Reconstitution of the protein kinase A response of the rat prolactin promoter: differential effects of distinct Pit-1 isoforms and functional interaction with Oct-1

PRL gene transcription is primarily regulated by dopamine, which lowers cAMP levels and inhibits protein kinase A (PKA) activity. Current data indicate that the cAMP/PKA response maps to the most proximal Pit-1/Pit-1beta binding site footprint I (FP I) on the rat PRL (rPRL) promoter. Pit-1, a POU-homeo domain transcription factor, is specifically expressed in the anterior pituitary and is required both for the normal development of anterior pituitary cell types, somatotrophs, lactotrophs, and thyrotrophs, and for the expression of their hormones: GH, PRL, and TSHbeta. Pit-1 has been shown to functionally interact, via FP I, with several transcription factors, including Oct-1, a ubiquitous homeobox protein, and thyrotroph embryonic factor, which is found in lactotrophs, to activate basal rPRL promoter activity. Pit-1beta/GHF-2, a distinct splice isoform of Pit-1, acts to inhibit Ras-activated transcription from the rPRL promoter, which is mediated by a functional interaction between Pit-1 and Ets-1 at the most distal Pit-1 binding site (FP IV). In this manuscript we show 1) that the Pit-1beta isoform not only fails to block PKA activation, but is, in fact, a superior mediator of the PKA response; 2) that the PKA response requires intact POU-specific and POU-homeo domains of Pit-1; and 3) that Oct-1, but not thyrotroph embryonic factor, functions as a Pit-1-interacting factor to mediate an optimal PKA response.

Human Cart-1: structural organization, chromosomal localization, and functional analysis of a cartilage-specific homeodomain cDNA

Homeoproteins control cell fates during development, specifying pattern formation and the ontogeny of specific tissues and organs in embryogenesis. Cart-1 cDNA was recently cloned from a rat chondrosarcoma tumor and it encodes a protein containing a paired-like homeodomain that is selectively expressed in cartilage during early chondrocyte differentiation. Here we report the molecular cloning of the human Cart-1 cDNA from a HeLa cervical carcinoma cDNA library. The human Cart-1 cDNA sequence is 88% identical and the deduced amino acid sequence is 95% identical to the rat sequence, indicating that Cart-1 structure is highly conserved. Northern and reverse transcriptase polymerase chain reaction (RT-PCR) analysis revealed Cart-1 mRNA expression in HeLa cervical carcinoma cells and human cervical tissue, but Cart-1 mRNA was not detected in GH3 rat pituitary cells and murine 10T1/2 one-half fibroblast cells. The Cart-1 gene was localized to human chromosome 12 and regionally mapped to the 12q21.3-q22 by PCR analysis of rodent-X-human somatic cell hybrid DNA and the CEPH megabase-insert YAC DNA pools, respectively. The Holt-Oram syndrome, characterized by upper limb and atrial septal dysplasias, also maps to the 12q21.3-q22 region. Cotransfection studies show that Cart-1 inhibits the rat prolactin promoter and that this repression is mediated by footprint II, an AT-rich element that functions as an inhibitory site of prolactin gene expression in nonpituitary cells and which was used to clone Cart-1. Taken together, these data indicate that Cart-1 may also influence cervix development, identify a putative DNA binding site for Cart-1, and, begin to define its functional role as modulator of gene expression.

Reconstitution of protein kinase A regulation of the rat prolactin promoter in HeLa nonpituitary cells: identification of both GHF-1/Pit-1-dependent and -independent mechanisms

Expression of the rat PRL (rPRL) gene is highly restricted to pituitary lactotroph cells and is induced by the cAMP-dependent protein kinase A (PKA) pathway. Current data indicate that this PKA effect requires at least one of the redundant pituitary-specific elements of the proximal rPRL promoter, suggesting the involvement of the pituitary-specific transcription factor, GHF-1/Pit-1. To directly determine whether GHF-1 is necessary and sufficient to mediate the PKA activation of the rPRL promoter, we established a cotransfection reconstitution assay whereby the activity of an intact and site-specific mutants of the (-425 to +73) rPRL promoter-luciferase reporter gene was reconstituted by cotransfecting expression vectors encoding for either the PKA beta catalytic subunit, GHF-1, or both, into HeLa nonpituitary cells. Cotransfection of PKA beta alone significantly stimulated rPRL promoter activity in HeLa cells in a GHF-1-independent manner, and this PKA beta effect was mapped to the most proximal GHF-1 site [footprint (FP) I; -67/-36]. Site-specific alterations of either FP II (-130/-120), or of the basal transcription element (BTE; -112/-80), did not significantly affect the PKA beta response. As expected, the transactivation effect of cotransfected GHF-1 mapped to the GHF-1/Pit-1 binding sites, FP I and/or FP III, of the rPRL promoter. Finally, cotransfection of PKA beta and GHF-1 resulted in a marked synergistic response of the rPRL promoter, and this response also localized to the FP I site. These data confirm not only that GHF-1/Pit-1 and the FP I site are involved in mediating the PKA response, but also imply that a distinct and possibly ubiquitous factor is involved by binding to FP I and functionally interacting with GHF-1 to modulate PKA beta regulation of the rPRL promoter.

Capacitative Ca2+ entry exclusively inhibits cAMP synthesis in C6-2B glioma cells. Evidence that physiologically evoked Ca2+ entry regulates Ca(2+)-inhibitable adenylyl cyclase in non-excitable cells

Elevation of cytosolic free Ca2+ inhibits the type VI adenylyl cyclase that predominates in C6-2B cells. However, it is not known whether there is any selective requirement for Ca2+ entry or release for inhibition of cAMP accumulation to occur. In the present study, the effectiveness of intracellular Ca2+ release evoked by three independent methods (thapsigargin, ionomycin, and UTP) was compared with the capacitative Ca2+ entry that was triggered by these treatments. In each situation, only Ca2+ entry could inhibit cAMP accumulation (La3+ ions blocked the effect); Ca2+ release, which was substantial in some cases, was without effect. A moderate inhibition, as was elicited by a modest degree of Ca2+ entry, could be rendered substantial in the absence of phosphodiesterase inhibitors. Such conditions more closely mimic the physiological situation of normal cells. These results are particularly significant, in demonstrating not only that Ca2+ entry mediates the inhibitory effects of Ca2+ on cAMP accumulation, but also that diffuse elevations in [Ca2+]i are ineffective in modulating cAMP synthesis. This property suggests that, as with certain Ca(2+)-sensitive ion channels, Ca(2+)-sensitive adenylyl cyclases may be functionally colocalized with Ca2+ entry channels.

Capacitative Ca2+ entry regulates Ca(2+)-sensitive adenylyl cyclases

A number of the currently described adenylyl cyclase species can be regulated by Ca2+ in the submicromolar concentration range in in vitro assays. The regulatory significance of these observations hinges on whether a physiological elevation in intracellular Ca2+ can regulate these cyclase activities in intact cells. However, achieving a physiological elevation in cytosolic Ca2+ is complicated by the fact that hormonal increases in cytosolic Ca2+ can be accompanied by additional effects, such as liberation of beta gamma-subunits of G-proteins and activation of protein kinase C, which can have disparate type-specific effects on cyclase activities. Therefore we have devised a strategy based on capacitative Ca2+ entry to show that, when types I and VI adenylyl cyclase are expressed in human embryonic kidney 293 cells, they are stimulated and inhibited respectively by Ca2+ entry. Blockade of Ca2+ entry by La3+ ions blocks the effects of Ca2+ entry on cyclic AMP synthesis. These studies establish that adenylyl cyclases deemed to be sensitive to Ca2+ in in vitro assays can be regulated by physiological Ca2+ entry, and therefore, such cyclases are poised to respond to changes in intracellular Ca2+ in tissues in which they are expressed.

Leukotriene B4 and leukotriene B5 have binding sites on lung membranes and cause contraction of bullfrog lung

Leukotriene (LT)B4 and LTB5 cause contraction of isolated bullfrog lung. LTB4 receptors were characterized in membranes prepared from bullfrog lung. Binding of [3H]LTB4 was maximal at 5 min and was reversible with the addition of 1000-fold excess LTB4. Scatchard analysis indicated a single class of binding sites with a Kd of 2.22 nM and a Bmax of 1228.86 fmol/mg protein. The Kd and the Bmax values in the presence of guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S) were 2.76 nM and 1289.61 fmol/mg protein, respectively. The Ki values for LTB4, LTB5 and 20(OH)-LTB4 were 5.5, 30.5 and 144.0 nM, respectively, whereas 20(COOH)-LTB4 was ineffective in preventing binding of [3H] LTB4 from 10(-9) to 10(-5) M. The peptide leukotrienes LTC4, LTD4 and LTE4 failed to inhibit the specific binding of [3H]LTB4. GTP gamma S in concentrations from 10(-10) to 10(-4) M did not affect the binding of 5 nM [3H]LTB4. Neither the mammalian LTD4 antagonist LY171883 nor the mammalian LTB4 antagonist LY255283 was an effective competitor for the bullfrog lung LTB4 receptor. In addition, sulfhydryl-modifying reagents NEM and PCMP did not affect LTB4 binding as they do in mammalian membrane preparations. The LTB4 receptor shows some differences from the described mammalian receptor. The cell type containing the LTB4 receptor remains to be determined.

Characterization and localization of leukotriene C4 receptors in bullfrog lung

Specific binding sites for [3H]leukotriene C4, ([3H]LTC4) were characterized in lung membrane preparations and localized to the septa of lung tissue from the American bullfrog (Rana catesbeiana). Binding assays were carried out at 23 degrees C for 30 min in the presence of serine (5 mM)-borate (10 mM) to prevent metabolism of LTC4 to LTD4. Specific binding reached steady state within 10 min and was completely reversible with the addition of 1000-fold excess unlabeled LTC4. Scatchard analysis predicted a single class of binding sites with a Kd of 55.7 nM and a Bmax of 61.9 pmol/mg protein. The Hill coefficient was 0.958. LTC4 was the most potent inhibitor of [3H]LTC4 binding, with a Ki of 33 nM. LTD4 and LTE4 are less potent inhibitors, with Ki values of 3350 and 5979 nM, respectively. Mammalian LTD4 antagonists LY171883, L-649,923 and ICI 198,615 only displaced [3H]LTC4 specific binding at concentrations in excess of 1 x 10(-4) M. Guanosine 5' triphosphate and its analog guanyl-5'-yl-imidodiphosphate did not affect specific binding of [3H]LTC4, suggesting that a Gi protein is not involved in the LTC4 transduction mechanism. Glutathione, S-decyl-glutathione and hematin had Ki values of 35,000, 280 and 29,000 nM, respectively, suggesting the binding site is not the enzyme LTC4-synthase. LTC4 contracted lung strips, with S-decyl-glutathione a partial agonist. Computer-enhanced autoradiographs localized the binding sites to the smooth muscle regions of the septa in lung tissue. These data suggest that LTC4 binds to distinct receptors in bullfrog lung.

Characterization of leukotriene C4 binding sites in the brain of the American bullfrog, Rana catesbeiana

In this study, specific binding sites for [3H]-LTC4 on membrane preparations from American bullfrog (Rana catesbeiana) brain were characterized. Binding assays were done in the presence of serine (5mM) borate (10 mM) for 30 min at 23 degrees C. Under these conditions, no metabolism of LTC4 to LTD4 occurred. Specific binding of [3H]-LTC4 reached steady state within 10 min, remained constant for 60 min, and was reversible with the addition of 1,000-fold excess unlabelled LTC4. Scatchard analysis of the binding data indicated a single class of binding sites with an estimated Kd of 89.83 nM and Bmax of 43.79 pmol/mg protein. Competition binding studies demonstrated that LTD4 and LTE4 were ineffective in displacing [3H]-LTC4 from its binding site. The Ki for LTC4 was 51 nM. S-decylglutathione, glutathione and hematin had Ki values of 44, 312,602, and 25,576 nM, respectively. The mammalian cysteinyl leukotriene antagonist L-660,711 inhibited specific binding of [3H]-LTC4, with a Ki of 87,149 nM. Guanosine-5'-0-3-thiotriphosphate (GTP gamma S) did not affect specific binding of [3H]-LTC4 indicating that, like mammalian LTC4 receptors, a Gi protein is not involved in the transduction mechanism. The LTC4 binding site in bullfrog brain demonstrates both similarities and differences from its mammalian counterpart.

Leukotriene C4 binds to receptors and has positive inotropic effects in bullfrog heart

Leukotriene (LT) C4, LTD4 and LTE4 have positive inotropic effects on contractility of the isolated perfused bullfrog heart. The effects of LTD4 and LTE4 but not LTC4 can be blocked by the mammalian antagonist L-649,923. Characterization of specific binding sites for [3H]LTC4 on membrane preparations from American bullfrog (Rana catesbeiana) ventricle was carried out. Binding assays were done in the presence of serine (5 mM) and borate (10 mM) for 30 min at 23 degrees C. Under these conditions, no metabolism of LTC4 to LTD4 occurred. Specific binding of [3H]LTC4 reached steady state within 10 min, remained constant for 60 min, and was reversible with the addition of 1000-fold excess unlabeled LTC4. Scatchard analysis of the binding data indicated a single class of binding sites with a Kd of 33.9 nM and maximal binding capacity of 51.6 pmol/mg of protein. Competition binding studies revealed an order of potency of LTC4 greater than LTD4 greater than LTE4 with Ki values of 47, 11766 and 32248 nM, respectively. Glutathione and hematin had Ki values of 50566 and 6014 nM, respectively, suggesting that the LTC4 receptor is not a site on glutathione transferase. Two mammalian LTD4 antagonists, L-649,923 and LY171883 failed to inhibit specific binding of [3H]LTC4, suggesting that the LTC4 receptor is distinct from the LTD4 receptor. Guanosine-5'-O-3-thiotriphosphate did not affect specific binding of [3H]LTC4 indicating that, like mammalian LTC4 receptors, a Gi protein is not involved in the transduction mechanism. LTC4 acts on bullfrog hearts through specific membrane receptors and is similar to its mammalian counterpart.