New therapeutic strategies have emerged along with our understanding that disturbances of iron homeostasis, impaired proliferation of erythroid progenitor cells, and blunted erythropoietin response occur in anemia of chronic disease. This review discusses advances in our knowledge of the causes and management of the condition.

https://www.nejm.org/doi/pdf/10.1056/NEJMra041809

Anemia of Chronic Disease

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

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Prolactin: Structure, Function, and Regulation of Secretion

PRL is an anterior pituitary hormone that, along with GH and PLs, forms a family of hormones that probably resulted from the duplication of an ancestral gene. The PRLR is also a member of a larger family, known as the cytokine class-1 receptor superfamily, which currently has more than 20 different members. PRLRs or binding sites are widely distributed throughout the body. In fact, it is difficult to find a tissue that does not express any PRLR mRNA or protein. In agreement with this wide distribution of receptors is the fact that now more than 300 separate actions of PRL have been reported in various vertebrates, including effects on water and salt balance, growth and development, endocrinology and metabolism, brain and behavior, reproduction, and immune regulation and protection. Clearly, a large proportion of these actions are directly or indirectly associated with the process of reproduction, including many behavioral effects. PRL is also becoming well known as an important regulator of immune function. A number of disease states, including the growth of different forms of cancer as well as various autoimmune diseases, appear to be related to an overproduction of PRL, which may act in an endocrine, autocrine, or paracrine manner, or via an increased sensitivity to the hormone. The first step in the mechanism of action of PRL is the binding to a cell surface receptor. The ligand binds in a two-step process in which site 1 on PRL binds to one receptor molecule, after which a second receptor molecule binds to site 2 on the hormone, forming a homodimer consisting of one molecule of PRL and two molecules of receptor. The PRLR contains no intrinsic tyrosine kinase cytoplasmic domain but associates with a cytoplasmic tyrosine kinase, JAK2. Dimerization of the receptor induces tyrosine phosphorylation and activation of the JAK kinase followed by phosphorylation of the receptor. Other receptor-associated kinases of the Src family have also been shown to be activated by PRL. One major pathway of signaling involves phosphorylation of cytoplasmic State proteins, which themselves dimerize and translocate to nucleus and bind to specific promoter elements on PRL-responsive genes. In addition, the Ras/Raf/MAP kinase pathway is also activated by PRL and may be involved in the proliferative effects of the hormone. Finally, a number of other potential mediators have been identified, including IRS-1, PI-3 kinase, SHP-2, PLC gamma, PKC, and intracellular Ca2+. The technique of gene targeting in mice has been used to develop the first experimental model in which the effect of the complete absence of any lactogen or PRL-mediated effects can be studied. Heterozygous (+/-) females show almost complete failure to lactate after the first, but not subsequent, pregnancies. Homozygous (-/-) females are infertile due to multiple reproductive abnormalities, including ovulation of premeiotic oocytes, reduced fertilization of oocytes, reduced preimplantation oocyte development, lack of embryo implantation, and the absence of pseudopregnancy. Twenty per cent of the homozygous males showed delayed fertility. Other phenotypes, including effects on the immune system and bone, are currently being examined. It is clear that there are multiple actions associated with PRL. It will be important to correlate known effects with local production of PRL to differentiate classic endocrine from autocrine/paracrine effects. The fact that extrapituitary PRL can, under some circumstances, compensate for pituitary PRL raises the interesting possibility that there may be effects of PRL other than those originally observed in hypophysectomized rats. The PRLR knockout mouse model should be an interesting system by which to look for effects activated only by PRL or other lactogenic hormones. On the other hand, many of the effects reported in this review may be shared with other hormones, cytokines, or growth factors and thus will be more difficult to study. (ABSTRACT TRUNCATED)

Prolactin (PRL) and Its Receptor: Actions, Signal Transduction Pathways and Phenotypes Observed in PRL Receptor Knockout Mice

Small molecule inhibitors have proven extremely useful for investigating signal transduction pathways and have the potential for development into therapeutics for inhibiting signal transduction pathways whose activities contribute to human diseases. Transforming growth factor beta (TGF-beta) is a member of a large family of pleiotropic cytokines that are involved in many biological processes, including growth control, differentiation, migration, cell survival, adhesion, and specification of developmental fate, in both normal and diseased states. TGF-beta superfamily members signal through a receptor complex comprising a type II and type I receptor, both serine/threonine kinases. Here, we characterize a small molecule inhibitor (SB-431542) that was identified as an inhibitor of activin receptor-like kinase (ALK)5 (the TGF-beta type I receptor). We demonstrate that it inhibits ALK5 and also the activin type I receptor ALK4 and the nodal type I receptor ALK7, which are very highly related to ALK5 in their kinase domains. It has no effect on the other, more divergent ALK family members that recognize bone morphogenetic proteins (BMPs). Consistent with this, we demonstrate that SB-431542 is a selective inhibitor of endogenous activin and TGF-beta signaling but has no effect on BMP signaling. To demonstrate the specificity of SB-431542, we tested its effect on several other signal transduction pathways whose activities depend on the concerted activation of multiple kinases. SB-431542 has no effect on components of the ERK, JNK, or p38 MAP kinase pathways or on components of the signaling pathways activated in response to serum.

SB-431542 is a potent and specific inhibitor of transforming growth factor-beta superfamily type I activin receptor-like kinase (ALK) receptors ALK4, ALK5, and ALK7.

Transforming growth factor-β utilizes a multitude of intracellular signaling pathways in addition to Smads to regulate a wide array of cellular functions. These non-canonical, non-Smad pathways are activated directly by ligand-occupied receptors to reinforce, attenuate, or otherwise modulate downstream cellular responses. These non-Smad pathways include various branches of MAP kinase pathways, Rho-like GTPase signaling pathways, and phosphatidylinositol-3-kinase/AKT pathways. This review focuses on recent advances in the understanding of the molecular and biochemical mechanisms of non-Smad pathways. In addition, functions of these non-Smad pathways are also discussed.

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Non-Smad pathways in TGF-β signaling

We have investigated whether human lymphoid cells are able to synthesize and secrete human PRL (hPRL) and to express PRL receptors. Metabolic labeling with [35S]methionine and immunoprecipitation of cell extracts from human mononuclear cells (MNC) and a human T lymphocyte cell line with an antiserum against hPRL revealed protein of M(r) 23,000, identical in size to pituitary hPRL. Dilution curves of lymphocyte immunoreactive hPRL were parallel to those obtained with pituitary hPRL in an immunoradiometric assay using two monoclonal antibodies against hPRL. Polymerase chain reaction experiments with primers located in the coding sequence of hPRL showed that the hPRL gene was expressed in MNC. Furthermore, cDNA cloning and sequence analysis indicated the presence of an extra 5' noncoding exon previously described for decidual hPRL. When MNCs were further separated into B cells, T cells, and monocytes, the expression of hPRL appeared to be mainly associated with the T lymphocyte fraction. The hPRL transcript ...

/pdf/expression-of-prolactin-and-its-receptor-in-human-lymphoid-55jkccsdv0.pdf

Expression of prolactin and its receptor in human lymphoid cells.

Human PRL receptor cDNA clones from hepatoma (Hep G2) and breast cancer (T-47D) libraries were isolated by using a rat PRL receptor cDNA probe. The nucleotide sequence predicts a mature protein of 598 amino acids with a much longer cytoplasmic domain than the rat liver PRL receptor. Although this extended region has additional segments of localized sequence identity with the human GH receptor, there is no identity with any consensus sequences known to be involved in hormonal signal transduction. This cDNA will be a valuable tool to better understand the role of PRL in the development and growth of human breast cancer.

Identification of a cDNA Encoding a Long Form of Prolactin Receptor in Human Hepatoma and Breast Cancer Cells

The screening of a size-selected cDNA library from the ovary revealed the existence of a second form of PRL receptor in the rat. The polypeptide sequence deduced from cDNAs has a much longer cytoplasmic domain (357 amino acids) than the form previously identified in the liver (57 amino acids). Nucleotide sequence analysis and comparison with rabbit, mouse, and human PRL receptor cDNAs suggests that the two forms of rat PRL receptor result from alternative splicing of a primary transcript. Complementary DNAs encoding the long form of the receptor were also found in a library prepared from estradiol-treated rat liver, although they represent a minor fraction of total PRL receptor cDNAs obtained from this tissue. DNA polymerase chain reaction amplification of cDNA confirmed the presence of the two receptor forms in both the ovary and liver. Northern analysis, using probes that specifically hybridize with either form of mRNA, indicates a major transcript of 1.8 kilobases (kb) in estradiol-treated liver, which...

/pdf/expression-of-two-forms-of-prolactin-receptor-in-rat-ovary-370drlah2k.pdf

Expression of two forms of prolactin receptor in rat ovary and liver.

Publisher Summary This chapter provides an overview of growth hormone (GH) and prolactin receptor (PRL) family. Sequence analysis has confirmed that GH and PRL have selected regions of strong homology and, along with placental lactogen (PL) or chorionic somatotropin (CS), form a family of polypeptide hormones that appear to have arisen by the duplication of an ancestral gene. Hormones act by first binding to a specific receptor on or inside the cell. Two general classes of hormone receptors exist, namely, DNA binding receptors and membrane receptors. The events responsible for the transfer of the hormonal message inside the cell occur just after the interaction of the ligand with the receptor; they remain, for the most part, unknown. In mammals, PRL is primarily responsible for the development of the mammary gland and lactogenesis. The best known effects of PRL are in mammary epithelial cells. PRL acts in association with insulin and glucocorticoids to stimulate milk protein gene expression at both the transcriptional and posttranscriptional levels, because the rate of gene transcription and the stability of milk protein mRNAs are increased under the influence of the hormone. GH, on the other hand, has a direct action on the differentiation of preadipocytes, where it is necessary for the initiation of the differentiation program for cells to become responsive to insulin-like growth factor-I and for its mitogenic effect.

Suhad Ali

Papers

Expression of prolactin and its receptor in human lymphoid cells.

Identification of a cDNA Encoding a Long Form of Prolactin Receptor in Human Hepatoma and Breast Cancer Cells

Expression of two forms of prolactin receptor in rat ovary and liver.

The growth hormone/prolactin receptor family.

Prolactin-induced proliferation of Nb2 cells involves tyrosine phosphorylation of the prolactin receptor and its associated tyrosine kinase JAK2.