3 (hypoxic) respiratory chain blockade (± 2 mmol/L glycine, to prevent plasma membrane disruption/cell death). Alternatively, oxidant injury was imposed (Fe2+ or cholesterol oxidase). Extra- and intracellular cyt c levels were quantified by Western blot. Plasma or urine cyt c levels were also determined after rhabdomyolysis or ischemic acute renal failure (ARF) (in mice), or clinical ARF. Results. AA, Rot, and hypoxia caused variable degrees of PT cyt c release (AA ≫ rot ≈ hypoxia), but at most, <20% of total cell content was involved. In contrast, Fe2+ evoked ∼65% cyt c efflux, and cholesterol oxidation caused ∼100% cyt c release. Glycine did not block cyt c efflux, dissociating this process from plasma membrane disruption/necrotic cell death. After rhabdomyolysis, plasma cyt c levels rose and correlated with the severity of ARF (r, 0.93 vs. BUNs). Cyt c was detected in urine after both experimental and clinical ARF. Conclusion. Cell cyt c release is dependent on the site and the type of mitochondrial injury sustained. Oxidative injury, in general, and cholesterol oxidation, in particular, seem particularly relevant in this regard. After mitochondrial release, cyt c traverses plasma membranes, eventuating in the extracellular space. The data suggest that plasma and/or urine cyt c appearance might function as a clinically useful in vivo marker of mitochondrial stress and the tissue injury sustained.