Nitric Oxide Derived from Cytoglobin-Deficient Hepatic Stellate Cells Causes Suppression of Cytochrome c Oxidase Activity in Hepatocytes.

Cell-cell interactions between hepatocytes (Hep) and other liver cells are key to maintaining liver homeostasis. Cytoglobin (CYGB), expressed exclusively by hepatic stellate cells (HSC), is essential in mitigating mitochondrial oxidative stress. CYGB absence causes Hep dysfunction and evokes hepatocarcinogenesis through an elusive mechanism. CYGB deficiency is speculated to hinder nitric oxide dioxygenase (NOD) activity, resulting in the elevated formation and release of nitric oxide (NO). Hence, we hypothesized that NO accumulation induced by the loss of NOD activity in CYGB-deficient HSC could adversely affect mitochondrial function in Hep, leading to disease progression. NO, a membrane-permeable gas metabolite overproduced by CYGB-deficient HSC, diffuses into the neighboring Hep to reversibly inhibit cytochrome oxidase (CcO), resulting in the suppression of respiratory function in an electron transport chain (ETC). The binding of NO to CcO is proved using purified CcO fractions from knockout () mouse liver mitochondria. Its inhibitory action toward CcO-specific activity is fully reversed by the external administration of oxyhemoglobin chasing away the bound NO. Thus, these findings indicate that the attenuation of respiratory function in ETC causes liver damage through the formation of excessive reactive oxygen species. Treating mice with an NO synthase inhibitor successfully relieved NO-induced inhibition of CcO activity . Our findings provide a biochemical link between CYGB-absence in HSC and neighboring Hep dysfunction; mechanistically the absence of CYGB in HSC causes mitochondrial dysfunction of Hep the inhibition of CcO activity by HSC-derived NO. 38, 463-479.