Differential expression of cytosolic and mitochondrial 3-hydroxy-3-methylglutaryl CoA synthases during adipocyte differentiation.

Mitochondrial and cytosolic 3-hydroxy-3-methylglutaryl CoA synthase (m-HMS and c-HMS) genes show high identity at the nucleotide and amino acid level, but no homology has been found in the promoter area. The main regulator for c-HMS is SREBP. The best known transcription factor that regulates m-HMS is PPAR alpha. Three types of PPAR, alpha, gamma and delta have been described in vertebrates. Here we found that they display distinct ligand response profiles in the m-HMS promoter. In some conditions PPAR gamma is a significant activator of m-HMS. Thus, the m-HMS gene is transiently expressed during the clonal expansion phase of 3T3-L1 differentiation. We found that C/EBP delta and PPAR gamma activate the m-HMS promoter in 3T3-L1 cells synergistically. This synergistic effect was only observed in the whole promoter (-1148 to +28). A small construct (-116 to +28) which contains the PPRE did not respond to C/EBP delta and/or PPAR gamma. This suggests that a putative C/EBP site lie somewhere between -1148 and -116 bp. We also show that C/EBP delta was more efficient that C/EBP alpha and C/EBP beta to activate the m-HMS promoter. The time course of c-HMS mRNA expression during 3T3-L1 differentiation was different, with a significant increase at terminal adipogenesis. We found that the transcription factor C/EBP alpha did not activate the c-HMS promoter. The differential pattern of expression shown by these two genes, which have a common ancestor, exemplifies refinements of transcriptional control during evolution.