Dysregulated lipid metabolism is associated with cardiovascular disease, obesity and type 2 diabetes. In the current report we explore the functional interactions between two im-portant regulators of lipid metabolism, sterol regulatory element-binding protein 1 and 2 (SREBP1/2) and PRDI-BF1 and RIZ homology domain containing 16 (PRDM16). The SREBP family of transcription factors regulate cholesterol and fatty acid synthesis and metabolism, primarily in liver but also in white adipose tissue. PRDM16 is a major regulator of brown adipose tissue (BAT) biogenesis and function, and an inhibitor of white adipogenesis. We find that PRDM16 interacts with the nuclear forms of SREBP1/2 and inhibits their transcriptional activities. Consequently, inactivation of PRDM16 enhances the expression of well-established SREBP target genes involved in fatty acid and cho-lesterol synthesis/metabolism. Importantly, PRDM16 inactivation increases the expression of LDL receptor mRNA and protein and augments the cellular uptake of LDL particles. Supporting these findings, PRDM16-deficient cells accumulate more neutral lipids in a SREBP1/2-dependent manner. Inactivation of PRDM16 in white and brown preadipocyte cell lines and human adipose-derived stem cells enhances the expression of SREBP target genes. In addition, the expression of adipogenic markers was increased in mature white adipocytes generated from PRDM16-knockdown preadipocytes. Thus, our study identifies PRDM16 as a novel inhibitor of SREBP-dependent lipid metabolism with implications for adipose biology and metabolic disease.

Dysregulated lipid metabolism is associated with cardiovascular disease, obesity and type 2 diabetes. In the current report, we explore the functional interactions between two important regulators of lipid metabolism, sterol regulatory element-binding protein 1 and 2 (SREBP1/2), and PRDI-BF1 and RIZ homology domain containing 16 (PRDM16). The SREBP family of transcription factors regulate cholesterol and fatty acid synthesis and metabolism, primarily in liver but also in white adipose tissue. PRDM16 is a major regulator of brown adipose tissue (BAT) biogenesis and function as well as an inhibitor of white adipogenesis. We find that PRDM16 interacts with the nuclear forms of SREBP1/2 and inhibits their transcriptional activities. Consequently, inactivation of PRDM16 enhances the expression of well-established SREBP target genes involved in fatty acid and cholesterol synthesis/metabolism. Importantly, PRDM16 inactivation increases the expression of LDL receptor mRNA (1.6-fold) and protein (1.7-fold) and augments the cellular uptake of LDL particles (2.3-fold). Supporting these findings, PRDM16-deficient cells accumulate more neutral lipids in a SREBP1/2-dependent manner. Inactivation of PRDM16 in white and brown preadipocyte cell lines and human adipose-derived stem cells enhances the expression of SREBP target genes. In addition, the expression of adipogenic markers was increased in mature white adipocytes generated from PRDM16-knockdown preadipocytes (1.8- to 3.9-fold). Thus, our study identifies PRDM16 as a novel inhibitor of SREBP-dependent lipid metabolism with implications for adipose biology and metabolic disease.

Colorectal cancer (CRC) remains a leading cause of cancer morbidity worldwide, highlighting the need for improved therapies. Metformin, a widely used antihyperglycemic agent, has gained attention for its potential antitumor properties. In this study, we evaluated the effects of the tumor suppressor genes TP53 and CDKN1A on metformin responsiveness in a KRAS-mutant CRC in vitro model using HCT116 cells harboring a G13D mutation in KRAS. Using parental (p53^+/+, p21^+/+) and isogenic knockout cell lines, we assessed cell-cycle distribution and transcriptomic responses following metformin treatment. In parental wild-type cells, metformin exposure was associated with a dose- and time-dependent reduction in cell viability and an increased proportion of cells in the G0/G1 phase, with significance levels across treatment conditions ranging from p = 0.03 to p < 0.0001. Loss of p53 or p21 was associated with attenuated cellular responses to metformin, with p21-deficient cells responding primarily at higher doses and prolonged exposure (72 h). Transcriptomic profiling revealed extensive differential gene expression in parental cells (1399 DEGs), compared with more limited responses in p53^−/− (270 DEGs) and p21^−/− cells (32 DEGs). Differentially expressed genes associated with MAPK signaling (DUSP5) and inflammatory regulation (TNFAIP3) were observed across genotypes, whereas pathway enrichment of DNA replication and chromatin organization was specific to p53-deficient cells. These findings provide a transcriptomic and phenotypic characterization of genotype-dependent cellular responses to metformin and establish a basis for future mechanistic and functional validation studies.

This report identifies a bidirectional signaling axis connecting lipid metabolism to nuclear mechanotransduction, with the potential to control fatty acid/triglyceride metabolism. The sterol regulatory element-binding (SREBP) family of transcription factors control fatty acid, triglyceride and cholesterol synthesis and metabolism. The family consists of three members: SREBP1a, SREBP1c, and SREBP2, that are regulated by intracellular cholesterol levels and insulin signaling. The SREBP2-dependent control of the LDL receptor gene is a well-established target for cholesterol-lowering therapeutics and the activity of SREBP1c is an attractive target in metabolic disease. In the current report, we identify SYNE4 (nesprin-4), a component of the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex, as a direct target of the SREBP family of transcription factors, and show that nesprin-4 in turn supports SREBP1c function. We identify functional SREBP binding sites in the human SYNE4 promoter and demonstrate that these are required for the sterol– and SREBP-dependent regulation of the promoter. Furthermore, we show that the endogenous SYNE4 gene is also regulated by SREBP1/2 and intracellular sterol levels. Interestingly, SREBP2 is responsible for the sterol regulation of the SYNE4 gene in HepG2 cells, while SREBP1 is the major regulator in MCF7 cells, demonstrating that diberent cell types use diberent SREBP paralogs to regulate the same promoter/gene. Importantly, we find that nesprin-4 is a positive regulator of SREBP1c expression and function in HepG2 cells and during the diberentiation of human adipose-derived stem cells. In summary, the current report identifies a novel regulatory interaction between lipid metabolism and the LINC complex. Importantly, we demonstrate that this signaling axis is bidirectional, forming a closed loop that has the potential to control SREBP1c activity and thereby fatty acid and triglyceride synthesis/metabolism. Based on our data, we propose that the nesprin-4-dependent regulation of SREBP1c could represent a novel therapeutic target in metabolic disease.