PUFAs regulate SREBP1c through phosphorylation of Insig2

The synthesis of fatty acids in mammalian liver is controlled in large measure by the transcription factor sterol regulatory element-binding protein 1c (SREBP1c). During activation, membrane-bound SREBP1c is transported from endoplasmicreticulum (ER) to Golgi and processed proteolytically to release the mature transcription factor. Mature SREBP1c moves to the nucleus where it binds the promoters of a battery of genes needed for lipid synthesis (1). Prior studies have shown that Polyunsaturatedfatty acids (PUFAs) inhibit the SERBP1c pathway at both the mRNA (messenger RNA) and protein levels to regulate lipogenesis (2). Transcriptional control of SREBP1c gene expression is known to involve several factors, including LXRs (liver X receptors), C/EBPs (CAAT-enhancer binding proteins), and BHLHE40 (3). The mechanism underlying the ability of PUFAs to affect SREBP1c proteolytic processing has been less clear. Tian et al. provide important insight by demonstrating that PUFAs suppress lipogenesis through cAMP (cyclic AMP)/PKA (protein kinase A)-dependent phosphorylation of the ER protein Insig2 (4).

Tian et al. used a human fibroblast model to study PUFA effects on the SREBP1c pathway. They first demonstrated that PUFAs decreased SREBP1c cleavage in a dose- and time-dependent manner, with only minor reductions in mRNA levels. Prior work has shown that PUFA treatment induces cAMP levels and activate PKA in myocytes and 3T3-L1 cells (5, 6). Tian et al. found similar results in human fibroblasts. They further showed that activation of PKA with a cell-permeable cAMP analog inhibited SREBP1c cleavage. Accordingly, treatment with a PKA inhibitor blocked suppression of SREBP1c cleavage by PUFA.

To gain further insight into the mechanism underlying PUFA effects, Tian et al. examined components of the ER-resident SREBP1c cleavage machinery. Insig1/2 and SCAP are critical for SREBP processing (7). SCAP escorts SREBP1c from the ER to the Golgi, while Insigs inhibit this process by retaining the SCAP–SREBP1c complex in the ER. The authors used mutant SV-589 cells lacking SCAP, Insig-1, or Insig-2 and found that Insig2 was essential for PUFA-mediated inhibition of SREBP1c cleavage.

The findings thus far led Tian et al. to hypothesize that PUFAs activate PKA to phosphorylate Insig2, but not Insig1. By searching for potential PKA phosphorylation sites, the authors identified a serine (S106) present in Insig2 but not Insig1. They then developed a specific antibody to detect phosphorylated Insig2 and used it to show that S106 was phosphorylated in response to PUFA or PKA agonist treatment. Furthermore, they found that mutation of S106 diminished the inhibitory effect of PUFAs SREBP1c cleavage, downstream gene expression, and fatty acid synthesis. The authors also showed that PUFA or PKA agonist treatment inhibited SREBP1c cleavage in freshly isolated hepatocytes (Fig. 1).

Fig. 1.

PUFAs cause phosphorylation of Insig2 and suppress SREBP1c cleavage. PUFAs activate the cAMP/PKA pathway, leading to the phosphorylation of Insig2 at serine 106. Phosphorylated Insig2 binds to the SCAP (SREBP cleavage activating protein)/SREBP1c complex, retaining it in the ER, thereby suppressing the expression of downstream lipogenic genes.

The work by Tian et al. provides a mechanistic basis for the inhibitory effects of PUFAs on fatty acid synthesis and highlights Insig2 as a pivotal regulatory node.

Insig1 and Insig2 were heretofore considered functionally redundant in inhibiting the processing of SREBPs (7). However, the study of Tian et al. provides compelling evidence that Insig2 has a distinct, nonredundant role in mediating the effects of PUFAs on fatty acid synthesis. The phosphorylation of Insig2 at S106 by PKA upon PUFA treatment reveals a targeted regulatory mechanism that selectively inhibits SREBP-1 processing without affecting SREBP-2.

The current study also greatly advances our understanding of PUFAs regulate SREBP1c cleavage (2). Previous studies have suggested that PUFA regulates SREBP1c cleavage in the ER through the PUFA-sensitive serine protease RHBDL4, independent of Insig1/2 and SCAP, in HEK 293 cells and mouse liver (8, 9). The results of Tian et al. showed that Insig2 KO (knockout) fibroblasts were dramatically impaired in their response to PUFA, arguing that direct ER cleavage plays a minor role in their system. Future in depth, head-to-head analysis of these proposed mechanisms in different cell types is warranted.

The study of Tian et al. has implications for metabolic health, particularly in the context of dietary interventions and therapeutic strategies aimed at controlling dyslipidemia and associated metabolic disorders. Fatty liver is a hallmark of conditions such as Metabolic Associated Steatosis Liver Disease. The identification of Insig2 as a critical mediator in PUFAs-dependent inhibition of fatty acid synthesis may inform the development of targeted treatments to reduce excessive lipid accumulation.

The current work raises interesting mechanistic questions for future investigation. For example, what is the basis of the specificity of phosphorylated Insig2 binding to SREBP1c but not SREBP2? Additionally, it will be important to further explore the implications of the PUFA-cAMP-Insig2 pathway for physiology. Food intake increases insulin and has been shown to downregulate Insig2 expression in the liver (10). The impact of PUFA in meals could potentially be impacted by this regulation.

In summary, the work of Tian et al. provides a mechanistic basis for the inhibitory effects of PUFAs on fatty acid synthesis and highlights Insig2 as a pivotal regulatory node. The findings open avenues for further research into how dietary components modulate lipid metabolism at the molecular level and have potential application to the management of metabolic diseases.