Multi-modal regulation of endogenous D1 dopamine receptor expression and function in the CAD catecholaminergic cell line

Dopamine receptors exhibit tissue- and cell type-specific expression that is modulated during development, aging and in diseases such as Parkinson's. The molecular mechanisms regulating expression of dopamine receptors are not well understood, in part due to the lack of a model cell line that not only expresses endogenous dopamine receptors but also has the requisite regulatory mechanisms. Here, we demonstrate that the CAD catecholaminergic cell line expresses D-1, D-2, D-3 and D-5 dopamine receptor subtypes and associated signaling proteins. CAD cell differentiation induced by serum withdrawal increases the levels of D-1 receptor mRNA by transcriptional up-regulation. This increase is also mimicked by the neurotrophin NT3. Interestingly, the increase of D-1 receptor mRNA does not result in increased levels of D-1 receptor protein in differentiated CAD cells. Furthermore, while the D-1 receptor protein is expressed in differentiated CAD cells, it loses its ability to activate adenylyl cyclase. We demonstrate that the post-transcriptional regulation is not due to decreased D-1 receptor mRNA stability or generation of a truncated D-1 receptor mRNA, and that the down-regulation of D-1 receptor function in differentiated CAD cells is mediated by post-translational mechanisms that decrease cell surface receptor expression by altering receptor processing and trafficking.