Apoptosis and necroptosis in the liver: a matter of life and death

RIPK1 and death receptor signaling drive biliary damage and early liver tumorigenesis in mice with chronic hepatobiliary injury

Hepatocyte apoptosis is intrinsically linked to chronic liver disease and hepatocarcinogenesis. Conversely, necroptosis of hepatocytes and other liver cell types and its relevance for liver disease is debated. Using liver parenchymal cell (LPC)-specific TGF-beta-activated kinase 1 (TAK1)-deficient (TAK1^LPC-KO) mice, which exhibit spontaneous hepatocellular and biliary damage, hepatitis, and early hepatocarcinogenesis, we have investigated the contribution of apoptosis and necroptosis in hepatocyte and cholangiocyte death and their impact on liver disease progression. Here, we provide in vivo evidence showing that TAK1-deficient cholangiocytes undergo spontaneous necroptosis induced primarily by TNFR1 and dependent on RIPK1 kinase activity, RIPK3, and NEMO. In contrast, TAK1-deficient hepatocytes die by FADD-dependent apoptosis, which is not significantly inhibited by LPC-specific RIPK1 deficiency, inhibition of RIPK1 kinase activity, RIPK3 deficiency or combined LPC-specific deletion of TNFR1, TRAILR, and Fas. Accordingly, normal mouse cholangiocytes can undergo necroptosis, while primary hepatocytes are resistant to it and die exclusively by apoptosis upon treatment with cell death-inducing stimuli in vitro, likely due to the differential expression of RIPK3. Interestingly, the genetic modifications that conferred protection from biliary damage also prevented the spontaneous lethality that was often observed in TAK1^LPC-KO mice. In the presence of chronic hepatocyte apoptosis, preventing biliary damage delayed but did not avert hepatocarcinogenesis. On the contrary, inhibition of hepatocyte apoptosis fully prevented liver tumorigenesis even in mice with extensive biliary damage. Altogether, our results suggest that using RIPK1 kinase activity inhibitors could be therapeutically useful for cholestatic liver disease patients.

Role of the Gut-Liver Axis in Liver Inflammation, Fibrosis, and Cancer: A Special Focus on the Gut Microbiota Relationship

The gut and the liver are anatomically and physiologically connected, and this “gut–liver axis” exerts various influences on liver pathology. The gut microbiota consists of various microorganisms that normally coexist in the human gut and have a role of maintaining the homeostasis of the host. However, once homeostasis is disturbed, metabolites and components derived from the gut microbiota translocate to the liver and induce pathologic effects in the liver. In this review, we introduce and discuss the mechanisms of liver inflammation, fibrosis, and cancer that are influenced by gut microbial components and metabolites; we include recent advances in molecular‐based therapeutics and novel mechanistic findings associated with the gut–liver axis and gut microbiota.

Ca2+/calmodulin-dependent protein kinase II regulation by inhibitor 1 of protein phosphatase 1 alleviates necroptosis in high glucose-induced cardiomyocytes injury

Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) plays an important role in the cardiovascular system. However, the potential protective role of inhibitor 1 of protein phosphatase 1 (I1PP1), which is able to regulate CaMKII, in high glucose-induced cardiomyocytes injury remains unknown. In the present study, cardiomyocytes were transfected with I1PP1 adenovirus to inhibit protein phosphatase 1 (PP1) expression. After the cardiomyocytes were subjected to high glucose stimulation for 48 h, quantitative real-time PCR was used to detect CaMKIIδ alternative splicing. Lactate dehydrogenase (LDH) release and adenosine triphosphate (ATP) level were measured to assess cell damage and energy metabolism respectively. CaMKII activity was represented as phospholamban (PLB) phosphorylation, CaMKII phosphorylation (p-CaMKII) and oxidation (ox-CaMKII). Dihydroethidium (DHE), MitoSOX and JC-1 staining were used to assess oxidative stress and mitochondrial membrane potential. Necroptosis was evaluated by receptor interacting protein kinase 3 (RIPK3) expression, TUNEL and cleaved-caspase 3 levels. RIPK3, mixed lineage kinase domain like protein (MLKL) and dynamin-related protein 1 (DRP1) expressions were also detected. We found that high glucose disordered CaMKIIδ alternative splicing. I1PP1 over-expression suppressed PLB phosphorylation, ox-CaMKII, DRP1, RIPK3 and cleaved-caspase 3 proteins expression, decreased LDH release, attenuated necroptosis, increased ATP level, inhibited oxidative stress, and elevated mitochondrial membrane potential in high glucose-stimulated cardiomyocytes. However, there was no effect on phosphorylation of MLKL (p-MLKL), p-CaMKII, and receptor interacting protein kinase 1 (RIPK1) expression. Altogether, I1PP1 over-expression alleviated CaMKIIδ alternative splicing disorder, suppressed CaMKII oxidation, reduced reactive oxygen species (ROS) accumulation and inhibited necroptosis to attenuate high glucose-induced cardiomyocytes injury.

Obeticholic acid differentially regulates hepatic injury and inflammation at different stages of D-galactosamine/lipopolysaccharide-evoked acute liver failure

The farnesoid X receptor (FXR) is a ligand-activated transcription factor that regulates genes involved in bile acid metabolism. Accumulating data demonstrate that FXR has an anti-inflammatory activity. The present study aimed to investigate the effect of obeticholic acid (OCA), a novel synthetic FXR agonist, on D-galactosamine (GalN)/lipopolysaccharide (LPS)-evoked acute liver injury. All mice except controls were intraperitoneally injected with GalN (300 mg/kg) plus LPS (2.5 μg/kg). Some mice were pretreated with OCA (10 mg/kg) 48, 24 and 1 h before GalN/LPS. As expected, pretreatment with OCA alleviated hepatocyte apoptosis at early and middle stages of GalN/LPS-induced acute liver failure. By contrast, pretreatment with OCA augmented hepatic injury and inflammatory cell infiltration at middle stage of GalN/LPS-induced acute liver failure. Additional experiment found that OCA inhibited hepatic NF-κB activation at early and middle stages of GalN/LPS-induced acute liver failure. Interestingly, OCA inhibited hepatic proinflammatory cytokine tnf-α and il-6 but upregulated hepatic anti-inflammatory cytokine il-10 at early stage of GalN/LPS-induced acute liver failure. By contrast, OCA suppressed hepatic anti-inflammatory cytokine tgf-β and il-10 at middle stage of GalN/LPS-induced acute liver injury. These results suggest that FXR agonist OCA differentially regulates hepatic injury and inflammation at different stages of GalN/LPS-evoked acute liver failure.