Role of sterol regulatory element binding proteins in the regulation of Galpha(i2) expression in cultured atrial cells.

We have previously demonstrated that growth of embryonic chick atrial cells in medium supplemented with lipoprotein-depleted serum (LPDS) resulted in a coordinate increase in the expression of genes involved in the parasympathetic response of the heart (the M2 muscarinic receptor; the alpha-subunit of the heterotrimeric G protein, Galpha(i2); and the inward rectifying K+ channel protein, GIRK1) and a marked increase in the negative chronotropic response of atrial cells to muscarinic stimulation. In the present study, we demonstrate that regulation of Galpha(i2) promoter activity by LPDS is mediated by the binding of a sterol regulatory element binding protein (SREBP) to a sterol regulatory element (SRE) in the Galpha(i2) promoter. Deletion and point mutation of this putative SRE interfered with the regulation of the Galpha(i2) promoter by SREBP and LPDS. Furthermore gel shift assays demonstrated that point mutations in the putative Galpha(i2) SRE markedly inhibited the binding of purified SREBP to oligonucleotides containing the Galpha(i2) SRE sequence. The expression of a dominant-negative SREBP mutant interfered with LPDS stimulation of Galpha(i2) promoter activity. Finally, we demonstrate that SREBP-1 is markedly more potent than SREBP-2 for the stimulation of Galpha(i2) promoter activity, suggesting that SREBP1 may play a role in the regulation of Galpha(i2) expression. These are the first data to demonstrate SREBP regulation of a protein not involved in lipid homeostasis and suggest a new relationship between lipid metabolism and the parasympathetic response of the heart.