Cyclin-dependent kinase inhibitors and basement membrane interact to regulate breast epithelial cell differentiation and acinar morphogenesis.

OBJECTIVE: The cyclin-dependent kinase inhibitors (CDKIs), p21(CIP1) and p27(KIP1) regulate growth and differentiation in diverse tissue types. We aimed to determine whether p21(CIP1) or p27(KIP1) could induce a terminally differentiated phenotype in breast cells, and to examine if CDKI expression is regulated by basement membrane interactions. MATERIALS AND METHODS: Effects of increased CDKI expression on the phenotype of MCF-10A breast epithelial cells were examined by retroviral transduction of p21(CIP1) or p27(KIP1) cDNA. RESULTS: Overexpression of p21(CIP1) or p27(KIP1) reduced MCF-10A growth rates in monolayer cultures, altered cellular morphology and stimulated accumulation of neutral lipid droplets, suggesting partial lactational differentiation. However, markers of luminal differentiation (oestrogen and progesterone receptors, alpha-lactalbumin, beta-casein and adipophilin) were absent when examined by reverse transcriptase-polymerase chain reaction and immunohistochemistry. Cell-basement membrane contacts are known to be essential for full mammary epithelial cell differentiation and therefore parental MCF-10A cells were cultured on a basement membrane preparation (Matrigel) in which they form acini. Immunocytochemistry showed that Ki67, the cell proliferation marker, was initially expressed at high levels and as growth decreased p27(KIP1) expression steadily increased. Surprisingly, p21(CIP1) was highest at the early stages of acinus growth and was detected in proliferating cells, as demonstrated by colocalization in dual Ki67/p21(CIP1) immunofluorescence. Overexpression of p21(CIP1) or p27(KIP1) impaired formation of acini, whereas their knockdown, using siRNA, increased acinus formation. CONCLUSION: We conclude that both p21(CIP1) and p27(KIP1) induce partial secretory differentiation of mammary cells in monolayer, but during acinus morphogenesis in 3D culture they have a highly regulated temporal expression pattern.

An abundant and specific binding site for the novel vasodilator adrenomedullin in the rat.

Rat adrenomedullin is a novel 50-amino acid peptide with structural similarities to the calcitonin family of peptides, calcitonin, calcitonin gene-related peptide (CGRP), and islet amyloid polypeptide (IAPP). Using rat [125I]adrenomedullin, specific binding sites were demonstrated in heart, lung, spleen, liver, soleus, diaphragm, gastrocnemius, and spinal cord membranes. The highest binding was present in heart and lung, which was further characterized. These sites exhibited saturation, dissociation, and competition. In rat lung, only rat (IC50 = 5.8 nM) and human (IC50 = 94 nM) adrenomedullin competed with [125I]adrenomedullin. However, in rat heart, rat (IC50 = 0.2 nM) and human (IC50 = 4.2 nM) adrenomedullin, IAPP (IC50 = 240 nM), and CGRP (IC50 = 1050 nM) all competed with [125I] adrenomedullin. Saturation analysis revealed binding capacities and dissociation constants of 2.8 +/- 0.3 pmol/mg protein and 1.3 +/- 0.3 nM, respectively, in lung and 0.47 +/- 0.11 pmol/mg protein and 0.41 +/- 0.14 nM in heart. Comparison with [125I]CGRP- and [125I]IAPP-binding sites in lung showed that rat adrenomedullin could potently inhibit at these sites (IC50 = 5 and 6 nM, respectively). Chemical cross-linking demonstrated a major band of 83,000 mol wt in lung, diaphragm, spleen, and liver and a band of 94,000 mol wt in heart, soleus, and gastrocnemius. Thus, [125I]adrenomedullin-binding sites in rat lung are abundant and can be differentiated from binding sites in rat heart, both pharmacologically and by mol wt.

Adrenomedullin stimulates DNA synthesis and cell proliferation via elevation of cAMP in Swiss 3T3 cells.

Our results demonstrate that the novel vasoactive regulatory peptide adrenomedullin is a potent mitogen for Swiss 3T3 cells. Acting via a specific adrenomedullin receptor, it stimulates a dose-dependent increase in DNA synthesis in synergy with insulin. Additionally, adrenomedullin stimulates further progression through the cell cycle resulting in cell proliferation, an effect that was further enhanced by the presence of insulin. Adrenomedullin rapidly induces accumulation of intracellular cAMP but does not stimulate an increase in intracellular Ca2+, activation of protein kinase C, or tyrosine phosphorylation of intracellular substrates. Adrenomedullin-stimulated mitogenesis is markedly enhanced in Swiss 3T3 cells stably transfected with a constitutively activated Gs alpha, which are highly sensitive to agents that elevate cAMP, and is inhibited by the PKA inhibitor H-89. Adrenomedullin is, thus, identified as a novel mitogenic regulatory peptide acting via cAMP.

The interaction of CGRP and adrenomedullin with a receptor expressed in the rat pulmonary vascular endothelium.

An abundant, seven trans-membrane domain receptor related to the calcitonin receptor has been studied by a number of groups without identification of its ligand. A recent report claimed that the receptor was a type 1 CGRP receptor (Aiyar et al J. Biol. Chem. 271 11325-11329 (1996)). We have studied the equivalent rat sequence in transfected cells. When expressed in 293 cells the receptor interacts with CGRP and adrenomedullin with KD values of 1.2 nM for CGRP and 11 nM for adrenomedullin. Both ligands cause an elevation of intracellular cAMP with EC50 values of 4 nM and 20 nM respectively and these effects are inhibited by the antagonist CGRP8-37. The receptor is expressed at high levels in the pulmonary vascular endothelium. Both the pharmacological data and the localisation are consistent with the conclusion that the orphan receptor is a type J CGRP receptor. However, when expressed in COS-7 cells, no receptor activity could be demonstrated suggesting that 293 cells contain a factor necessary for functional receptor expression.

CGRP and adrenomedullin binding correlates with transcript levels for calcitonin receptor-like receptor (CRLR) and receptor activity modifying proteins (RAMPs) in rat tissues.

1. Putative receptors for CGRP and adrenomedullin have been investigated in the rat. Calcitonin Receptor-Like Receptor (CRLR), in combination with Receptor Activity Modifying Proteins (RAMPs) is hypothesized to bind either CGRP or adrenomedullin. The receptors known as RDC1 and L1 have also been shown to bind CGRP and adrenomedullin respectively. 2. In this study it is shown that rat CRLR cDNA specifies a CGRP receptor when co-transfected with RAMP-1 cDNA and an adrenomedullin receptor when co-transfected with either RAMP-2 or RAMP-3 cDNA in human embryonic kidney 293 cells. 3. CRLR, RAMP, RCD1 and L1 mRNA levels and CGRP and adrenomedullin receptor densities have been measured and correlated with each other in eight rat tissues selected for their distinctive patterns of CGRP and adrenomedullin binding. 4. The data are consistent with the predictions of the CRLR/RAMP model. CGRP binding correlates well with RAMP-1 mRNA levels (R=1.0, P=0.007), adrenomedullin binding shows a tendency to vary with RAMP-2 mRNA levels (R=0.85, P=0.14) and total binding is correlated with CRLR mRNA levels (R=0.94, P=0.03). The data do not support the hypothesis that RDC1 and L1 account for the majority of CGRP and adrenomedullin binding respectively.

Effects of intracerebroventricular injection of glucagon like peptide-1 and its related peptides on serotonin metabolism and on levels of amino acids in the rat hypothalamus.

High concentrations of glucagon-like peptide-1 (7-36) amide (GLP-1) and its specific receptor (GLP-1R) have been found in the rat hypothalamus. In this study the actions of GLP-1 and its related peptides, exendin-4 (GLP-1R agonist), exendin (9-39) (GLP-1R antagonist) and GLP-1 (9-36) amide (the major GLP-1 metabolite) on levels of serotonin (5-HT), 5-hydroxyindolacetic acid (5-HIAA) and amino acids (Glu, Asp, Gln, Gly, Tyr, Trp, GABA) in the hypothalamus were investigated. Intracerebroventricular (ICV) injection of GLP-1 (4 nmol) produced a significant reduction in levels of 5-HT (54%) and all measured amino acids (34 to 56%) compared with saline injected controls, whereas exendin (9-39) (4 nmol) was ineffective. ICV injection of exendin-4 produced a significant reduction in the levels of 5-HT, 5-HIAA, Trp, Glu, and Tyr. ICV injection of GLP-1(9-36) amide showed a statistically significant increase in the level of 5-HT, 5-HIAA and all the amino acids tested in this study. Prior administration of exendin (9-39) or GLP-1 (9-36) amide blocked the effects of GLP-1 on the levels of 5-HT and the amino acids. These data are consistent with exendin-4 being a GLP-1R agonist and exendin (9-39) being a specific GLP-1R antagonist. GLP-1 (9-36) amide, a primary metabolite of GLP-1, appears to act as an endogenous antagonist at the GLP-1R.

A rat skeletal muscle cell line (L6) expresses specific adrenomedullin binding sites but activates adenylate cyclase via calcitonin gene-related peptide receptors.

We have previously demonstrated specific binding sites for adrenomedullin, a novel member of the calcitonin family of peptides, in rat muscles. It is unclear whether these receptors are vascular or muscular. Receptors for the structurally similar calcitonin gene-related peptide (CGRP) are present on myocytes and might be involved in the regulation of myocyte glucose metabolism and control by motor neurons. We investigated whether adrenomedullin binding sites were present on L6 myocytes. Specific [125I]adrenomedullin binding sites were demonstrated where adrenomedullin competed with an IC50 of 0.22 +/- 0.04 nM (mean +/- S.E.M.) and a concentration of binding sites (Bmax) of 0.95 +/- 0.19 pmol/mg of protein (mean +/- S.E.M.). CGRP and the specific CGRP receptor antagonist CGRP(8-37) competed weakly at this site (IC50 > 10 and 601 +/- 298 nM respectively). Binding studies with [125I]CGRP revealed a binding site for CGRP (IC50 = 0.13 +/- 0.01 nM; Bmax = 0.83 +/- 0.10 pmol/mg of protein) where both CGRP(8-37) and adrenomedullin competed with [125I]CGRP with IC50 values of 1.15 +/- 0.12 and 8.68 +/- 0.98 nM respectively. Chemical cross-linking showed the CGRP and adrenomedullin binding site-ligand complexes to have approximate molecular masses of 82 and 76 kDa respectively. Both CGRP and adrenomedullin increased adenylate cyclase activity with similar potencies. In both cases adenylate cyclase activation was blocked by CGRP(8-37). Stimulation with 10 nM adrenomedullin or CGRP caused an increase in the percentage of total activated cellular cAMP-dependent protein kinase from 38% in resting cells to 100% and 98% respectively. Therefore in L6 cells adrenomedullin can bind to CGRP receptors, activating adenylate cyclase and cAMP-dependent protein kinase.

Adrenomedullin: receptor and signal transduction.

Adrenomedullin is a vascular tissue peptide and a member of the calcitonin family of peptides, which includes calcitonin, calcitonin-gene-related peptide (CGRP) and amylin. Its many biological actions are mediated via CGRP type 1 (CGRP(1)) receptors and by specific adrenomedullin receptors. Although the pharmacology of these receptors is distinct, they are both represented in molecular terms by the type II family G-protein-coupled receptor, calcitonin-receptor-like receptor (CRLR). The specificity here is defined by co-expression of receptor-activity-modifying proteins (RAMPs). CGRP(1) receptors are represented by CRLR and RAMP1, and specific adrenomedullin receptors by CRLR and RAMP2 or 3. Here we discuss how CRLR/RAMP2 relates to adrenomedullin binding, pharmacology and pathophysiology, and how chemical cross-linking of receptor-ligand complexes in tissue relates to that in CRLR/RAMP2-expressing cells. CRLR, like other type II family G-protein-coupled receptors, signals via G(s) and adenylate cyclase activation. We demonstrated that adrenomedullin signalling in cell lines expressing specific adrenomedullin receptors followed this expected pattern.

Rat-2 fibroblasts express specific adrenomedullin receptors, but not calcitonin-gene-related-peptide receptors, which mediate increased intracellular cAMP and inhibit mitogen-activated protein kinase activity.

Rat-2 fibroblasts demonstrate specific binding of 125I-labelled rat adrenomedullin (KD=0.43 nM; Bmax=50 fmol/mg of protein) in the absence of 125I-labelled calcitonin-gene-related peptide (CGRP) binding. Therefore Rat-2 cells were used to examine the pharmacology and signal transduction pathways of adrenomedullin receptors. We examined the effects of adrenomedullin, the CGRP receptor antagonist CGRP-(8-37) and the amylin antagonists AC187 and AC253 on receptor binding and cAMP production. AC253, AC187 and CGRP-(8-37) inhibited 125I-adrenomedullin binding, with respective IC50 values of 25+/-8, 129+/-39 and 214+/-56 nM. Adrenomedullin dose-dependently increased intracellular cAMP (approximate EC50=1.0 nM). CGRP-(8-37), AC253 and AC187 antagonized adrenomedullin-stimulated cAMP production at micromolar concentrations. Using kinase-substrate assays, Mono Q FPLC and 'phospho-specific' Western blotting, we found that adrenomedullin alone abolished basal mitogen-activated protein kinase (MAPK) activity and dose-dependently inhibited platelet-derived-growth-factor-stimulated MAPK activity. Radioimmunoassay for adrenomedullin of media from Rat-2 cells showed a linear release of adrenomedullin-like immunoreactivity of 3.1 fmol/h per 2x10(6) cells. Gel-filtration chromatography showed that this adrenomedullin-like immunoreactivity co-eluted with synthetic rat adrenomedullin. Northern blotting with a rat adrenomedullin cDNA probe was used to confirm the presence of adrenomedullin mRNA. However, neither Northern blotting nor reverse transcriptase-PCR showed the presence of the cloned adrenomedullin receptor (L1). We conclude that the Rat-2 cell line expresses a specific adrenomedullin receptor (coupled to cAMP production and regulation of MAPK) and secretes adrenomedullin, which may participate in a regulatory control loop.