TP53INP2 at the crossroad of transcription, autophagy and liver metabolism

Abstract

[eng] Metabolic-associated steatotic liver disease (MASLD) is the leading global liver chronic disease and it is estimated to affect 30% of the world population. MASLD is a progressive multi-stage disease characterized by an abnormal accumulation of fat in liver, also known as steatosis. Major risk factors for developing MASLD include obesity and metabolic syndrome. Patients with MASLD are at risk of developing co-morbidities such as cardiovascular disease, diabetes and chronic kidney disease. Unfortunately, there is no approved pharmacological treatment for MASLD. PPARα is a nuclear receptor that is activated upon fasting to coordinate the hepatic response against this cue. It promotes the expression of genes that are essential for fatty acid oxidation (FAO) and ketone body (KB) synthesis. It emerged as a promising therapeutic target for MASLD due to its strong protective role against the development of MASLD in mouse models. In addition, treatment of MASLD mouse models with PPARα agonists improves steatosis. In this thesis, we have identified TP53INP2 as a novel regulator of PPARα. TP53INP2 is a multifaceted protein that regulates many aspects of metabolic homeostasis through its action in different tissues. It also modulates many cellular processes, including autophagy and apoptosis. Essentially, liver-specific TP53INP2 ablation impaired the activation of PPARα upon fasting, which caused a blunted upregulation of FAO and KB synthesis genes. This resulted in increased hepatic fat accumulation. These effects were more pronounced in aged mice. By using PPARα agonists, we uncovered that TP53INP2 is required to fully activate PPARα. Interestingly, TP53INP2 ablation also impaired the transcriptional activation of autophagy during fasting. This may be a direct effect of impaired PPARα activation, as PPARα was shown to be a master regulator of hepatic autophagy in response to fasting. Autophagy has also been established to be a protective against MASLD. We used a methionine- and choline-deficient (MCD) diet to promote MASLD in control and TP53INP2-deficient mice and treated them with the PPARα agonist GW7647 to assess the potential role of TP53INP2 in the development and treatment of MASLD. Interestingly, female TP53INP2-deficient mice subjected to a one-week regime of MCD diet showed impaired activation of PPARα by the agonist GW7647. In addition, male TP53INP2-deficient mice fed with MCD diet during 3 weeks and treated chronically with GW7647 during the diet showed lower response to GW7647. To elucidate how TP53INP2 regulates PPARα activation we used the BioID technology to map the binding partners of TP53INP2. Most of the identified interactors are involved in the regulation of chromatin dynamics and epigenetics, with coregulator activity and nuclear receptor binding functions. In keeping with this, we have also shown that TP53INP2 is a chromatin-binding protein that interacts with RING1A, a ubiquitin ligase (E3) that acts as the catalytic component of the Polycomb repressive complex 1 (PRC1). The PRC1 is an epigenetic modulator that ubiquitinates histone H2A to suppress gene expression. In addition, TP53INP2 interacts with UBR5, another E3 that is involved in the turnover of nuclear

receptors. These results indicate that TP53INP2 may regulate PPARα-dependent gene expression through its interaction with players of the epigenetic machinery. ChIP-Seq experiments will be key to confirm this hypothesis. In conclusion, TP53INP2 is required for the full activation of PPARα in response to fasting and to agonists. When TP53INP2 is ablated in the liver, upregulation of FAO, KB synthesis and autophagy genes in response to fasting is blunted. This renders TP53INP2- deficient hepatocytes incapable of ridding of the fat overload coming from adipose tissue lipolysis, leading to increased hepatic fat accumulation.