Involvement of Intracellular or Extracellular Calcium in Activation of Tyrosine Hydroxylase Gene Expression in PC12 Cells

Abstract: The relationship between elevations in intracellular free Ca²⁺ concentration (Ca²⁺]_i) by different mechanisms and tyrosine hydroxylase (TH) gene expression was examined. Depolarization by an elevated K⁺ concentration triggered rapid and sustained increases in Ca²⁺]_i from a basal level of ～50 to 110–150 nM and three- to fourfold elevations in TH mRNA levels, requiring extracellular calcium but not inositol 1,4,5-trisphosphate (IP₃). On the other hand, bradykinin or thapsigargin, both of which induce release of intracellular calcium stores via IP₃ or inhibition of Ca²⁺-ATPase, rapidly elevated Ca²⁺]_i to >200 nM and increased TH gene expression (three-to fivefold). Confocal imaging showed that the elevations in Ca²⁺]_i in each case occurred throughout the cyto- and nucleoplasm. The initial rise in Ca²⁺]_i due to either bradykinin or thapsigargin, which did not require extracellular calcium, was sufficient to initiate the events leading to increased TH expression. Consistent with this, the effects of bradykinin on TH expression were inhibited by 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid or 3,4,5-trimethoxybenzoic acid 8-(diethylamino)-octyl ester which chelates or inhibits the release of intracellular calcium, respectively. Bradykinin required a rise in Ca²⁺]_i for <10 min, as opposed to 10–30 min for depolarization to increase TH mRNA levels. These results demonstrate that although each of these treatments increased TH gene expression by raising Ca²⁺]_i, there are important differences among them in terms of the magnitude of elevated Ca²⁺]_i, requirements for extracellular calcium or release of intracellular calcium stores, and duration of elevated Ca²⁺]_i, indicating the involvement of different calcium signaling pathways leading to regulation of TH gene expression.