Purification, cloning, and expression of the prolactin receptor

The rat liver prolactin receptor has been purified to homogeneity, and partial amino acid sequences have been obtained. The structure of the receptor has been deduced from a single complementary DNA clone. The mature protein of 291 amino acids has a relatively long extracellular region, a single transmembrane segment, and a short (57 amino acids) cytoplasmic domain. With the rat cDNA used as a probe, the prolactin receptor in rabbit mammary gland and human hepatoma cells has also been isolated. These tissues contain a second, longer form of the receptor (592 and 598 amino acids, respectively). Both the short and long forms of the prolactin receptor show regions of strong sequence identity with the human and rabbit growth hormone receptors, suggesting that the prolactin and growth hormone receptors originate from a common ancestor.     

Isolation and characterization of two novel rat ovarian lactogen receptor cDNA species

Two novel lactogen receptor cDNA clones (2.1 and 1.2 kb) were isolated from a rat ovarian cDNA library. Nucleotide sequence of the 2.1 kb clone codes for a 610 aa receptor (nonglycosylated mol. wgt. 66,000 D) with an extracellular domain, a transmembrane region and an intracellular domain, and exhibited significant overall similarity with the rat liver receptor (310 aa) and both rabbit mammary and human hepatoma receptors (616 and 622 aa). However, the ovarian lactogen receptor sequence contains a unique cytoplasmic domain of 110 aa and consensus sequences for both a tyrosine phosphorylation site and an ATP/GTP type A binding site, and thus has potential for signal transduction and mitogenic activity. The 1.2 kb clone codes for a truncated binding form of 150 aa that is identical with the ovarian long form over only the first 130 residues, and lacks the transmembrane region. Differences between long and short forms of the ovarian lactogen receptors and the truncated liver species may result from alternative splicing. The prolactin holoreceptor gene(s) has the potential for producing several receptor subtypes that differ in tissue-specific expression, size, compartmentalization and mode of signal transduction, and may subserve the divergent functions of prolactin in its several target cells.     

Expression and genomic structure of the genes encoding FdI, the major allergen from the domestic cat.

The genes encoding chain 1 (Ch1) and chain 2 (Ch2) of the major allergen of the domestic cat, Felis domesticus I, have been analyzed by genomic cloning and by polymerase chain reaction (PCR). Ch1 has two potential leader sequences, designated A and B. Analysis of a genomic clone encoding Ch1 demonstrated that one structural gene contains sequences corresponding to both leaders, which utilize different Met start codons. PCR analysis showed that genes encoding Ch1 and Ch2 are co-expressed in both the salivary glands and the skin, and that leader sequence A of Ch1 is utilized preferentially in both tissues. Ch2 was shown to have two dominant forms that are differentially expressed in the aforementioned tissues. The long form (Ch2L), composed of 92 amino acids (aa), is preferentially expressed in the salivary glands, while the short form (Ch2S), composed of 90 aa, is preferentially expressed in the skin. There is minor sequence polymorphism in both forms of Ch2. A genomic clone for Ch2 only contained sequences for Ch2S, suggesting that Ch2L is encoded by an exon not contained within this genomic clone.     

The Nb2 form of prolactin receptor is able to activate a milk protein gene promoter.

We have recently cloned a cDNA encoding a mutant form of PRL receptor (PRL-R) from Nb2 cells, a PRL-dependent T lymphocyte-derived cell line. This cDNA is identical to the long form of the rat PRL-R, except for a deletion of 594 base pairs in the cytoplasmic domain, resulting in a mature receptor protein of 393 amino acids. Although a segment containing three cytoplasmic regions of moderate to high amino acid sequence identity with members of the PRL/GH receptor family is missing in this receptor form, the region of highest (70%) identity is retained. In the following studies, a homologous functional assay was developed to test the activity of three forms of receptor with respect to their ability to transmit a lactogenic signal. In this system, CHO cells were transiently transfected with a construct containing 2300 base pairs of the 5'-flanking sequence of the rat beta-casein gene fused to the chloramphenicol acetyltransferase (CAT) gene and an expression vector containing the various forms of rat PRL-R cDNA. The transfected cells were grown in serum-free medium in the absence or presence of PRL. In cells transfected with the long form of the PRL-R and beta-casein/CAT construct, a 7.2- +/- 0.9-fold induction (n = 3) of CAT activity was seen when cells were cultured in the presence of 400 ng/ml PRL and 1 micrograms/ml hydrocortisone. This level of stimulation was similar to that observed for the ovine beta-lactoglobulin/CAT construct in which a 5.7- +/- 1.2-fold (n = 3) effect was found.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plakophilins 2a and 2b: constitutive proteins of dual location in the karyoplasm and the desmosomal plaque

Using antibodies and recombinant DNA techniques, we have identified plakophilin 2, a novel desmosomal plaque protein of M(r) 100,000 (estimated from SDS-PAGE), which is a member of the arm-repeat family of proteins and can occur in two splice forms (2a and 2b) because of the insertion of a 44 amino acid (aa)-encoding exon. In its aa sequence (837 and 881 aa, calculated pIs: 9.33 and 9.38, mol wts 92,750 and 97,410 kD), it is conspicuously related to the 80-kD plakophilin 1, with which it shares a central region of 9 repeats of the arm-motif, preceeded by a long head region and followed by a very short (11 aa) carboxy-terminal sequence. Plakophilin 2 and its mRNA have been detected in a wide range of tissues and cell types, including cells devoid of desmosomes. By light and electron microscopical immunolocalization, plakophilin 2 has been localized to plaques of desmosomes of one-layered ("simple") and complex epithelia, carcinomas, diverse epithelium-derived cell culture lines, as well as cardiac tissue and the dendritic reticulum cells of lymphatic germinal centers, i.e., desmosomes in which plakophilin 1 is not detected. However, plakophilin 2 has also been localized in the desmosomes of certain but not all stratified epithelia where it coexists with plakophilin 1. Remarkably, plakophilin 2 is also enriched in the karyoplasm of a wide range of cell types, including many that lack desmosomes and in which, therefore, the nuclear state is the only locally enriched form of plakophilin 2 present. We conclude that plakophilins 2a and 2b are basic nuclear proteins that in certain cell types additionally assemble with other proteins to form the desmosomal plaque and serve general nuclear functions as well as a function specific to many but not all desmosomes.     

A prolactin-dependent immune cell line (Nb2) expresses a mutant form of prolactin receptor.

The Nb2 cell line is a pre-T rat lymphoma that is dependent on prolactin (PRL) for mitogenesis. Two forms of PRL receptor (PRL-R), which differ in the length of their cytoplasmic domains have been identified in different tissues and species. In the present study we have cloned the cDNA and characterized the mitogenic form of PRL-R in Nb2 cells. Polymerase chain reaction amplification of first strand cDNA prepared from Nb2-11C (PRL-dependent) and Nb2-Sp (PRL-independent) cell lines was performed using oligonucleotide primers specific for the binding domain, the short form of the PRL-R, and the cytoplasmic domain of the long form of the PRL-R. These studies indicate that both cell lines express a novel form of PRL-R. A cDNA was isolated from an Nb2-Sp cDNA library, which contains 1446 base pairs identical to the nucleotide sequence of the long form of the rat PRL-R. However, the cDNA sequence is missing 594 base pairs in the cytoplasmic domain compared with the long form of the PRL-R. The cDNA encodes a protein of 393 amino acids, lacking 198 amino acids in the cytoplasmic domain. Scatchard analysis of 125I-labeled ovine prolactin (oPRL) binding to microsomes prepared from transiently transfected COS-7 cells with either PRL-R long form cDNA or Nb2 PRL-R cDNA indicates that the long form of PRL-R binds oPRL with high affinity (K alpha = 8.8 x 10(9) M-1), while the Nb2 PRL-R showed a 3.3-fold increased affinity for PRL (K alpha = 29.1 x 10(9) M-1). In addition, immunoblot analysis of these microsomes using 125I-labeled monoclonal antibody (U6) to the PRL-R demonstrates a Mr of approximately 82,000 for the long form and approximately 62,000 for the Nb2 form of PRL-R. Polymerase chain reaction amplification of genomic DNA prepared from PRL-dependent and -independent cell lines suggests that this form of PRL-R results from a deletion in the PRL-R gene. The identification of a modified long form of PRL-R in the Nb2 cell line should help localize domains of the PRL-R involved in signal transduction and further the investigation of prolactin's role in immune cell proliferation.     

Prolactin stimulates activation of c-jun N-terminal kinase (JNK)

In recent years the mitogen-activated protein (MAP) kinase family has expanded to include both c-jun N-terminal kinases (JNKs), and the p38/HOG1 family in addition to the extracellular regulated kinase (ERK) family. These kinases are activated by a variety of growth factors, as well as extra- and intracellular insults such as osmotic stress, UV light, and chemotherapeutic agents. Stimulation of the PRL-dependent Nb2 cell line with PRL results in the rapid activation of JNK as determined by the glutathione-S-transferase (GST)-jun kinase assay. Activation was maximal 30 min after stimulation with 50 nM rat PRL (rPRL) and decreased after that time. Dose response studies indicated that concentrations as low as 10 nM rPRL resulted in maximal activation. The interleukin-3 (IL-3)-dependent myeloid progenitor cell line 32Dcl3 was transfected with the long, Nb2, and short forms of the rat PRL receptor (rPRLR), as well as the long form of the human PRLR (hPRLR). The long and Nb2 forms of the PRLR were able to stimulate activation of JNK; however, the short form of the rPRLR was not. This corresponds with the inability of the short form of the rPRLR to stimulate proliferation of 32Dcl3 cells. Activation of JNK in 32Dcl3 cells expressing the long form of the hPRLR was maximal at 30 min after stimulation with 100 nM ovine PRL (oPRL) and declined after that time. Dose response studies indicated that activation of JNK was maximal after 30 min at a concentration of 10 nM, and the amount of activated JNK declined at the highest concentration of oPRL, 100 nM. Immunoblot analysis with an antibody that recognizes the activated (phosphorylated) forms of JNK1 and JNK2 indicated that both JNK1 and JNK2 isoforms were activated in 32D/hPRLR cells stimulated with oPRL. A recombinant human adenovirus expressing a kinase-inactive mutant of JNK1 (APF mutant) was used to determine the biological effect of blocking JNK activity in Nb2 cells. Expression of the JNK1-APF mutant inhibited cellular proliferation and induced DNA fragmentation typical of cells undergoing apoptosis. These data suggest that activation of JNKs may be important in mitogenic signaling and/or suppression of apoptosis in Nb2 cells.     

Mutational analysis of human hydroxysteroid sulfotransferase SULT2B1 isoforms reveals that exon 1B of the SULT2B1 gene produces cholesterol sulfotransferase,whereas exon 1A yields pregnenolone sulfotransferase

As a result of an alternative exon 1, the gene for human hydroxysteroid sulfotransferase (SULTB1) encodes for two peptides differing only at their amino termini. The SULT2B1b isoform preferentially sulfonates cholesterol. Conversely, the SULT2B1a isoform avidly sulfonates pregnenolone but not cholesterol. The outstanding structural feature that distinguishes the SULT2B1 isoforms from the prototypical SULT2A1 isozyme is the presence of extended amino- and carboxyl-terminal ends in the former. Investigating the functional significance of this unique characteristic reveals that removal of 53 amino acids from the relatively long carboxyl-terminal end that is common to both SULT2B1 isoforms has no effect on the catalytic activity of either isoform. On the other hand, removal of 23 amino acids from the amino-terminal end that is unique to SULT2B1b results in loss of cholesterol sulfotransferase activity, whereas removal of 8 amino acids from the amino-terminal end that is unique to SULT2B1a has no effect on pregnenolone sulfotransferase activity. Deletion analysis along with site-directed mutagenesis of SULT2B1b reveal that the amino acid segment 19-23 residues from the amino terminus and particularly isoleucines at positions 21 and 23 are crucial for cholesterol catalysis. In the gene for SULT2B1, exon 1B encodes for only the unique amino-terminal region of SULT2B1b; however, exon 1A encodes for the unique amino-terminal end of SULT2B1a plus an additional 48 amino acids. Thus, if the gene for SULT2B1 employs exon 1B, cholesterol sulfotransferase is synthesized, whereas if exon 1A is used, pregnenolone sulfotransferase is produced.     

Identification and characterization of membralin, a novel tumor-associated gene, in ovarian carcinoma

Identification of new genes in cancer is the key to understand the molecular basis of tumor development as well as provide potential diagnostic markers and therapeutic targets. A novel gene, membralin (GeneBank accession number: DQ005958), was cloned from a human ovarian cancer cell line. Human membralin is unique and does not share significant sequence homology with other human genes, only membralins of other species. The gene contains 11 exons which encode at least two spliced variants in human cancer. The long form of membralin (membralin-1) comprises all 11 exons, encoding a protein of 620-amino acids long and the short form of membralin (membralin-3) contains all exons except for exon 10, encoding a protein of 408 amino acids. Expression of different membralin isoforms depends on tissue type. The long form, membralin-1, is expressed in ovarian and colorectal carcinomas but not in breast or pancreatic carcinomas, which express only the short splice form, membralin-3. Membralin-1-GFP fusion protein demonstrates exclusive cytoplasmic localization. Based on quantitative real-time PCR, in situ hybridization and Western blot analysis, membralin was highly expressed in ovarian serous carcinomas as compared to ovarian surface epithelium (P<0.001). Ovarian carcinomas in effusions demonstrated a significantly higher level of membralin expression than in solid tumors (P<0.001). In conclusion, these findings represent the first characterization of human membralin and suggest that membralin is a novel tumor-associated marker in ovarian serous carcinomas.