Protein kinase Cδ negatively regulates tyrosine hydroxylase activity and dopamine synthesis by enhancing protein phosphatase-2A activity in dopaminergic neurons

Tyrosine hydroxylase ( TH), the rate-limiting enzyme in dopamine synthesis, can be regulated by phosphorylation at multiple serine residues, including serine-40. In the present study, we report a novel interaction between a key member of the novel PKC family, protein kinase C delta( PKC delta), and TH, in which the kinase modulates dopamine synthesis by negatively regulating TH activity via protein phosphatase 2A ( PP2A). We observed that PKC delta is highly expressed in nigral dopaminergic neurons and colocalizes with TH. Interestingly, suppression of PKC delta activity with the kinase inhibitor rottlerin, PKC delta-small interfering RNA, or with PKC delta dominant-negative mutant effectively increased a number of key biochemical events in the dopamine pathway, including TH-ser40 phosphorylation, TH enzymatic activity, and dopamine synthesis in neuronal cell culture models. Additionally, we found that PKC delta not only physically associates with the PP2A catalytic subunit ( PP2Ac) but also phosphorylates the phosphatase to increase its activity. Notably, inhibition of PKC delta reduced the dephosphorylation activity of PP2A and thereby increased TH-ser40 phosphorylation, TH activity, and dopamine synthesis. To further validate our findings, we used the PKC delta knock-out ( PKC delta-/-) mouse model. Consistent with other results, we found greater TH-ser40 phosphorylation and reduced PP2A activity in the substantia nigra of PKC delta-/- mice than in wild-type mice. Importantly, this was accompanied by an increased dopamine level in the striatum of PKC delta-/- mice. Collectively, these results suggest that PKC delta phosphorylates PP2Ac to enhance its activity and thereby reduces TH-ser40 phosphorylation and TH activity and ultimately dopamine synthesis.