PXR-dependent induction of SREBP1a largely determines induction of lipogenic SREBP1 target genes

Primary human hepatocyte cultures of 23 donors were treated with 30 μM rifampicin for 48 h. Linear regression analysis of log2-transformed values of fold induction by rifampicin of the mRNA expression of SREBP1a and the indicated genes in primary human hepatocytes, as shown in Fig. 3a in the article »Pregnane X receptor activation and silencing promote steatosis of human hepatic cells by distinct lipogenic mechanisms« (see Suppl. material).

Details about the proposed molecular mechanisms of PXR-dependent induction of SREBP1a can be found below the Animation Scheme

First mechanism

1. Both LSD1 and PXR interact with SIRT1

A: Mulligan et al.  A SIRT1-LSD1 corepressor complex regulates Notch target gene expression and development. Mol Cell. 2011 Jun 10;42(5):689-99.

B: Biswas et al. Acetylation of pregnane X receptor protein determines selective function independent of ligand activation. Biochem Biophys Res Commun. 2011 Mar 18;406(3):371-6.

C: Buler et al. Energy sensing factors PGC-1α and SIRT1 modulate PXR expression and function. Biochem Pharmacol. 2011 Dec 15;82(12):2008-15.

2. LSD1 increases transcription (maybe through     demethylation of Sp1) and DNA binding affinity of     SREBP1a

A: Abdulla et al. Regulation of lipogenic gene expression by lysine-specific histone demethylase-1 (LSD1). J Biol Chem. 2014 Oct 24;289(43):299937-47.

B: Chuang et al. Hydrogen peroxide induces Sp1 methylation and thereby suppresses cyclin B1 via recruitment of Suv39H1 and HDAC1 in cancer cells. Free Radic Biol Med. 2011 Dec 15;51(12):2309-18.

C: Zhang et al. A simple promoter containing two Sp1 sites controls the expression of sterol-regulatory-element-binding protein 1a (SREBP-1a). Biochem J. 2005 Feb 15;386(Pt 1):161-8.

D: Fernández-Alvarez et al. Reciprocal regulation of the human sterol regulatory element binding protein (SREBP)-1a promoter by Sp1 and EGR-1 transcription factors. FEBS Lett. 2008 Jan 23;582(2):177-84.

3. SREBP1a is acetylated by p300 and deacetylated by     SIRT1

A: Giandomenico et al. Coactivator-dependent acetylation stabilizes members of the SREBP family of transcription factors. Mol Cell Biol. 2003 Apr;23(7):2587-99.

B: Walker et al. Conserved role of SIRT1 orthologs in fasting-dependent inhibition of the lipid/cholesterol regulator SREBP. Genes Dev. 2010 Jul 1;24(13):1403-17.

Second mechanism

1. PXR induces p38 MAPK phosphorylation

Kodama et al. Pregnane X receptor PXR activates the GADD45beta gene, eliciting the p38 MAPK signal and cell migration. J Biol Chem. 2011 Feb 4;286(5):3570-8.

2. Relocation of hnRNP A1 and the activation of SREBP1a     are dependent on the p38 MAPK signal pathway

A: Siculella et al. Lipid accumulation stimulates the cap-independent translation of SREBP-1a mRNA by promoting hnRNP A1 binding to its 5'-UTR in a cellular model of hepatic steatosis. Biochim Biophys Acta. 2016 May;1861(5):471-81.

B: Damiano et al. hnRNP A1 mediates the activation of the IRES-dependent SREBP-1a mRNA translation in response to endoplasmic reticulum stress. Biochem J. 2013 Jan 15;449(2):543-53.

A crucial point to note:

SIRT1 is an important hub interacting with a complex network of proteins. However, there are plausible reasons to assume that the interaction of PXR with SIRT1 not only activates SREBP1a but also influences total body energy metabolism (see animation below) .

Furthermore, I suppose that PXR not only activates SREBP1a but also represses the transcription of SREBP1c via SIRT1 in a LXR-dependent manner (SREBP1a-dependent induction of 1c remains!):

A: Li et al. SIRT1 deacetylates and positively regulates the nuclear receptor LXR. Mol Cell. 2007 Oct 12;28(1):91-106.

B: Bitter et al. Pregnane X receptor activation and silencing promote steatosis of human hepatic cells by distinct lipogenic mechanisms. Arch Toxicol. 2015 Nov;89(11):2089-103. [See figure 3a and supplementary figure S4]

What about LXR

One important question remains:

 Which ligand (or combination of ligands!) could be responsible for an elevated activation of the pregnane X receptor PXR?

Finally, the ligands must have properties to activate PXR chronically and deliver the performance to activate PXR in an additive or Synergistic Way

 And in this context, the following proposed mechanisms (and the combination of both!) seem to be highly relevant:

1. Exogenous PXR ligands

1. Exogenous PXR ligands: Lipophilic chemical toxicants activate PXR. DNL, de novo lipogenesis; LD, lipid droplet; ER, endoplasmic reticulum; CYP, cytochrome P450; EDC, endocrine disrupting chemical. Numbers indicate the proposed sequence of events.

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The central role of storing chemical toxicants in lipid droplets for the physiology and pathophysiology of pregnane X receptor-induced human hepatocellular steatosis

Hypothesis

To explain the apparent contradiction of the parallel induction of detoxification and de novo lipogenesis, in terms of competition for the NADPH pool, it can be postulated that the steatogenic effect of acute PXR activation is a protective physiological mechanism to provide more buffer capacity/safe storage locations for potentially toxic compounds in the induced lipid droplets of hepatocytes and to promote regulated detoxification by the cytochrome P450 enzyme system (Upper figure, left part). This enzyme system is localized in the endoplasmatic reticulum, to which lipid droplets are closely associated or even connected. The hypothesis newly ascribes a physiological function to de novo lipogenesis and lipid droplet formation due to the acute activation of PXR, which may also be the cause of the pathophysiological situation potentially arising in chronic PXR activation. If PXR activation is maintained for long by continuous exposure to e.g. environmental pollutants being PXR ligands, this is suggested to result in lipid droplets steadily growing both in size and numbers, thereby leading to progressive accumulation of lipophilic toxic chemicals in the lipid droplets. A vicious cycle of further PXR activation, increasing de novo lipogenesis/steatosis and accumulation of PXR activating toxic lipophilic chemicals is thus predicted (Upper figure, right part). Finally, this should result in oxidative stress by exhausting NADPH pools, which are essential for e.g. the enzymatic activity of the glutathione reductase, and consequent cellular damage.

The concept is solely based on theoretical considerations and thus contains a certain risk of failure. However, it is completely new and groundbreaking and if true will not only provide an explanation for the starting observation, but will also open new views on detoxification as well as on the etiology of NAFLD/NASH.

2. Endogenous PXR ligands

2. Endogenous PXR ligands: Retinol activates PXR. DNL, de novo lipogenesis; LD, lipid droplet; ER, endoplasmic reticulum; HSC, hepatic stellate cell; GSR, glutathione reductase. Numbers indicate the proposed sequence of events.

One Example for Future Research

Already in 2012, I received the opportunity to discuss about my work and hypotheses in form of presentations:

(May 20 – 22) 1^st IKP Stuttgart Scientific Retreat: The link between intestinal microbiota, hepatic PXR and NAFLD.

(February 29 – March 1) EASL Basic School of Hepatology Course 7: Hepatocyte Damage & Liver Metabolism: The role of PXR in human hepatic steatosis: Establishment of a cellular model.

If you are interested in the full version of the possible future research project presented below, please contact me (non-profit-oriented).

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The pathophysiological role of the PXR-dependent elevation of de novo lipogenesis in non-alcoholic steatohepatitis (NASH)

Abstract

Non-alcoholic steatohepatitis (NASH), the chronic inflammation of the liver with concomitant fatty liver (steatosis), represents, besides alcohol abuse and hepatic viral infections, the leading risk factor for developing liver cancer. Hepatic oxidative stress is regarded as the key component in the pathogenesis of NASH. Furthermore, recent investigations have shown that the pathological increase of newly synthesized fatty acids in liver (hepatic de novo lipogenesis) represents a pathophysiological process, which contributes in a decisive manner to the development of steatosis and the progression of non-alcoholic fatty liver disease (NAFLD). However, the molecular mechanisms in the pathogenesis and progression of NASH are still remain largely unknown. My recently published work, concerning the role of the pregnane X receptor PXR and SREBP1a in human hepatic de novo lipogenesis and NASH, suggests that both PXR and its target gene SREBP1, the master transcriptional regulator of fatty acid synthesis, constitute a key function in the development and progression of NASH. I propose that the cause of the permanent and significantly increased hepatic de novo lipogenesis in NASH patients is based on the combination of PXR-deficiency and a retinol-dependent increased ligand-induced activity of the still existing PXR. In addition to the investigation of the role of PXR in the retinol-mediated interaction of hepatic stellate cells (HSCs) and hepatocytes, the main attention shall lie on the analysis about the pathophysiologically relevant consequence of a PXR-mediated chronically elevated de novo lipogenesis, the origin of oxidative stress. The resulting findings will contribute to a better understanding of the role of PXR and SREBP1a in NASH and may help devising new therapeutic approaches.

Look inside (~45.000 characters and 5 figures)

SREBP1a: A promising new molecular target for the treatment of NAFLD/NASH?