Endocytosis and degradation of prolactin and its receptor in Chinese hamster ovary cells stably transfected with prolactin receptor cDNA

Immunohistochemical localization of prolactin receptor (PRLR) to Hodgkin's and Reed-Sternberg cells of Hodgkin's lymphoma

Prolactin receptor (PRLR), a type-1 cytokine receptor, is overexpressed in a number of cancer types. It has attracted much attention for putative pro-oncogenic roles, which however, remains controversial in some malignancies. In this study, we reported the localization of PRLR to the Hodgkin's and Reed-Sternberg (HRS) cells of Hodgkin's lymphoma (HL), a neoplasm of predominantly B cell origin. Immunohistochemistry performed on 5-μm thick FFPE sections revealed expression of PRLR in HRS cells. Cellular immunofluorescence experiments showed that the HL-derived cell lines, Hs445, KMH2 and L428 overexpressed PRLR. The PRLR immunofluorescent signal was depleted after treating the cell lines with 10 μM of siRNA for 48 h. We also tested whether PRLR is involved in the growth of HL, in vitro. One-way analysis of variance (ANOVA) on cell growth data obtain from WST-1 cell proliferation assay and trypan blue exclusion assay and hemocytometry showed that siRNA-depletion of PRLR expression resulted in decreased growth in all three cell lines. These results offered only a short insight into the involvement of PRLR in HL. As a result, further investigation is required to decipher the precise role(s) of PRLR in the pathogenesis of HL.

Growth Hormone Receptor Regulation in Cancer and Chronic Diseases

The GHR signaling pathway plays important roles in growth, metabolism, cell cycle control, immunity, homeostatic processes, and chemoresistance via both the JAK/STAT and the SRC pathways. Dysregulation of GHR signaling is associated with various diseases and chronic conditions such as acromegaly, cancer, aging, metabolic disease, fibroses, inflammation and autoimmunity. Numerous studies entailing the GHR signaling pathway have been conducted for various cancers. Diverse factors mediate the up- or down-regulation of GHR signaling through post-translational modifications. Of the numerous modifications, ubiquitination and deubiquitination are prominent events. Ubiquitination by E3 ligase attaches ubiquitins to target proteins and induces proteasomal degradation or starts the sequence of events that leads to endocytosis and lysosomal degradation. In this review, we discuss the role of first line effectors that act directly on the GHR at the cell surface including ADAM17, JAK2, SRC family member Lyn, Ubc13/CHIP, proteasome, βTrCP, CK2, STAT5b, and SOCS2. Activity of all, except JAK2, Lyn and STAT5b, counteract GHR signaling. Loss of their function increases the GH-induced signaling in favor of aging and certain chronic diseases, exemplified by increased lung cancer risk in case of a mutation in the SOCS2-GHR interaction site. Insight in their roles in GHR signaling can be applied for cancer and other therapeutic strategies.

Leptin, Leptin Receptor, KHDRBS1 (KH RNA Binding Domain Containing, Signal Transduction Associated 1), and Adiponectin in Bone Metastasis from Breast Carcinoma: An Immunohistochemical Study

Breast cancer patients are at a high risk of complications from bone metastasis. Molecular characterization of bone metastases is essential for the discovery of new therapeutic targets. Here, we investigated the expression and the intracellular distribution of KH RNA binding domain containing, signal transduction associated 1 (KHDRBS1), leptin, leptin receptor (LEPR), and adiponectin in bone metastasis from breast carcinoma and looked for correlations between the data. The expression of these proteins is known in breast carcinoma, but it has not been investigated in bone metastatic tissue to date. Immunohistochemical analysis was carried out on bone metastasis specimens, then semiquantitative evaluation of the results and the Pearson test were performed to determine eventual correlations. KHDRBS1 expression was significantly higher in the nuclei than in the cytosol of metastatic cells; LEPR was prevalently observed in the cytosol and the nuclei; leptin and adiponectin were found in metastatic cells and stromal cells; the strongest positive correlation was between nuclear KHDRBS1 and nuclear LEPR expression. Taken together, our findings support the importance of the leptin/LEPR/KHDRBS1 axis and of adiponectin in the progression of bone metastasis and suggest their potential application in pharmacological interventions.

An Emerging Role for Prolactin in Female-Selective Pain

Women experience many pain conditions more frequently when compared with men, but the biological mechanisms underlying sex differences in pain remain poorly understood. In particular, little is known about possible sex differences in peripheral nociceptors, the fundamental building blocks of pain transmission. Emerging evidence reveals that prolactin (PRL) signaling at its cognate prolactin receptor (PRLR) in primary afferents promotes nociceptor sensitization and pain in a female-selective fashion. In this review, we summarize recent progress in understanding the female-selective role of PRL/PRLR in nociceptor sensitization and in pathological pain conditions, including postoperative, inflammatory, neuropathic, and migraine pain, as well as opioid-induced hyperalgesia. The clinical implications of the peripheral PRL/PRLR system for the discovery of new therapies for pain control in women are also discussed.

Prolactin Receptor-Mediated Internalization of Imaging Agents Detects Epithelial Ovarian Cancer with Enhanced Sensitivity and Specificity

Poor prognosis of ovarian cancer, the deadliest of the gynecologic malignancies, reflects major limitations associated with detection and diagnosis. Current methods lack high sensitivity to detect small tumors and high specificity to distinguish malignant from benign tissue, both impeding diagnosis of early and metastatic cancer stages and leading to costly and invasive surgeries. Tissue microarray analysis revealed that >98% of ovarian cancers express the prolactin receptor (PRLR), forming the basis of a new molecular imaging strategy. We fused human placental lactogen (hPL), a specific and tight binding PRLR ligand, to magnetic resonance imaging (gadolinium) and near-infrared fluorescence imaging agents. Both in tissue culture and in mouse models, these imaging bioconjugates underwent selective internalization into ovarian cancer cells via PRLR-mediated endocytosis. Compared with current clinical MRI techniques, this targeted approach yielded both enhanced signal-to-noise ratio from accumulation of signal via selective internalization and improved specificity conferred by PRLR upregulation in malignant ovarian cancer. These features endow PRLR-targeted imaging with the potential to transform ovarian cancer detection. Cancer Res; 77(7); 1684-96. ©2017 AACR.

Contribution of individual histidines to the global stability of human prolactin

A member of the family of hematopoietic cytokines human prolactin (hPRL) is a 23k kDa polypeptide hormone, which displays pH dependence in its structural and functional properties. The binding affinity of hPRL for the extracellular domain of its receptor decreases 500-fold over the relatively narrow, physiologic pH range from 8 to 6; whereas, the affinity of human growth hormone (hGH), its closest evolutionary cousin, does not. Similarly, the structural stability of hPRL decreases from 7.6 to 5.6 kcal/mol from pH 8 to 6, respectively, whereas the stability of hGH is slightly increased over this same pH range. hPRL contains nine histidines, compared with hGH's three, and they are likely responsible for hPRL's pH-dependent behavior. We have systematically mutated each of hPRL's histidines to alanine and measured the effect on pH-dependent global stability. Surprisingly, a vast majority of these mutations stabilize the native protein, by as much as 2-3 kcal/mol. Changes in the overall pH dependence to hPRL global stability can be rationalized according to the predominant structural interactions of individual histidines in the hPRL tertiary structure. Using double mutant cycles, we detect large interaction free energies within a cluster of nearby histidines, which are both stabilizing and destabilizing to the native state. Finally, by comparing the structural locations of hPRL's nine histidines with their homologous residues in hGH, we speculate on the evolutionary role of replacing structurally stabilizing residues with histidine to introduce pH dependence to cytokine function.

SRC family kinases accelerate prolactin receptor internalization, modulating trafficking and signaling in breast cancer cells

Despite the growing body of evidence supporting prolactin (PRL) actions in human breast cancer, little is known regarding PRL regulation of its own receptor in these cells. Ligand-initiated endocytosis is a key process in the regulation of receptor availability and signaling cascades that may lead to oncogenic actions. Although exposure to exogenous PRL accelerates degradation of the long isoform of the PRL receptor (lPRLR), neither the signals initiated by PRL that lead to lPRLR internalization and subsequent down-regulation, nor the relationship to downstream pathways are understood in breast cancer cells. In this study, we showed that PRL-induced down-regulation of the lPRLR was reduced by inhibition of src family kinases (SFKs), but not Janus kinase 2, in MCF-7 cells. Inhibition of SFKs also resulted in accumulation of a PRL-induced PRLR fragment containing the extracellular domain, which appeared to be generated from newly synthesized PRLR. lPRLR was constitutively associated with SFKs in lipid rafts. PRL-induced SFK activation led to recruitment of the guanosine triphosphatase, dynamin-2, to an internalization complex, resulting in endocytosis. Inhibition of endocytosis by small interfering RNA-mediated knockdown of dynamin-2 blocked PRL-induced down-regulation of lPRLR, confirming that internalization is essential for this process. Endocytosis also was required for optimal phosphorylation of ERK1/2 and Akt, but not for Janus kinase 2 or signal transducer and activator of transcription 5, indicating that internalization selectively modulates signaling cascades. Together, these data indicate that SFKs are key mediators of ligand-initiated lPRLR internalization, down-regulation, and signal transduction in breast cancer cells, and underscore the importance of target cell context in receptor trafficking and signal transduction.

Regulation of prolactin receptor levels and activity in breast cancer

From its traditional identity as a hormone involved in growth and differentiation of mammary epithelium and in lactation, to having a pertinent role in the development of mammary carcinoma, the peptide hormone/cytokine prolactin (PRL) has emerged as a versatile signaling molecule. There has been significant progress in our understanding of the fine working of PRL in the past several years. Notably, much effort has been concentrated on the mediator of PRL action, namely, the prolactin receptor (PRLr). The causal link between increased PRLr expression and breast cancer is being increasingly appreciated. Considering that the level of the receptor on the surface is a critical determinant of signaling output in response to PRL, the uncovering of regulatory elements that control receptor expression becomes important. The principle focus of this review is on the regulation of PRLr expression and activity in breast cancer with a brief overview of different isoforms of PRLr, their expression, signaling capabilities and the biological outcomes of PRL/PRLr signaling.

Proteasomes mediate prolactin-induced receptor down-regulation and fragment generation in breast cancer cells

Prolactin regulates a variety of physiological processes, including mammary gland growth and differentiation, and recent findings support an important role in breast cancer development and progression. However, little is known about the trafficking of its receptor, a member of the cytokine receptor superfamily. In the present study, we examined the effect of ligand on the endogenous "long" isoform of the prolactin receptor in breast cancer cells. We found that prolactin caused rapid and prolonged down-regulation of this receptor. The prolactin-induced increase in degradation was blocked by inhibitors of both proteasomes and lysosomes. However, the ubiquitin-conjugating system was not required for internalization. Prolactin also resulted in the concomitant appearance of a cell-associated prolactin receptor fragment containing the extracellular domain. This latter process required proteasomal, but not metalloprotease, activity, distinguishing it from ectodomain "shedding" of other membrane receptors, which are secreted as binding proteins. The prolactin receptor fragment was labeled by surface biotinylation and independent of protein synthesis. Together, these data indicated that prolactin binding initiates limited proteasomal cleavage of its receptor, generating a cell-associated fragment containing the extracellular domain. Our findings described a new potential mediator of prolactin action and a novel mechanism whereby proteasomes modulate cellular processes.

Distinctive roles for prolactin and growth hormone in the activation of signal transducer and activator of transcription 5 in pancreatic islets of langerhans

Although the beta-cells of the pancreatic islets of Langerhans express both prolactin (PRL) and GH receptors, we have observed that PRL is considerably more effective than GH in the up-regulation of islet function in vitro. This study examined whether differences in the activation of the Janus kinase 2/signal transducer and activator of transcription (STAT) 5 signaling pathway by these closely related receptors may be involved in this disparity. The activation of STAT5B by PRL was biphasic, with an initial peak within 30 min, a nadir between 1 and 3 h, and prolonged activation after 4 h. In contrast, the response to GH was transient for 1 h. The importance of the long-term activation of STAT5B by PRL was supported by the similar dose response curves for STAT5B activation and the PRL-induced increases in insulin secretion and islet cell proliferation. Because the pulsatile secretion of GH affects its actions in other target tissues, the ability of pretreatment with either hormone to affect subsequent stimulation was also examined. Surprisingly, the response to PRL was inhibited by prior exposure for less than 3 h to either PRL or GH and disappeared with a longer pretreatment with either hormone. Similar to other tissues, the response to GH was inhibited by any length of prior exposure to GH. However, pretreatment with PRL had no effect. These experiments are the first demonstration of the transient desensitization of the PRL receptor by either PRL or GH pretreatment in any tissue and the desensitization of GH stimulation in islet cells. These observations provide insight into the mechanisms that regulate the desensitization of these receptors and, more importantly, allow the long-term activation of STAT5B by the PRL receptor. These results may apply to other members of the cytokine superfamily of receptors. We also demonstrate that the increase in islet cell proliferation required continuous stimulation with PRL, whereas the smaller effect with GH occurred with either continuous or pulsatile stimulation. In summary, this study demonstrates that islets are sensitive to the temporal pattern of stimulation by these hormones and provides a new basis for understanding their physiological roles in the regulation of islet function.

A role for the cytoskeleton in prolactin-dependent mammary epithelial cell differentiation

The function of exocrine glands depends on signals within the extracellular environment. In the mammary gland, integrin-mediated adhesion to the extracellular matrix protein laminin co-operates with soluble factors such as prolactin to regulate tissue-specific gene expression. The mechanism of matrix and prolactin crosstalk and the activation of downstream signals are not fully understood. Because integrins organize the cytoskeleton, we analysed the contribution of the cytoskeleton to prolactin receptor activation and the resultant stimulation of milk protein gene expression. We show that the proximal signalling events initiated by prolactin (i.e. tyrosine phosphorylation of receptor and the associated kinase Jak2) do not depend on an intact actin cytoskeleton. However, actin networks and microtubules are both necessary for continued mammary cell differentiation, because cytoskeletal integrity is required to transduce the signals between prolactin receptor and Stat5, a transcription factor necessary for milk protein gene transcription. The two different cytoskeletal scaffolds regulate prolactin signalling through separate mechanisms that are specific to cellular differentiation but do not affect the general profile of protein synthesis.

Negative regulation of prolactin receptor stability and signaling mediated by SCF(beta-TrCP) E3 ubiquitin ligase

Ubiquitin-dependent degradation of hormone receptors is emerging as a key mechanism that regulates the magnitude and duration of hormonal effects on cells and tissues. The pituitary hormone prolactin (PRL) is involved in regulating cell differentiation, proliferation, and survival. PRL engages its receptor (PRLR) to initiate various signaling cascades, including the phosphorylation and activation of Stat5. We found that PRL promotes interaction between PRLR and the F-box protein beta-TrCP2, which functions as a substrate recognition subunit of the SCF(beta-TrCP) E3 ubiquitin ligase. This interaction requires PRLR phosphorylation and the integrity of serine 349 within a conserved motif, which is similar to conserved motifs present in other substrates of SCF(beta-TrCP). The PRLR(S349A) mutant is resistant to ubiquitination and is more stable than its wild-type counterpart. Phosphorylated PRLR undergoes ubiquitination by SCF(beta-TrCP) in vitro. Knockdown of beta-TrCP expression inhibits the ubiquitination and degradation of PRLR and promotes PRL-dependent phosphorylation of Stat5 as well as Stat5-dependent transcription in cells. Furthermore, the activation of Stat5 and the stimulation of cell growth by PRL are augmented in cells expressing the PRLR(S349A) mutant. These data indicate that PRLR is a novel SCF(beta-TrCP) substrate and implicate beta-TrCP as an important negative regulator of PRL signaling and cellular responses to this hormone.

Seasonal environmental changes modulate the prolactin receptor expression in an eurythermal fish

Eurythermal fish have evolved compensatory responses to the cyclical seasonal changes of the environment. The complex adaptive mechanisms include the transduction of the physical parameters variations into molecular signals. Studies in carp have indicated that prolactin and growth hormone expression is associated with acclimatization, suggesting that the pituitary gland is a relevant physiological node in the generation of the homeostatic rearrangement that occurs in this adaptive process. Here, we report the cloning and characterization of a full-length carp prolactin receptor cDNA, which codes for the long form of the protein resembling that found in mammalian prolactin receptors. We identified up to three receptor transcript isoforms in different tissues of the teleost and assessed cell- and temporal-specific transcription and protein expression in carp undergoing seasonal acclimatization. The distinctive pattern of expression that carp prolactin receptor (cPRLr) depicts upon seasonal acclimatization supports the hypothesis that prolactin and its receptor are clearly involved in the new homeostatic stage that the eurythermal fish needs to survive during the cyclical changes of its habitat.

Endogenous human prolactin and not exogenous human prolactin induces estrogen receptor alpha and prolactin receptor expression and increases estrogen responsiveness in breast cancer cells

Prolactin (PRL) and estrogen act synergistically to increase mammary gland growth, development, and differentiation. Based on their roles in the normal gland, these hormones have been studied to determine their interactions in the development and progression of breast cancer. However, most studies have evaluated only endocrine PRL and did not take into account the recent discovery that PRL is synthesized by human mammary cells, permitting autocrine/paracrine activity. To examine the effects of this endogenous PRL, we engineered MCF7 cells to inducibly overexpress human prolactin (hPRL). Using this Tet-On MCF7hPRL cell line, we studied effects on cell growth, PRLR, ER alpha, and PgR levels, and estrogen target genes. Induced endogenous hPRL, but not exogenous hPRL, increased ER alpha levels as well as estrogen responsiveness in these cells, suggesting that effects on breast cancer development and progression by estrogen may be amplified by cross-regulation of ER alpha levels by endogenous hPRL. The long PRLR isoform was also upregulated by endogenous, but not exogenous PRL. This model will allow investigation of endogenous hPRL in mammary epithelial cells and will enable further dissection of PRL effects on other hormone signaling pathways to determine the role of PRL in breast cancer.

The role of prolactin in mammary carcinoma

The contribution of prolactin (PRL) to the pathogenesis and progression of human breast cancer at the cellular, transgenic, and epidemiological levels is increasingly appreciated. Acting at the endocrine and autocrine/paracrine levels, PRL functions to stimulate the growth and motility of human breast cancer cells. The actions of this ligand are mediated by at least six recognized PRL receptor isoforms found on, or secreted by, human breast epithelium. The PRL/PRL receptor complex associates with and activates several signaling networks that are shared with other members of the cytokine receptor superfamily. Coupled with the recently identified intranuclear function of PRL, these networks are integrated into the in vitro and in vivo actions induced by ligand. These findings indicate that antagonists of PRL/PRL receptor interaction or PRL receptor-associated signal transduction may be of considerable utility in the treatment of human breast cancer.

The role of prolactin in fish osmoregulation: a review

The protein hormone prolactin (PRL) was first discovered as an anterior pituitary factor capable of stimulating milk production in mammals. We now know that PRL has over 300 different functions in vertebrates. In fish, PRL plays an important role in freshwater osmoregulation by preventing both the loss of ions and the uptake of water. This paper will review what is currently known about the structure and evolution of fish PRL and its mechanisms of action in relation to the maintenance of hydromineral balance. Historically, functional studies of fish PRL were carried out using heterologous PRLs and the results varied greatly between experiments and species. In some cases this variability was due to the ability of these PRLs to bind to both growth hormone and PRL receptors. In fact, a recurring theme in the literature is that the actions of PRL cannot be generalized to all fish due to marked differences between species. Many of the effects of PRL on hydromineral balance are specific to euryhaline fish, which is appropriate given that they frequently experience sudden changes in environmental salinity. Much of the recent work has focused on the isolation and characterization of fish PRLs and their receptors. These studies have provided the necessary tools to obtain a better understanding of the evolution of PRL and its role in osmoregulation.

Structural and functional effects of high prolactin levels on injured endothelial cells: evidence for an endothelial prolactin receptor

Stress has been linked to health problems such as atherosclerosis and prolonged wound healing, which involve the responses of injured endothelial cells. Though prolactin (PRL) levels become increased during the physiological response to stress, the significance and effects of these increases are largely unknown. Here we examined the effects of elevated, though physiological, concentrations of PRL on the responses of cultured endothelial cells after mechanical injury to cell monolayers. When treated at the time of injury with PRL levels of 62.5-1000 ng/mL, cells at the wound front became abnormal in shape and had reductions in f-actin staining in comparison to controls that were not PRL-treated. High PRL concentrations also inhibited the adhesion of cells to their growth surface in a dose-dependent manner. Using rhodamine-labeled PRL, we observed specific PRL uptake by these cells that suggested the presence of a PRL receptor. Finally, mRNA for the long form of the PRL receptor was detected by RT-PCR. To our knowledge, this is the first report demonstrating that (1) high PRL concentrations alter the actin cytoskeleton and adhesion of injured endothelial cells and (2) endothelial cells express the transcript for the PRL receptor. Thus, we report novel effects of PRL that may be mediated by activation of an endothelial cell PRL receptor.

Increased expression of growth hormone and prolactin receptors in hepatocellular carcinomas

The liver is an essential target tissue for growth hormone (GH) and prolactin (PRL). The aim of this study was to determine the in situ expression of growth hormone receptor (GHR) and prolactin receptor (PRLR) in hepatocellular carcinomas and to compare the results with normal liver. For this purpose, in situ hybridization (ISH) and immunohistochemical techniques were performed and several tests were conducted to validate the results. By radioactive ISH, all the hepatocellular carcinomas studied showed labeling for GHR and PRLR mRNAs. Relative expression levels, determined by computer-assisted microdensity, were higher in hepatocellular carcinomas than in normal liver. Immunohistochemistry led us to confirm the constant expression of both receptor proteins in hepatocellular carcinomas and normal liver and to demonstrate their localization not only in the cytoplasm but also in the nucleus. These results confirm that the liver is a major GH and PRL target tissue and suggest that in hepatocellular carcinomas the proliferative effects of these hormones may be increased by a higher expression of their receptors.

Mapping the bacterial cell architecture into the chromosome

A genome is not a simple collection of genes. We propose here that it can be viewed as being organized as a 'celluloculus' similar to the homunculus of preformists, but pertaining to the category of programmes (or algorithms) rather than to that of architectures or structures: a significant correlation exists between the distribution of genes along the chromosome and the physical architecture of the cell. We review here data supporting this observation, stressing physical constraints operating on the cell's architecture and dynamics, and their consequences in terms of gene and genome structure. If such a correlation exists, it derives from some selection pressure: simple and general physical principles acting at the level of the cell structure are discussed. As a first case in point we see the piling up of planar modules as a stable, entropy-driven, architectural principle that could be at the root of the coupling between the architecture of the cell and the location of genes at specific places in the chromosome. We propose that the specific organization of certain genes whose products have a general tendency to form easily planar modules is a general motor for architectural organization in the bacterial cell. A second mechanism, operating at the transcription level, is described that could account for the efficient building up of complex structures. As an organizing principle we suggest that exploration by biological polymers of the vast space of possible conformation states is constrained by anchoring points. In particular, we suggest that transcription does not always allow the 5'-end of the transcript to go free and explore the many conformations available, but that, in many cases, it remains linked to the transcribing RNA polymerase complex in such a way that loops of RNA, rather than threads with a free end, explore the surrounding medium. In bacteria, extension of the loops throughout the cytoplasm would therefore be mediated by the de novo synthesis of ribosomes in growing cells. Termination of transcription and mRNA turnover would accordingly be expected to be controlled by sequence features at both the 3'- and 5'-ends of the molecule. These concepts are discussed taking into account in vitro analysis of genome sequences and experimental data about cell compartmentalization, mRNA folding and turnover, as well as known structural features of protein and membrane complexes.

Big prolactin 60 kDa is overexpressed in salivary glandular epithelial cells from patients with Sjögren's syndrome

Characterization of endogenous synthesis of prolactin (PRL) proteins and their cellular localization in labial salivary glands of patients with Sjogren's syndrome (SS) were achieved. PRL, PRL-receptors (PRL-R), and S100A6 protein were detected by immunohistochemistry. In situ prolactin synthesis was investigated in controls and SS patients by ex vivo incubation of minor salivary glands biopsies and immunoprecipitation assay. Increased PRL-immunoreactivity was found in cytoplasmic acinar epithelial cells in SS patients compared with normal subjects. PRL-R was distributed only in ductal epithelial cells in which S100A6 protein (a PRL-R-associated protein) was also present. PRL, PRL-R, or S100A6-immunoreactivity was not detected in infiltrating mononuclear cells. Immunoprecipitation demonstrated that PRL synthesis occurred in minor salivary glands with increased synthesis of two distinct PRL-like proteins (one major band at 60 kDa and a minor at 16 kDa) in SS glands compared with normal glands. Expression of PRL gene was demonstrated in SS salivary glands using RT-PCR. A positive correlation was found between the presence of PRL-like proteins in acinar epithelial cells of SS patients and clinical extraglandular manifestations. The presence of anti-Ro and anti-La antibodies also positively correlated with a higher percentage of PRL in acinar epithelial cells. In conclusion, PRL-like proteins are synthetized and overexpressed in glandular epithelial cells of labial salivary glands from SS patients and correlate with the aggressiveness of the disease.

Subcellular localization and internalization of the four human leptin receptor isoforms

There are four known isoforms of the human leptin receptor (HLR) with different C-terminal cytoplasmic domains (designated by the number of unique C-terminal amino acids). In cells expressing HLR-5, -15, or -274, 15-25% of the leptin binding sites were located at the plasma membrane. In contrast, in cells expressing HLR-67, only 5% of the total binding sites were at the plasma membrane. Immunofluorescent microscopy showed that all four isoforms partially co-localized with calnexin and beta-COP, markers of the endoplasmic reticulum and the Golgi, respectively. All isoforms were also detected in an unidentified punctate compartment. All isoforms were internalized via clathrin-mediated endocytosis, but at different rates. After 20 min at 37 degrees C, 45% of a bound cohort of labeled ligand had been internalized by HLR-15, 30% by HLR-67, 25% by HLR-274, and 15% by HLR-5. Degradation of internalized leptin occurred in lysosomes. Overnight exposure to leptin down-regulated all isoforms, but to a variable extent. HLR-274 displayed the greatest down-regulation and also appeared to reach lysosomes more quickly than the other isoforms. The faster degradation of HLR-274 may help to terminate leptin signaling.

Functional properties of leptin receptor isoforms containing the gln-->pro extracellular domain mutation of the fatty rat

Mutations of the leptin receptor have been found to cause obesity in rodents. The fa mutation that is responsible for obesity in Zucker rats is a missense mutation (269 gln-->pro) in the extracellular domain of the leptin receptor. We have characterized the effects of this mutation on the two major isoforms of the leptin receptor, Ob-Rb and Ob-Ra, by studying cell-surface expression, leptin binding affinity, signaling capacity, and receptor-mediated internalization and degradation of leptin in transfected mammalian cell lines. Both Ob-Rb(269 gln-->pro) and Ob-Ra(269 gln-->pro) have decreased cell-surface expression and decreased leptin binding affinity. Ob-Rb(269 gln-->pro) was shown to have defective signaling to the JAK-STAT pathway and markedly diminished ability to activate transcription of the egr-1 promoter. Constitutive ligand-independent activation of Ob-Rb(269 gln-->pro) was observed for activation of egr-1-luc but only under conditions when JAK2 was coexpressed with Ob-Rb(269 gln-->pro), Finally, Ob-Ra(269 gln-->pro) has an increased ability to internalize leptin but is less efficient at degrading leptin, as compared with Ob-Ra. In conclusion, both Ob-Ra(269 gln-->pro) and Ob-Rb(269 gln-->pro) have multiple functional defects.

Transferrin and prolactin transcytosis in the lactating mammary epithelial cell

The mammary epithelial cell ferries constituents originating from blood and from stromal cells, into milk, by transcytosis. Morphological analysis of a membrane marker of transcytosis in the lactating mammary epithelial cell showed that very rapid endocytosis of surface membrane occurs from both the basolateral and the apical side of the cell. In both cases, membrane trafficking between endosomes and the Golgi complex allows communication between the endocytic and the biosynthetic pathways. Transferrin and prolactin are internalized in mammary cells and transported through multivesicular bodies and Golgi stacks. They are released into milk via different types of secretory vesicles, prolactin being carried in secretory vesicles containing casein micelles. Consequences of the intracellular transport of these proteins and physiological benefits for cell function are discussed.

Identification of cytoplasmic motifs required for short prolactin receptor internalization

Cloning of rat prolactin receptor (PRLR) cDNAs revealed the existence of two isoforms, termed short and long according to the length of their cytoplasmic domain. Internalization studies show, first, that PRLR internalization is hormone-dependent and, second, that ligand-receptor complexes of the short PRLR are internalized to a larger extent compared to the long form. In order to identify regions within the cytoplasmic domain of the short PRLR required for efficient internalization, serial truncations of the cytoplasmic tail were performed by inserting a stop codon in place of those encoding residues 282, 273, 262, 253, 244, or 237 (wild type short PRLR contains 291 amino acids). Our data show that two motifs, lying within residues 253-261 and 273-281, are involved in internalization. Both regions contain a consensus feature identified within other receptors as internalization signals, namely a di-leucine peptide (amino acids 259-260) and a tetrapeptide predicted to adopt a beta-turn structure (amino acids 276-279). We propose these two motifs are involved in PRLR endocytosis. Finally, we show that alpha-adaptin, a component of adaptor protein AP-2, coprecipitates with short PRLR complexes upon PRL stimulation, which strongly suggests that PRLR internalization is mediated by the clathrin-coated pits endocytotic pathway.