Figure 4. SREBP-Related Pathway Regulates Fatty Acid Desaturase and Palmitate-Induced Lipotoxicity.

(A) Schematic representation of the SREBP pathway in which GP78 degrades INSIG1 to free the SCAP-SREBP1 complex for trafficking from the ER to the Golgi apparatus, where cleavage liberates the SREBP1 transcription factor fragment from the membrane. This, in turn, leads to induction of target genes that have a sterol-response element (SRE) sequence in their promoter, such as SCD1. Aggravating hits are shown in red, protective hits are shown in blue.

(B) INSIG1 knockdown elevates FAS and SCD1 and reduces palmitate-induced upregulation of BIP. Western blot of WT and INSIG1-kd cells under basal condition or after palmitate treatment (0.15 mM, 20 h).

(C) INSIG1 knockdown modestly protects cells from palmitate-induced cell death, as measured by PI staining, and this protection is lost with SCD inhibition (4 μM). **p < 0.001.

(D) Unfolded protein response gene expression levels of WT and INSIG1-kd cells.

(E) Relative quantification for phosphatidic acid (PA, left panel) and diacylglycerol (DAG, right panel) in INISIG1-kd cells compared to WT cells before after after palmitate treatment, as identified by LC-MS². K562 cells untreated or treated with 0.2 mM palmitate. n=3–4 for each treatment. ***p < 0.001.

(F) INSIG1-kd cells exhibit increased incorporation of radiolabeled palmitate into TG. Cells were treated for 6 h with 0.2 mM nonradiolabeled palmitate and 0.15μCi ¹⁴C-palmitate. Triacsin C (10uM, an inhibitor of ACSL1, ACSL3 and ACSL4) was added confirm ACSL-specific fatty acid uptake. Untreated and triacsin C values are also shown in Figure 1H. n=3 for each treatment. **p < 0.01; ***p < 0.001.

See also Figures S4.