Establishing model mechanism‐based causal linkages between APAP‐induced hepatic necrosis and serum ALT
Authors: Hunt C. Anthony , Smith Andrew K. , Ropella Glen E. P. 
Published: June 2021

Alanine aminotransferase (ALT) is a biomarker for hepatocellular injury. For injury caused by the reactive metabolite (NAPQI) of acetaminophen (APAP), improved insight into the responsible causal mechanism for ALT release is expected to improve the clinical usefulness of serum ALT measurements. We are assessing the scientific utility of experimenting on model mechanisms (doi:10.3390/pr6050056) to posit plausible causal links between APAP within individual hepatocytes and ALT release. Early results suggest a strategy to further improve their explanatory power.To be scientifically useful, objects of virtual experiments, such as virtual mice, livers, and hepatocytes (vHPCs) must demonstrate credibility and meet demanding requirements, including the following. Model mechanism components are concrete. Component organization produces a strong behavioral analogy to the referent biology in its experimental context. Within reasonable constraints, events occurring during execution are biomimetic. Measurements of different phenomena generated during execution are quantitatively similar to wet‐lab measurements.Smith et al. (PMID27984590) demonstrate the scientific usefulness of virtual experiments by falsifying the prevailing explanation for the early pericentral (PC) pattern of APAP‐induced necrosis in mice. They also discovered a model mechanism that provides plausible quantitative causal explanations for major features of APAP hepatotoxicity in mice. We are reusing their virtual mice, livers, vHPCs, and experiment protocols. Key features include NAPQI formation rate increases periportal (PP)‐to‐PC; probabilities of GSH depletion and damage mitigation events decrease PP‐to‐PC; in vivo damage products are represented parsimoniously by virtual mitochondrial (mitoD) and non‐mitochondrial (nonMD) damage products. Accumulation of mitoD triggers necrosis.We extend those features by implementing parsimonious ALT release mechanisms in which all vHPCs contain the same amount of ALT. Within a vHPC, the stress resulting from accumulation of membrane damage products above a threshold value triggers ALT release. Each simulation cycle after membrane damage is above threshold, an ALT‐release event is scheduled to occur after a delay of D simulation cycles (1 cyc = 1 sec). D is a random draw from the uniform [1 h, 3 h] distribution. When a vHPC becomes necrotic, all remaining ALT is released. We experimented on three mechanism variants: membrane damage is triggered by either 1) mitoD, 2) nonMD, or 3) the combination. Released ALT accumulates in Mouse Body (ALT‐MB), which maps to serum ALT. Following a toxic APAP dose, ALT‐MB increases after 1.5 h and plateaus after 5.5 h for all three mechanisms. For mechanisms‐1 & ‐2, ALT‐MB values are approximately the same. Mechanism‐1 causes more ALT release from PC vHPCs, whereas mechanism‐2 causes more PP release. ALT‐MB values are only about 50% larger (rather than 2x) for mechanisms‐3, which is a consequence of differences in PP and PC amounts of ALT released. Results of wet‐lab experiments that determine relative PP and PC ALT release will enable selecting one model mechanism for further refinement.Support or Funding InformationUCSF Biosystems GroupThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.