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Molecular attribution and the hepatic SREBP1 isoform hierarchy: The sequence defines the experiment

Andreas Bitter
Independent Researcher

Preprint · Version 1.0 · 5 November 2026

Abstract

SREBP1c occupies a central position in prevailing models of hepatic lipogenesis, whereas SREBP1a is commonly treated as the quantitatively less important hepatic isoform. That hierarchy has strong biological foundations, yet several influential observations change meaning when the underlying molecular identity is resolved. A prominent antisense intervention reported as selective SREBP1c inhibition targeted a sequence within the Srebf1a-specific 5′ architecture. Independent human studies used an assay under an SREBP1c designation that instead interrogates SREBP1a-class transcript architecture. Previous human liver and primary hepatocyte data show that SREBP1a is quantitatively substantial, is detectable as mature protein and contributes to endogenous lipogenic regulation. Genuine SREBP1c-specific evidence remains intact under the same sequence standard. Orthogonal evidence comes from a transactivation-interface inhibitor that preferentially affects SREBP1a transactivation domain activity. Finally, acute pan-SREBP1 silencing can suppress directly measured hepatic de novo lipogenesis by approximately 70%, yet that flux has not been quantitatively partitioned between SREBP1a and SREBP1c. Together, these observations suggest that the hepatic SREBP1 hierarchy may reflect both biology and the way that biology was experimentally attributed.

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DOI 10.5281/zenodo.23894065

Citation
Bitter, A. (2026). Molecular attribution and the hepatic SREBP1 isoform hierarchy: The sequence defines the experiment (Version 1.0). Zenodo. https://doi.org/10.5281/zenodo.23894065

CC BY 4.0

Molecular identity can diverge from the label

SREBP1c became central to hepatic lipogenesis for compelling biological reasons. It is strongly regulated by nutritional state, insulin and liver X receptor (LXR) signaling, is abundant in fed rodent liver, and selective first-exon genetics demonstrate important physiological functions [1–7]. The emphasis on SREBP1c is therefore biologically grounded.

But abundance, annotation and causal contribution are not interchangeable. SREBP1a and SREBP1c arise from alternative first exons of the same gene and share most downstream sequence [2]. They are also not functionally equivalent: SREBP1a has a substantially stronger transactivation domain (TAD), differs in promoter behavior and produces greater transcriptional output at comparable nuclear abundance [3,8–10]. A smaller SREBP1a pool therefore need not make a proportionally smaller functional contribution.

Pnpla3 provides a concrete hepatic example. In transgenic mouse livers expressing similar levels of active SREBPs, Pnpla3 mRNA was reported to rise >100-fold in the SREBP1a line, versus 12-fold in the SREBP1c line and 4-fold in the SREBP2 line [11]. Independent promoter, EMSA and ChIP experiments using expressed SREBP1a demonstrated SREBP1a binding to the Pnpla3 promoter and SRE-dependent transcriptional activation [12]. Together, these findings illustrate why relative hepatic isoform abundance cannot be assumed to predict transcriptional output [3,11,12].

Sequence assignment. Reported oligonucleotide and assay target regions were compared with the canonical alternative-first-exon architecture of SREBP1. For mouse, Srebf1a NM_011480.4 and Srebf1c NM_001358314.1 were used as reference classes; targets present identically in both were classified as shared/pan-SREBP1, whereas class-specific assignments required discrimination within the alternative 5′ architecture. Corresponding human assignments followed the same first-exon logic. These classifications define intended molecular identity; they do not establish transcriptome-wide absence of off-target binding or equal in vivo knockdown of every compatible transcript.

The clearest targeting example is the 2011 study by Frederico and colleagues, which reported reversal of diet-induced fatty liver after short-term antisense inhibition of ‘SREBP-1c’ [13]. The final published article gives the phosphorothioate sense sequence 5′-CGGTGAGGCGGCTCTGGAAC-3′ and cites NM_011480 as the Srebf1 sequence used for oligonucleotide design. Exact comparison with the alternative-first-exon references places this sequence in the unique Srebf1a 5′ region and outside the Srebf1c-specific start architecture.

After 14 days of treatment in mice with established diet-induced obesity, total liver fat fell by approximately 70% and plasma triglycerides by approximately 61% relative to sense-treated obese controls [13]. In established diet-induced fatty liver, this is the earliest published in vivo therapeutic SREBP1 antisense experiment identified in the literature; its published target sequence defines an Srebf1a-class intervention, not an Srebf1c-selective one. The phenotype remains. The isoform attribution does not. A sequence-defined Srebf1a-class intervention cannot serve as clean evidence for selective Srebf1c dependence merely because the historical label says SREBP1c [13,14].

The problem is not confined to one historical experiment. In a 2023 study of established metabolic dysfunction-associated steatohepatitis (MASH), an siRNA described as targeting SREBP1c used the sequence 5′-UACAUCUUUAAAGCAGCGGGT-3′; its target is present identically in the shared coding region of Srebf1a and Srebf1c [15]. The transcript architecture also explains how such drift can arise: isoform-specific target space is concentrated in the alternative first-exon regions, whereas most of Srebf1 is shared [2,16]. A potent common-region reagent can therefore be selected without ever isolating an isoform; if hepatic SREBP1 is conceptually equated with SREBP1c, a pan-SREBP1 reagent can then inherit an SREBP1c label.

Attribution drift also occurs at the measurement step. The commercially available human assay Hs00231674_m1 has been used in the literature as an SREBP1c readout, although its probe targets the SREBP1a-specific exon 1a/exon 2 junction. Its isoform selectivity was experimentally validated with more than 3 × 10⁴-fold discrimination between matching and non-matching isoform cDNA [17]. Lima-Cabello et al. used the assay in 109 human liver samples as an SREBP1c readout [18]. In human hepatic cells, the same assay was used as an SREBP1c readout by Duniec-Dmuchowski et al. [19], whereas Ochiai et al. explicitly described it as recognizing both SREBP1a and SREBP1c [20]. The RNA signal remains; the isoform attribution changes. The correction is deliberately narrow: unrelated protein measurements and the surrounding experiments are not automatically reclassified.

Molecular attribution can therefore fail at both ends of experimental inference: in what is perturbed and in what is measured. This is not a nomenclature problem. It is a causal attribution problem. It creates a route by which SREBP1a can become less visible in the literature without becoming less important in the biology.

The isoform obscured by attribution is biologically substantial

An attribution problem would matter little if hepatic SREBP1a were merely a trace transcript. Human data argue otherwise. Across 65 human liver samples, the median SREBP1a:SREBP1c ratio was approximately 0.35; across primary human hepatocytes from 22 donors, it was approximately 0.59 [17]. At the median, these values correspond to roughly one quarter of the measured canonical SREBP1 pool in intact liver and more than one third in primary hepatocytes.

Mature SREBP1a and SREBP1c were physically resolved in primary human hepatocytes [17]. Functionally, an siRNA directed against the unique SREBP1a first exon reduced SREBP1a by approximately 80% in hepatocytes from three donors, accompanied by roughly 20–40% reductions in ACSS2, ACACA, ACACB, FASN and SCD [17]. SREBP1c fell secondarily by about 30%, so the downstream changes cannot be assigned arithmetically and exclusively to SREBP1a. The experiment nevertheless establishes that perturbing the SREBP1a arm alters the endogenous human lipogenic program.

In the studied human non-alcoholic steatohepatitis (NASH) cohort, median SREBP1a expression increased across histological disease stages alongside a coordinated lipogenic program extending from ACLY and ACACA through FASN to SCD and ELOVL6; SREBP1c increased as well [17,21]. This is not an isoform switch. It shows that both arms of SREBP1 biology can be engaged in human disease.

Because SREBP1a and SREBP1c are not equivalent transcriptional units [3,8,9], relative RNA abundance is an incomplete proxy for relative functional weight. A smaller pool is not necessarily a smaller driver.

A symmetric reassessment preserves genuine SREBP1c biology

A credible molecular reassessment must be symmetric. It should not move evidence from SREBP1c to SREBP1a by default; it should restore each experiment to the molecular resolution its reagent actually permits.

Selective deletion of the SREBP1c-specific first exon is genuine isoform-specific evidence and clearly alters hepatic responses to fasting/refeeding and LXR activation [4]. Sequence-defined SREBP1c-directed antisense targeting is also technically possible: in an in vitro screen, ISIS 219634 maps to the unique Srebf1c 5′ start region [16]. When the sequence isolates SREBP1c, the evidence remains SREBP1c. When the target is shared, the evidence is pan-SREBP1. When the target lies in the unique Srebf1a 5′ region, it is Srebf1a-class. That symmetry is the methodological core of the reassessment.

Causal timescale and independent perturbation change the functional question

Chronic genetics and acute perturbation answer related but non-identical causal questions. Selective SREBP1c deletion retains SREBP1a and is accompanied by increased SREBP2 [4], illustrating that long-term loss can be followed by regulatory remodeling. Such adaptation is biologically real, but it changes the causal question: chronic deletion tests what state can eventually exist without a regulator, whereas acute perturbation tests what an established adult state acutely depends on [22].

Mechanistically independent perturbation provides an orthogonal line of evidence. BF175 directly disrupted MED15-KIX/SREBP1a-TAD binding in biochemical pull-down experiments and inhibited SREBP1a-TAD activity more strongly than SREBP1c-TAD or SREBP2-TAD activity [23]. BF175 is not a selective chemical SREBP1a knockout. Its value is orthogonal: a compound shown directly to disrupt MED15-KIX/SREBP1a-TAD binding also suppressed SREBP-dependent lipogenic output in hepatocyte systems and diet-induced obese mice [23].

The unallocated function is large enough to matter

The decisive question is quantitative: how much acute hepatic SREBP1 function belongs to each isoform? Ruiz and colleagues provide an unusually useful scale. Using adult hepatic pan-SREBP1 silencing and direct ²H₂O tracing, they measured an approximately 70% reduction in de novo hepatic palmitate synthesis [24]. Because the underlying shRNA targets a sequence shared by Srebf1a and Srebf1c, the experiment cannot allocate that lost flux between the two isoforms.

That limitation is precisely why the result matters here. The unresolved allocation is not semantic; a major fraction of hepatic lipogenic flux is at stake.

The decisive experiment is therefore straightforward: matched acute adult interventions against the unique SREBP1a first exon, the unique SREBP1c first exon and shared/pan-SREBP1 sequence; verification of both isoforms after every intervention, including mature nuclear proteins; and direct stable isotope measurement of hepatic de novo lipogenesis (DNL) before chronic adaptation dominates the phenotype.

SREBP1c may account for most acute hepatic DNL. SREBP1a may instead be functionally predominant across conditions, despite its lower abundance. The relative contributions may also vary with nutritional state and disease. Any of these patterns would be informative. What should no longer be assumed is that this quantitative comparison has already been made.

Conclusion

The conventional hepatic SREBP1 hierarchy rests on genuine biology. The problem is not that SREBP1c was recognized as important. It is that molecular labels have sometimes outrun molecular specificity, while relative abundance has too easily stood in for functional weight.

Across distinct inferential layers, the same pattern emerges: target identity can diverge from label, measurement identity can drift, abundance can understate functional weight, and chronic genetics can answer a different causal question from acute dependence. Once these layers are separated, SREBP1a repeatedly re-emerges as a substantial component of hepatic SREBP1 biology.

SREBP1c is important. SREBP1a has been experimentally underestimated. Those statements are compatible.

The hepatic SREBP1 hierarchy may therefore reflect both biology and the way that biology was experimentally attributed.

The remaining question is quantitative: how much acute hepatic SREBP1 function belongs to each isoform?

For RNA-targeting experiments, the sequence defines the experiment. The label does not.

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