Follicle-stimulating hormone regulation of A-kinase anchoring proteins in granulosa cells

Structural basis of binding the unique N-terminal domain of microtubule-associated protein 2c to proteins regulating kinases of signaling pathways

Isoforms of microtubule-associated protein 2 (MAP2) differ from their homolog Tau in the sequence and interactions of the N-terminal region. Binding of the N-terminal region of MAP2c (N-MAP2c) to the dimerization/docking domains of the regulatory subunit RIIα of cAMP-dependent protein kinase (RIIDD2) and to the Src-homology domain 2 (SH2) of growth factor receptor-bound protein 2 (Grb2) have been described long time ago. However, the structural features of the complexes remained unknown due to the disordered nature of MAP2. Here, we provide structural description of the complexes. We have solved solution structure of N-MAP2c in complex with RIIDD2, confirming formation of an amphiphilic α-helix of MAP2c upon binding, defining orientation of the α-helix in the complex and showing that its binding register differs from previous predictions. Using chemical shift mapping, we characterized the binding interface of SH2-Grb2 and rat MAP2c phosphorylated by the tyrosine kinase Fyn in their complex and proposed a model explaining differences between SH2-Grb2 complexes with rat MAP2c and phosphopeptides with a Grb2-specific sequence. The results provide the structural basis of a potential role of MAP2 in regulating cAMP-dependent phosphorylation cascade via interactions with RIIDD2 and Ras signaling pathway via interactions with SH2-Grb2.

AKAP13 Enhances CREB1 Activation by FSH in Granulosa Cells

Granulosa cells (GCs) must respond appropriately to follicle-stimulating hormone (FSH) for proper follicle maturation. FSH activates protein kinase A (PKA) leading to phosphorylation of the cyclic AMP response element binding protein-1 (CREB1). We identified a unique A-kinase anchoring protein (AKAP13) containing a Rho guanine nucleotide exchange factor (RhoGEF) region that was induced in GCs during folliculogenesis. AKAPs are known to coordinate signaling cascades, and we sought to evaluate the role of AKAP13 in GCs in response to FSH. Aromatase reporter activity was increased in COV434 human GCs overexpressing AKAP13. Addition of FSH, or the PKA activator forskolin, significantly enhanced this activity by 1.5- to 2.5-fold, respectively (p < 0.001). Treatment with the PKA inhibitor H89 significantly reduced AKAP13-dependent activation of an aromatase reporter (p = 0.0067). AKAP13 physically interacted with CREB1 in co-immunoprecipitation experiments and increased the phosphorylation of CREB1. CREB1 phosphorylation increased after FSH treatment in a time-specific manner, and this effect was reduced by siRNA directed against AKAP13 (p = 0.05). CREB1 activation increased by 18.5-fold with co-expression of AKAP13 in the presence of FSH (p < 0.001). Aromatase reporter activity was reduced by inhibitors of the RhoGEF region, C3 transferase and A13, and greatly enhanced by the RhoGEF activator, A02. In primary murine and COV43 GCs, siRNA knockdown of Akap13/AKAP13 decreased aromatase and luteinizing hormone receptor transcripts in cells treated with FSH, compared with controls. Collectively, these findings suggest that AKAP13 may function as a scaffolding protein in FSH signal transduction via an interaction with CREB, resulting in phosphorylation of CREB.

cAMP-Dependent Signaling and Ovarian Cancer

cAMP-dependent pathway is one of the most significant signaling cascades in healthy and neoplastic ovarian cells. Working through its major effector proteins-PKA and EPAC-it regulates gene expression and many cellular functions. PKA promotes the phosphorylation of cAMP response element-binding protein (CREB) which mediates gene transcription, cell migration, mitochondrial homeostasis, cell proliferation, and death. EPAC, on the other hand, is involved in cell adhesion, binding, differentiation, and interaction between cell junctions. Ovarian cancer growth and metabolism largely depend on changes in the signal processing of the cAMP-PKA-CREB axis, often associated with neoplastic transformation, metastasis, proliferation, and inhibition of apoptosis. In addition, the intracellular level of cAMP also determines the course of other pathways including AKT, ERK, MAPK, and mTOR, that are hypo- or hyperactivated among patients with ovarian neoplasm. With this review, we summarize the current findings on cAMP signaling in the ovary and its association with carcinogenesis, multiplication, metastasis, and survival of cancer cells. Additionally, we indicate that targeting particular stages of cAMP-dependent processes might provide promising therapeutic opportunities for the effective management of patients with ovarian cancer.

Molecular Signatures Correlated With Poor IVF Outcomes: Insights From the mRNA and lncRNA Expression of Endometriotic Granulosa Cells

The outcomes of in vitro fertilization (IVF) for endometriotic women are significantly worse than for patients without ovarian endometriosis (OEM), as shown by fewer retrieved oocytes. However, the exact pathophysiological mechanism is still unknown. Thus, we conducted a prospective study that analyzed mRNA and lncRNA transcriptome between granulosa cells (GCs) from patients with fewer retrieved oocytes due to OEM and GCs from controls with male factor (MF) infertility using an RNA sequencing approach. We found a group of significantly differentially expressed genes (DEGs), including NR5A2, MAP3K5, PGRMC2, PRKAR2A, DEPTOR, ITGAV, KPNB1, GPC6, EIF3A, and SMC5, which were validated to be upregulated and negatively correlated with retrieved oocyte numbers in GCs of patients with OEM, while DUSP1 demonstrated the opposite. The molecular functions of these DEGs were mainly enriched in pathways involving mitogen-activated protein kinase (MAPK) signaling, Wnt signaling, steroid hormone response, apoptosis, and cell junction. Furthermore, we performed lncRNA analysis and identified a group of differentially expressed known/novel lncRNAs that were co-expressed with the validated DEGs and correlated with retrieved oocyte numbers. Co-expression networks were constructed between the DEGs and known/novel lncRNAs. These distinctive molecular signatures uncovered in this study are involved in the pathological regulation of ovarian reserve dysfunction in OEM patients.

Molecular Mechanisms of Action of FSH

The glycoprotein follicle-stimulating hormone (FSH) acts on gonadal target cells, hence regulating gametogenesis. The transduction of the hormone-induced signal is mediated by the FSH-specific G protein-coupled receptor (FSHR), of which the action relies on the interaction with a number of intracellular effectors. The stimulatory Gαs protein is a long-time known transducer of FSH signaling, mainly leading to intracellular cAMP increase and protein kinase A (PKA) activation, the latter acting as a master regulator of cell metabolism and sex steroid production. While in vivo data clearly demonstrate the relevance of PKA activation in mediating gametogenesis by triggering proliferative signals, some in vitro data suggest that pro-apoptotic pathways may be awakened as a "dark side" of cAMP/PKA-dependent steroidogenesis, in certain conditions. P38 mitogen-activated protein kinases (MAPK) are players of death signals in steroidogenic cells, involving downstream p53 and caspases. Although it could be hypothesized that pro-apoptotic signals, if relevant, may be required for regulating atresia of non-dominant ovarian follicles, they should be transient and counterbalanced by mitogenic signals upon FSHR interaction with opposing transducers, such as Gαi proteins and β-arrestins. These molecules modulate the steroidogenic pathway via extracellular-regulated kinases (ERK1/2), phosphatidylinositol-4,5-bisphosphate 3-kinases (PI3K)/protein kinase B (AKT), calcium signaling and other intracellular signaling effectors, resulting in a complex and dynamic signaling network characterizing sex- and stage-specific gamete maturation. Even if the FSH-mediated signaling network is not yet entirely deciphered, its full comprehension is of high physiological and clinical relevance due to the crucial role covered by the hormone in regulating human development and reproduction.

Follicle-Stimulating Hormone (FSH)-dependent Regulation of Extracellular Regulated Kinase (ERK) Phosphorylation by the Mitogen-activated Protein (MAP) Kinase Phosphatase MKP3

Within the ovarian follicle, granulosa cells (GCs) surround and support immature oocytes. FSH promotes the differentiation and proliferation of GCs and is essential for fertility. We recently reported that ERK activation is necessary for FSH to induce key genes that define the preovulatory GC. This research focused on the phosphoregulation by FSH of ERK within GCs. FSH-stimulated ERK phosphorylation on Thr(202)/Tyr(204) was PKA-dependent, but MEK(Ser(217)/Ser(221)) phosphorylation was not regulated; rather, MEK was already active. However, treatment of GCs with the EGF receptor inhibitor AG1478, a dominant-negative RAS, an Src homology 2 domain-containing Tyr phosphatase inhibitor (NSC 87877), or the MEK inhibitor PD98059 blocked FSH-dependent ERK(Thr(202)/Tyr(204)) phosphorylation, demonstrating the requirement for upstream pathway components. We hypothesized that FSH via PKA enhances ERK phosphorylation by inhibiting the activity of a protein phosphatase that constitutively dephosphorylates ERK in the absence of FSH, allowing MEK-phosphorylated ERK to accumulate in the presence of FSH because of inactivation of the phosphatase. GCs treated with different phosphatase inhibitors permitted elimination of both Ser/Thr and Tyr phosphatases and implicated dual specificity phosphatases (DUSPs) in the dephosphorylation of ERK. Treatment with MAP kinase phosphatase (MKP3, DUSP6) inhibitors increased ERK(Thr(202)/Tyr(204)) phosphorylation in the absence of FSH to levels comparable with ERK phosphorylated in the presence of FSH. ERK co-immunoprecipitated with Myc-FLAG-tagged MKP3(DUSP6). GCs treated with MKP3(DUSP6) inhibitors blocked and PKA inhibitors enhanced dephosphorylation of recombinant ERK2-GST in an in vitro phosphatase assay. Together, these results suggest that FSH-stimulated ERK activation in GCs requires the PKA-dependent inactivation of MKP3(DUSP6).

Dephosphorylation of MAP2D enhances its binding to vimentin in preovulatory ovarian granulosa cells

Preovulatory granulosa cells express the low-molecular-mass MAP2D variant of microtubule-associated protein 2 (MAP2). Activation of the luteinizing hormone choriogonadotropin receptor by human choriogonadotropin (hCG) promotes dephosphorylation of MAP2D on Thr256 and Thr259. We sought to evaluate the association of MAP2D with the cytoskeleton, and the effect of hCG on this association. MAP2D partially colocalized, as assessed by confocal immunofluorescence microscopy, with the vimentin intermediate filament and microtubule cytoskeletons in naive cells. In vitro binding studies showed that MAP2D bound directly to vimentin and β-tubulin. Phosphorylation of recombinant MAP2D on Thr256 and Thr259, which mimics the phosphorylation status of MAP2D in naive cells, reduces binding of MAP2D to vimentin and tubulin by two- and three-fold, respectively. PKA-dependent phosphorylation of vimentin (Ser32 and Ser38) promoted binding of vimentin to MAP2D and increased contraction of granulosa cells with reorganization of vimentin filaments and MAP2D from the periphery into a thickened layer surrounding the nucleus and into prominent cellular extensions. Chemical disruption of vimentin filament organization increased progesterone production. Taken together, these results suggest that hCG-stimulated dephosphorylation of MAP2D at Thr256 and Thr259, phosphorylation of vimentin at Ser38 and Ser72, and the resulting enhanced binding of MAP2D to vimentin might contribute to the progesterone synthetic response required for ovulation.

Extracellular Signal-regulated Kinase (ERK)-dependent Phosphorylation of Y-Box-binding Protein 1 (YB-1) Enhances Gene Expression in Granulosa Cells in Response to Follicle-stimulating Hormone (FSH)

Within the ovarian follicle, immature oocytes are surrounded and supported by granulosa cells (GCs). Stimulation of GCs by FSH leads to their proliferation and differentiation, events that are necessary for fertility. FSH activates multiple signaling pathways to regulate genes necessary for follicular maturation. Herein, we investigated the role of Y-box-binding protein-1 (YB-1) within GCs. YB-1 is a nucleic acid binding protein that regulates transcription and translation. Our results show that FSH promotes an increase in the phosphorylation of YB-1 on Ser(102) within 15 min that is maintained at significantly increased levels until ∼8 h post treatment. FSH-stimulated phosphorylation of YB-1(Ser(102)) is prevented by pretreatment of GCs with the PKA-selective inhibitor PKA inhibitor (PKI), the MEK inhibitor PD98059, or the ribosomal S6 kinase-2 (RSK-2) inhibitor BI-D1870. Thus, phosphorylation of YB-1 on Ser(102) is PKA-, ERK-, and RSK-2-dependent. However, pretreatment of GCs with the protein phosphatase 1 (PP1) inhibitor tautomycin increased phosphorylation of YB-1(Ser(102)) in the absence of FSH; FSH did not further increase YB-1(Ser(102)) phosphorylation. This result suggests that the major effect of RSK-2 is to inhibit PP1 rather than to directly phosphorylate YB-1 on Ser(102) YB-1 coimmunoprecipitated with PP1β catalytic subunit and RSK-2. Transduction of GCs with the dephospho-adenoviral-YB-1(S102A) mutant prevented the induction by FSH of Egfr, Cyp19a1, Inha, Lhcgr, Cyp11a1, Hsd17b1, and Pappa mRNAs and estradiol-17β production. Collectively, our results reveal that phosphorylation of YB-1 on Ser(102) via the ERK/RSK-2 signaling pathway is necessary for FSH-mediated expression of target genes required for maturation of follicles to a preovulatory phenotype.

Hyaluronic acid prevents immunosuppressive drug-induced ovarian damage via up-regulating PGRMC1 expression

Chemotherapy treatment in women can frequently cause damage to the ovaries, which may lead to primary ovarian insufficiency (POI). In this study, we assessed the preventative effects of hyaluronic acid (HA) in immunosuppressive drug-induced POI-like rat models and investigated the possible mechanisms. We found that HA, which was reduced in primary and immunosuppressant-induced POI patients, could protect the immunosuppressant-induced damage to granulosa cells (GCs) in vitro. Then we found that HA blocked the tripterygium glycosides (TG) induced POI-like presentations in rats, including delayed or irregular estrous cycles, reduced 17 beta-estradiol(E2) concentration, decreased number of follicles, destruction of follicle structure, and damage of reproductive ability. Furthermore, we investigated the mechanisms of HA prevention effects on POI, which was associated with promotion of GC proliferation and PGRMC1 expression. In conclusion, HA prevents chemotherapy-induced ovarian damage by promoting PGRMC1 in GCs. This study may provide a new strategy for prevention and treatment of POI.

Hyaluronic acid promotes the expression of progesterone receptor membrane component 1 via epigenetic silencing of miR-139-5p in human and rat granulosa cells

Primary ovarian insufficiency (POI) is a serious reproductive dysfunction in which the follicle pool is reduced and depleted. Abnormal apoptosis of ovarian granulosa cells (GCs) is believed to result in follicle loss. Progesterone receptor membrane component 1 (PGRMC1), which is critical for GC survival, was reported to be reduced in POI patients, but the mechanism is unknown. In the present study, we found that PGRMC1 expression was correlated with the level of hyaluronic acid (HA) in POI patients. HA up-regulated PGRMC1 expression in GCs via suppression of miR-139-5p, which was proven by Western blotting and luciferase reporter assays to target PGRMC1. Consistent with these findings, levels of miR-139-5p were significantly increased and presented an inverse correlation with PGRMC1 in POI patients. Noticeably, HA inhibited CD44-mediated miR-139-5p expression but had no effect on luciferase activity after insertion of miR-139 promoter into luciferase plasmid. Interestingly, miR-139-5p was significantly up-regulated in KGN cells (GC tumor cell line) by the histone deacetylase inhibitor trichostatin A, indicating that HA down-regulated miR-139-5p expression via histone deacetylation. Taken together, we report an unrecognized mechanism of HA in the promotion of PGRMC1 expression, suggesting that HA may be a potential molecule for the prevention and treatment of POI.

PKA and GAB2 play central roles in the FSH signaling pathway to PI3K and AKT in ovarian granulosa cells

Controlled maturation of ovarian follicles is necessary for fertility. Follicles are restrained at an immature stage until stimulated by FSH secreted by pituitary gonadotropes. FSH acts on granulosa cells within the immature follicle to inhibit apoptosis, promote proliferation, stimulate production of steroid and protein hormones, and induce ligand receptors and signaling intermediates. The phosphoinositide 3-kinase (PI3K)/AKT (protein kinase B) pathway is a pivotal signaling corridor necessary for transducing the FSH signal. We report that protein kinase A (PKA) mediates the actions of FSH by signaling through multiple targets to activate PI3K/AKT. PKA uses a route that promotes phosphorylation of insulin receptor substrate-1 (IRS-1) on Tyr(989), a canonical binding site for the 85-kDa regulatory subunit of PI3K that allosterically activates the catalytic subunit. PI3K activation leads to activation of AKT through phosphorylation of AKT on Thr(308) and Ser(473). The adaptor growth factor receptor bound protein 2-associated binding protein 2 (GAB2) is present in a preformed complex with PI3K heterodimer and IRS-1, it is an A-kinase anchoring protein that binds the type I regulatory subunit of PKA, and it is phosphorylated by PKA on Ser(159). Overexpression of GAB2 enhances FSH-stimulated AKT phosphorylation. GAB2, thus, seems to coordinate signals from the FSH-stimulated rise in cAMP that leads to activation of PI3K/AKT. The ability of PKA to commandeer IRS-1 and GAB2, adaptors that normally integrate receptor/nonreceptor tyrosine kinase signaling into PI3K/AKT, reveals a previously unrecognized route for PKA to activate a pathway that promotes proliferation, inhibits apoptosis, enhances translation, and initiates differentiation of granulosa cells.

Functional Anchoring of the cAMP-Dependent Protein Kinase

The precision of cAMP-responsive events is controlled in part through compartmentalization of the signal transduction machinery. Recent evidence suggests that the cAMP-dependent protein kinase (PKA) is localized to specific subcellular compartments through association with A Kinase Anchoring Proteins (AKAPs). The AKAPs now represent a functionally related family of regulatory proteins that contain a conserved PKA binding domain and unique targeting sequences that direct the PKA-AKAP complex to subcellular structures. In this review, the recent evidence suggesting that AKAPs facilitate PKA anchoring close to key membrane substrates, such as glutamate receptors, calcium-activated potassium channels, and skeletal or cardiac muscle calcium channels, is surveyed.

Bigger, better, faster: principles and models of AKAP anchoring protein signaling

A kinase anchoring proteins (AKAPs) bind multiple signaling proteins and have subcellular targeting domains that allow them to greatly impact cellular signaling. AKAPs localize, specify, amplify, and accelerate signal transduction within the cell by bringing signaling proteins together in space and time. AKAPs also organize higher-order network motifs such as feed forward and feedback loops that may create complex network responses, including adaptation, oscillation, and ultrasensitivity. Computational models have begun to provide an insight into how AKAPs regulate signaling dynamics and cardiovascular pathophysiology. Models of mitogen-activated protein kinase and epidermal growth factor receptor scaffolds have revealed additional design principles and new methods for representing signaling scaffolds mathematically. Coupling computational modeling with quantitative experimental approaches will be increasingly necessary for dissecting the diverse information processing functions performed by AKAP signaling complexes.

Estrogen-dependent activation of neutral cholesterol ester hydrolase underlying gender difference of atherogenesis in apoE-/- mice

OBJECTIVE

Mechanisms underlying gender difference of atherogenesis were investigated focusing on direct effects of estrogen on the artery.

METHODS

First, male and female apoE(-/-) mice were fed an atherogenic diet for 16 weeks from 10 weeks of age. Second, female apoE(-/-) mice were ovariectomized (ovx) or sham operated at 8 weeks of age, and 2-weeks afterwards, one-third of each ovx-group received conjugated equine estrogens (CEE) (0, 2.5 or 5.0 μg/day) for 16 weeks. Atherosclerotic lesions were examined after experimental periods. To clarify anti-atherogenic effect of 17β-estradiol (E2) on artery, neutral cholesteryl ester hydrolase (N-CEase) activity in aorta and peritoneal macrophages, and E2-treated J774A.1 cells were measured.

RESULTS

First, atherosclerotic lesion in female mice was significantly less than male mice without any changes in serum lipids and lipoprotein profile. N-CEase activity in aorta and peritoneal macrophages in female mice was significantly higher than male mice. Second, atherosclerotic lesion in ovx-group was significantly greater than sham-group. CEE-replacement to ovx-group decreased atherosclerotic lesion in a dose-dependent manner. N-CEase activity in aorta and peritoneal macrophages was decreased in ovx-group compared to sham-group, and restored by CEE-replacement in macrophages. To study detailed mechanisms, J774A.1 cells were treated with E2. E2 significantly increased N-CEase activity, and cAMP-dependent protein kinase (A-kinase) type II activity and the protein in cytosol fraction without any changes of total protein of A-kinase type II.

CONCLUSION

These results suggest that gender difference of atherogenesis is partly accounted for activation of N-CEase through estrogen-dependent translocation of A-kinase type II in macrophages.

Luteinizing hormone receptor activation in ovarian granulosa cells promotes protein kinase A-dependent dephosphorylation of microtubule-associated protein 2D

The actions of LH to induce ovulation and luteinization of preovulatory follicles are mediated principally by activation of cAMP-dependent protein kinase (PKA) in granulosa cells. PKA activity is targeted to specific locations in many cells by A kinase-anchoring proteins (AKAPs). We previously showed that FSH induces expression of microtubule-associated protein (MAP) 2D, an 80-kDa AKAP, in rat granulosa cells, and that MAP2D coimmunoprecipitates with PKA-regulatory subunits in these cells. Here we report a rapid and targeted dephosphorylation of MAP2D at Thr256/Thr259 after treatment with human chorionic gonadotropin, an LH receptor agonist. This event is mimicked by treatment with forskolin or a cAMP analog and is blocked by the PKA inhibitor myristoylated-PKI, indicating a role for cAMP and PKA signaling in phosphoregulation of granulosa cell MAP2D. Furthermore, we show that Thr256/Thr259 dephosphorylation is blocked by the protein phosphatase 2A (PP2A) inhibitor, okadaic acid, and demonstrate interactions between MAP2D and PP2A by coimmunoprecipitation and microcystin-agarose pull-down. We also show that MAP2D interacts with glycogen synthase kinase (GSK) 3beta and is phosphorylated at Thr256/Thr259 by this kinase in the basal state. Increased phosphorylation of GSK3beta at Ser9 and the PP2A B56delta subunit at Ser566 is observed after treatment with human chorionic gonadotropin and appears to result in LH receptor-mediated inhibition of GSK3beta and activation of PP2A, respectively. Taken together, these results show that the phosphorylation status of the AKAP MAP2D is acutely regulated by LH receptor-mediated modulation of kinase and phosphatase activities via PKA.

Disruption of the RIIbeta subunit of PKA reverses the obesity syndrome of Agouti lethal yellow mice

Agouti lethal yellow (A(y)) mice express agouti ectopically because of a genetic rearrangement at the agouti locus. The agouti peptide is a potent antagonist of the melanocortin 4 receptor (MC4R) expressed in neurons, and this leads to hyperphagia, hypoactivity, and increased fat mass. The MC4R signals through Gs and is thought to stimulate the production of cAMP and activation of downstream cAMP effector molecules such as PKA. Disruption of the RIIbeta regulatory subunit gene of PKA results in release of the active catalytic subunit and an increase in basal PKA activity in cells where RIIbeta is highly expressed. Because RIIbeta is expressed in neurons including those in the hypothalamic nuclei where MC4R is prominent we tested the possibility that the RIIbeta knockout might rescue the body weight phenotypes of the A(y) mice. Disruption of the RIIbeta PKA regulatory subunit gene in mice leads to a 50% reduction in white adipose tissue and resistance to diet-induced obesity and hyperglycemia. The RIIbeta mutation rescued the elevated body weight, hyperphagia, and obesity of A(y) mice. Partial rescue of the A(y) phenotypes was even observed on an RIIbeta heterozygote background. These results suggest that the RIIbeta gene mutation alters adiposity and locomotor activity by modifying PKA signaling pathways downstream of the agouti antagonism of MC4R in the hypothalamus.

Mitochondrial A-kinase anchoring protein 121 binds type II protein kinase A and enhances steroidogenic acute regulatory protein-mediated steroidogenesis in MA-10 mouse leydig tumor cells

The expression of the steroidogenic acute regulatory protein (STAR) is regulated by PKA in response to trophic hormone stimulation through the second messenger cAMP. However, in steroidogenic cells, the concentrations of hormone necessary to maximally induce cAMP synthesis and PKA activity are often significantly higher than is necessary to achieve maximum steroidogenesis. One general mechanism believed to make PKA signaling more effective is the use of A-kinase anchoring proteins (AKAPs) to recruit PKA to discrete subcellular compartments, which coordinates and focuses PKA action with respect to its substrates. The characterization of AKAP121 has suggested that it enhances the posttranscriptional regulation of STAR by recruiting both Star mRNA and PKA to the mitochondria, thereby permitting more effective translation and phosphorylation of STAR. Testing this hypothesis revealed that cAMP-induced STAR expression and steroidogenesis closely followed AKAP121 abundance when this AKAP was silenced or overexpressed in MA-10 cells but that these changes were effected posttranscriptionally. Moreover, silencing AKAP121 expression in these cells specifically altered the localization of type II PKA regulatory subunit alpha (PKAR2A) at the mitochondria but did not affect its relative expression within the cell. Affinity purification experiments showed that PKAR2A preferentially associated with AKAP121, and cAMP analogs that activate type II PKA induced STAR phosphorylation more efficiently than analogs stimulating type I PKA. This suggests that AKAP121 and PKAR2A serve to enhance steroidogenesis by directing the synthesis and activation of STAR at the mitochondria in response to cAMP.

Insulin disrupts beta-adrenergic signalling to protein kinase A in adipocytes

Hormones mobilize intracellular second messengers and initiate signalling cascades involving protein kinases and phosphatases, which are often spatially compartmentalized by anchoring proteins to increase signalling specificity. These scaffold proteins may themselves be modulated by hormones. In adipocytes, stimulation of beta-adrenergic receptors increases cyclic AMP levels and activates protein kinase A (PKA), which stimulates lipolysis by phosphorylating hormone-sensitive lipase and perilipin. Acute insulin treatment activates phosphodiesterase 3B, reduces cAMP levels and quenches beta-adrenergic receptor signalling. In contrast, chronic hyperinsulinaemic conditions (typical of type 2 diabetes) enhance beta-adrenergic receptor-mediated cAMP production. This amplification of cAMP signalling is paradoxical because it should enhance lipolysis, the opposite of the known short-term effect of hyperinsulinaemia. Here we show that in adipocytes, chronically high insulin levels inhibit beta-adrenergic receptors (but not other cAMP-elevating stimuli) from activating PKA. We measured this using an improved fluorescent reporter and by phosphorylation of endogenous cAMP-response-element binding protein (CREB). Disruption of PKA scaffolding mimics the interference of insulin with beta-adrenergic receptor signalling. Chronically high insulin levels may disrupt the close apposition of beta-adrenergic receptors and PKA, identifying a new mechanism for crosstalk between heterologous signal transduction pathways.

Peptide inhibitors of protein kinases-discovery, characterisation and use

Protein kinases are now the second largest group of drug targets, and most protein kinase inhibitors in clinical development are directed towards the ATP-binding site. However, these inhibitors must compete with high intracellular ATP concentrations and they must discriminate between the ATP-binding sites of all protein kinases as well the other proteins that also utilise ATP. It would therefore be beneficial to target sites on protein kinases other than the ATP-binding site. This review describes the discovery, characterisation and use of peptide inhibitors of protein kinases. In many cases, the development of these peptides has resulted from an understanding of the specific protein-binding partners for a particular protein kinase. In addition, novel peptide sequences have been discovered in library screening approaches and have provided new leads in the discovery and/or design of peptide inhibitors of protein kinases. These approaches are therefore providing exciting new opportunities in the development of ATP non-competitive inhibitors of protein kinases.

Induction of cyclin D2 in rat granulosa cells requires FSH-dependent relief from FOXO1 repression coupled with positive signals from Smad

Ovarian follicles undergo exponential growth in response to follicle-stimulating hormone (FSH), largely as a result of the proliferation of granulosa cells (GCs). In vitro under serum-free conditions, rat GCs differentiate in response to FSH but do not proliferate unless activin is also present. In the presence of FSH plus activin, GCs exhibit enhanced expression of cyclin D2 as well as inhibin-alpha, aromatase, steroidogenic factor-1 (SF-1), cholesterol side chain (SCC), and epiregulin. In this report we sought to identify the signaling pathways by which FSH and activin promote GC proliferation and differentiation. Our results show that these responses are associated with prolonged Akt phosphorylation relative to time-matched controls and are dependent on phosphatidylinositol 3-kinase (PI 3-kinase) and Smad2/3 signaling, based on the ability of the PI 3-kinase inhibitor LY294002 or infection with adenoviral dominant negative Smad3 (DN-Smad3) mutant to attenuate induction of cyclin D2, inhibin-alpha, aromatase, SCC, SF-1, and epiregulin. The DN-Smad3 mutant also abolished prolonged Akt phosphorylation stimulated by FSH plus activin 24 h post-treatment. Infection with the adenoviral constitutively active forkhead box-containing protein, O subfamily (FOXO)1 mutant suppressed induction of cyclin D2, aromatase, inhibin-alpha, SF-1, and epiregulin. Transient transfections of GCs with constitutively active FOXO1 mutant also suppressed cyclin D2, inhibin-alpha, and epiregulin promoter-reporter activities. Chromatin immunoprecipitation results demonstrate in vivo the association of FOXO1 with the cyclin D2 promoter in untreated GCs and release of FOXO1 from the cyclin D2 promoter upon addition of FSH plus activin. These results suggest that proliferation and differentiation of GCs in response to FSH plus activin requires both removal of FOXO1-dependent repression and positive signaling from Smad2/3.

Inhibition of extracellular signal-regulated protein kinase-2 phosphorylation by dihydrotestosterone reduces follicle-stimulating hormone-mediated cyclin D2 messenger ribonucleic acid expression in rat granulosa cells

Granulosa cell mitogenesis is critical for the development of normal ovarian follicles. FSH and other mitogenic stimuli play a crucial role in this process. We have shown that exposing granulosa cells to 5alpha-dihydrotestosterone (DHT) reduces forskolin-stimulated cyclin D2 mRNA expression, which leads to cell cycle arrest resulting in reduced cell proliferation. The present study investigated the signaling molecules upstream of cyclin D2 in FSH-mediated, cAMP-dependent signaling pathway that may be negatively affected by DHT, leading to inhibition of cell cycle progression. Because ERK is an important molecule in mitogenic signaling, the possible effect of DHT on its phosphorylation was examined. Granulosa cells from 3-d estradiol-primed immature rats were treated with DHT (90 ng/ml) for 24 h and subsequently stimulated with forskolin. DHT treatment reduced forskolin stimulation of ERK phosphorylation. Although DHT exposure did not affect cellular cAMP production in response to forskolin, treating the cells with DHT for 24 h significantly reduced protein kinase A activity. DHT also caused a reduction in ERK-2 phosphorylation in response to FSH similar to that seen with forskolin. Furthermore, blocking ERK phosphorylation as well as DHT treatment resulted in a reduction in FSH-stimulated cyclin D2 mRNA expression. From these results, we conclude that DHT treatment reduces the FSH-mediated ERK phosphorylation in granulosa cells, leading to reduced cyclin D2 mRNA expression that culminates in cell cycle arrest.

Neuronal microtubule-associated protein 2D is a dual a-kinase anchoring protein expressed in rat ovarian granulosa cells

A-kinase anchoring proteins (AKAPs) function to target protein kinase A (PKA) to specific locations within the cell. AKAPs are functionally identified by their ability to bind the type II regulatory subunits (RII) of PKA in an in vitro overlay assay. We previously showed that follicle-stimulating hormone (FSH) induces the expression of an 80-kDa AKAP (AKAP 80) in ovarian granulosa cells as they mature from a preantral to a preovulatory phenotype. In this report, we identify AKAP 80 as microtubule-associated protein 2D (MAP2D), a low molecular weight splice variant of the neuronal MAP2 protein. MAP2D is induced in granulosa cells by dexamethasone and by FSH in a time-dependent manner that mimics that of AKAP 80, and immunoprecipitation of MAP2D depletes extracts of AKAP 80. MAP2D is the only MAP2 protein present in ovaries and is localized to granulosa cells of preovulatory follicles and to luteal cells. MAP2D is concentrated at the Golgi apparatus along with RI and RII and, based on coimmunoprecipitation results, appears to bind both RI and RII in granulosa cells. Reduced expression of MAP2D resulting from treatment of granulosa cells with antisense oligonucleotides to MAP2 inhibited the phosphorylation of cAMP-response element-binding protein. These results suggest that this classic neuronal RII AKAP is a dual RI/RII AKAP that performs unique functions in ovarian granulosa cells that contribute to the preovulatory phenotype.

Localized effects of cAMP mediated by distinct routes of protein kinase A

More than 20% of the human genome encodes proteins involved in transmembrane and intracellular signaling pathways. The cAMP-protein kinase A (PKA) pathway is one of the most common and versatile signal pathways in eukaryotic cells and is involved in regulation of cellular functions in almost all tissues in mammals. Various extracellular signals converge on this signal pathway through ligand binding to G protein-coupled receptors, and the cAMP-PKA pathway is therefore tightly regulated at several levels to maintain specificity in the multitude of signal inputs. Ligand-induced changes in cAMP concentration vary in duration, amplitude, and extension into the cell, and cAMP microdomains are shaped by adenylyl cyclases that form cAMP as well as phosphodiesterases that degrade cAMP. Different PKA isozymes with distinct biochemical properties and cell-specific expression contribute to cell and organ specificity. A kinase anchoring proteins (AKAPs) target PKA to specific substrates and distinct subcellular compartments providing spatial and temporal specificity for mediation of biological effects channeled through the cAMP-PKA pathway. AKAPs also serve as scaffolding proteins that assemble PKA together with signal terminators such as phosphatases and cAMP-specific phosphodiesterases as well as components of other signaling pathways into multiprotein signaling complexes that serve as crossroads for different paths of cell signaling. Targeting of PKA and integration of a wide repertoire of proteins involved in signal transduction into complex signal networks further increase the specificity required for the precise regulation of numerous cellular and physiological processes.

Bioinformatic design of A-kinase anchoring protein-in silico: a potent and selective peptide antagonist of type II protein kinase A anchoring

Compartmentalization of the cAMP-dependent protein kinase (PKA) is coordinated through association with A-kinase anchoring proteins (AKAPs). A defining characteristic of most AKAPs is a 14- to 18-aa sequence that binds to the regulatory subunits (RI or RII) of the kinase. Cellular delivery of peptides to these regions disrupts PKA anchoring and has been used to delineate a physiological role for AKAPs in the facilitation of certain cAMP-responsive events. Here, we describe a bioinformatic approach that yields an RII-selective peptide, called AKAP-in silico (AKAP-IS), that binds RII with a K(d) of 0.4 nM and binds RI with a K(d) of 277 nM. AKAP-IS associates with the type II PKA holoenzyme inside cells and displaces the kinase from natural anchoring sites. Electrophysiological recordings indicate that perfusion of AKAP-IS evokes a more rapid and complete attenuation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor currents than previously described anchoring inhibitor peptides. Thus, computer-based and peptide array screening approaches have generated a reagent that binds PKA with higher affinity than previously described AKAPs.

Follicle-stimulating hormone activates extracellular signal-regulated kinase but not extracellular signal-regulated kinase kinase through a 100-kDa phosphotyrosine phosphatase

In this report we sought to elucidate the mechanism by which the follicle-stimulating hormone (FSH) receptor signals to promote activation of the p42/p44 extracellular signal-regulated protein kinases (ERKs) in granulosa cells. Results show that the ERK kinase MEK and upstream intermediates Raf-1, Ras, Src, and L-type Ca(2+) channels are already partially activated in vehicle-treated cells and that FSH does not further activate them. This tonic stimulatory pathway appears to be restrained at the level of ERK by a 100-kDa phosphotyrosine phosphatase that associates with ERK in vehicle-treated cells and promotes dephosphorylation of its regulatory Tyr residue, resulting in ERK inactivation. FSH promotes the phosphorylation of this phosphotyrosine phosphatase and its dissociation from ERK, relieving ERK from inhibition and resulting in its activation by the tonic stimulatory pathway and consequent translocation to the nucleus. Consistent with this premise, FSH-stimulated ERK activation is inhibited by the cell-permeable protein kinase A-specific inhibitor peptide Myr-PKI as well as by inhibitors of MEK, Src, a Ca(2+) channel blocker, and chelation of extracellular Ca(2+). These results suggest that FSH stimulates ERK activity in immature granulosa cells by relieving an inhibition imposed by a 100-kDa phosphotyrosine phosphatase.

Regulation of granulosa cell-derived insulin-like growth factor binding proteins (IGFBPs): role for protein kinase-C in the pre- and posttranslational modulation of IGFBP-4 and IGFBP-5

A growing body of information suggests antigonadotropic and atretogenic roles for granulosa cell-derived insulin-like growth factor binding proteins (IGFBPs) 4 and 5 during ovarian folliculogenesis. Activation of protein kinase-A (PKA) in rat granulosa cells has been shown to modulate the relative expression of IGFBP-4 and -5 transcripts and proteins. In this article, we assess the role of protein kinase-C (PKC) in this regard. Provision of granulosa cells with phorbol 12-myristate 13-acetate (PMA) (but not 4alphaPMA, an inert analogue), a tumor-promoting phorbol ester and an established activator of PKC, was without significant effect on the expression of IGFBP-4 transcripts but resulted in biphasic dose-dependent alterations in IGFBP-5 transcripts and in the accumulation of the IGFBP-4 and -5 proteins. Comparable effects were noted for GnRH, an established PKC agonist. Provision of staurosporine, a potent inhibitor of the catalytic subunit of PKC, produced significant dose-dependent decrements in the relative expression of IGFBP-5 transcripts. Treatment with FSH (presumptively PKA-mediated) markedly attenuated the ability of PMA or GnRH to upregulate the accumulation of the IGFBP-5 (but not IGFBP-4) protein. Taken together, our present findings indicate that the modulation of rat ovarian IGFBP-4 and -5 is PKC as well as PKA dependent and that these two signaling pathways interact in a diametrically opposed and antagonistic fashion.

A-kinase anchor proteins as potential regulators of protein kinase A function in oocytes

In the mammalian oocyte, the cAMP-dependent protein kinase (PKA) has critical functions in the maintenance of meiotic arrest and oocyte maturation. Because PKA is spatially regulated, its localization was examined in developing oocytes. Both regulatory subunits (RI and RII) and the catalytic subunit (C) of PKA were found in oocytes and metaphase II-arrested eggs. In the oocyte, RI and C were predominantly localized in the cortical region, while RII showed a punctate distribution within the cytoplasm. After maturation to metaphase II, RI remained in the cortex and was also localized to the meiotic spindle, while RII was found adjacent to the spindle. C was diffuse within the cytoplasm of the egg but was enriched in the cytoplasm surrounding the metaphase spindle, much like RII. The polarized localization and redistribution of RI, RII, and C suggested that PKA might be tethered by A-kinase anchor proteins (AKAPs), proteins that tether PKA close to its physiological substrates. An AKAP, AKAP140, was identified that was developmentally regulated and phosphorylated in oocytes and eggs. AKAP140 was shown to be a dual-specific AKAP, having the ability to bind both RI and RII. By compartmentalizing PKA, AKAP140 and/or other AKAPs could spatially regulate PKA activity during oocyte development.

Acute signaling by the LH receptor is independent of protein kinase C activation

LH receptor activation leads to the phosphorylation/activation of p42/44 MAPK in preovulatory granulosa cells. As the LH receptor can activate both adenylyl cyclase and phospholipase C, we hypothesized that the LH receptor could elicit phosphorylation of p42/44 MAPK through activation of protein kinase A (PKA) and/or protein kinase C (PKC). Preovulatory granulosa cells in serum-free primary cultures were treated with ovulatory concentrations of human chorionic gonadotropin (hCG), an LH receptor agonist, with or without various inhibitors. The PKA inhibitor H89 as well as the myristoylated PKA inhibitor peptide PKI strongly inhibited hCG-stimulated p42/44 MAPK phosphorylation, whereas the PKC inhibitor GF109203X had no effect on p42/44 MAPK phosphorylation. LH receptor-stimulated phosphorylation of cAMP response element-binding protein (CREB), histone H3, and MAPK kinase (MEK) was also strongly inhibited by H89 and not by GF109203X. The extent of PKC activation was assessed in preovulatory granulosa cells using three criteria: translocation of PKC isoforms to the membrane fraction, phosphorylation of a known PKC substrate, and autophosphorylation of PKC delta on an activation-related site. By all three criteria PKCs were partially activated before hCG stimulation, and hCG treatment failed to elicit further PKC activation, in vitro or in vivo. Taken together, these results indicate that, under primary culture conditions where physiological levels of signaling proteins are present, hCG signals to activate MEK, p42/44 MAPK, CREB, and histone H3 in a predominantly PKA-dependent and PKC-independent manner. Unexpectedly, PKCs were partially activated in the absence of LH receptor activation, and LH receptor activation did not elicit further detectable PKC activation.

A-kinase anchor proteins in endocrine systems and reproduction

Over the past few years, significant progress has been made in characterizing the expression and localization of proteins that act as scaffolds for cAMP-dependent protein kinase (PK-A). These A-kinase anchor proteins (AKAPs) tether PK-A to intracellular organelles and structures, sequestering the kinase near its physiological substrates. The compartmentalization of distinct pockets of PK-A activity serves to provide spatial regulation of this signaling pathway. In addition, other signaling proteins bind to AKAPs, as do some newly described proteins of unknown function, suggesting that proteins of various pathways are anchored through AKAPs.

Follicle-stimulating hormone stimulates protein kinase A-mediated histone H3 phosphorylation and acetylation leading to select gene activation in ovarian granulosa cells

We examined the phosphorylation and acetylation of histone H3 in ovarian granulosa cells stimulated to differentiate by follicle-stimulating hormone (FSH). We found that protein kinase A (PKA) mediates H3 phosphorylation on serine 10, based on inhibition exclusively by PKA inhibitors. FSH-stimulated H3 phosphorylation in granulosa cells is not downstream of mitogen-activated protein kinase/extracellular signal-regulated kinase, ribosomal S6 kinase-2, mitogen- and stress-activated protein kinase-1, p38 MAPK, phosphatidylinositol-3 kinase, or protein kinase C. Transcriptional activation-associated H3 phosphorylation on serine 10 and acetylation of lysine 14 leads to activation of serum glucocorticoid kinase, inhibin alpha, and c-fos genes. We propose that phosphorylation of histone H3 on serine 10 by PKA in coordination with acetylation of H3 on lysine 14 results in reorganization of the promoters of select FSH responsive genes into a more accessible configuration for activation. The unique role of PKA as the physiological histone H3 kinase is consistent with the central role of PKA in initiating granulosa cell differentiation.

Role and identification of protein kinase A anchoring proteins in vasopressin-mediated aquaporin-2 translocation

The antidiuretic hormone arginine vasopressin (AVP) regulates water reabsorption in renal principal cells by inducing a cAMP/protein kinase A-dependent translocation of water channels [aquaporin-2 (AQP2)] from intracellular vesicles into the apical cell membranes. Using primary cultured rat inner medullary collecting duct (IMCD) cells, it has been shown that AQP2 translocation in response to AVP stimulation occurs only if protein kinase A (PKA) is anchored to PKA anchoring proteins (AKAPs), which are present in various subcellular compartments. The identity of the AKAPs involved has not yet been elucidated. One potential candidate is a new splice variant of AKAP18, namely AKAP18 delta.

The biological functions of A-kinase anchor proteins

cAMP-dependent protein kinase is targeted to discrete subcellular locations by a family of specific anchor proteins (A-kinase anchor proteins, AKAPs). Localization recruits protein kinase A (PKA) holoenzyme close to its substrate/effector proteins, directing and amplifying the biological effects of cAMP signaling.AKAPs include two conserved structural modules: (i) a targeting domain that serves as a scaffold and membrane anchor; and (ii) a tethering domain that interacts with PKA regulatory subunits. Alternative splicing can shuffle targeting and tethering domains to generate a variety of AKAPs with different targeting specificity. Although AKAPs have been identified on the basis of their interaction with PKA, they also bind other signaling molecules, mainly phosphatases and kinases, that regulate AKAP targeting and activate other signal transduction pathways. We suggest that AKAP forms a "transduceosome" by acting as an autonomous multivalent scaffold that assembles and integrates signals derived from multiple pathways. The transduceosome amplifies cAMP and other signals locally and, by stabilizing and reducing the basal activity of PKA, it also exerts long-distance effects. The AKAP transduceosome thus optimizes the amplitude and the signal/noise ratio of cAMP-PKA stimuli travelling from the membrane to the nucleus and other subcellular compartments.

The ADP ribosylation factor nucleotide exchange factor ARNO promotes beta-arrestin release necessary for luteinizing hormone/choriogonadotropin receptor desensitization

Desensitization of guanine nucleotide binding protein-coupled receptors is a ubiquitous phenomenon characterized by declining effector activity upon persistent agonist stimulation. The luteinizing hormone/choriogonadotropin receptor (LH/CGR) in ovarian follicles exhibits desensitization of effector adenylyl cyclase activity in response to the mid-cycle surge of LH. We have previously shown that uncoupling of the agonist-activated LH/CGR from the stimulatory G protein (G(s)) is dependent on GTP and attributable to binding of beta-arrestin present in adenylyl cyclase-rich follicular membrane fraction to the third intracellular (3i) loop of the receptor. Here, we report that LH/CGR-dependent desensitization is mimicked by ADP ribosylation factor nucleotide-binding site opener, a guanine nucleotide exchange factor of the small G proteins ADP ribosylation factors (Arfs) 1 and 6, and blocked by synthetic N-terminal Arf6 peptide, suggesting that the GTP-dependent step of LH/CGR desensitization is receptor-dependent Arf6 activation. Arf activation by GTP and ADP ribosylation factor nucelotide-binding site opener promotes the release of docked beta-arrestin from the membrane, making beta-arrestin available for LH/CGR; Arf6 but not Arf1 peptides block beta-arrestin release from the membrane. Thus, LH/CGR appears to activate two membrane delimited signaling cascades via two types of G proteins: heterotrimeric G(s) and small G protein Arf6. Arf6 activation releases docked beta-arrestin necessary for receptor desensitization, providing a feedback mechanism for receptor self-regulation.

Regulation of PKC delta expression by estrogen and rat placental lactogen-1 in luteinized rat ovarian granulosa cells

Protein kinase C (PKC) delta is dramatically upregulated in the corpus luteum in the second half of pregnancy in the rat. To gain insight into the hormonal regulation of PKC delta expression, studies were undertaken to analyze the regulation of PKC delta expression in a luteinized rat granulosa cell model. PKC delta protein expression was evaluated in luteinized granulosa cells, isolated from human (h)CG-treated immature female rats 7 h after the injection of an ovulatory dose of hCG and cultured up to 12 days. Cytochrome P450 cholesterol side chain cleavage enzyme expression was observed throughout the culture period, and a majority of the cells expressed steroidogenic acute regulatory protein and responded to rat placental lactogen (rPL)-1 by exhibiting hypertrophy, consistent with maintenance of the luteal phenotype. Both PKC delta protein and mRNA expression increased 3.5-4-fold with time of culture, and PKC delta mRNA expression could be eliminated by treatment of cells with the PKC inhibitor GF109203X. E(2) caused a specific dose- and time-dependent increase in expression of PKC delta protein of twofold, whereas PKC delta mRNA was unaffected by E(2) over a 12-day culture period. Treatment of cells with 500 ng/ml rPL-1 for the final 4 days of a 12-day culture in the absence of E(2) had no effect on PKC delta protein or mRNA expression, while treatment with 500 or 3000 ng/ml rPL-1 in the presence of E(2) significantly enhanced both PKC delta protein and mRNA expression (up to threefold). These results show that two of the major regulators of luteal function in the second half of pregnancy in the rat, E(2) and rPL-1, cooperate to regulate PKC delta expression in luteinized granulosa cells.

Alternative splicing regulates the subcellular localization of A-kinase anchoring protein 18 isoforms

The cAMP-dependent protein kinase (PKA) is localized to specific subcellular compartments by association with A-kinase anchoring proteins (AKAPs). AKAPs are a family of functionally related proteins that bind the regulatory (R) subunit of PKA with high affinity and target the kinase to specific subcellular organelles. Recently, AKAP18, a low molecular weight plasma membrane AKAP that facilitates PKA-mediated phosphorylation of the L-type Ca(2+) channel, was cloned. We now report the cloning of two additional isoforms of AKAP18, which we have designated AKAP18beta and AKAP18gamma, that arise from alternative mRNA splicing. The AKAP18 isoforms share a common R subunit binding site, but have distinct targeting domains. The original AKAP18 (renamed AKAP18alpha) and AKAP18beta target the plasma membrane when expressed in HEK-293 cells, while AKAP18gamma is cytosolic. When expressed in epithelial cells, AKAP18alpha is targeted to lateral membranes, whereas AKAP18beta is accumulated at the apical membrane. A 23-amino acid insert, following the plasma membrane targeting domain, facilitates the association of AKAP18beta with the apical membrane. The data suggest that AKAP18 isoforms are differentially targeted to modulate distinct intracellular signaling events. Furthermore, the data suggest that plasma membrane AKAPs may be targeted to subdomains of the cell surface, adding additional specificity in intracellular signaling.

Activation of PKC delta in the rat corpus luteum during pregnancy. Potential role of prolactin signaling

Maintenance of pregnancy in the rat requires the corpus luteum. At a time when rat placental lactogens (rPLs) are required to support progesterone production by the corpus luteum and when relaxin expression is initiated, expression of a specific protein kinase C (PKC) isoform, PKC delta, is dramatically increased. We therefore assessed whether prolactin (PRL) receptor activation promotes activation of PKC delta in a luteinized granulosa cell model. We also assessed the activation status of PKC delta in corpora lutea obtained when the corpus luteum is exposed to chronically high concentrations of rPLs. The activity of PKC delta was assessed by two means: an immune complex (IC) assay and Western blotting with a phospho-epitope-specific antibody that detects PKC delta phosphorylated on serine 662. PKC delta activation in the IC kinase assay was determined by the ability of immunoprecipitated PKC delta to phosphorylate the PKC delta-preferential substrate small heat shock protein (HSP-27). Treatment of luteinized rat granulosa cells with phorbol myristate acetate, a known activator of PKC, promoted a 7-fold increase in HSP-27 phosphorylation by PKC delta. Similarly, immunoreactivity with the phospho-epitope-specific PKC delta antibody was increased in extracts prepared from luteinized granulosa cells treated with phorbol myristate acetate or following in vitro activation of recombinant PKC delta. Using these assays, we assessed whether PRL receptor agonists were capable of activating PKC delta in luteinized granulosa cells. PRL receptor agonists induced translocation PKC delta from the cytosolic to the Triton-soluble membrane fraction and increased PKC delta activity assessed by both IC kinase assay and Western blotting with phospho-epitope-specific PKC delta antibody. Analysis of PKC delta activity in corpora lutea obtained during pregnancy by both the IC kinase assay and Western blotting with the phospho-epitope-specific PKC delta antibody revealed that PKC delta activity was increased throughout the second half of pregnancy. These results demonstrate that PRL receptor activation promotes the acute activation of PKC delta in luteinized rat granulosa cells. At a time when the rat is exposed to chronically high concentrations of rPLs, PKC delta is increasingly expressed and active.

mAKAP: an A-kinase anchoring protein targeted to the nuclear membrane of differentiated myocytes

The compartmentalization of second messenger-activated protein kinases contributes to the fidelity of hormone-mediated signal transduction events. For example, the cAMP-dependent protein kinase is tethered at specific intracellular locations through association with A-kinase anchoring proteins (AKAPs). We now report the cloning of mAKAP, an anchoring protein found predominantly in heart, skeletal muscle and brain, and whose expression is induced in neonatal ventriculocytes by treatment with hypertrophic stimuli. mAKAP is targeted to the nuclear membrane of differentiated myocytes. Analysis of mAKAP-green fluorescent protein (GFP) fusion constructs revealed that nuclear membrane targeting is conferred by two regions of the protein, between residues 772–915 and 915–1065, which contain spectrin-like repeat sequences. Heterologous expression of the mAKAP targeting sequences displaced the endogenous anchoring protein from the nuclear membrane, demonstrating that mAKAP targeting is saturable. Collectively, these data suggest that a domain containing spectrin-like repeats mediates targeting of the anchoring protein mAKAP and the cAMP-dependent protein kinase holoenzyme to the nuclear membrane in response to differentiation signals.

Heterotrimetric Gi/o proteins control cyclic AMP oscillations and cytoskeletal structure assembly in primary human granulosa-lutein cells

In granulosa cells, the luteinising hormone (LH) and the follicle-stimulating hormone (FSH) receptors are coupled to the adenylyl cyclase-cAMP pathway. We identified at least eight different G proteins belonging to three families--Gs, Gq, and Gi/o--in primary human granulosa-lutein cells. By exploring the function of Gi/o by time-lapse and digital-imaging microscopy of live cells, we found that the reversible actin stress fibre-dependent cytoplasmic retraction of pre-luteinised cells in primary culture is a highly sensitive and quite rapid system allowing detection of an intracellular cAMP surge. This morphology was characterised by maintenance of connexin43-dependent cell-cell contacts and that of microtubule-directed cell processes attached to the substrate and to neighbouring cells. Inhibitors of cyclic nucleotide phosphodiesterase subfamily type 4 (PDE-4), hLH and hFSH provoked this reversible cAMP-dependent phenotype in a temporal-, spatial- and dose-dependent manner. Gi/o inhibited adenylyl cyclase in membranes, and cell treatment with islet-activating protein (IAP) caused the cAMP-dependent retracted phenotype. It is concluded that the basal intracellular cAMP level is kept within a narrow range of concentrations, below the threshold for disassembly of stress fibres, through Gs, Gi/o, adenylyl cyclases and phosphodiesterase-4. This work supports the paradigm that switching of the agonist-occupied receptors to Gs and Gi/o would control both the intracellular bursts of cAMP (through the gonadotropin-catalysed activation of Gs) and the basal cAMP (through a Gi/o-mediated braking effect).

Differential localization of protein kinase A type II isozymes in the Golgi-centrosomal area

Selectivity in the action of cAMP may be mediated by compartmentalized pools of cyclic AMP-dependent protein kinase (PKA). PKA type II is directed to different subcellular loci by interaction of the type II regulatory subunits (RIIalpha, RIIbeta) with A-kinase anchoring proteins. In order to separately investigate the subcellular localization of PKA type II isozymes, monospecific antibodies to human RIIalpha and RIIbeta subunits of PKA were developed. We demonstrate that centrosomes bind both RIIalpha and RIIbeta. Centrosomes were the preferred intracellular anchoring site for RIIbeta. However, centrosomal localization of RIIbeta was observed only in some highly differentiated cells such as keratinocytes, granulosa cells, and macrophages and in all neoplastic cell lines examined. Centrosomal localization of RIIbeta was not observed in normal undifferentiated cells such as fibroblasts, myoblasts, and T and B cells. In contrast, RIIalpha was abundant in the Golgi area and in the trans-Golgi network (TGN). Furthermore, although RIIalpha appeared to colocalize with microtubules in the Golgi/TGN, extractions with nonionic detergent demonstrated that RIIalpha was mainly membrane-associated. In addition, alterations of microtubule dynamics with Nocodazole or Taxol affected the distribution of the detergent-extractable pool of RIIalpha, indicating that RIIalpha may localize with microtubule-associated vesicles. Thus, RIIalpha and RIIbeta clearly localize differently in the Golgi-centrosomal region. This indicates specific roles for PKA isozymes containing either RIIalpha or RIIbeta.

The type and the localization of cAMP-dependent protein kinase regulate transmission of cAMP signals to the nucleus in cortical and cerebellar granule cells

cAMP signals are received and transmitted by multiple isoforms of cAMP-dependent protein kinases, typically determined by their specific regulatory subunits. In the brain the major regulatory isoform RIIbeta and the RII-anchor protein, AKAP150 (rat) or 75 (bovine), are differentially expressed. Cortical neurons express RIIbeta and AKAP75; conversely, granule cerebellar cells express predominantly RIalpha and RIIalpha. Cortical neurons accumulate PKA catalytic subunit and phosphorylated cAMP responsive element binding protein very efficiently into nuclei upon cAMP induction, whereas granule cerebellar cells fail to do so. Down-regulation of RIIbeta synthesis by antisense oligonucleotides inhibited cAMP-induced nuclear signaling in cortical neurons. Expression in cerebellar granule cells of RIIbeta and AKAP75 genes by microinjection of specific expression vectors, markedly stimulated cAMP-induced transcription of the lacZ gene driven by a cAMP-responsive element promoter. These data indicate that the composition of PKA in cortical and granule cells underlies the differential ability of these cells to transmit cAMP signals to the nucleus.

Follicle-stimulating hormone promotes histone H3 phosphorylation on serine-10

FSH promoted the rapid phosphorylation of the nuclear protein histone H3 in immature rat ovarian granulosa cells under experimental conditions that lead to cellular differentiation and not proliferation. FSH-stimulated histone H3 phosphorylation correlated with cAMP-dependent protein kinase A (PKA) activation and translocation of the PKA catalytic subunit to a nuclear-enriched fraction and was inhibited by the PKA inhibitor H89, and histone H3 phosphorylation was stimulated in cells treated with agents that raise intracellular cAMP levels such as forskolin and 8-bromo-cAMP. FSH-stimulated histone H3 phosphorylation in granulosa cells mapped to ser-10, a site previously identified as the PKA phosphorylation site in various mitotically active cells as the mitosis-specific phosphorylation site. Injection of the FSH analog PMSG to immature rats, which is known to stimulate granulosa cell proliferation as well as differentiation, also promoted histone H3 phosphorylation on ser-10 in granulosa cells. These results establish that FSH-stimulated histone H3 phosphorylation in granulosa cells is linked not only to granulosa cell mitosis but also to granulosa cell differentiation and that FSH-stimulated histone H3 phosphorylation on ser-10 in isolated granulosa cells is mediated by PKA. These results also identify the PKA-dependent histone H3 phosphorylation as an early nuclear protein marker for FSH-stimulated differentiation of granulosa cells. Based on the recently described function of histone H3 as a coactivator of transcription, these results are consistent with the hypothesis that phosphorylated histone H3 may facilitate PKA-dependent gene transcription in granulosa cells leading to the preovulatory phenotype.

Expression of a kinase anchor protein 121 is regulated by hormones in thyroid and testicular germ cells

Distinct A Kinase Anchor Proteins (AKAPs) immobilize and concentrate protein kinase A II (PKAII) isoforms at specific intracellular locations. AKAP121 binds and targets PKAIIalpha to the cytoplasmic surface of mitochondria. Mechanisms that control expression of this mitochondrial AKAP are unknown. We have cloned cDNA for rat AKAP121 and show that AKAP121 protein expression is regulated by thyroid stimulating hormone (TSH) and cAMP. Differentiated thyroid cells (TL5) accumulate AKAP121 upon incubation with TSH or a cAMP analog. Levels of total and newly synthesized AKAP121 mRNA also increased after treatment. AKAP121 mRNA accumulated in the presence of cycloheximide, suggesting that transcription of the anchor protein gene is directly controlled by cAMP and PKA. AKAP121 is induced with similar kinetics when an unrelated, spermatocyte-derived cell line (GC-2) is incubated with 8-chlorophenylthio-cAMP. Thus, AKAP121 concentration may be controlled by hormones that activate adenylate cyclase. This mode of regulation could provide a general mechanism for (a) enhancing the sensitivity of distal organelles to cAMP and (b) shifting the focus of cAMP-mediated signaling from cytoplasm to organelles.

Association of protein kinase A type I with detergent-resistant structures of mammalian sperm cells

The finding that flagellar movement in detergent-permeabilized sperm cells is restored when Mg ATP and cAMP are added implicated detergent-resistant protein kinase A (PKA) in the regulation of sperm motility. It is widely believed that only the PKA regulatory subunit RII can associate with the cytoskeleton and/or organelles. In this paper we used monoclonal antibodies against the PKA catalytic subunit and RI subunit and demonstrated that PKA type I is also associated with the sperm cytoskeleton. To our knowledge, this is the first report showing anchored PKA type I. This association was found in sperm of nonrodent mammalian species and, to a lesser extent, also in mouse sperm. The PKA catalytic subunit is bound to the cytoskeleton secondarily via its complex with the regulatory subunit. The detergent-resistant complexes of RI and catalytic subunits localize predominantly to the flagellum. Ultrastructural immunogold labeling revealed the association of detergent-resistant PKA type I with outer dense fibers (ODF) and the fibrous sheath (FS) but not with microtubules. This location is consistent with a proposed role of PKA in regulation of FS sliding on underlying ODF.

The cyclic adenosine monophosphate-dependent protein kinase (PKA) is required for the sustained activation of mitogen-activated kinases and gene expression by nerve growth factor

Induction of neuronal differentiation of the rat pheochromocytoma cell line, PC12 cells, by nerve growth factor (NGF) requires activation of the mitogen-activated protein (MAP) kinase or extracellular signal-regulated kinase (ERK). cAMP-dependent protein kinase (protein kinase A (PKA)) also can induce differentiation of these cells. Like NGF, the ability of PKA to differentiate PC12 cells is associated with a sustained activation of ERKs. Here we show that maximal sustained activation of ERK1 by NGF requires PKA. Inhibitors of PKA partially blocked activation of ERK1 by NGF but had no effect on activation of ERK1 by EGF. Inhibition of PKA also reduced the ability of NGF and cAMP, but not EGF, to activate the transcription factor Elk-1, reduced the induction of both immediate early and late genes after NGF treatment, and blocked the nuclear translocation of ERK1 induced by NGF. We propose that PKA is an important contributor to the activation of ERK1 by NGF and is required for maximal induction of gene expression by NGF.

D-AKAP2, a novel protein kinase A anchoring protein with a putative RGS domain

Subcellular localization directed by specific A kinase anchoring proteins (AKAPs) is a mechanism for compartmentalization of cAMP-dependent protein kinase (PKA). Using a two-hybrid screen, a novel AKAP was isolated. Because it interacts with both the type I and type II regulatory subunits, it was defined as a dual specific AKAP or D-AKAP1. Here we report the cloning and characterization of another novel cDNA isolated from that screen. This new member of the D-AKAP family, D-AKAP2, also binds both types of regulatory subunits. A message of 5 kb pairs was detected for D-AKAP2 in all embryonic stages and in all adult tissues tested. In brain, skeletal muscle, kidney, and testis, a 10-kb mRNA was identified. In testis, several small mRNAs were observed. Therefore, D-AKAP2 represents a novel family of proteins. cDNA cloning from a mouse testis library identified the full length D-AKAP2. It is composed of 372 amino acids which includes the R binding fragment, residues 333-372, at its C-terminus. Based on coprecipitation assays, the R binding domain interacts with the N-terminal dimerization domain of RIalpha and RIIalpha. A putative RGS domain was identified near the N-terminal region of D-AKAP2. The presence of this domain raises the intriguing possibility that D-AKAP2 may interact with a Galpha protein thus providing a link between the signaling machinery at the plasma membrane and the downstream kinase.

Cloning and characterization of a testis-specific, developmentally regulated A-kinase-anchoring protein (TAKAP-80) present on the fibrous sheath of rat sperm

cAMP is important for the initiation of mammalian sperm motility. Previously we established that a type II cAMP-dependent protein kinase is tightly associated with the fibrous sheath of rat sperm. This unique cytoskeletal structure surrounds the 9+2 axonemal network in the principal piece of the flagellum. Association of the kinase to the fibrous sheath is mediated via its regulatory subunit, RII. An RII-binding overlay procedure was used to document that RII could specifically associate with fibrous sheath polypeptides of 120 and 80 kDa. In this study, we report the cloning of a rat testis-specific, developmentally regulated, RII-binding protein (TAKAP-80). A 1.2-kb cDNA clone, isolated by screening a rat testis expression library with 32P-labeled RII, hybridized to a 1.8-kb mRNA transcript present exclusively in testis. This transcript appeared at detectable levels at 30 days after birth. Over the next 10 days the mRNA levels increased greatly. This time interval corresponds to the initiation of spermiogenesis. The complete nucleotide sequence of TAKAP-80 cDNA was obtained by polymerase chain reaction and contained a continuous open reading frame of 502 amino acids. The deduced amino acid sequence showed a clear demarcation of charged and hydrophobic amino acid residues. Amino acids 1-147 of the protein contained 45% charged residues, with lysine and arginine predominating. Similarly, amino acids 268-502 also contained a high percentage of charged amino acids (35%). In contrast, amino acids 148-267 were mostly hydrophobic and contained clusters of a repeating PXXP motif where X was predominantly valine and alanine or sometimes proline. The 1.2-kb cDNA clone was inserted into the pRSET vector and expressed as a His6 tag fusion protein in Escherichia coli. The recombinant protein was soluble and bound RIIalpha, RIIbeta and type IIalpha holoenzyme by the RII-binding overlay procedure. Deletion analysis revealed that the high-affinity interaction site for RII was contained within amino acids 258-378 of TAKAP-80. Antibodies prepared against the fusion protein recognized an 80-kDa protein present in the urea-insoluble particulate fraction of rat testis and in purified fibrous sheath preparations isolated from rat epididymal sperm. Levels of the 80-kDa immunoreactive protein were significantly higher in mature (60 days old) compared with immature (30 days old) rat testis, correlating with the mRNA levels.

Protein kinase A-anchoring inhibitor peptides arrest mammalian sperm motility

Cyclic AMP-dependent protein kinase (PKA) is anchored at specific subcellular sites through the interaction of the regulatory subunit (R) with protein kinase A-anchoring proteins (AKAPs) via an amphipathic helix binding motif. Synthetic peptides containing this amphipathic helix domain competitively disrupt PKA binding to AKAPs and cause a loss of PKA modulation of cellular responses. In this report we use S-Ht31, a cell-permeant anchoring inhibitor peptide, to study the role of PKA anchoring in sperm. Our analysis of three species of mammalian sperm detected three isoforms of PKA (RIIalpha, RIIbeta, and RIbeta) and one 110-kDa AKAP. The addition of S-Ht31 to bovine caudal epididymal sperm inhibits motility in a time- and concentration-dependent manner. A control peptide, S-Ht31-P, identical to S-Ht31 except for a proline for isoleucine substitution to prevent amphipathic helix formation, had no effect on motility. The inhibition of motility by S-Ht31 is reversible but only if calcium is present in the suspension buffer, suggesting a role for PKA anchoring in regulating cellular calcium homeostasis. Surprisingly, inhibition of PKA catalytic activity had little effect on basal motility or motility stimulated by agents previously thought to work via PKA activation. These data suggest that the interaction of the regulatory subunit of PKA with sperm AKAPs, independent of PKA catalytic activity, is a key regulator of sperm motility and that disruption of this interaction using cell-permeable anchoring inhibitor peptides may form the basis of a sperm-targeted contraceptive.

Membrane localization of cAMP-dependent protein kinase amplifies cAMP signaling to the nucleus in PC12 cells

The A126 cell line, in contrast to its PC12 parent, does not differentiate, accumulate nuclear cAMP-dependent protein kinase A (PKA) catalytic subunit, or transcribe cAMP-dependent promoters in response to cAMP. Total PKA is reduced by 50% and is partly resistant to cAMP-induced dissociation in vivo. Unlike PC12, where PKAII is membrane-associated, PKAII is exclusively cytosolic in A126. Cotransfection with the RII anchor protein (AKAP75) and the PKA catalytic subunit (C-PKA) restored cAMP-induced transcription to levels found in PC12. These data indicate that membrane-bound PKAII amplifies cAMP signaling to the nucleus and suggest that cAMP-mediated responses are specified by the type and cellular localization of the PKA isoform.