Reply
Authors: Zoe Hall, Fiona Oakley, Michele Vacca, Julian L. Griffin
Journal: Hepatology (2022)

We thank Fuchen Liu and colleagues for their interest in our work and their comments. We would like to take this opportunity to clarify that the aim of our study was not to describe the dynamics of liver regeneration (LR) per se but to more broadly describe metabolic rewiring in proliferating hepatocytes. We clearly showed that independently from the injury/model/time point/species studied, in multiple models of synchronous/asynchronous regeneration and HCC, proliferating hepatocytes rewire metabolism similarly leading to the formation of monounsaturated fatty acid phosphatidylcholine (MUFA‐PC). However, we do not disagree that different metabolic needs will prompt the activation/inhibition of specific metabolic programs at different stages of cell cycle and/or LR processes. As Michalopoulos and Bhushan highlighted in their review, LR is a highly dynamic process, which can be variable depending on species and experimental settings (e.g., feeding, circadian rhythm, species, genetic background).[1] The method used to assess proliferation will further contribute to apparent variability in the assessment of peak proliferation because bromodeoxyuridine, iododeoxyuridine, proliferating cell nuclear antigen (PCNA), and KI67 have slightly different dynamics. We therefore disagree that the dynamics of different experiments can be meticulously compared. In Supporting Figure S1A of our paper,[2] we provided the dynamics of our experimental setup, showing that PCNA expression peaked at Day 3 after partial hepatectomy (PH). We considered nuclei size/roundness to ensure that we were reasonably focusing on hepatocyte proliferation, not other cell types. The fact that cell proliferation is robustly activated at Day 3 is also confirmed in our RNA‐sequencing data (Supporting File 1). Nevertheless, we agree with Liu and colleagues that multiple cell types dynamically contribute to LR: We are now addressing this interesting question as part of ongoing studies using spatial lipidomics and imaging mass cytometry. Finally, we checked our main findings at Day 2 (48 h) post‐PH. In agreement with the dynamics of PCNA expression in our experiment, we confirmed that MUFA‐PC tended to increase at 48 h without reaching statistical significance. We therefore believe that our data are sound and that the study design was appropriate; the apparent discrepancies that the authors pointed out (transient regeneration–associated steatosis, β‐oxidation) are most likely due to slightly slower dynamics in our experiments compared to those the authors mentioned (for the reasons explained above). Our study was a first attempt to integrate multiple "omics" layers to study the rewiring of metabolism in hepatocyte proliferation and HCC. The commonality in metabolic rewiring in the context of biologically diverse liver injuries suggests that MUFA‐PCs are associated with hepatocyte proliferation, but our data do not allow conclusions to be drawn on LR dynamics. Although exploring LR dynamics was out of scope, we agree with Liu and colleagues that the integrated methodology we used can be exploited to other fields of hepatology (including LR and other liver cancers) for future work. CONFLICT OF INTEREST Dr. Oakley is employed by and owns stock in FibroFind. She received grants from GlaxoSmithKline.