Signaling Pathway Involved in the Pro-Apoptotic Effect of Dopamine in the GH3 Pituitary Cell Line

Abstract

Besides its physiological role as a neurotransmitter, dopamine (DA) induces apoptosis in the central nervous system. This effect is mediated partly by the DA transporter (DAT) and involves reactive oxygen species (ROS) formation as well as oxidative stress. In the pituitary, the inhibitory control by DA of prolactin release and synthesis and lactotrope cell proliferation is well known, while the pro-apoptotic effect of DA remains unclear. Our aim was to study the pro-apoptotic effect of DA in the GH3 mammosomatotrope cell line and determine the DA mechanism that leads to apoptosis in these cells. Using flow cytometry, Western blot, and confocal microscopy, we showed for the first time that DA induced: (1) loss of mitochondrial potential; (2) relocation of Bax to the mitochondria; (3) cytochrome c release; (4) caspase-3 activation, and (5) nuclear fragmentation, resulting in apoptosis. We observed that DAT was expressed in GH3 cells and participated in the DA effect, as apoptosis was significantly reversed in the presence of DAT inhibitors. Direct measurement showed that DA rapidly increased the formation of intracellular ROS. The antioxidant N-acetyl-L-cysteine (NAC) effectively blocked DA-induced ROS formation and apoptosis. Neither JNK nor p38 were involved in this process, so we suggest that the mitochondrial pore of transition is the likely target of the ROS generated by DA. These data provide the first evidence that DA triggers apoptosis in pituitary cells via a mechanism involving DAT and oxidative stress. These findings may be particularly relevant in understanding lactotrope apoptosis during postnatal life.

Reversal of Endogenous Dopamine Receptor Silencing in Pituitary Cells Augments Receptor-Mediated Apoptosis

“…Our conclusion, therefore, from that study, in the absence of a detectable D2R or DAT were that the mechanism by which DA induces apoptosis had not been definitively identified (12). However, in these studies (11,12) we had also noted that high doses of DA or BC were required to induce an apoptotic response, and we proposed several possible explanation to account for these observations (see Ref. 12).…”

“…These studies, in cells that differed only in D2R status, showed an augment and presumed D2R mediated response to drug challenges with either DA or BC. In these cases these agents had been shown previously to induce apoptosis in these cells (11,12).…”

“…Early studies suggested that resistance to DA but not bromocriptine (BC)-mediated apoptosis was a consequence of these cells not expressing a D2R (10). However, more recent studies by Jaubert and colleagues (11) suggest that DAinitiated apoptotic responses are mediated through the DA transporter (DAT). Although this transporter represents an attractive alternate mechanism a recent study from our laboratory, using this same model system, namely, the rodent pituitary cell line GH3, reached a different conclusion.…”

Apoptosis of Lactotrophs Induced by D2 Receptor Activation Is Estrogen Dependent

“…DA has been reported to induce apoptosis of anterior pituitary cells [13,14,15], but the mechanism that triggers this process remains controversial. Jaubert et al[ 14, 15] reported that in anterior pituitary cells from postlactating rats and GH3 cells, a somatolactotroph cell line lacking D2 receptors, DA-induced apoptosis is mediated by DAT.…”

“…Also, D2 receptor activation inhibits prolactin gene expression and reduces lactotroph proliferation [1, 12]. Although DA induces apoptosis of GH3 cells, either the D2 receptor or DAT has been implicated in this action [13, 14]. This transporter was identified in anterior pituitary cells [15, 16].…”