Sterol mediated regulation of SREBP-1a,1b,1c and SREBP-2 in cultured human cells.

Under conditions of cholesterol depletion and SREBP-1 accumulation, changes in the levels of sterol regulatory element binding protein(s) (SREBPs) and sterol regulated gene mRNA were studied in Hep G2 cells by RNase protection assay. Cholesterol depletion increased the expression of mRNAs for cholesterol biosynthetic enzymes and low density lipoprotein (LDL) receptor. mRNAs levels for SREBP-1c and SREBP-2 were also increased by the cholesterol depletion. In contrast, levels for SREBP-1a and 1b (1a/b) mRNA increased transiently and then decreased. To examine the effect of SREBP-1 accumulation, Hep G2 cells were incubated with a SREBP-1 degradation inhibitor, N-acetyl-leucyl-leucyl-norleucinal (ALLN). The ALLN treatment increased the LDL receptor mRNA significantly, and also increased mRNA levels for HMG-CoA reductase, SREBP-1a/b and SREBP-2. The mRNA level for squalene synthase was not changed, and for SREBP-1c was decreased by the treatment. In conclusion, the regulation of differential expression of SREBP mRNA may be involved in sterol mediated regulation of gene expression. Moreover, the regulation of the SREBP-1 level may be a critical step in the regulation of sterol mediated LDL receptor expression.