NudC associates with Lis1 and the dynein motor at the leading pole of neurons

NUDC is a highly conserved protein important for nuclear migration and viability in Aspergillus nidulans. Mammalian NudC interacts with Lis1, a neuronal migration protein important during neocorticogenesis, suggesting a conserved mechanism of nuclear movement in A. nidulans and neuronal migration in the developing mammalian brain (S. M. Morris et al., 1998). To further investigate this possibility, we show for the first time that NudC, Lis1, and cytoplasmic dynein intermediate chain (CDIC) colocalize at the microtubule organizing center (MTOC) around the nucleus in a polarized manner facing the leading pole of cerebellar granule cells with a migratory morphology. In neurons with stationary morphology, NudC is distributed throughout the soma and colocalizes with CDIC and tubulin in neurites as well as at the MTOC. At the subcellular level, NudC, CDIC, and p150 dynactin colocalize to the interphase microtubule array and the MTOC in fibroblasts. The observed colocalization is confirmed biochemically by coimmunoprecipitation of NudC with CDIC and cytoplasmic dynein heavy chain (CDHC) from mouse brain extracts. Consistent with its expression in individual neurons, a high level of NudC is detected in regions of the embryonic neocortex undergoing extensive neurogenesis as well as neuronal migration. These data suggest a biochemical and functional interaction of NudC with Lis1 and the dynein motor complex during neuronal migration in vivo.

Differential roles for signal transducers and activators of transcription 5a and 5b in PRL stimulation of ERalpha and ERbeta transcription

PRL has been shown to stimulate mRNA expression of both ERalpha and ERbeta in the rat corpus luteum and decidua of pregnancy. To investigate whether PRL may stimulate ER expression at the level of transcription and which transcription factors may mediate this stimulation, we have cloned the 5'-flanking regions of both rat ER genes. A constitutively active PRL receptor (PRL-R(CA)) stimulated both ERalpha and ERbeta promoter activity, indicating that PRL is acting to stimulate ER transcription. Putative signal transducer and activator of transcription (Stat)5 response elements were identified at -189 in the ERalpha promoter and at -330 in the ERbeta promoter. Mutation of these response elements or overexpression of dominant negative Stat5 prevented stimulation of ERalpha and ERbeta promoter activity, indicating that PRL regulation of ER expression requires both intact Stat5 binding sites as well as functional Stat5. Interestingly, either Stat5a or Stat5b could stimulate ERalpha transcription while stimulation of ERbeta occurred only in the presence of Stat5b. Through mutational analysis, a single nucleotide difference between the ERalpha and ERbeta Stat5 response elements was shown to be responsible for the lack of Stat5a-mediated stimulation of ERbeta. These findings indicate that PRL stimulation of ER expression occurs at the level of transcription and that PRL regulation of ERalpha can be mediated by either Stat5a or Stat5b, while regulation of ERbeta appears to be mediated only by Stat5b.

Sp1 is required for prolactin activation of the interferon regulatory factor-1 gene

Transcription of the interferon regulatory factor-1 gene (IRF-1) is induced in a biphasic manner (G1 and G1/S phase) in Nb2 T cells in response to prolactin (PRL) stimulation. Signal transducer and activator of transcription 1 (Stat1) is required for PRL activation of the IRF-1 promoter. Mutation of a -200 bp Sp1 site in the IRF-1 promoter results in a loss of G1 but not G1/S IRF-1 transcriptional activity in response to PRL. These studies illustrate that the temporal transcription of the IRF-1 gene is mediated by not only Stat1 but also Sp1 in response to PRL stimulation.

NUDC expression during amphibian development

To identify gene products important for gastrulation in the amphibian Pleurodeles waltl, a screen for regional differences in new protein expression at the early gastrula stage was performed. A 45 kDa protein whose synthesis was specific for progenitor endodermal cells was identified. Microsequencing and cDNA cloning showed that P45 is highly homologous to rat NUDC, a protein suggested to play a role in nuclear migration. Although PNUDC can be detected in all regions of the embryo, its de novo synthesis is tightly regulated spatially and temporally throughout oogenesis and embryonic development. New PNUDC synthesis in the progenitor endodermal cells depends on induction by the mesodermal cells in the gastrula. During development, PNUDC is localized in the egg cortical cytoplasm, at the cleavage furrow during the first embryonic division, around the nuclei and cortical regions of bottle cells in the gastrula, and at the basal region of polarized tissues in the developing embryo. These results show for the first time the expression and compartmentalization of PNUDC at distinct stages during amphibian development.

Effects of the estrous cycle, pregnancy, and interferon tau on 2',5'-oligoadenylate synthetase expression in the ovine uterus

The enzymes which comprise the 2',5'-oligoadenylate synthetase (OAS) family are interferon (IFN) stimulated genes which regulate ribonuclease L antiviral responses and may play additional roles in control of cellular growth and differentiation. This study characterized OAS expression in the endometrium of cyclic and pregnant ewes as well as determined effects of IFNtau and progesterone on OAS expression in cyclic or ovariectomized ewes and in endometrial epithelial and stromal cell lines. In cyclic ewes, low levels of OAS protein were detected in the endometrial stroma (S) and glandular epithelium (GE). In early pregnant ewes, OAS expression increased in the S and GE on Day 15. OAS expression in the lumenal epithelium (LE) was not detected in uteri from either cyclic or pregnant ewes. Intrauterine administration of IFNtau stimulated OAS expression in the S and GE, and this effect of IFNtau was dependent on progesterone. Ovine endometrial LE, GE, and S cell lines responded to IFNtau with induction of OAS proteins. In all three cell lines, the 40/46-kDa OAS forms were induced by IFNtau, whereas the 100-kDa OAS form appeared to be constitutively expressed and not affected by IFNtau. The 69/71-kDa OAS forms were induced by IFNtau in the S and GE cell lines, but not in the LE. Collectively, these results indicate that OAS expression in the endometrial S and GE of the early pregnant ovine uterus is directly regulated by IFNtau from conceptus and requires the presence of progesterone.

Interferon-tau activates multiple signal transducer and activator of transcription proteins and has complex effects on interferon-responsive gene transcription in ovine endometrial epithelial cells

Interferon-tau (IFNtau), a type I IFN produced by sheep conceptus trophectoderm, is the signal for maternal recognition of pregnancy. Although it is clear that IFNtau suppresses transcription of the estrogen receptor alpha and oxytocin receptor genes and induces expression of various IFN-stimulated genes within the endometrial epithelium, little is known of the signal transduction pathway activated by the hormone. This study determined the effects of IFNtau on signal transducer and activator of transcription (STAT) activation, expression, DNA binding, and transcriptional activation using an ovine endometrial epithelial cell line. IFNtau induced persistent tyrosine phosphorylation and nuclear translocation of STAT1 and -2 (10 min to 48 h), but transient phosphorylation and nuclear translocation of STAT3, -5a/b, and -6 (10 to <60 min). IFNtau increased expression of STAT1 and -2, but not STAT3, -5a/b, and -6. IFN-stimulated gene factor-3 and STAT1 homodimers formed and bound an IFN-stimulated response element (ISRE) and gamma-activated sequence (GAS) element, respectively. IFNtau increased transcription of GAS-driven promoters at 3 h, but suppressed their activity at 24 h. In contrast, the activity of an ISRE-driven promoter was increased at 3 and 24 h. These results indicate that IFNtau activates multiple STATs and has differential effects on ISRE- and GAS-driven gene transcription.

Prolactin activates interferon regulatory factor-1 expression in normal lympho-hemopoietic cells

It has been proposed that prolactin (PRL) is a lympho-hemopoietic growth and differentiation factor. We show here by Western blotting that PRL-receptors (PRL-R) are expressed in normal rat bone marrow and spleen cells. We also show that PRL stimulates the phosphorylation of the PRL-R-associated Janus tyrosine kinase (JAK)-2 in rat bone marrow and spleen cells. This leads to the activation and subsequent binding of signal transducer and activator of transcription (Stat) 5b to an interferon regulatory factor-1 (IRF-1) gamma activation sequence (GAS) as visualized by electromobility shift assay. As shown after reverse transcription of mRNA by polymerase chain reaction, PRL, at physiological concentrations (0.01 microg/ml), stimulates the expression of the IRF-1 gene in these normal cells. PRL could thus affect several aspects of the immune response.

Paired Stat6 C-terminal transcription activation domains required both for inhibition of an IFN-responsive promoter and trans-activation

The cytokines IL-4 and IFN-gamma exert biologically antagonistic effects that in part reflect opposing influences on gene transcription. While the molecular mechanisms for IL-4-mediated transcription activation have been extensively studied, little is known about molecular mechanisms required for IL-4 inhibition of IFN-gamma signaling. We have investigated IL-4 inhibition of the IFN-gamma-inducible promoter for IFN regulatory factor-1 (IRF-1). In a cell line with low endogenous Stat6, increasing levels of activated Stat6 at constant doses of IFN-gamma and IL-4 leads to inhibition of the IRF-1 promoter. The Stat1-dependent IFN-gamma activation sequence element of the IRF-1 promoter is a target for Stat6-mediated inhibition despite apparently normal Stat1 DNA binding. However, our data are inconsistent with competition between Stat1 and Stat6 for access to the IRF-1 IFN-gamma activation sequence or for an essential coactivator as a mechanism for this Stat6-mediated inhibition. Instead, the data demonstrate that a threshold of Stat6 transcription activation domains is required for IL-4-dependent inhibition. The findings provide evidence of a novel mechanism in which the Stat6 transcription activation domains play a critical role in the IL-4-mediated inhibition of an IFN-gamma-inducible promoter.

Localization of interferon regulatory factor-1 (IRF-1) in nonpregnant human endometrium: expression of IRF-1 is up-regulated by prolactin during the secretory phase of the menstrual cycle

PRL expression in the human uterus is up-regulated during the mid to late secretory phase of the menstrual cycle. This coincides with up-regulation of the expression of the PRL receptor, which is localized primarily to the endometrial glandular epithelial cells. Recent data have demonstrated activation of the Jak (Janus kinase)/Stat (signal transducer and activator of transcription) signaling pathway in the secretory endometrium after stimulation with exogenous PRL. However, the target genes for the action of PRL on the endometrial epithelial cells have not been elucidated. In this study we have investigated the pattern/site of expression of the transcription factor interferon regulatory factor-1 (IRF-1) as well as the effect of exogenous PRL on the transcription of IRF-1 in the human endometrium during the mid to late secretory phase of the menstrual cycle. Expression of the IRF-1 gene was confirmed by RNase protection assays using a 260-bp homologous [alpha-32P]UTP-labeled IRF-1 complementary ribonucleic acid (RNA) probe and 10 microg total RNA extracted from human endometrium (n = 5) collected between days 19 and 26 of the menstrual cycle. Northern and Western blot analyses were conducted on secretory phase human endometrium (n = 3) using human [alpha-32P]dCTP-labeled IRF-1 complementary DNA and antihuman IRF-1 antibody. Expression of the IRF-1 gene in the secretory phase endometrium was encoded by a RNA transcript of approximately 2.1 kb and a protein of 48 kDa. Furthermore, expression of the IRF-1 gene in the secretory phase endometrium was localized by immunohistochemistry predominantly to the glandular epithelial cells as has been shown previously for the PRL receptor. To investigate the effect of PRL on expression of IRF-1, human endometrial biopsies (n = 3) collected between days 24-26 of the menstrual cycle were cultured in the presence of cycloheximide with or without 100 ng/mL human PRL for 2 and 4 h. Culture of endometrial tissue with PRL for 2 and 4 h resulted in 2.9 +/- 0.3-fold (P < 0.01) and 1.7 +/- 0.1-fold induction of expression of the IRF-1 gene, respectively. These data demonstrate the expression of the transcription factor IRF-1 in the glandular epithelium of the endometrium and its regulation by PRL during the secretory phase of the menstrual cycle. Previous observations of the temporal up-regulation of expression of both PRL and PRL receptors in the secretory human endometrium and their localization to the stromal and glandular compartments, respectively, suggest that endometrial PRL mediates transcription of the IRF-1 gene in a paracrine fashion.

A functional DNA binding domain is required for growth hormone-induced nuclear accumulation of Stat5B

The mechanisms regulating the cellular distribution of STAT family transcription factors remain poorly understood. To identify regions of Stat5B required for ligand-induced nuclear accumulation, we constructed a cDNA encoding green fluorescent protein (GFP) fused to the N terminus of Stat5B and performed site-directed mutagenesis. When co-expressed with growth hormone (GH) receptor in COS-7 cells, GFP-Stat5B is tyrosyl-phosphorylated, forms dimers, and binds DNA in response to GH in a manner indistinguishable from untagged Stat5B. In multiple cell types, laser scanning confocal imaging of GFP-Stat5B co-expressed with GH receptor shows that GFP-Stat5B undergoes a rapid, dramatic accumulation in the nucleus upon GH stimulation. We introduced alanine substitutions in several regions of Stat5B and assayed for GH-dependent nuclear localization. Only the mutation that prevented binding to DNA (466VVVI469) abrogated GH-stimulated nuclear localization. This mutant fusion protein is tyrosyl-phosphorylated and dimerizes in response to GH. These results suggest that either high affinity binding to DNA contributes to nuclear accumulation of Stat5B or that this region is crucial for two functions, namely accumulation of Stat5B in the nucleus and DNA binding. Thus, we have identified a mutant Stat5 defective in nuclear localization despite its ability to be tyrosyl-phosphorylated and to dimerize.

The lissencephaly gene product Lis1, a protein involved in neuronal migration, interacts with a nuclear movement protein, NudC

Important clues to how the mammalian cerebral cortex develops are provided by the analysis of genetic diseases that cause cortical malformations [1-5]. People with Miller-Dieker syndrome (MDS) or isolated lissencephaly sequence (ILS) have a hemizygous deletion or mutation in the LIS1 gene [3,6]; both conditions are characterized by a smooth cerebral surface, a thickened cortex with four abnormal layers, and misplaced neurons [7,8]. LIS1 is highly expressed in the ventricular zone and the cortical plate [9,10], and its product, Lis1, has seven WD repeats [3]; several proteins with such repeats have been shown to interact with other polypeptides, giving rise to multiprotein complexes [11]. Lis1 copurifies with platelet-activating factor acetylhydrolase subunits alpha 1 and alpha 2 [12], and with tubulin; it also reduces microtubule catastrophe events in vitro [13]. We used a yeast two-hybrid screen to isolate new Lis1-interacting proteins and found a mammalian ortholog of NudC, a protein required for nuclear movement in Aspergillus nidulans [14]. The specificity of the mammalian NudC-Lis1 interaction was demonstrated by protein-protein interaction assays in vitro and by co-immunoprecipitation from mouse brain extracts. In addition, the murine mNudC and mLis1 genes are coexpressed in the ventricular zone of the forebrain and in the cortical plate. The interaction of Lis1 with NudC, in conjunction with the MDS and ILS phenotypes, raises the possibility that nuclear movement in the ventricular zone is tied to the specification of neuronal fates and thus to cortical architecture.

Lactogenic hormone signal transduction

The peptide hormone prolactin (PRL) is known to regulate numerous target tissues. Among the less well-known targets are cells of the immune system, including T cells, B cells, and macrophages. Our laboratory has cloned a panel of PRL-inducible T-cell activation genes for use in studies investigating how PRL modulates the biology of cells of the immune system. This article focuses on two such PRL-inducible genes. One is a transcription factor called interferon regulatory factor-1, whose expression is regulated by signaling molecules along the PRL-inducible JAK/Stat signaling pathway. These signaling molecules include Stat1 and CBP as positive mediators and, unexpectedly, Stat5b as a negative mediator. A second PRL-inducible gene is c15/RNUDC, a novel nuclear movement protein, which may provide a link between PRL signaling and signaling via the lipid second messenger, platelet activating factor.

Expression of RNUDC, a potential nuclear movement protein, in mammalian cells: localization to the Golgi apparatus

Prolactin and other cytokines regulate lymphocyte proliferation through the activation of a number of genes, one of which was identified as RnudC from a prolactin-dependent rat T cell line, Nb2. RnudC encodes a 45-kDa protein whose carboxy terminal 94 amino acids are similar to the carboxy terminus of the Aspergillus nidulans nuclear movement protein NUDC. In Nb2 T cells, RNUDC protein levels are induced two- to threefold by prolactin stimulation. This prolactin-inducible increase in RNUDC protein levels is due in part to an increase in RNUDC protein synthesis between 8 and 12 h, during the G1/S transition. Newly synthesized RNUDC protein is very stable, exhibiting a half-life of greater than 8 h. RNUDC has also been identified in many different cell types and species, ranging from fibroblasts to neuronal cells and from mouse to human, suggesting a highly conserved function. Immunocytochemical studies, using affinity-purified rabbit anti-rat RNUDC antibodies, have localized a significant fraction of the RNUDC protein to the region of the Golgi apparatus in interphase rat Nb2 T cells, monkey kidney fibroblast COS-1 cells, and human 2AG10 adenocarcinoma cells. Treatment of 2AG10 cells with the microtubule-depolymerizing drug nocodazole, which causes dispersion of the Golgi apparatus, led to a diffuse pattern of RNUDC staining. Removal of nocodazole, which allowed the reformation of the Golgi apparatus, led to the reconcentration of RNUDC staining to the Golgi region. Taken together, these studies suggest that a fraction of RNUDC is tightly associated with the Golgi apparatus in many different mammalian cell types.

Growth hormone-induced tyrosyl phosphorylation and deoxyribonucleic acid binding activity of Stat5A and Stat5B

GH is known to activate JAK2 tyrosine kinase and members of the Stat family of transcription factors, including Stats 1, 3, and 5. The recent observation that at least two Stat5 proteins (Stat5A and Stat5B) exist in mouse and human, raises the question of whether GH activates both Stat5A and Stat5B and, if so, whether the requirements for activation are the same. An initial report investigating this issue demonstrated GH-dependent activation of Stat5A but not Stat5B. In this paper, we demonstrate (in COS cells expressing rat GH receptor (rGHR) and either Stat5A or Stat5B, 3T3-F442A fibroblasts, and CHO cells expressing rGHR) that GH induces tyrosyl phosphorylation of both Stat5A and Stat5B. Similar time courses of phosphorylation were observed for the two proteins. Interestingly, the pattern of observed bands differs for the two forms of Stat5. Two closely migrating Stat5A bands can be detected in cells treated with or without GH. Both of these bands become tyrosyl phosphorylated in response to GH. Three species of Stat5B are observed in untreated cells. An additional, more slowly migrating Stat5B band, appears upon treatment with GH. The three more slower migrating Stat5B bands observed in response to GH contain phosphorylated tyrosyl residues. We further demonstrate that GH induces binding of Stat5A and Stat5B, as well as Stat1, to the GAS-like element in the beta-casein promoter. We and others have demonstrated previously that specific regions of GHR are required for GH-dependent activation of what is here identified as Stat5B. To gain insight into the mechanism by which GH promotes tyrosyl phosphorylation of Stat5A, GH-dependent tyrosyl phosphorylation of Stat5A was examined in CHO cells expressing truncated and mutated rGHR. The results indicate that Stat5A and Stat5B require the same regions of rGHR for maximal activation by GH: the C-terminal half of the cytoplasmic domain; tyrosines 333 and/or 338 in the N-terminal half of the cytoplasmic domain; and the regions required for JAK2 activation. To dissect further the mechanism by which GH activates Stat5A and B, the requirement for JAK2 in GH-dependent Stat5 tyrosyl phosphorylation was assessed using JAK2-deficient cells expressing GHR (gamma2A-GHR) and the wild-type parental cell line expressing GHR (2C4-GHR). GH-induced tyrosyl phosphorylation of Stat5B in 2C4-GHR cells but not in the JAK2 deficient, gamma2A-GHR cells, indicating that JAK2 is required for GH-dependent tyrosyl phosphorylation of Stat5B. Western blotting revealed that Stat5A is not expressed in this cell type. Taken together, these findings suggest that: 1) GH activates both Stat5A and Stat5B in several cell types; 2) the pattern of bands observed differs for Stat5A and Stat5B; 3) GH-dependent tyrosyl phosphorylation of Stat5A requires specific regions of GHR, and these requirements are the same as for Stat5B; and 4) JAK2 kinase is required for GH-dependent tyrosyl phosphorylation of Stat5B and, most likely, Stat5A.

Molecular actions of prolactin in the immune system

The immunoregulatory properties of prolactin, a pituitary peptide hormone, have received renewed attention. The prolactin receptor, a member of the hematopoietin/cytokine receptor superfamily, is ubiquitously expressed by cells in the immune system. Certain subpopulations of lymphocytes synthesize and secrete biologically active prolactin, which suggests that prolactin can act as an autocrine and/or paracrine factor to modulate the activities of cells of the immune system. This review focuses on the molecular actions of prolactin in the immune system. Emphasis is given to recent information about the molecular mechanisms of prolactin receptor signal transduction, and the signaling molecules and prolactin-inducible target genes that participate in these responses. In particular, the prolactin-inducible interferon regulatory factor-1 gene and its roles in mediating diverse immune responses are highlighted.

Multiple stat complexes interact at the interferon regulatory factor-1 interferon-gamma activation sequence in prolactin-stimulated Nb2 T cells

Interferon regulatory factor-1 (IRF-1) is a major immediate early gene induced by prolactin (PRL) in a biphasic, cell cycle-dependent manner in Nb2 T cells. This biphasic expression (30 min and 10 h) is mediated in part by an interferon-gamma activation sequence (GAS) in the IRF-1 promoter which binds factors belonging to the Signal Transducers and Activators of Transcription (Stat) family. By electrophoretic mobility shift assays (EMSA), Stat1 alpha was found to be the major and Stat5a a minor component of the 30 min complex. At 10 h, Stat-like factors were again found at the IRF-1 GAS. Western blot analyses show that Stat5a was rapidly induced by PRL to enter the nucleus, but unexpectedly, Stat1 alpha and the alternatively-spliced Stat1 beta were already present in the uninduced nucleus. Further, Stat1 alpha but not Stat1 beta is preferentially tyrosine phosphorylated in response to PRL stimulation. Our studies suggest that multiple Stat complexes may contribute to the biphasic transcription of the IRF-1 gene in PRL-stimulated T cells.

Multiple cis-trans conformers of the prolactin receptor proline-rich motif (PRM) peptide detected by reverse-phase HPLC, CD and NMR spectroscopy

An eight-amino-acid synthetic peptide (IIe1-Phe2-Pro3-Pro4-Val5-Pro6-Gly7-Pro8) corresponding to the conserved proline-rich motif (PRM) of the intracellular domain of the prolactin receptor (PRL-R) was studied by one- and two-dimensional (1D and 2D) proton NMR spectroscopy in water and DMSO in order to characterize its conformational dynamics. The purified PRL-R PRM peptide eluted as two partially resolved peaks in equilibrium on reverse-phase HPLC (RP-HPLC) at 20 degrees C with a ratio of 60:40. At 30 degrees C, the two peaks coalesced into a single peak The two RP-HPLC peaks correspond to two peptide conformers resulting from the slow cis-trans isomerization of one of the four proline amide bonds. Although the peptide has only three amide (NH) protons, its ID NMR spectrum in water contains approximately 15 discernible NH region peaks, providing evidence for multiple conformers. The amide resonances were assigned on the basis of 2D-COSY spectra, chemical shift values resonance splitting patterns and temperature coefficients. The cis:trans ratio for each proline in water, calculated from integrated intensities and/or peak heights of the appropriate resonances, were Phe2-Pro3 (35:65), Pro3-Pro4 (40:60), Val5-Pro6 (70:30), and Gly7-Pro8 (30:70). Temperature studies (25-70 degrees C) were used to semi-quantitatively estimate the rates of isomerization for the different prolines. In water, Pro8 undergoes rapid isomerization; Pro3 isomerizes at an intermediate rate; while Pro4 and Pro6 both appear to isomerize very slowly since no coalescence of amide resonances was observed. In DMSO, only Pro4 displayed slow isomerization. Slow kinetics combined with a similar 60:40 ratio of conformers determined by RP-HPLC and NMR suggests that isomerization of the Pro3-Pro4 bond generates the two RP-HPLC peaks. Both proximal and distal proline isomerization effects were observed in NMR experiments. All of the 16 theoretical (24 = 16) proline configurations appear to exist in equilibrium in water The predominant (19%) conformation, trans3-trans4-cis6-trans8, may reflect the configuration of the PRM prolines in the native PRL-R. Isomerization of Pro6 from cis to trans generates an interaction between the peptide N-and C-termini, suggesting an overall pseudo-cyclic conformation. This all-trans proline configuration may play an important biochemical role in the function of cytokine/haematopoietin receptors. A model is proposed which suggests that isomerization of the PRM by an immunophilin such as the FK 506-binding protein (FKBP) serves as an on-off switch for cytokine receptor activation.

Biphasic transcriptional regulation of the interferon regulatory factor-1 gene by prolactin: involvement of gamma-interferon-activated sequence and Stat-related proteins

Stimulation of quiescent Nb2 T cells by PRL leads to the rapid transcriptional activation of a T cell activation gene, interferon regulatory factor-1 (IRF-1). IRF-1 is induced twice by PRL in a single cell cycle, first during G1 at 30-60 min and again over early S phase at 10-12 h. By nuclear run-on transcription analysis of IRF-1 promoter-chloramphenicol acetyl transferase (CAT) constructs, the -1.7 kilobase (kb) 5'-flanking IRF-1 DNA was shown to contain elements that mediate both G1 and S phase expression. The -200 bp IRF-1 promoter DNA contains elements that respond to G1 PRL stimulation in a protein synthesis independent manner, suggesting the involvement of pre-existing factors. Further promoter deletion analysis delineated a minimal PRL responsive region between -112 and -205 bp. Within this region is a Gamma Interferon Activated Sequence or GAS, consisting of two inverted GAAA motifs (-123/-113), which confers PRL-inducible expression to a reporter gene, suggesting that GAS can function as a PRL responsive element. Further, GAS exhibits binding with nuclear proteins in a PRL-inducible, cell cycle-dependent manner. One of these proteins appears to be related to the emerging family of Signal Transducer and Activator of Transcription or Stat factors. These studies suggest that the GAS site and Stat-like proteins participate in PRL receptor signal transduction to regulate the biphasic expression of the IRF-1 gene in PRL-stimulated T cells.

Characterization of a prolactin-inducible gene, clone 15, in T cells

To examine how PRL regulates lymphocyte proliferation, a number of PRL-activated genes were identified from a PRL-dependent rat T lymphoma cell line, Nb2. One of the downstream genes in the PRL signaling cascade was identified as clone 15 (c15). PRL stimulation of quiescent Nb2 T cells results in the expression of a 1.7-kilobase c15 mRNA, which reaches maximum levels between 8 and 10 h after stimulation. Corresponding [3H]thymidine incorporation experiments show that the maximum level of c15 mRNA expression correlates with the G1/S transition phase of the cell cycle. Sequencing of approximately 1.3-kilobase cDNA revealed one open reading frame that predicts a 332-amino acid protein. In vitro transcription/translation of c15 cDNA resulted in the production of a 45-kilodalton protein. Sequence analysis revealed that the c15 open reading frame contains a potential nuclear localization signal, a very acidic region, and a carboxy-terminal region of 94 amino acids which are 68% identical and 78% similar to the nuclear movement protein, NUDC, found in Aspergillus nidulans. Such a high degree of conservation suggests that the NUDC-like motif in c15 has been conserved through evolution for an important structure and/or function.

Coordinate gene expression of luteinizing hormone-releasing hormone (LHRH) and the LHRH-receptor after prolactin stimulation in the rat Nb2 T-cell line: implications for a role in immunomodulation and cell cycle gene expression

PRL has been shown to induce a number of genes after the stimulation of quiescent Nb2 T-cells, including c-fos, c-myc, ornithine decarboxylase, interferon regulatory factor-1, and others. One of these genes, LHRH, has not previously been reported to respond in this manner, although we and others have reported its presence in rat and human T- and B-cells. Furthermore, recent evidence suggests that LHRH functions as an immunoregulator in a cytokine-like manner. Using the rat immature T-cell line Nb2, we present data showing for the first time that 1) the LHRH gene is regulated by PRL at various times during the cell cycle; 2) an alternatively spliced LHRH messenger RNA exists in Nb2 cells and may produce a new truncated GnRH-associated peptide (alternatively called PIF for PRL-inhibiting factor); 3) the LHRH receptor is expressed in lymphocytes in a manner similar to the LHRH gene after PRL addition, and its complementary DNA sequence is identical to that of the pituitary receptor; 5) the SH gene, found on the opposite strand of the LHRH gene, is expressed in lymphocytes at the same time and in the same manner as the LHRH gene; 6) the LHRH messenger RNA has a very short half-life in these cells; and 7) the lymphocyte LHRH transcription start site is essentially the same as the hypothalamic site. These data strengthen the relationship between PRL and LHRH expression in the immune system and further support our contention that LHRH is an important immunoregulator, on par with other known cytokines.

Differential signal transduction of the short, Nb2, and long prolactin receptors. Activation of interferon regulatory factor-1 and cell proliferation

An Nb2 prolactin receptor (PRL-R) cDNA has been cloned from the PRL-dependent Nb2-11C cell line, and the protein-coding region is identical to that of the PRL-R isolated from the PRL-independent cell line Nb2-Sp. Short, Nb2, and long forms of the PRL-R were analyzed for signal transduction to the immediate-early gene, interferon regulatory factor-1 (IRF-1) and for cellular proliferation. Receptor and IRF-1-CAT reporter constructs were transiently cotransfected into the interleukin-3-dependent cell lines FDC-P1 and BaF3. The Nb2 PRL-R induced IRF-1-CAT 14.3-fold on addition of PRL, while the long PRL-R induced IRF-1-CAT 5.6-fold in FDC-P1 cells. The short PRL-R did not activate the IRF-1 promoter. Stable transfectants were also generated by selecting for growth in PRL. Only the Nb2 and long forms were able to convert the IL-3-dependent cells to PRL-dependence. IRF-1-CAT was induced in these cell lines by the Nb2 PRL-R 10- to 12-fold and long PRL-R 3- to 3.5-fold. Overall, the Nb2 form is more efficient than the long form by about 3-fold at inducing IRF-1-CAT. A PRL dose-response growth curve showed that the Nb2 form requires 20-fold less PRL for half maximal growth than the long form. A PRL dose-response for IRF-1-CAT activity gave similar results, indicating a tight correlation between IRF-1 induction and cell proliferation. These results show that the short PRL-R does not signal to IRF-1 or for growth, and that the Nb2 PRL-R signals more efficiently than the long PRL-R.

Structure-function studies of anti-3-fucosyllactosamine (Le(x)) and galactosylgloboside antibodies

We are studying murine mAbs against two carbohydrate epitopes, 3-fucosyllactosamine (Le(x), CD15) and galactosylgloboside. The VH domains of both panels of Ab are encoded by VH441, a member of the X24 family of Ig genes. To evaluate the contribution of the heavy chain CDR3 to the affinity of the anti-3-fucosyllactosamine Ab, CDR3-H of PMN6, a low affinity Ab, was replaced by the CDR3 of PM81, a higher affinity Ab. The affinity of the chimeric 6/81 Ab was increased when the heavy chain was paired with the PM81 light chain, but not when paired with another light chain (M5), which differs from PM81 light chain by three amino acids. To evaluate the contribution of somatic mutations to the binding of GalGb4, the 3A9 VH sequence, which contains three amino acid substitutions, was replaced by a germ-line sequence encoded by either VH441 or VHX24. The chimeric Ab, 441/3A9 and X24/3A9, bound Ag as well as the wild-type 3A9 Ab. Computer models of the Fv fragments of PM81 and 3A9 were compared with the crystal structure of the Fv fragment of J539, a galactan-binding myeloma protein that is encoded by the same VH and VK genes as 3A9. The surfaces of 3A9 and J539 have shallow pockets that are potential Ag-binding sites. Replacement of CDR3-H Tyr99, which is a prominent component of the pocket, by Ala abolished the binding of Ag. In contrast, the Fv surface of PM81 contains a large cleft rather than a pocket. These models indicate how the same VH gene segment can be used to encode Abs that exhibit different specificities.

Structure-function studies of anti-3-fucosyllactosamine (Le(x)) and galactosylgloboside antibodies

We are studying murine mAbs against two carbohydrate epitopes, 3-fucosyllactosamine (Le(x), CD15) and galactosylgloboside. The VH domains of both panels of Ab are encoded by VH441, a member of the X24 family of Ig genes. To evaluate the contribution of the heavy chain CDR3 to the affinity of the anti-3-fucosyllactosamine Ab, CDR3-H of PMN6, a low affinity Ab, was replaced by the CDR3 of PM81, a higher affinity Ab. The affinity of the chimeric 6/81 Ab was increased when the heavy chain was paired with the PM81 light chain, but not when paired with another light chain (M5), which differs from PM81 light chain by three amino acids. To evaluate the contribution of somatic mutations to the binding of GalGb4, the 3A9 VH sequence, which contains three amino acid substitutions, was replaced by a germ-line sequence encoded by either VH441 or VHX24. The chimeric Ab, 441/3A9 and X24/3A9, bound Ag as well as the wild-type 3A9 Ab. Computer models of the Fv fragments of PM81 and 3A9 were compared with the crystal structure of the Fv fragment of J539, a galactan-binding myeloma protein that is encoded by the same VH and VK genes as 3A9. The surfaces of 3A9 and J539 have shallow pockets that are potential Ag-binding sites. Replacement of CDR3-H Tyr99, which is a prominent component of the pocket, by Ala abolished the binding of Ag. In contrast, the Fv surface of PM81 contains a large cleft rather than a pocket. These models indicate how the same VH gene segment can be used to encode Abs that exhibit different specificities.

Multiple prolactin-responsive elements mediate G1 and S phase expression of the interferon regulatory factor-1 gene

The interferon regulatory factor-1 (IRF-1) gene is both an immediate-early G1 phase gene and an S phase gene inducible by PRL in rat Nb2 T lymphocytes. To understand the mechanism by which PRL regulates the biphasic expression of IRF-1, we cloned the rat IRF-1 gene and functionally characterized the IRF-1 promoter. Upon transfection into Nb2 T cells, 1.7 kilobases (kb) of IRF-1 5'-flanking DNA linked to a chloramphenicol acetyl transferase (CAT) reporter gene mediated a 30-fold induction of CAT enzyme activity in response to 24 h of PRL stimulation. Deletion mutants containing 1.3, 0.6, and 0.2 kb 5'-flanking DNA were incrementally less transcriptionally active, although 0.2 kb still mediated a 12-fold induction by PRL. The sequence between -1.7 and -0.2 kb linked to a heterologous thymidine kinase promoter failed to respond to PRL stimulation, suggesting that the activity of upstream PRL response elements may require an interaction with promoter-proximal elements. By assaying CAT enzyme activity across a 24-h PRL induction time course, we were able to assign G1 vs. S phase PRL responses of the IRF-1 gene to different regions of the IRF-1 5'-flanking and promoter DNA. The 0.2-kb IRF-CAT construct was induced by PRL stimulation during the G1 phase of the cell cycle. In contrast, the 1.7-kb IRF-CAT construct was inducible by PRL during both G1 and S phase of the cell cycle. Hence, the PRL-induced biphasic expression of the IRF-1 gene appears to be controlled by separate PRL-responsive elements: elements in the first 0.2 kb of the IRF-1 promoter region act during early activation, and elements between 0.2 and 1.7 kb act in concert with the proximal 0.2-kb region during S phase progression.

The proline-rich motif (PRM): a novel feature of the cytokine/hematopoietin receptor superfamily

Members of the cytokine receptor superfamily have been grouped together by function and by the presence of conserved amino acids in the extracellular domain, including four cysteine residues and the Trp-Ser-X-Trp-Ser (WSXWS) motif. However, no consensus sequence motif has been described in the intracellular domain of the cytokine receptors. We now report the presence of a proline-rich consensus sequence motif, eight amino acids in length, which is found in the intracellular domain of all the cytokine receptors. The proline-rich motif (PRM) can be divided into two complementary families that have superimposable consensus sequences. The consensus sequences were found by allowing similar amino acids (aliphatic = Al, aromatic = Ar) to be grouped together. The first motif (PRM1) has the sequence Al-Ar-Pro-X-Al-Pro-X-Pro, while the second (PRM2) is Ar-X-X-X-Al-Pro-X-Pro. An overall consensus sequence for the PRM (PRM1 and PRM2) is derived by allowing aromatic and aliphatic residues to be considered hydrophobic (psi): psi-X-X-X-Al-Pro-X-Pro. Several alternative cytokine receptor isoforms contain two copies of the PRM within the same intracellular domain. The conservation of the proline-rich motif in cytokine receptors suggests that it plays a critical role in receptor function and defines a new feature of the cytokine receptor superfamily.

The transcription factor interferon regulatory factor-1 is expressed during both early G1 and the G1/S transition in the prolactin-induced lymphocyte cell cycle

PRL induces quiescent Nb2 rat T-lymphoma cells to undergo mitogenesis. Upon PRL stimulation, the transcription factor interferon regulatory factor-1 (IRF-1) is induced as a novel T-cell activation gene in Nb2 cells. Surprisingly, IRF-1 is expressed twice during a single PRL-induced growth cycle: first during the early G1 phase, in an immediate transient peak from 15 min to 2 h, and second during the G1/S phase transition, in a broader peak beginning at 8 h. The unusual biphasic expression of IRF-1 mRNA is accompanied both times by de novo IRF-1 protein synthesis. However, the rate of IRF-1 protein turnover appears to be different in G1 and S phases. IRF-1 protein expressed in G1 exhibits a half-life of about 25 min, whereas in the S phase, the half-life is about 60 min. By washing out PRL at various times during G1, we found a direct correlation among the length of PRL exposure, the second peak of IRF-1 mRNA expression, and DNA synthesis. Our data suggest that PRL and one putative nuclear mediator, IRF-1, may be important in two distinct phases of the cell cycle: first in cell cycle activation, and then in S phase progression.

Structure and specificities of anti-ganglioside autoantibodies associated with motor neuropathies

Autoantibodies that bind to GM1 ganglioside and asialo GM1 (GA1) have been implicated in the pathogenesis of motor neuropathies. To investigate the structure and specificity of these autoantibodies, peripheral blood B cells from patients with motor neuron diseases and from normal individuals were immortalized by EBV, and B cells secreting anti-GM1 or GA1 antibodies were cloned. We report an analysis of the structure and specificities of eight autoantibodies from patients with motor neuropathy, and two from normal individuals. Four antibodies were IgM, six were IgG, and all bound predominantly to GA1. The sequences of V domains of H and L chains were determined by a reverse transcription-polymerase chain reaction procedure. A variety of V genes were used to encode these antibodies: four VH1, two VH3, three VH4, one VH5, two V kappa I, two V kappa II, three V kappa III, and two V lambda II. Most V genes (13/19) exhibited less than 95% similarity to known germ-line genes, which suggests that somatic mutation was required to generate these autoantibodies, or that the relevant germ-line genes have not been identified. The average length of the H chain CDR3 was 16 amino acids, and in three antibodies this segment contained more than 20 amino acids. It was not possible to identify amino acid sequences that were encoded by germ-line D segments by conventional alignment of sequences. Partial analogies could be identified by introducing gaps, allowing mismatches and searching for D-D fusions and inversions. These results indicate that anti-GA1 antibodies can be encoded by a variety of VH-VL pairs, that the antibodies exhibit extensive somatic mutation, and that the CDR3 segments are generated by a number of nonconventional mechanisms.

Structure and specificities of anti-ganglioside autoantibodies associated with motor neuropathies

Autoantibodies that bind to GM1 ganglioside and asialo GM1 (GA1) have been implicated in the pathogenesis of motor neuropathies. To investigate the structure and specificity of these autoantibodies, peripheral blood B cells from patients with motor neuron diseases and from normal individuals were immortalized by EBV, and B cells secreting anti-GM1 or GA1 antibodies were cloned. We report an analysis of the structure and specificities of eight autoantibodies from patients with motor neuropathy, and two from normal individuals. Four antibodies were IgM, six were IgG, and all bound predominantly to GA1. The sequences of V domains of H and L chains were determined by a reverse transcription-polymerase chain reaction procedure. A variety of V genes were used to encode these antibodies: four VH1, two VH3, three VH4, one VH5, two V kappa I, two V kappa II, three V kappa III, and two V lambda II. Most V genes (13/19) exhibited less than 95% similarity to known germ-line genes, which suggests that somatic mutation was required to generate these autoantibodies, or that the relevant germ-line genes have not been identified. The average length of the H chain CDR3 was 16 amino acids, and in three antibodies this segment contained more than 20 amino acids. It was not possible to identify amino acid sequences that were encoded by germ-line D segments by conventional alignment of sequences. Partial analogies could be identified by introducing gaps, allowing mismatches and searching for D-D fusions and inversions. These results indicate that anti-GA1 antibodies can be encoded by a variety of VH-VL pairs, that the antibodies exhibit extensive somatic mutation, and that the CDR3 segments are generated by a number of nonconventional mechanisms.

Prolactin gene expression in human thymocytes

Recent evidence suggests that lymphocytes produce prolactin (PRL). Here, we report the cDNA cloning and expression of PRL from normal human thymocytes. Sequence analysis showed that the thymocyte cDNA encodes a 23 kDa protein which is identical to pituitary PRL. RNA blot analysis showed that the thymocyte PRL mRNA is approximately 170 nucleotides larger than the pituitary PRL message. PRL message was also detected in several non-pituitary human cell lines including Jurkat T, HeLa, and JEG cells. Furthermore, PRL gene expression in JEG cells was inhibited by glucocorticoid treatment. Our data support the hypothesis that PRL is a T cell-derived cytokine.

Interferon regulatory factor-1 is inducible by prolactin, interleukin-2 and concanavalin A in T cells

Interferon regulatory factor-1 (IRF-1) gene expression is rapidly upregulated in the prolactin (PRL)-activated Nb2 rat T lymphoma cell line. To further elucidate its role as a T cell activation molecule, IRF-1 gene expression in response to various T cell stimuli was examined. In Nb2 T cells, PRL induced two peaks of IRF-1 gene expression: a rapid, transient peak at 1 h and a sustained peak at 12 h. PRL subsequently induced interferon-gamma (IFN-gamma) gene expression at 3-6 h. However, the early induction of IRF-1 and IFN-gamma does not appear to be interdependent. Interleukin-2 (IL-2) also induced IRF-1 gene expression in Nb2 T cells but only one broad peak at 10 h was observed. In primary mouse splenocytes, concanavalin A induced rapid and transient expression of the IRF-1 gene; maximal expression occurred by 6 h, and then returned to basal levels by 12-15 h. These results provide additional evidence for the importance of IRF-1 in T cell activation.

Polymorphism of human immunoglobulin VH4 germ-line genes

The human immunoglobulin VH4 gene family is thought to contain approximately 10 germ-line genes and to exhibit little polymorphism. We report here an analysis of VH4 germ-line genes that were amplified from DNA of two unrelated individuals. Ten unique (non-repetitive) sequences were obtained from individual A and 11 from individual B. Nine of these sequences represent new germ-line genes, and 8/9 exhibit only 89%-96% similarity to genes identified previously. Subsets of VH4 genes displayed distinctive nucleotide motifs that account for most of the differences between them. This observation suggests that diversity in the VH4 gene family arose from the acquisition of blocks of nucleotides, rather than by accumulation of point mutations. These nucleotide blocks could have been acquired by gene conversion or by homologous recombination. All of the VH4 genes have a potential N-linked glycosylation site at Asn 60, and some genes encode a second site at Asn 52. The VH4 gene family is larger and more polymorphic than appreciated previously. Immunoglobulin gene polymorphism may make a significant contribution to hereditary variations in the immune response and to the genetic predisposition to autoimmune diseases.

Prolactin receptor gene expression in lymphoid cells

To understand the role of pituitary prolactin (PRL) and its receptor (PRL-R) in the growth and differentiation of lymphoid cells, PRL-R gene expression was analyzed in various lymphoid tissues and in a rat T lymphoma cell line, Nb2, which requires PRL for growth. The technique of reverse transcription coupled to polymerase chain reaction (RT-PCR) was used to detect the low abundance PRL-R transcripts. Within 30 min to 1 h, PRL stimulates a rapid but transient increase in PRL-R mRNA levels in Nb2 T cells. By 4 h, PRL-R mRNA returned to near basal levels and then gradually declined to a new steady-state level by 12 h. Significant increases in receptor RNA levels were observed in the presence of protein synthesis inhibitors, which suggests that PRL-R mRNA levels are under negative regulation. PRL-R gene expression was also demonstrated in normal mouse thymocytes, splenocytes, and in several lymphoid cell lines. The expression of the PRL-R gene in stimulated lymphoid cells provides additional evidence for the role of PRL as an immunomodulatory molecule.

Heavy and light chain sequences of four monoclonal antibodies that bind galactosylgloboside (GalGb4)

We recently described IgM monoclonal antibodies directed against the glycospingolipid galactosylgloboside (GalGb4; Marcus, M. D. et al., Arch. Biochem. Biophys, 1988.262: 620). We now present the nucleotide and deduced amino acid sequences of the heavy and light chains of these antibodies. The antibodies were generated in a single fusion, their heavy as well as their light chains are almost identical, and they appear to be clonally related. The light chains were encoded by J kappa 5 and a V kappa gene belonging to the Ox1 family, but they are only 93% homologous to the most closely related germ-line gene, and they are probably encoded by a germ-line gene that has not yet been identified. The heavy chains were all encoded by VH441 and JH2, and have identical N segments. The VH441 germ-line gene encodes a potential glycosylation site at Asn58 in the complementarity-determining region 2. This site, which has been retained in all VH441-encoded monoclonal antibodies sequenced previously, was mutated out by a single base change in all four anti-GalGb4 antibodies.

Interferon-regulatory factor 1 is an immediate-early gene under transcriptional regulation by prolactin in Nb2 T cells

The pituitary peptide hormone prolactin (Prl) is a potent inducer of Nb2 T lymphoma cell proliferation. To analyze the early genetic response to the mitogenic signals of Prl, a cDNA library was constructed from Nb2 T cells stimulated for 4 h with Prl and the protein synthesis inhibitor cycloheximide. Of 26 distinct clones isolated by differential screening, one clone, designated c25, exhibited extremely rapid but transient kinetics of induction by Prl and superinduction by Prl plus cycloheximide. Run-on transcription analysis indicated that c25 gene transcription was induced greater than 20-fold within 30 to 60 min of Prl stimulation. Surprisingly, DNA sequence analysis of c25 cDNA revealed that this Prl-inducible early-response gene is the rat homolog of the mouse transcription factor interferon-regulatory factor 1 (IRF-1), sharing 91% coding sequence similarity with mouse IRF-1. At the protein level, rat IRF-1 shares 97% and 92% homology with mouse IRF-1 and human IRF-1, respectively, suggesting that this molecule has been functionally conserved throughout evolution. Our studies show that the gene for IRF-1 is an immediate-early gene in Prl-stimulated T cells, which suggests that IRF-1 is a multifunctional molecule. In addition to its role in regulating growth-inhibitory interferon genes, IRF-1 may, therefore, also play a stimulatory role in cell proliferation. The gene for IRF-1 is one of the earliest genes known to be transcriptionally regulated by Prl.

Prolactin stimulates transcription of growth-related genes in Nb2 T lymphoma cells

The pituitary peptide hormone prolactin exerts a profound effect on various physiological processes involving both cellular proliferation and differentiation. The rat Nb2 T lymphoma cell line has been used as a model system for studying prolactin regulation of cell proliferation. Several genes associated with cell growth (c-myc, ornithine decarboxylase (ODC), heat shock protein 70 (hsp 70)-homologue, and beta-actin) are induced rapidly within 4 h after prolactin addition. Nuclear run-on transcription assays indicate that prolactin induction of these growth-related genes occurs primarily at the transcriptional level. According to the different kinetics of transcriptional response to prolactin, these growth-related genes can be divided into immediate-early (actin, c-myc), early (ODC) and mid-G1 (hsp 70-homologue) genes. Thus, prolactin may regulate Nb2 T cell-proliferative responses by modulating the transcriptional induction of various growth-related genes. These studies also represent a first report of a transcriptional cascade set off in rapid response to prolactin in cultured T cells.

A transfected alpha-casein minigene bypasses posttranscriptional control by hormones, but retains cell-substratum regulation in mammary epithelial cells

DNA-mediated gene transfection using an alpha-casein minigene cloned into a bovine papilloma virus (BPV)-based neomycin-selectable expression vector has been employed to study the mechanisms by which hormonal and cell-substratum interactions regulate milk protein gene expression. Permanently transformed clones and pooled populations of normal midpregnant mouse mammary epithelial cells (COMMA-D) containing the minigene express an authentic rat alpha-casein mRNA, as well as a series of larger cytoplasmic RNA transcripts. These transcripts are correctly initiated and spliced; however, a large proportion also contain additional sequences at the 3'-end. Constitutive expression of the minigene in the absence of PRL and glucocorticoids in COMMA-D cells grown on floating type I collagen gels is observed. Thus, the minigene-BPV construct apparently overrides the normal posttranscriptional regulatory mechanisms which are responsible for the expression of unstable casein gene transcripts in the absence of PRL and glucocorticoids. In contrast, this minigene-BPV construct is regulated appropriately by cell-substratum interactions in pooled transfectants. Minigene expression is undetectable when pooled transfectants are plated on a plastic substratum, and readily detectable when cells are grown on floating type I collagen gels. Thus, hormones and cell-substratum interactions may regulate different steps in the same differentiation pathway leading to increased casein gene expression.

Evolution of the casein multigene family: conserved sequences in the 5' flanking and exon regions

The rat alpha- and bovine alpha s1-casein genes have been isolated and their 5' sequences determined. The rat alpha-, beta-, gamma- and bovine alpha s1-casein genes contain similar 5' exon arrangements in which the 5' noncoding, signal peptide and casein kinase phosphorylation sequences are each encoded by separate exons. These findings support the hypothesis that during evolution, the family of casein genes arose by a process involving exon recruitment followed by intragenic and intergenic duplication of a primordial gene. Several highly conserved regions in the first 200 base pairs of the 5' flanking DNA have been identified. Additional sequence homology extending up to 550 base pairs upstream of the CAP site has been found between the rat alpha- and bovine alpha s1-casein sequences. Unexpectedly, the 5' flanking promoter regions are conserved to a greater extent than both the entire mature coding and intron regions of these genes. These conserved 5' flanking sequences may contain potential cis regulatory elements which are responsible for the coordinate expression of the functionally-related casein genes during mammary gland development.

The rat casein multigene family. Fine structure and evolution of the beta-casein gene

Eight overlapping phage clones, spanning 34.4 kilobase pairs of genomic DNA, containing the 7.2-kilobase pair rat beta-casein gene have been isolated and characterized. The first 510 base pairs (bp) of 5' flanking, 110 bp of 3' flanking, and all the exon/intron junctions have been sequenced. The beta-casein gene contains 9 exons ranging in size from 21 to 525 bp. We have attempted to identify potential regulatory elements by searching for regions of sequence homology shared between milk protein genes which respond similarly to lactogenic hormones and by searching for previously reported hormone receptor-binding sites. Within the conserved first 200 bp of 5' flanking sequences 3 regions of greater than 70% homology were observed between the rat beta- and gamma-casein genes. One of these contains a region 90% homologous to the chicken progesterone receptor-binding site. The conserved 5' noncoding region, the highly conserved signal peptide, and the hydrophobic carboxyl-terminal region of the protein are each encoded by a separate exon. In contrast the evolutionarily conserved phosphorylation site of beta-casein is formed by an RNA-splicing event. The exons which encode the phosphorylation sites of beta-casein appear to have resulted from an intragenic duplication. Based upon the exon structure of the casein genes, an evolutionary model of intragenic and intergenic exon duplications for this gene family is proposed.

Tissue- and cell-specific casein gene expression. II. Relationship to site-specific DNA methylation

The relationship between DNA methylation and the expression of the gamma- and beta-casein genes was investigated in both expressing and nonexpressing tissues and in isolated tumor cell subpopulations displaying differential casein gene expression. MspI/HpaII digestions of DNA isolated from liver, a totally nonexpressing tissue, indicated that specific sites of hypermethylation existed in these genes as compared to the DNA isolated from casein-producing lactating mammary gland. The positions of these sites were mapped in the gamma-casein gene by comparing total genomic DNA Southern blots to the restriction digests of several overlapping phage clones constituting the gamma-casein gene. In contrast, the methylation status of the HhaI sites in the gamma-casein gene was found to be invariant regardless of the expression status of the gene. The inverse correlation between the hypermethylation of certain MspI/HpaII restriction sites in the casein genes and their potential expressibility was further substantiated by studies in 7,12-dimethylbenz(a)anthracene- and N-nitrosomethylurea-induced mammary carcinomas, which have an attenuated casein gene expression, and in cell subpopulations isolated from the 7,12-dimethylbenz(a)-anthracene tumor which were either depleted or enriched in casein-producing cells. Analysis of total tumor DNAs indicated that the casein genes were hypermethylated at the same sites observed in liver. However, a very faint hybridization signal was observed in the HpaII digests, suggesting cell-specific methylation differences. We have confirmed the hypomethylation of at least two of these MspI/HpaII sites within the subpopulation containing the casein-producing cells at a level consistent with the relative enrichment in that fraction. These results demonstrate differential site-specific casein gene methylation not only between tissues but also between cell subpopulations within a single tissue.

The rat casein multigene family. I. Fine structure of the gamma-casein gene

A region approximately 35 kilobase pairs (kb) in length containing the hormonally regulated rat gamma-casein gene has been characterized by examining overlapping clones of genomic rat DNA obtained from two Charon 4A libraries. The entire gamma-casein structural gene is contained in a single 17-kb phage clone. R-loop and restriction enzyme mapping analyses revealed that the gamma-casein gene is approximately 15 kb long and is, therefore, 17.4 times larger than the mature gamma-casein mRNA. The coding regions of the gamma-casein gene are split into at least nine small segments, interspersed with long intervening sequences. Sequence analysis of the 5' end of the gamma-casein gene revealed the presence of a TATA sequence which may play a role in the initiation of gene transcription. The first exon is 44 nucleotides long and encodes part of the 5' noncoding sequences of the gamma-casein mRNA. The first intron was found to contain a short interspersed repeated DNA sequences which shares a 92% homology with a cloned rat repeated DNA sequence found at the 3' end of several other rat genes. In addition, the gamma-casein gene contains several families of highly repeated sequences interspersed throughout the intervening and flanking regions, including a family of evolutionary conserved repeats. Thus, the gamma-casein gene represents an unusually large and complex split mammalian gene.