Coordination of fatty acid synthesis and cell proliferation by transcription factor Sp1 in MCF-7 human breast cancer cells

LB-237 Many human cancers including breast cancer exhibit increased denovo fatty acid synthesis with overexpression of fatty acid synthase (FAS). Unlike normal cells and tissues that preferentially utilize circulating fatty acids derived from the diet, cancers synthesize fatty acids endogenously for membrane biosynthesis to sustain cell proliferation. The transcription factor Sp1 is highly expressed in a variety of cancers. Sp1 regulates gene expression by interacting with GC-rich promoter sequences. Genes that regulate cell cycle progression often contain such promoter sequences, and Sp1 is critical for their expression. The promoter region of FAS also has Sp1 binding sites, and Sp1 together with sterol regulatory element-binding protein-1 (SREBP-1) has been shown to regulate FAS expression in hepatocytes. Here, we hypothesize that Sp1 coordinately regulates FAS and cell cycle progression in estrogen-responsive MCF-7 breast cancer cells. Based on previous studies, to up-regulate Sp1 activity we treated MCF-7 cells with 100 nM 17β-estradiol (E2) for 48 h. To down-regulate Sp1 activity, we treated the cells for 48 h with 0.025 or 0.05 nM mithramycin, a known inhibitor of Sp1, or we transfected the cells with Sp1 siRNA, then maintained them for 72 h. Cell proliferation, cell cycle progression and expression of FAS and cell cycle regulatory proteins were measured. E2 treatment stimulated cell proliferation, accelerated the G0/G1 to S phase transition, and increased FAS and cyclin D1 expression. In contrast, mithramycin treatment or Sp1 siRNA transfection inhibited cell proliferation, led to G0/G1 cell cycle arrest, and decreased FAS and cyclin D1 expression. Furthermore, the effects of E2 were mitigated by mithramycin or Sp1 siRNA, and the effects of mithramycin or Sp1 siRNA were mitigated by E2. Other G1 cell cycle regulatory proteins such as cdk2, cdk4 and cyclin E were unaffected by these various treatments. We have also shown that inhibition of fatty acid synthesis by 5-(tetradecyloxy)-2-furoric acid (TOFA), a known inhibitor of acetyl-CoA carboxylase (ACC), decreased cell proliferation, caused G0/G1 cell cycle arrest, and suppressed the expression of Sp1 and cyclin D1. Our results indicate that Sp1 coordinately regulates the expression of FAS and cyclin D1 in MCF-7 cells, suggesting that Sp1 is involved in synchronizing fatty acid synthesis with cell cycle progression. The synchronization of these two biological processes by Sp1 could be exploited as a strategy for cancer therapy.

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