Characterization of clone D2: An orphan protein that interacts with peroxisome proliferator-activated receptor gamma

Phosphoenolpymvate carboxykinase (PEPCK) expression is regulated in a tissue specific manner. However, understanding the expression pattern in different tissues is not well documented. We are interested in the adipocyte specific regulation and expression of PEPCK. Our lab had previously identified an adipocyte specific enhancer (ASE) which regulates PEPCK expression through the transcription factor PPARy. To further characterize this effect, the yeast-2-hybrid system was used to characterize what factors bind PPARy in the adipocyte. The screen led to the isolation of a novel protein we have named D2.

Our initial hypothesis was that D2 is a cofactor for PPARy. The studies presented here show that D2 is a putative coiled-coil protein whose expression partem does not correlate with that of PPARy. In fact, D2 expression is extremely low in all tissues screened except for several cancer tissues and cell lines. The translated product is highly conserved in various species which suggests an important function. There also seem to be multiple isoforms that potentially arise by the use of an alternative promoter and/or alternative splicing. We also show that D2 expression in both prokaryotic and eukaryotic cells is toxic. The pattern of expression, its conservation among species, and its toxic effects led us to propose that it could be involved in cell cycle control and/or act as a tumor suppressor. Indeed, further investigation revealed that the D2 gene is located in a region on chromosome 9 that is frequently deleted in multiple cancers. Interestingly, there are multiple genes involved in cell cycle control and tumor suppression near the D2 locus. In fact it has been proposed that this chromosomal region could potentially have more tumor suppressors than those previously described. These studies have led to the new working hypothesis that D2 is involved in cell cycle regulation as a novel tumor suppressor