Retinoic acid controls PHOX2A expression by means of a dual regulatory mechanism

The specification of neuronal identity is the result of the interaction between two distinct classes of determining factors: extrinsic factors, including secreted or cell membrane-associated signals in the local environment, and intrinsic factors that often consist of transcription factors cascades. PHOX2A is a homedomain protein that participates in the network regulating the development of autonomic ganglia. We used an undifferentiated human neuroblastoma cell line to show that retinoic acid, a well-established extrinsic factor that profoundly affects the differentiation of sympathetic neurons at different developmental stages, regulates PHOX2A expression by means of a dual effect: it starts by acting as a positive regulator of gene expression, and later triggers a process culminating in the complete disappearance of the transcription factor. The early effects of retinoic acid were related to an increase in the amount of specific mRNA that lead to an increase in the amount of the corresponding protein. The delayed effects, which were completely evident after 48 hours of treatment, consisted of the selective degradation of the PHOX2A protein, whereas the corresponding mRNA remained up-regulated. The persistence of PHOX2A protein, iduced by treatment with proteasome inhibitors, resulted in a selective dis-regulation of the transcription of the dopamine-beta-hydroxylase, a well characterized PHOX2A target gene. This suggests that the expression of PHOX2A must be finely regulated during development in order to direct neurons towards the terminal noradrenergic differentiation.