Analysis of the role of the mitogen-activated protein kinase in mediating cyclic-adenosine 3',5'-monophosphate effects on prolactin promoter activity

Ras-mutant cancer cells display B-Raf binding to Ras that activates extracellular signal-regulated kinase and is inhibited by protein kinase A phosphorylation

The small G protein Ras regulates proliferation through activation of the mitogen-activated protein (MAP) kinase (ERK) cascade. The first step of Ras-dependent activation of ERK signaling is Ras binding to members of the Raf family of MAP kinase kinase kinases, C-Raf and B-Raf. Recently, it has been reported that in melanoma cells harboring oncogenic Ras mutations, B-Raf does not bind to Ras and does not contribute to basal ERK activation. For other types of Ras-mutant tumors, the relative contributions of C-Raf and B-Raf are not known. We examined non-melanoma cancer cell lines containing oncogenic Ras mutations and express both C-Raf and B-Raf isoforms, including the lung cancer cell line H1299 cells. Both B-Raf and C-Raf were constitutively bound to oncogenic Ras and contributed to Ras-dependent ERK activation. Ras binding to B-Raf and C-Raf were both subject to inhibition by the cAMP-dependent protein kinase PKA. cAMP inhibited the growth of H1299 cells and Ras-dependent ERK activation via PKA. PKA inhibited the binding of Ras to both C-Raf and B-Raf through phosphorylations of C-Raf at Ser-259 and B-Raf at Ser-365, respectively. These studies demonstrate that in non-melanocytic Ras-mutant cancer cells, Ras signaling to B-Raf is a significant contributor to ERK activation and that the B-Raf pathway, like that of C-Raf, is a target for inhibition by PKA. We suggest that cAMP and hormones coupled to cAMP may prove useful in dampening the effects of oncogenic Ras in non-melanocytic cancer cells through PKA-dependent actions on B-Raf as well as C-Raf.

Each individual isoform of the dopamine D2 receptor protects from lactotroph hyperplasia

Dopamine acting through D2 receptors (D2Rs) controls lactotroph proliferation and prolactin (PRL) levels. Ablation of this receptor in mice results in lactotroph hyperplasia and prolactinomas in aged females. Alternative splicing of the Drd2 gene generates 2 independent isoforms, a long (D2L) and a short (D2S) isoform, which are present in all D2R-expressing cells. Here, we addressed the role of D2L and D2S on lactotroph physiology through the generation and analysis of D2S-null mice and their comparison with D2L-null animals. These mice represent a valuable tool with which to investigate dopamine-dependent isoform-specific signaling in the pituitary gland. We sought to assess the existence of a more prominent role of D2L or D2S in controlling PRL expression and lactotroph hyperplasia. Importantly, we found that D2L and D2S are specifically linked to independent transduction pathways in the pituitary. D2L-mediated signaling inhibits the AKT/protein kinase B kinase activity whereas D2S, in contrast, is required for the activation of the ERK 1/2 pathway. Under normal conditions, presence of only 1 of the 2 D2R isoforms in vivo prevents hyperprolactinemia, formation of lactotroph's hyperplasia, and tumorigenesis that is observed when both isoforms are deleted as in D2R-/- mice. However, the protective function of the single D2R isoforms is overridden when single isoform-knockout mice are challenged by chronic estrogen treatments as they show increased PRL production and lactotroph hyperplasia. Our study indicates that signaling from each of the D2R isoforms is sufficient to maintain lactotroph homeostasis in physiologic conditions; however, signaling from both is necessary in conditions simulating pathologic states.

Distinct pools of cAMP centre on different isoforms of adenylyl cyclase in pituitary-derived GH3B6 cells

Microdomains have been proposed to explain specificity in the myriad of possible cellular targets of cAMP. Local differences in cAMP levels can be generated by phosphodiesterases, which control the diffusion of cAMP. Here, we address the possibility that adenylyl cyclases, the source of cAMP, can be primary architects of such microdomains. Distinctly regulated adenylyl cyclases often contribute to total cAMP levels in endogenous cellular settings, making it virtually impossible to determine the contribution of a specific isoform. To investigate cAMP dynamics with high precision at the single-isoform level, we developed a targeted version of Epac2-camps, a cAMP sensor, in which the sensor was tagged to a catalytically inactive version of the Ca(2+)-stimulable adenylyl cyclase 8 (AC8). This sensor, and less stringently targeted versions of Epac2-camps, revealed opposite regulation of cAMP synthesis in response to Ca(2+) in GH(3)B(6) pituitary cells. Ca(2+) release triggered by thyrotropin-releasing hormone stimulated the minor endogenous AC8 species. cAMP levels were decreased by inhibition of AC5 and AC6, and simultaneous activation of phosphodiesterases, in different compartments of the same cell. These findings demonstrate the existence of distinct adenylyl-cyclase-centered cAMP microdomains in live cells and open the door to their molecular micro-dissection.

Docosahexaenoic acid and arachidonic acid release in rat brain astrocytes is mediated by two separate isoforms of phospholipase A2 and is differently regulated by cyclic AMP and Ca2+

1. Docosahexaenoic acid (DHA) and arachidonic acid (AA), polyunsaturated fatty acids (PUFAs), are important for central nervous system function during development and in various pathological states. Astrocytes are involved in the biosynthesis of PUFAs in neuronal tissue. Here, we investigated the mechanism of DHA and AA release in cultured rat brain astrocytes. 2. Primary astrocytes were cultured under standard conditions and prelabeled with [(14)C]DHA or with [(3)H]AA. Adenosine 5'-triphosphate (ATP) (20 micro M applied for 15 min), the P2Y receptor agonist, stimulates release of both DHA (289% of control) and AA (266% of control) from astrocytes. DHA release stimulated by ATP is mediated by Ca(2+)-independent phospholipase A(2) (iPLA(2)), since it is blocked by the selective iPLA(2) inhibitor 4-bromoenol lactone (BEL, 5 micro M) and is not affected either by removal of Ca(2+) from extracellular medium or by suppression of intracellular Ca(2+) release through PLC inhibitor (U73122, 5 micro M). 3. AA release, on the other hand, which is stimulated by ATP, is attributed to Ca(2+)-dependent cytosolic PLA(2) (cPLA(2)). AA release is abolished by U73122 and, by removal of extracellular Ca(2+), is insensitive to BEL and can be selectively suppressed by methyl arachidonyl fluorophosphonate (3 micro M), a general inhibitor of intracellular PLA(2) s. 4. Western blot analysis confirms the presence in rat brain astrocytes of 85 kDa cPLA(2) and 40 kDa protein reactive to iPLA(2) antibodies. 5. The influence of cAMP on regulation of PUFA release was investigated. Release of DHA is strongly amplified by the adenylyl cyclase activator forskolin (10 micro M), and by the protein kinase A (PKA) activator dibutyryl-cAMP (1 mM). In contrast, release of AA is not affected by forskolin or dibutyryl-cAMP, but is almost completely blocked by 2,3-dideoxyadenosine (20 micro M) and inhibited by 34% by H89 (10 micro M), inhibitors of adenylyl cyclase and PKA, respectively. 6. Other neuromediators, such as bradykinin, glutamate and thrombin, stimulate release of DHA and AA, which is comparable to the release stimulated by ATP. 7. Different sensitivities of iPLA(2) and cPLA(2) to Ca(2+) and cAMP reveal new pathways for the regulation of fatty acid release and reflect the significance of astrocytes in control of DHA and AA metabolism under normal and pathological conditions in brain.