The stimulated C-kinase activity in estradiol-treated rat pituitaries is reduced by chronic treatment with the dopamine agonist CV 205-502

Effect of oestradiol on dopamine receptors and protein kinase C activity in the rat pituitary: binding of oestradiol to pituitary membranes

Oestradiol exerts an important modulatory influence on the release of prolactin which is accomplished partly through disruption of the inhibitory influence of dopamine. We have focused on the status of the anterior pituitary D2 dopamine receptor in female rats treated chronically with oestradiol or progesterone. A direct membrane effect of these steroids on the dopamine system was also investigated in vitro. Both steroids affected the status of the D2 receptor, oestradiol decreasing the number of sites in vitro and progesterone increasing it both in vitro and in vivo. The in vitro studies demonstrated that these steroids exert a direct membrane effect on the D2 receptor. These results correlated with an in vitro short-term physiological effect of oestradiol and progesterone on the dopaminergic inhibition of prolactin release, oestradiol decreasing it while progesterone had the opposite effect. Binding studies with [3H] oestradiol on pituitary membranes revealed a site for oestradiol of high affinity and low capacity, indicating that oestradiol's membrane effects could be mediated by a specific receptor. In vivo treatment with oestradiol also induces proliferation of prolactin-secreting cells (lactotrophs). We focused on the effect of oestradiol on protein kinase C activity, which is involved in both secretion and proliferation. In female rats treated with oestradiol total protein kinase C activity was increased by 74% (particulate 90%, soluble 71%) in comparison with controls. This effect was reversed by concomitant treatment with a dopamine agonist. Thus in the pituitary oestradiol and progesterone affect the characteristics of membrane components that are implicated in the physiological control of the cell. Whether these effects are post-transcriptional only or are also mediated through direct membrane mechanisms needs further investigation.

Membrane targeting and cytoplasmic sequestration in the spatiotemporal localization of human protein kinase C alpha

In order to map the molecular determinants that dictate the subcellular localization of human protein kinase C alpha (hPKCalpha), full-length and deletion mutants of hPKCalpha were tagged with the green fluorescent protein (GFP) and transiently expressed in GH3B6 cells. We found that upon thyrotropin-releasing hormone (TRH) or phorbol 12-myristate 13-acetate stimulation, hPKCalpha-GFP was localized exclusively in regions of cell-cell contacts. Surprisingly, PKCalpha failed to translocate in single cells despite the presence of TRH receptors, as attested by the TRH-induced rise in intracellular calcium concentration in these cells. TRH-stimulated translocation of hPKCalpha-GFP from the cytoplasm to cell-cell contacts was a biphasic process: a fast (measured in seconds) and transient phase, followed by a slower (approximately 1 hour) and long lasting phase. The latter and the translocation induced by phorbol 12-myristate 13-acetate absolutely required the N-terminal V1 region. In contrast to the full-length hPKCalpha, the N-terminal regulatory domain alone or associated with the V3 hinge region was spontaneously and uniformly localized at the plasma membrane of single and apposed cells. However, treatment with the calcium chelator BAPTA/AM induced a differential cytoplasmic/nuclear redistribution of the regulatory domain, depending on its association with V3, which suggests the existence of a mechanism controlling the cytoplasmic sequestration of inactive hPKCalpha and involving the V3 region. By using other deletion mutants, we were able to map the sequence required for this sequestration to the C2+V3 regions. This work points to the existence of a complex interplay between membrane targeting and cytoplasmic sequestration in the control of the spatiotemporal localization of hPKCalpha.

Selective loss of substrate recognition induced by the tumour-associated D294G point mutation in protein kinase Calpha

The tumour-associated D294G mutant of protein kinase Calpha (PKCalpha) was recently shown not to be translocated to the plasma membrane on stimulation with PMA, in contrast with the wild-type enzyme. Using recombinant wild-type and mutant PKCalpha, we establish here that, although the PKCalpha intrinsic lipid-dependent catalytic activity remains unaltered by the D294G mutation, the mutant enzyme exhibits a selective loss of substrate recognition. Indeed, whereas the mutant enzyme is still able to phosphorylate histone IIIS with comparable efficiency to that of the wild-type enzyme, it exhibits a lack of kinase activity towards the previously cloned 35F and 35H substrates for PKC. Overlay experiments demonstrate that this selective loss of kinase activity is correlated with a decrease in binding of D294G PKCalpha to the 35F and 35H proteins compared with that of the wild-type enzyme. Because the 35H and 35F proteins are predicted to be PKCalpha-anchoring proteins, these findings suggest a selective loss of PKCalpha-protein interactions that might fail to stabilize the location of the PKCalpha mutant at the plasma membrane.

Ca(2+)-dependent protein kinase C isoforms in rat pituitary hyperplasia: effect of in vivo treatment with quinagolide

Ca(2+)-dependent protein kinase C (PKC) activity, diacylglycerol levels and PKC alpha, beta I, beta II and gamma expression were analyzed in the pituitary of female rats treated with estradiol alone (2 months) or in combination with quinagolide in the second month. Polymerase chain reaction (PCR) and Western blot analysis revealed the presence of PKC alpha, beta I and beta II isoenzymes in the rat pituitary gland but not of PKC gamma isoenzymes. Increases in pituitary weight and plasma prolactin levels induced by estradiol were associated with an increase in diacylglycerol pituitary content (1.55 +/- 0.06 versus 1.12 +/- 0.17 nmol diacylglycerol/mg protein in controls, P < 0.01). Cotreatment with quinagolide reversed these effects. Changes in PKC activity were accompanied by parallel changes in PKC alpha and beta I expressions. Estradiol treatment increased the expression of these isoforms whereas cotreatment with quinagolide antagonized these effects. PKC beta II expression was not affected. In conclusion, Ca(2+)-dependent PKC activity and protein expression are increased in hyperplastic pituitary cells, suggesting the involvement of this class of PKCs in the rat pituitary cell proliferation and/or hormonal secretion. This is further assessed by the fact that the dopamine receptor agonist treatment decreases activity and expression of these PKCs in parallel with the decrease in hormonal secretion and cell proliferation.

Immunohistochemical expression of protein kinase C type III in human pituitary adenomas

Immunohistochemical expression of the three major isozymes of protein kinase C--Types I, II, and III--was studied in 32 cases of human pituitary adenomas, and the results were compared in detail with their clinical data. Immunoreactivity for the Type I and Type II isozymes was negative in tumor cells of all pituitary adenomas. Moderate to strong cytoplasmic immunoreactivity for the Type III isozyme was constantly seen in acromegaly, Cushing's disease, and nonfunctioning adenomas, which indicated overexpression of the isozyme, since only slight cytoplasmic immunoreactivity was observed in the normal human anterior pituitary cells. Among 13 prolactinomas, 5 cases showed positive immunoreactivity for Type III in all tumor cells, whereas 8 cases showed negative immunoreactivity for the isozyme in all or more than 85% of tumor cells. The sizes of the tumors in this protein kinase C Type III negative group of prolactinomas tended to be smaller than those of the Type III positive prolactinomas. Also, the negative immunoreactivity for Type III was predominantly observed in those cases where prolactinomas were relatively well controlled by continuous oral dosage of dopamine agonists before operation. These results suggest that protein kinase C Type III is closely involved in human pituitary adenomas. The exceptional negativity for the isozyme in prolactinomas may be relevant to the suppression of tumor growth by dopamine agonists.

Protein kinase C activity and expression in normal and adenomatous human pituitaries

Protein kinase C (PKC) activity and expression were measured in 54 adenomas (prolactin (PRL)-, growth hormone (GH)- and non-secreting), 1 of them obtained from a patient treated with the dopamine agonist bromocriptine and 2 from patients treated with the somatostatin analog octreotide. They were also measured in normal human and rat pituitaries. Total PKC activity was measured by incorporation of 32P into histones, and PKC expression by dot blot immunoquantification using purified PKC as a standard. Both enzyme activity and expression were higher in adenomatous pituitaries than in normal human or rat pituitaries. PKC expression in GH-secreting and non-secreting tumors was significantly higher than that in PRL-secreting tumors. Furthermore, it was significantly higher invasive tumors than in non-invasive tumors. In the 3 adenomas which were obtained from patients treated with bromocriptine or octreotide and which were used for PKC-activity measurement, particulate- and soluble-PKC activities were significantly lower than those measured in non-treated adenomas.