SREBP1a is back at the center of the PXR story.

More than ten years ago, our 2015 Archives of Toxicology study identified SREBP1a induction as a key mechanism linking PXR activation to the SREBP1-dependent lipogenic pathway in human hepatic cells.

A new Archives of Toxicology study now provides independent in vivo support for that axis.

The key signal is clear:

Atorvastatin induced hepatic Srebf1a in Pxr+/+ mice.
In Pxr−/− mice, this response disappeared.

That is the genetic point.

PCN adds the biological weight. As the prototypical mouse PXR agonist, PCN also induced Srebf1a in wild-type liver and activated nuclear SREBP1.

Together, these findings place SREBP1a within the PXR-shaped hepatic transcriptional state space.

This is not just another lipid gene response.

It is independent in vivo support for a mechanism that has been central to srebp1a.com from the beginning: the PXR–SREBP1a axis.

The timing is remarkable.

Recent work has already pushed AKR1B10 beyond marker status and strengthened it as a functional lipogenic stress-state node. Now, SREBP1a also receives independent support as a second molecular anchor of a detoxification–lipogenic hepatic state.

This does not prove Detoxification State Fixation as a whole.

But it changes the weight of interpretation.

SREBP1a is no longer "only" an early human hepatic cell observation.

More than ten years later, it is independently supported in vivo — downstream of PXR.

References

Nabil, H. et al. Atorvastatin regulates hepatic transcriptome PXR dependently but distinct from pregnenolone 16α-carbonitrile and does not induce PXR-mediated liver steatosis. Archives of Toxicology 100, 1443-1463 (2026). https://doi.org/10.1007/s00204-025-04280-0

Bitter, A. et al. Pregnane X receptor activation and silencing promote steatosis of human hepatic cells by distinct lipogenic mechanisms. Archives of Toxicology 89, 2089-2103 (2015). https://doi.org/10.1007/s00204-014-1348-x

AKR1B10 is rising.

CYP2C19 is falling.

That may be more than a biomarker pattern.

It may be a sign that detoxification is changing direction.

The previous note argued that AKR1B10 has moved beyond passive marker status in NAFLD. The next question is where this AKR1B10 signal sits within disease progression. The AKR1B10–CYP2C19 divergence reported by Liu et al. provides a sharper frame: AKR1B10 does not rise in isolation. It rises while a classical CYP drug-metabolizing output falls.

A recent integrative multi-omics study by Liu et al. mapped the progressive disease landscape of metabolic dysfunction-associated steatotic liver disease and identified four recurrent progression-associated candidates: AKR1B10, COL1A2, SPP1 and CYP2C19. Among them, AKR1B10 and CYP2C19 are particularly interesting because both were predominantly localized to hepatocytes and both were functionally interrogated in hepatocyte models.

The direction of change was striking.

AKR1B10 increased during disease progression.

CYP2C19 decreased.

At first glance, this could be read simply as one marker going up and another going down. But that would miss the deeper biological signal. AKR1B10 and CYP2C19 do not represent random hepatocyte proteins. They point into different regions of hepatic detoxification-state biology.

AKR1B10 is an NADPH-dependent aldo-keto reductase involved in carbonyl and aldehyde handling, retinoid metabolism, lipid-associated stress biology and redox adaptation. CYP2C19, by contrast, represents one classical cytochrome P450 drug-metabolizing output. The divergence between AKR1B10 and CYP2C19 therefore does not look like a simple “detoxification up” or “detoxification down” event.

It looks like redistribution.

AKR1B10 is not a passive marker

The importance of AKR1B10 has been increasing steadily.

In fatty liver disease, AKR1B10 has repeatedly appeared as a disease-associated hepatic signal. More recent work has moved it beyond marker status. Yang et al. showed that berberine directly targets AKR1B10 protein and that AKR1B10 perturbation influences lipid and glucose metabolic outputs in experimental NAFLD models. This does not make berberine the central point. The important point is more general: AKR1B10 can be touched, and touching it can alter metabolic disease-relevant outputs.

A recent pathway-level review further strengthens this interpretation. It places AKR1B10 at the interface of carbonyl detoxification, retinoid metabolism, redox control, ACCα-linked lipogenesis, DAG/PKC/ERK signaling, inflammatory signaling and hepatocellular carcinoma biology. In that framework, AKR1B10 is not merely a diagnostic label attached to injured liver tissue. It is a metabolic-redox-detoxification node.

This matters for fatty liver disease progression.

If AKR1B10 rises together with lipogenic, inflammatory and oxidative stress programs, the signal is not just “AKR1B10 is present.” The signal may be that hepatocytes are entering a different mode of stress handling: one in which aldehyde detoxification, retinoid routing, NADPH use, carbonyl stress buffering and lipogenic output become increasingly connected.

AKR1B10 should not be read as intrinsically pathological. In a transient stress context, its carbonyl-detoxifying, retinaldehyde-reducing and redox-buffering functions may be protective. A transient lipogenic output may also be adaptive, for example by expanding lipid-buffering capacity for lipophilic stress mediators and by supporting membrane and lipid requirements during repair and regeneration. The disease-relevant question is whether this AKR1B10-linked stress-handling mode resolves — or whether it becomes persistently co-maintained with lipogenic output.

That is precisely the type of biology that a state-based framework should notice.

The CYP2C19 decrease changes the interpretation

The decrease in CYP2C19 makes the pattern sharper.

If AKR1B10 alone increased, one could describe it as a stress marker, a lipogenic contributor, or a disease-associated enzyme. But when AKR1B10 rises while CYP2C19 falls, the interpretation changes.

This is not simply detoxification becoming stronger.

It is not simply detoxification failing.

It is a directional reorganization of hepatocyte output.

One arm associated with redox-retinoid-carbonyl-lipogenic stress handling rises. One classical CYP drug-metabolizing output falls. In Liu et al., AKR1B10 knockdown and CYP2C19 overexpression both reduced lipid droplet accumulation and intracellular triglyceride levels in FFA-treated hepatocytes, with the combined intervention showing the strongest effect. The same perturbations also reduced oxidative stress and inflammatory readouts.

That functional direction is important.

It suggests that the AKR1B10/CYP2C19 pattern is not only descriptive. It may be linked to the hepatocyte state itself.

Not global CYP loss

There is an important caveat.

The fall of CYP2C19 must not be misunderstood as global cytochrome P450 collapse.

The CYP system is not a single output. Some CYP-linked routes may remain active or even increase under steatohepatitic stress, including lipid-oxidation and omega-oxidation pathways such as those involving CYP2E1 and CYP4A. These routes are often discussed in relation to microsomal oxidative stress, lipid peroxidation and fatty-acid stress metabolism.

Therefore, the key observation is not global CYP loss.

The key observation is selective output redistribution.

CYP2C19 decreases as one classical drug-metabolizing CYP output within the progression landscape. Other CYP-linked stress routes may behave differently. That is not a weakness of the interpretation. It is the point.

Fatty liver disease progression may not switch hepatic detoxification simply on or off. It may redistribute detoxification-state outputs across different enzymatic arms.

PXR-connected, but not simple PXR activation

This pattern is also PXR-relevant.

Pregnane X receptor biology has long been linked to hepatic xenobiotic metabolism, but it is not restricted to a simple drug-detoxification switch. In human hepatic cells, PXR perturbation has been connected to steatotic lipogenic output and to AKR1B10-associated mechanisms. Notably, PXR activation and PXR silencing can both promote steatosis, but through distinct lipogenic routes.

That matters here.

The AKR1B10/CYP2C19 divergence should not be reduced to simple PXR activation or repression. It is more consistent with a PXR-connected imbalance in detoxification-state biology: selected stress-adaptive outputs rise, while selected classical CYP outputs fall.

This is exactly where simple on/off language becomes insufficient.

The pattern asks for state language.

NADPH allocation, not simple energy failure

There is another layer.

AKR1B10 is NADPH-dependent. Lipogenesis is NADPH-demanding. Carbonyl detoxification, antioxidant defense and lipid peroxide handling also draw on redox capacity. The relevant question is therefore not whether the liver can generate NADPH. The relevant question is how NADPH-dependent outputs are chronically allocated across detoxification, lipogenesis, carbonyl handling, retinoid routing and antioxidant defense.

In a transient adaptive state, this allocation may be useful.

In a fixed state, the same allocation may become self-maintaining.

That is where detoxification-state biology begins to overlap with disease persistence.

A DSF-compatible reading

Detoxification State Fixation (DSF) proposes that progressive fatty liver disease may involve fixation of an originally adaptive hepatic detoxification-lipogenic rescue state beyond its useful window. In that view, pathology does not arise because detoxification is simply active. It arises when a protective mode loses reversibility and becomes partly self-maintaining.

The AKR1B10/CYP2C19 pattern fits this logic remarkably well.

AKR1B10 rises as a redox-retinoid-carbonyl-lipogenic stress node.

CYP2C19 falls as a classical CYP drug-metabolizing output.

Other CYP-linked stress routes may remain active or increase.

The result is not uniform detoxification activation.

It is not uniform detoxification failure.

It is output-selective disequilibrium.

That is the important conceptual step.

A hepatocyte under progressive fatty liver disease pressure may not simply detoxify more or less. It may detoxify differently. It may allocate redox capacity, NADPH use, lipid handling and carbonyl stress buffering into a new pattern. If that pattern becomes persistent, it may help stabilize the disease state itself.

Why this matters

The Liu et al. study does not prove DSF.

It does something more specific and, in some ways, more useful: it provides a modern progression-associated pattern that requires better language than “marker up” and “marker down.”

AKR1B10 rising while CYP2C19 falls is a hepatocyte-centered clue. It suggests that fatty liver disease progression may involve directional remodeling of detoxification-related outputs. When combined with evidence that AKR1B10 is functionally linked to lipid metabolism, redox stress, retinoid metabolism and ACCα-associated lipogenic biology, the signal becomes difficult to dismiss as passive.

This is why AKR1B10 matters.

This is why CYP2C19 matters.

And this is why the divergence between them may matter even more than either marker alone.

Detoxification does not disappear.

It changes direction.

Related framework

Detoxification State Fixation (DSF)

Related note

AKR1B10 moves beyond marker status in NAFLD

References

Liu, K. et al. Integrative Multi-Omics Analysis Elucidates the Progressive Disease Landscape and Reveals Dynamic Protein Biomarkers for MASLD Surveillance. The FASEB Journal 40, e71998 (2026). https://doi.org/10.1096/fj.202601011R

Yang, S. et al. Berberine directly targets AKR1B10 protein to modulate lipid and glucose metabolism disorders in NAFLD. Journal of Ethnopharmacology 332, 118354 (2024). https://doi.org/10.1016/j.jep.2024.118354

Wang, C. et al. The pathway network of aldo-keto reductase 1B10: a new perspective on gene-targeted therapy. npj Gut and Liver 3, 20 (2026). https://doi.org/10.1038/s44355-026-00064-0

Bitter, A. et al. Pregnane X receptor activation and silencing promote steatosis of human hepatic cells by distinct lipogenic mechanisms. Archives of Toxicology 89, 2089-2103 (2015). https://doi.org/10.1007/s00204-014-1348-x

Lin, X.-L. et al. Nicotinate-curcumin improves NASH by inhibiting the AKR1B10/ACCα-mediated triglyceride synthesis. Lipids in Health and Disease 23, 201 (2024). https://doi.org/10.1186/s12944-024-02162-5

Ma, J. et al. Aldo-keto reductase family 1 B10 affects fatty acid synthesis by regulating the stability of acetyl-CoA carboxylase-alpha in breast cancer cells. Journal of Biological Chemistry 283, 3418-3423 (2008). https://doi.org/10.1074/jbc.M707650200

AKR1B10 is no longer only a marker.

It can be touched.

A recent study by Yang et al., published in the Journal of Ethnopharmacology, identifies AKR1B10 as a direct berberine-binding target in experimental NAFLD models.

The important point is not berberine as a supplement.

The important point is AKR1B10.

In this study, AKR1B10 is chemically engaged, enzymatically inhibited, genetically perturbed and functionally connected to lipid and glucose metabolic outputs.

That changes the weight of the argument.

From marker to functional node

AKR1B10 has often been read as a disease-associated marker in fatty liver disease, steatohepatitis, fibrosis and liver cancer risk.

That view is becoming too small.

AKR1B10 is a NADPH-dependent aldo-keto reductase. It belongs to carbonyl, aldehyde and retinoid-related stress-handling biology.

In a diseased liver, that matters.

AKR1B10 does not only mark stress.

It may help shape the metabolic state that follows from stress.

That is why recent AKR1B10 work is important. Earlier studies linked AKR1B10 to ACCα stability and the lipogenic machinery that drives de novo lipogenesis and triglyceride synthesis. More recent NASH work placed the AKR1B10/ACCα axis directly within hepatic triglyceride accumulation.

Yang et al. now add another layer.

AKR1B10 appears pharmacologically addressable.

What Yang et al. show

Yang et al. used high-fat diet-fed mice and oleic acid-treated HepG2 cells as experimental NAFLD models.

Berberine improved several metabolic readouts, including hepatic steatosis, triglyceride accumulation, glucose-related parameters and insulin resistance-associated changes.

But the key point is target engagement.

Using a berberine-derived probe and click-chemistry proteomics, the authors identified AKR1B10 among candidate berberine-binding proteins.

They then supported AKR1B10 engagement with additional approaches, including co-localization, molecular docking, SPR, DARTS and CETSA.

Berberine also inhibited AKR1B10 enzymatic activity.

This makes the result stronger than an expression observation.

AKR1B10 is not only increased in disease-associated settings.

It is directly engaged by a small molecule.

The strongest signal is dependence

The most important part of the study is not the target list.

It is the perturbation logic.

In oleic acid-treated HepG2 cells, AKR1B10 knockdown itself increased glucose consumption and reduced triglyceride content.

After AKR1B10 knockdown, berberine produced little or no additional effect on these readouts.

Pharmacological AKR1B10 inhibition supported the same dependency pattern.

AKR1B10 overexpression moved the system the other way: glucose consumption decreased and triglyceride content increased. Berberine counteracted this overexpression phenotype.

The in vivo data point in the same direction. In high-fat diet-fed mice, AAV-mediated AKR1B10 knockdown weakened the additional metabolic effects of berberine.

This is best read as functional dependence.

When AKR1B10 is already reduced or blocked, there is less room for berberine to act.

That is what one expects from a relevant metabolic control point.

Why this matters

A marker can describe disease.

A functional node can help explain how a disease state is maintained.

That is the step AKR1B10 is beginning to take.

Yang et al. do not show that AKR1B10 alone drives NAFLD.

They do not show that berberine treats Detoxification State Fixation.

They do not test the full DSF architecture.

But they do show something important:

AKR1B10 is perturbable.

That matters for a disease-state framework in which stress handling, redox chemistry, lipid remodeling, de novo lipogenesis and lipogenic output may become linked in a self-maintaining hepatic state.

AKR1B10 sits close to that interface.

It connects detoxification-state biology with lipid metabolism.

That is why AKR1B10 should not be read only as a marker of where fatty liver disease has been.

It may be one of the places where the diseased hepatic state can be touched.

A broader pattern is emerging

AKR1B10 may not be standing alone.

A broader progression-level pattern is now emerging.

That pattern deserves its own discussion.

For now, the message is already clear:

AKR1B10 has moved beyond marker status in NAFLD.

Related framework:

Detoxification State Fixation (DSF)

References

Yang, S. et al. Berberine directly targets AKR1B10 protein to modulate lipid and glucose metabolism disorders in NAFLD. Journal of Ethnopharmacology 332, 118354 (2024). https://doi.org/10.1016/j.jep.2024.118354

Lin, X.-L. et al. Nicotinate-curcumin improves NASH by inhibiting the AKR1B10/ACCα-mediated triglyceride synthesis. Lipids in Health and Disease 23, 201 (2024). https://doi.org/10.1186/s12944-024-02162-5

Ma, J. et al. Aldo-keto reductase family 1 B10 affects fatty acid synthesis by regulating the stability of acetyl-CoA carboxylase-alpha in breast cancer cells. Journal of Biological Chemistry 283, 3418-3423 (2008). https://doi.org/10.1074/jbc.M707650200

Bitter, A. et al. Pregnane X receptor activation and silencing promote steatosis of human hepatic cells by distinct lipogenic mechanisms. Archives of Toxicology 89, 2089-2103 (2015). https://doi.org/10.1007/s00204-014-1348-x

Related note:
AKR1B10 as a lipogenic control node in NASH

Mechanistic note

AKR1B10 has often been discussed as a disease-associated marker in steatohepatitis and liver injury. However, an increasingly important question is whether AKR1B10 is only a marker of disease state, or whether it can actively contribute to the persistence of a lipogenic hepatic state.

A 2024 study by Lin et al. adds a particularly relevant functional layer to this question. The authors investigated nicotinate-curcumin in experimental NASH models and focused on the AKR1B10/ACCα pathway. In a high-fat/high-fructose rat model and in Ox-LDL/high-glucose-stressed HepG2 cells, they report increased AKR1B10 and ACCα together with changes in Malonyl-CoA, free fatty acids and triglycerides. Treatment with nicotinate-curcumin reduced AKR1B10/ACCα signaling and was associated with lower Malonyl-CoA, FFA and TG levels.

This supports the view that AKR1B10 is more than a passive NASH-associated readout. ACCα catalyzes the formation of Malonyl-CoA, a central building block for fatty acid synthesis. If AKR1B10 helps to maintain ACCα-dependent lipid synthesis, it may participate in stabilizing the metabolic output of the diseased hepatocyte.

This connects back to my earlier PXR/steatosis work in human hepatic cells. In 2015, we reported that ligand-dependent PXR activation and reduced PXR signaling can both promote steatosis, but through distinct mechanisms. PXR activation induced SREBP1a and lipogenic SREBP1 target genes, whereas PXR knockdown increased AKR1B10 and an ACC-dependent branch of de novo lipogenesis. In histologically classified human NASH liver samples, PXR protein was reduced, while AKR1B10, SREBP1a and lipogenic target genes were increased.

The work by Lin et al. therefore does not stand isolated. It strengthens a mechanistic line in which AKR1B10 links stress-associated hepatic remodeling to ACCα, Malonyl-CoA, FFA and triglyceride synthesis.

This is also relevant for the Detoxification State Fixation (DSF) framework. In the DSF model, AKR1B10 is proposed as one of the molecular anchors that may help stabilize an originally adaptive detoxification-lipogenic hepatic state beyond its normal resolution window. The important point is not that AKR1B10 alone “causes” NASH. Rather, AKR1B10 may help maintain one part of a self-reinforcing state: persistent lipogenic output under disease-shaped stress conditions.

From this perspective, AKR1B10 becomes more than a biomarker. It becomes a candidate state-stabilizing node.

This does not prove DSF as a whole. But it provides independent functional support for one of its proposed molecular anchors: AKR1B10 as a lipogenic control node connecting detoxification-associated stress biology with persistent hepatic lipid synthesis.

Related framework:
Detoxification State Fixation (DSF)
https://srebp1a.com/state-fixation/

References

Lin, X.-L. et al. Nicotinate-curcumin improves NASH by inhibiting the AKR1B10/ACCα-mediated triglyceride synthesis. Lipids in Health and Disease 23, 201 (2024). https://doi.org/10.1186/s12944-024-02162-5

Bitter, A. et al. Pregnane X receptor activation and silencing promote steatosis of human hepatic cells by distinct lipogenic mechanisms. Archives of Toxicology 89, 2089-2103 (2015). https://doi.org/10.1007/s00204-014-1348-x

In biomedical research, accurate nomenclature is not a cosmetic detail. It determines whether relevant biology remains visible to researchers, databases, search engines, and automated text-mining systems.

The history of human SREBP1a illustrates this point. SREBP1a and SREBP1c are not interchangeable names. They are distinct SREBF1-derived isoforms with different biological and experimental implications. Nevertheless, for many years, human database entries made SREBP1c highly visible while SREBP1a was much harder to find.

The historical oversight

Until early 2016, the official alias structure for the human SREBF1 gene did not properly reflect SREBP1a. Historical database entries listed SREBP1, bHLHd1, and SREBP-1c, but SREBP1a was missing from the visible alias field.

Figure 1 | Historical NCBI Gene entry for human SREBF1, archived in July 2015, showing SREBP1, bHLHd1, and SREBP-1c as aliases, while SREBP1a was not listed.

This was more than a naming inconvenience. If a human protein isoform is missing from alias structures, database searches, literature searches, and automated semantic tools may fail to connect that protein to its gene.

The 2016 correction

In February 2016, I contacted NCBI and HGNC regarding the absence of SREBP1a from the human SREBF1 alias listings. Shortly thereafter, SREBP1a was added, restoring a direct database link between the human SREBP1a protein and the SREBF1 gene.

Figure 2 | NCBI RefSeq curator confirmation from February 1, 2016 documenting that SREBP1a was added as an additional alias to the human SREBF1 gene record (GeneID: 6720).

 Figure 3 | HGNC curator confirmation from February 3, 2016 documenting that SREBP1a was added as an additional alias to the human SREBF1 gene record.

This correction mattered because SREBP1a is not a synonym for SREBP1c. Both arise from SREBF1, but they represent distinct isoform biology. Making SREBP1a visible in official alias systems improved the connection between human experimental protein research and genomic database structure.

Current status

In recent years, database displays have changed. Some resources no longer show all transcript- or isoform-specific names in the most prominent summary fields. This can make rapid searches less straightforward.

HGNC remains especially important because it provides the official curated framework for human gene nomenclature. For SREBF1, this means that SREBP1a remains connected to the human SREBF1 gene in an official nomenclature context.

Figure 4 | HGNC symbol report for human SREBF1, accessed in June 2026, showing SREBP1a as a listed alias symbol and thereby confirming continued visibility of the isoform in an official gene nomenclature resource.

Conclusion

The 2016 correction helped preserve an important semantic link: human SREBP1a belongs visibly to the SREBF1 gene context.

That may sound like a small database detail. It is not. When nomenclature is incomplete, biology becomes harder to find. When nomenclature is corrected, human protein isoform research becomes more searchable, more transparent, and easier to integrate across literature, databases, and automated discovery tools.

For human SREBP1a research, this link matters.

Related: Detoxification State Fixation (DSF)

Please use the Internet Archive...

Jung et al. Role of the AMPK/SREBP-1 pathway in the development of orotic acid-induced fatty liver. J Lipid Res. 2011 Sep;52(9):1617-25.

 »Whereas decreased phosphorylation of AMPK and modulation of a series of downstream events leading to fatty acid synthesis and lipogenesis were observed in rat hepatocytes and human hepatoma cell lines, mouse hepatocytes were resistant to OA with regard to the AMPK/SREBP-1-dependent lipogenic pathway. Similar results were observed in animal studies using SD rats and C57BL/6 mice. These two species showed completely different responses to OA in terms of AMPK/SREBP-1 signaling and development of steatosis, indicating that not only pharmacokinetic but also pharmacodynamic factors participate in determining interspecies differences.«

Ohno et al. Transgenic Mice Overexpressing SREBP-1a in Male ob/ob Mice Exhibit Lipodystrophy and Exacerbate Insulin Resistance. Endocrinology. 2018 Jun 1;159(6):2308-2323.

What is going on?

Q?

Future proves Past

Be strong (anons)

HMGCR-dependent SREBP1a maturation (simplified scheme). 1a, mature SREBP1a protein; ER, endoplasmic reticulum. (see section »PXR versus LXR«)

The dogma of the SREBP1 protein:

»Sterols inhibit the cleavage of the precursor, and the mature nuclear form is rapidly catabolized, thereby reducing transcription.« (see ref 1, ref 2, ref 3)

However, with all due respect, I would like to mention that the results of dozens of scientific articles contradict and/or do not support this dogma. The literature analysis on srebp1a.com and the results of the following mentioned article reveal it.

Hwang et al. Contribution of Accelerated Degradation to Feedback Regulation of 3-Hydroxy-3-methylglutaryl Coenzyme A Reductase and Cholesterol Metabolism in the Liver. J Biol Chem. 2016 Jun 24;291(26):13479-94.

Before the literature analysis on srebp1a.com and the analysis of the article mentioned above, please consider the following two points:

1. HMGCR is the rate-limiting enzyme of sterol biosynthesis.

2. HMGCR protein/activity induces the maturation of SREBP1.

Alarming predictions:

Estes et al. Modeling the epidemic of nonalcoholic fatty liver disease demonstrates an exponential increase in burden of disease. Hepatology. 2017 Aug 12. doi: 10.1002/hep.29466. [Epub ahead of print]

Abstract

One further newly published peer-reviewed article:

Tajima-Shirasaki et al. Eicosapentaenoic Acid Downregulates Expression
of the Selenoprotein P Gene by Inhibiting SREBP-1c Independently of the AMPK
Pathway in H4IIEC3 Hepatocytes. J Biol Chem. 2017 May 2. pii: jbc.M116.747006.

Introduction:

According to the purpose of the study, it was the aim to find out if hepatic SREBP1c induces the transcripition of SELENOP eicosapentaenoic acid (EPA)-dependently or rather if EPA, a polyunsaturated fatty acid (PUFA), inhibits the activity of SREBP1c.

Evaluation:

Without going into great detail, it is useful to consider the following three issues:

1. For knock-down experiments the authors used siRNA which simultaneously targets both, the SREBP1c and SREBP1a transcript (see here). What about SREBP1a?
2. In overexpression experiments the authors only transfected the precursor 1a isoform of SREBP1 but transfected separately both, the precursor AND mature isoform SREBP1c. (Only the mature SREBP1 protein is transcriptionally active). However, they have concluded: »The mature SREBP-1c overexpression especially inhibited the suppressive effect of EPA on SELENOP promoter activity (Fig. 3C); however, SREBP-1a overexpression had no effect (data not shown). These results suggest an association of SREBP-1c activity with the EPA-induced suppression of SELENOP expression.« What about the mature SREBP1a?
3. In regard to the ChIP assay they have used, the authors assume: »...treatment with EPA was found to decrease the binding of SREBP-1c to Selenop promoter (Fig.5B), indicating that EPA decreases SELENOP promoter activity and gene expression via SREBP-1c inactivation...« It is important to know that SREBPs function as dimers in which the individual molecules can form homo- or heteromers (see here). What about the mature SREBP1a?

While Cho et al. have written a modern article in regard to hepatic SREBP1a, it may well be that Rong et al. (“Horton Lab”) have done everything to maintain the conventional view about SREBP1c and SREBP1a in hepatic lipogenesis.

*** First article: Cho et al. ***

ENOblock, a unique small molecule inhibitor of the non-glycolytic functions of enolase, alleviates the symptoms of type 2 diabetes. Sci Rep. 2017 Mar 8;7:44186.

Statements:

»…the 12 mg/kg ENOblock treatment decreased the serum level of free fatty acid compared to untreated db/db mice (Fig. 2G). This could be due to the observed inhibition of Srebp-1a and -1c, which are key regulators of lipid homeostasis.«

»ENOblock treatment normalized Srebp-1a expression to the level observed in normal B6 mice and also reduced the expression of Srebp-1c (Fig. 3R,S).«

*** Second article: Rong et al. ***

Expression of SREBP-1c Requires SREBP-2-mediated Generation of a Sterol Ligand for LXR in Livers of Mice. Elife. 2017 Feb 28;6. pii: e25015.

The following statements must be considered very critically:

• Statement 1: »Expression of SREBP-1c Requires SREBP-2-mediated Generation of a Sterol Ligand for LXR in Livers of Mice.« (The title of this article)

• Statement 2: »The only molecular signature we found that differed between hepatocyte-Srebf-2^-/- and hepatocyte-Scap^-/- livers was the retained expression of SREBP-1a and ACC2 in the livers of hepatocyte-Srebf-2^-/- mice. These studies confirm that SREBP-1a has only a minor role in regulating basal and stimulated cholesterol and fatty acid synthesis in the liver.«

Evaluation of statement 1:

In my opinion, the title is not supported by the results – Au contraire:

1. They have examined liver tissue extracts for a possible endogenous sterol LXR ligand whose concentration is greatly reduced in hepatocyte-Srebf-2^-/- livers. However, they have not found any. In contrast, the liver tissue concentrations of the potent endogenous sterol LXR ligands desmosterol (Ref. 1) and 24,25-epoxycholesterol (Ref. 2; and references therein) were elevated 3-fold and 7-fold, respectively, in hepatocyte-Srebf-2^-/- livers of 12-13 week-old mice (see here).
2. The studies with T0901317 and 0,2% Cholesterol are far from being sufficient in order to support the conclusion/title of this article. (I will get back to this topic in more detail soon; see section »PXR versus LXR«)

Evaluation of statement 2:

1. As they have already written in the Results section: »…ACC2 expression, which is primarily regulated by SREBP-1a, was only slightly (-30%) lower (in hepatocyte-Srebf-2^-/- livers).« Furthermore, in SREBP1a-deficient mice and SREBP1a-deficient human hepatocytes, ACC2 mRNA expression is only 40-50% lower (Ref. 3, Ref. 4).
2. ACC2 was only 40% lower in S1P-inhibited human hepatocytes (Ref. 4). (Inhibition of S1P blocks the proteolytic activation of SREBP proteins)
3. ACC2 was not measured in livers of mice that lack Scap in hepatocytes (Ref. 5, Ref. 6 and this article).
4. Beside the lower mRNA expression of SREBP1a in hepatocyte-Srebf-2^-/- livers (-20%; see here), they do not mention the 40% and 70% reduced hepatic SREBP1a mRNA level in hypomorphic SREBP-2 and Srebf-2^-/- mice which was observed by Vergnes et al. (Ref. 7). Furthermore, the reduced mRNA expression of SREBP1a in livers of hepatocyte-Srebf-2^-/- mice gives no indication of whether there is an even more pronounced reduction in the amount of the mature SREBP1a protein. (Also see section »SREBP1a«)

Mary K. Bennett, Timothy F. Osborne and co-authors nailed it:

»Although the existence of gene families and overlapping mRNAs in higher eukaryotes has been known since the 1970s, the extensive clusters of highly related genes and the almost ubiquitous nature of alternative mRNA processing was not fully realized until the sequence of the human genome was reported 7 years ago. The sequencing of genomes from several other species has confirmed this as a basic feature of all complex eukaryotic organisms. Thus, a major goal now is to define precisely the unique and common roles for the different proteins produced from overlapping transcripts and for closely related proteins in the same family. This is complicated when the proteins are co-expressed in the same cells and function as dimers/multimers in which the individual molecules can form homo- or heteromers as in the case of the mammalian SREBPs.« [Bennett et al. Selective binding of sterol regulatory element-binding protein isoforms and co-regulatory proteins to promoters for lipid metabolic genes in liver. J Biol Chem. 2008 Jun 6;283(23):15628-37.]

2 decades ago, Nobel Laureates Goldstein & Brown discovered SREBPs (SREBP1a, SREBP1c and SREBP2) as key transcriptional regulators of lipid metabolism. However, until January 2016, one year ago, the only "synonyms" for the SREBF1 gene were SREBP1, bHLHd1 and SREBP-1c (provided by NCBI and HGNC). SREBP1a could not be found! This “mistake” was corrected in February 2016, after I have mentioned it. See here: NCBI and HGNC.

Not only SREBP1c, also SREBP1a is a transcript of SREBF1 and SREBP-1c is NOT a synonym for SREBP1a.