Tannin-controlled micelles and fibrils of κ-casein.

Effects of green tea tannin epigallocatechin-gallate (EGCG) on thermal-stress-induced amyloid fibril formation of reduced carboxymethylated bovine milk protein κ-casein were studied by dynamical light scattering and small angle X-ray scattering (SAXS). Two populations of aggregates, micelles, and fibrils dominated the time evolution of light scattering intensity and of effective hydrodynamic diameter. SAXS experiments allowed us to resolve micelles and fibrils so that the time dependence of the scattering profile revealed the structural evolution of the two populations. The low-Q scattering intensity prior to an expected increase in time due to fibril growth shows an intriguing rapid decrease, which is interpreted as the release of monomers from micelles. This phenomenon, observed both in the absence and in the presence of EGCG, indicates that under thermal stress free conditions, native monomers are converted to amyloid-prone monomers that do not form micelles. The consumption of free native monomers results in a release of native monomers from micelles because only native proteins participate in micelle-monomer (quasi)equilibrium. This release is reversible, indicating also that native-to-amyloid-prone monomer conversion is reversible as well. We show that EGCG does not bind to protein in fibrils, neither does it affect/prevent the proamyloid conversion of monomers. EGCG hinders the addition of monomers to growing fibrils. These facts allowed us to propose the kinetics model for EGCG-controlled amyloid aggregation of micellar proteins. Therein, we introduced the growth-rate inhibition function, which quantitatively accounts for the effect of EGCG on the fibril growth at any degree of thermal stress.

Inhibition of arginase via jugular infusion of Nω-hydroxy-nor-l-arginine inhibits casein synthesis in lactating dairy cows.

A previous in vitro study revealed that Arg elicits positive effects on casein synthesis through alterations of the Arg-ornithine pathway in bovine mammary epithelial cells. The main purpose of this work was to determine the effects of arginase inhibition using Nω-hydroxy-nor-l-arginine (nor-NOHA) on milk protein synthesis in vivo. Six healthy Chinese Holstein cows with similar body weight (550.0 ± 20 kg; means ± standard deviation), parity (4), body condition score (3.0), milk yield (21.0 ± 1.0 kg), and days in milk (80 ± 2) were selected and randomly assigned to 3 treatments in a replicated 3 × 3 Latin square design with 22 d for each period (7 d for infusion and 15 d for washout). The treatments were (1) control: saline infusion; (2) nor-NOHA: infusion of 125 mg/L of nor-NOHA; (3) nor-NOHA + Arg: infusion of 125 mg/L of nor-NOHA with 9.42 g/L of Arg. The activity of enzymes related to Arg metabolism, milk protein synthesis, and expression of AA transporters was determined. The infusion of nor-NOHA decreased the activity of arginase but had no effect on the activity of ornithine decarboxylase and nitric oxide synthase in serum, and these responses were the same at the gene expression level in mammary gland. In addition, the infusion of nor-NOHA also reduced protein and fat synthesis in milk but had no effect on milk yield. When Arg was infused with nor-NOHA, the activity of total arginase, ornithine decarboxylase, and nitric oxide synthase, and the concentration of casein, protein, and fat in milk did not change compared with the nor-NOHA group, but the milk protein yield, the expression of some Arg transporters (SLC7A5 and SLC7A8), and milk yield increased. Overall, results verified previous in vitro findings indicating that synthesis of casein protein is closely regulated by the Arg-ornithine pathway in bovine mammary gland.

Trimethylamine N-oxide abolishes the chaperone activity of α-casein: an intrinsically disordered protein.

Osmolytes (small molecules that help in circumventing stresses) are known to promote protein folding and prevent aggregation in the case of globular proteins. However, the effect of such osmolytes on the structure and function of intrinsically disordered proteins (IDPs) has not been clearly understood. Here we have investigated the effect of methylamine osmolytes on α-casein (an IDP present in mammalian milk) and discovered that TMAO (Trimethylamine-N-oxide) but not other methylamines renders α-casein functionless. We observed that the loss of chaperone activity of α-casein in presence of TMAO was due to the induction of an unstable aggregation-prone intermediate. The results indicate that different osmolytes may have different structural and functional consequences on IDPs, and therefore might have clinical implications for a large number of human diseases (e.g., amyloidosis, cancer, diabetes, and neurodegeneration) where IDPs are involved.

Inhibitory effect of fluvoxamine on ß-casein expression via a serotonin-independent mechanism in human mammary epithelial cells.

Selective serotonin reuptake inhibitors (SSRIs) are widely used as a first-line therapy in postpartum depression. The objective of this study was to determine the mechanism underlying the inhibitory effects of the SSRI, fluvoxamine, on ß-casein expression, an indicator of lactation, in MCF-12A human mammary epithelial cells. Expression levels of serotonin (5-hydroxytryptamine; 5-HT) transporter, an SSRI target protein, and tryptophan hydroxylase 1, a rate-limiting enzyme in 5-HT biosynthesis, were increased in MCF-12A cells by prolactin treatment. Treatment with 1 µM fluvoxamine for 72 h significantly decreased protein levels of ß-casein and phosphorylated signal transducer and activator transcription 5 (pSTAT5). Extracellular 5-HT levels were significantly increased after exposure to 1 µM fluvoxamine, in comparison with those of untreated and vehicle-treated cells; however, extracellular 5-HT had little effect on the decrease in ß-casein expression. Expression of glucose-related protein 78/binding immunoglobulin protein, a regulator of endoplasmic reticulum (ER) stress, was significantly increased after treatment with 1 µM fluvoxamine for 48 h. Exposure to tunicamycin, an inducer of ER stress, also decreased expression of ß-casein and pSTAT5 in a manner similar to fluvoxamine. Our results indicate that fluvoxamine suppresses ß-casein expression in MCF-12A cells via inhibition of STAT5 phosphorylation caused by induction of ER stress. Further studies are required to confirm the effect of fluvoxamine on the function of mammary epithelial cells.

Inhibitory activity of YKL-40 in mammary epithelial cell differentiation and polarization induced by lactogenic hormones: a role in mammary tissue involution.

We previously reported that a secreted glycoprotein YKL-40 acts as an angiogenic factor to promote breast cancer angiogenesis. However, its functional role in normal mammary gland development is poorly understood. Here we investigated its biophysiological activity in mammary epithelial development and mammary tissue morphogenesis. YKL-40 was expressed exclusively by ductal epithelial cells of parous and non-parous mammary tissue, but was dramatically up-regulated at the beginning of involution. To mimic ductal development and explore activity of elevated YKL-40 during mammary tissue regression in vivo, we grew a mammary epithelial cell line 76N MECs in a 3-D Matrigel system in the presence of lactogenic hormones including prolactin, hydrocortisone, and insulin. Treatment of 76N MECs with recombinant YKL-40 significantly inhibited acinar formation, luminal polarization, and secretion. YKL-40 also suppressed expression of E-cadherin but increased MMP-9 and cell motility, the crucial mechanisms that mediate mammary tissue remodeling during involution. In addition, engineering of 76N MECs with YKL-40 gene to express ectopic YKL-40 recapitulated the same activities as recombinant YKL-40 in the inhibition of cell differentiation. These results suggest that YKL-40-mediated inhibition of cell differentiation and polarization in the presence of lactogenic hormones may represent its important function during mammary tissue involution. Identification of this biophysiological property will enhance our understanding of its pathologic role in the later stage of breast cancer that is developed from poorly differentiated and highly invasive cells.

Carboxymethylated-kappa-casein: a convenient tool for the identification of polyphenolic inhibitors of amyloid fibril formation.

Reduced and carboxymethylated-kappa-casein (RCM-kappa-CN) is a milk-derived amyloidogenic protein that readily undergoes nucleation-dependent aggregation and amyloid fibril formation via a similar pathway to disease-specific amyloidogenic peptides like amyloid beta (Abeta), which is associated with Alzheimer's disease. In this study, a series of flavonoids, many known to be inhibitors of Abeta fibril formation, were screened for their ability to inhibit RCM-kappa-CN fibrilisation, and the results were compared with literature data on Abeta inhibition. Flavonoids that had a high degree of hydroxylation and molecular planarity gave good inhibition of RCM-kappa-CN fibril formation. IC(50) values were between 10- and 200-fold higher with RCM-kappa-CN than literature results for Abeta fibril inhibition, however, with few exceptions, they showed a similar trend in potency. The convenience and reproducibility of the RCM-kappa-CN assay make it an economic alternative first screen for Abeta inhibitory activity, especially for use with large compound libraries.

Molecular docking studies and anti-snake venom metalloproteinase activity of Thai mango seed kernel extract.

Snakebite envenomations cause severe local tissue necrosis and the venom metalloproteinases are thought to be the key toxins involved. In this study, the ethanolic extract from seed kernels of Thai mango (Mangifera indica L. cv. 'Fahlun') (Anacardiaceae) and its major phenolic principle (pentagalloylglucopyranose) exhibited potent and dose-dependent inhibitory effects on the caseinolytic and fibrinogenolytic activities of Malayan pit viper and Thai cobra venoms in in vitro tests. molecular docking studies revealed that the binding orientations of the phenolic principles were in the binding pockets of snake venom metalloproteinases (SVMPs). The phenolic principles could form hydrogen bonds with the three histidine residues in the conserved zinc-binding motif and could chelate the Zn(2+) atom of the SVMPs, which could potentially result in inhibition of the venom enzymatic activities and thereby inhibit tissue necrosis.

Presence of a lethal protease in the extracellular products of Vibrio splendidus-Vibrio lentus related strains.

The presence of a lethal extracellular 39-kDa protease, a virulence determinant of a Listonella pelagia strain which produces vibriosis in turbot, was determined in the extracellular products (ECP) of 33 Vibrionaceae strains. Both immunological and enzymatic techniques distinguished this specific protease from other Vibrionaceae proteins. It was detected in 15% (5/33) of the ECPs assayed belonging to strains of the Vibrio splendidus-V. lentus related group isolated in Galician aquaculture systems (NW Spain). As these strains were associated with diseased octopus and cultured turbot, were able to colonize the internal organs of fish and produced a lethal ECP for fish, they are a potential risk for the health of reared aquatic organisms.

Characterization and study of a kappa-casein-like chymosin-sensitive linkage.

The present report is dealing with the identification, in various unrelated proteins, of protein fragments sharing local sequence and structure similarities with the chymosin-sensitive linkage surrounding the Phe-Met/Ile bond of kappa-caseins. In all these proteins, this linkage is observed within an exposed beta-strand-like structure, as also predicted for kappa-caseins. The structure of one of these fragments, included in glutamine synthetase, particularly superimposes well with the conformation observed for a chymosin inhibitor (CP-113972) within the complex it forms with chymosin and can be similarly accommodated by specificity pockets within the enzyme substrate binding cleft. The effect of the enzyme activity of chymosin was thus tested on glutamine synthetase. Chymosin cut the latter at the Phe-Met linkage, suggesting that this system may locally resemble the kappa-casein/chymosin complex.

Oral administration of D-003, a mixture of very long chain fatty acids prevents casein-induced endogenous hypercholesterolemia in rabbits.

D-003 is a mixture of very long chain saturated fatty acids (VLCSFA) purified from sugar cane wax with cholesterol-lowering effects proven in animal models and healthy volunteers. D-003 inhibits cholesterol biosynthesis through the regulation of HMG-CoA reductase activity. Rabbits fed diets enriched with casein develop endogenous hypercholesterolemia (EH), making them a very useful model for determining the mechanism of action of drugs affecting lipids. We examined whether D-003 prevented EH. Rabbits were fed a casein diet for 4 weeks, administered simultaneously with D-003 (5, 50, and 100 mg.kg-1.day-1). As expected, nontreated rabbits became hypercholesterolemic; however, as early as 15 days following administration, the treated group (50 and 100 mg.kg-1.day-1) had significantly decreased total cholesterol and low-density lipoprotein cholesterol (LDL-C). Triglycerides were not affected; however, at study completion, HDL-C levels significantly increased at all the doses assayed. D-003 inhibited de novo synthesis of cholesterol, since the incorporation of 3H2O into sterols in the liver and proximal small bowel was significantly depressed. Also, D-003 significantly raised the rate of removal of [125I]-LDL from serum and significantly elevated [125I]-LDL binding activity to liver homogenates. Taken together, these results show that the efficacy of D-003 in reducing casein-derived hypercholesterolemia could involve, at least partially, an inhibition of hepatic cholesterol biosynthesis, which may elicit a decreased cholesterol concentration in hepatocytes, preventing the loss of hepatic LDL receptors induced by casein administration. However, since casein-induced hypercholesterolemia is also a consequence of a stimulation of cholesterol absorption in the lumen and an increase of the output of cholesterol associated with LDL, the effect of D-003 on cholesterol absorption and LDL synthesis by the liver should be investigated.

The liver-enriched inhibitory protein isoform of CCAAT/enhancer-binding protein beta, but not nuclear factor-kappaB, mediates the transcriptional inhibition of beta-casein by tumor necrosis factor-alpha.

TNF-alpha is a physiological regulator of mammary gland development that stimulates the growth of both normal and malignant mammary epithelial cells in primary culture and inhibits functional differentiation. To understand how TNF exerts its effects, the current study examined the mechanism by which TNF down-regulates expression of the beta-casein and whey acidic protein (WAP) genes. TNF treatment markedly decreased activity of the beta-casein and WAP promoters in transiently transfected HC11 mammary epithelial cells. Overexpression of the nuclear factor-kappaB (NFkappaB) p50 and/or p65 proteins increased the transcriptional activity of the beta-casein and WAP promoters in HC11 cells, suggesting that the inhibitory effect of TNF on transcription of these genes is not mediated by NFkappaB. This was further confirmed in experiments in which an NFkappaB super-repressor was overexpressed, and by deletion of an NFkappaB binding site in the beta-casein promoter. In contrast, we found that TNF induced both nuclear expression and the DNA-binding activity of liver-enriched inhibitory protein (LIP) isoform of CCAAT/enhancer-binding protein beta. Moreover, cotransfection of LIP and beta-casein expression vectors showed that LIP suppressed the transcriptional activity of the beta-casein promoter. Together, these results suggest that LIP plays a critical role in mediating TNF-induced down-regulation of the beta-casein gene.

Purification and partial characterisation of a fish lethal extracellular protease from Vibrio pelagius.

The purification, characterisation and lethal effect of an extracellular protease present in the extracellular products (ECPs) of a pathogenic Vibrio pelagius (7P) strain are described. The extracellular protease was purified by size-exclusion high-performance liquid chromatography and characterised by enzymatic assays. The lethal effect was evaluated by injection into fish. The native protease had a molecular mass of 39 kDa, was active on casein and L-leucyl-beta-naphthylamide (LNA) and its metalloprotease nature was shown by the LNA inhibition profile. Kinetic studies on the hydrolysis of casein and LNA confirmed a competitive inhibition of one substrate with respect to the other. The temperature assays showed that both aminopeptidolytic and caseinolytic activities were labile at 70 degrees C for 3 min. The N-terminal amino acid sequence of 7P protease revealed a high degree of homology with other metalloproteases of Vibrio species that are implicated in virulence. The purified 7P protease showed an LD(50) of 1.77 microgprotein/g fish for turbot. The quick lethal effect (<24h) and the macroscopic damage (external haemorrhagic areas, principally on fins and mouth, petechial haemorrhages in internal organs, but with no external or internal apparent necrotic areas) detected in the host were similar to those obtained by injection of total ECP and live cells of 7P strain. An extracellular protease with endopeptidolytic and exopeptidolytic activities, responsible for the lethal effect of ECP and clinical signs of vibriosis in turbot was purified from a pathogenic V. pelagius (7P) strain.

Impairment of proteasome function upon UVA- and UVB-irradiation of human keratinocytes.

The major environmental influence for epidermal cells is sun exposure and the harmful effect of UV radiation on skin is related to the generation of reactive oxygen species that are altering cellular components including proteins. It is now well established that the proteasome is responsible for the degradation of oxidized proteins. Therefore, the effects of UV-irradiation on proteasome have been investigated in human keratinocyte cultures. Human keratinocytes were irradiated with 10 J/cm(2) of UVA and 0.05 J/cm(2) of UVB and proteasome peptidase activities were measured in cell lysates using fluorogenic peptides. All three peptidase activities were decreased as early as 1 h and up to 24 h after irradiation of the cells. Increased levels of oxidized and ubiquitinated proteins as well as proteins modified by the lipid peroxidation product 4-hydroxy-2-nonenal were also observed in irradiated cells. However, immunopurified 20S proteasome exhibited no difference in both peptidase specific activities and 2D gel pattern of subunits in irradiated cells, ruling out the possibility that the 20S proteasome could be a target for the UV-induced damage. Finally, extracts from irradiated keratinocytes were able to inhibit degradation by the proteasome, demonstrating the presence of endogeneous inhibitors, including 4-hydroxy-2-nonenal modified proteins, generated upon UV-irradiation.

Detection and localization of Cripto-1 binding in mouse mammary epithelial cells and in the mouse mammary gland using an immunoglobulin-cripto-1 fusion protein.

Human Cripto-1 (CR-1), a member of the epidermal growth factor-CFC (EGF-CFC) family of peptides, is expressed in the developing mouse mammary gland and can modulate mammary epithelial cell migration, branching morphogenesis and milk protein expression in vitro. In order to screen for a CR-1 receptor and to identify potential CR-1 target tissues, we constructed a fusion protein comprising the EGF-like domain of CR-1 and the Fc domain of a human IgG1. The recombinant CR-1 fusion protein (CR-1-Fc) was biologically active as it was able to activate the ras/raf/mitogen activated protein kinase (MAPK) pathway and to inhibit transcription of the milk protein beta-casein in NMuMG and HC-11 mouse mammary epithelial cells. By using immunocytochemistry and by an in situ enzyme-linked immunosorbent assay (ELISA), CR-1-Fc was found to specifically bind to NMuMG and HC-11 cells. Finally, immunohistochemical analysis using CR-1-Fc showed a specific localization of CR-1 binding to tissue sections from mouse mammary gland. In particular, more than 60% of the epithelial cells were intensely stained with the CR-1-Fc fusion protein in the lactating mouse mammary gland, whereas approximately 25% of the mammary epithelial cells were stained in the gland from pregnant mouse. Since expression of mouse cripto-1 (Cr-1) in the pregnant and lactating mouse mammary gland as well as its presence in milk has been previously demonstrated, these data strongly suggest that an autocrine pathway involving Cr-1 and its putative receptor is operating in the mouse mammary gland during pregnancy and lactation.

Trichostatin A inhibits beta-casein expression in mammary epithelial cells.

Many aspects of cellular behavior are defined by the content of information provided by association of the extracellular matrix (ECM) and with cell membrane receptors. When cultured in the presence of laminin-containing ECM and prolactin (Prl), normal mammary epithelial cells express the milk protein beta-casein. We have previously found that the minimal ECM- and Prl-responsive enhancer element BCE-1 was only active when stably integrated into chromatin, and that trichostatin A (TSA), a reagent that leads to alterations in chromatin structure, was able to activate the integrated enhancer element. We now show that endogenous beta-casein gene, which is controlled by a genetic assembly that is highly similar to that of BCE-1 and which is also activated by incubation in ECM and Prl, is instead inhibited by TSA. We provide evidence that the differing response of beta-casein and BCE-1 to TSA is neither due to an unusual effect of TSA on mammary epithelial cells, nor to secondary consequences from the expression of a separate gene, nor to a particular property of the BCE-1 construct. As a component of this investigation, we also showed that ECM mediated rapid histone deacetylation in mammary epithelial cells. These results are discussed in combination with previous work showing that TSA mediates the differentiation of many types of cancer cells but inhibits differentiation of some nonmalignant cell types.

Vanadate disrupts mammary gland development in whole organ culture.

Protein tyrosine kinases and phosphatases are signaling molecules involved in all aspects of development, including proliferation, differentiation, and apoptosis. How disruption of protein tyrosine phosphatase affects mammary gland development is not entirely clear. We examined the effects of sodium vanadate, which is known to primarily inhibit tyrosine phosphatases, in mouse mammary gland development in whole organ culture. Mammary epithelial differentiation was effectively inhibited by vanadate in a dose-dependent manner as indicated by lack of epithelial alveoli compared to the contralateral non-treated gland controls. Mammary glands in the differentiation medium after four days in the presence of vanadate did not differentiate into alveoli. Instead, they exhibited prominent terminal end buds and lost the distinctive epithelial structures. The inhibitory effect of vanadate on mammary epithelial cell differentiation was irreversible after one day of treatment. Immunohistochemical staining for PCNA (Proliferating Cell Nuclear Antigen) showed that vanadate-treated glands exhibited elevated proliferation signals in the differentiation medium. Expression of beta-casein protein in the vanadate-treated glands decreased dramatically and progressively. Short-term exposure (up to 72 hours) of mammary glands to vanadate resulted in an increase in mammary epithelial cell density and loss of organization of the mammary structures. TUNEL assay of mammary glands with prolonged exposure to vanadate revealed widespread apoptosis. Furthermore, some cells were still proliferating or expressing beta-casein after prolonged exposure to vanadate. Taken together, these data indicate that vanadate treatment blocks mammary epithelial cell differentiation and promotes abnormal proliferation and apoptosis, likely through the inhibition of protein tyrosine phosphatase-mediated signaling.

Isolation and characterization of DM40 and DM43, two snake venom metalloproteinase inhibitors from Didelphis marsupialis serum.

From Didelphis marsupialis serum, two antihemorrhagic proteins were isolated by DEAE-Sephacel, Phenyl-Sepharose and Superdex 200 and characterized. Their masses by mass spectrometry were 40318 AMU for DM40 and 42373 and 43010 AMU for DM43, indicating the presence of isoforms for the last. Molecular masses of 44.8 and 47.3 were obtained by SDS-PAGE, respectively for DM40 and DM43. Both inhibitors showed isoelectric points lower than 3.5 and glycosylation percentages varying from 20.5 to 29.0%, as estimated by chemical deglycosylation and amino acid analysis. N-terminal sequences of the first 17 residues of DM40 and DM43 were identical except for the exchange of R9 for P9. Both were homologous to oprin, a similar inhibitor from Didelphis virginiana serum. No evidence of complex formation between DM40 and DM43 was observed either by native PAGE or gel filtration chromatography. In addition to the antihemorrhagic activity, DM40 and DM43 inhibited the hydrolysis of casein, fibrinogen and fibronectin by Bothrops jararaca venom. DM43 also showed antilethal, antiedematogenic and antihyperalgesic activities. None of the inhibitors showed enzymatic activity on casein. Both proteins formed stable complexes with jararhagin and inhibited its hemorrhagic effect as well as the enzymatic activity of this toxin on fluorogenic substrate.

Nidogen-1 regulates laminin-1-dependent mammary-specific gene expression.

Nidogen-1 (entactin) acts as a bridge between the extracellular matrix molecules laminin-1 and type IV collagen, and thus participates in the assembly of basement membranes. To investigate the role of nidogen-1 in regulating cell-type-specific gene expression in mammary epithelium, we designed a culture microecosystem in which each component, including epithelial cells, mesenchymal cells, lactogenic hormones and extracellular matrix, could be controlled. We found that primary and established mesenchymal and myoepithelial cells synthesized and secreted nidogen-1, whereas expression was absent in primary and established epithelial cells. In an epithelial cell line containing mesenchymal cells, nidogen-1 was produced by the mesenchymal cells but deposited between the epithelial cells. In this mixed culture, mammary epithelial cells express (beta)-casein in the presence of lactogenic hormones. Addition of either laminin-1 plus nidogen-1, or laminin-1 alone, to mammary epithelial cells induced (beta)-casein production. We asked whether recombinant nidogen-1 alone could signal directly for (beta)-casein. Nidogen-1 did not induce (beta)-casein synthesis in epithelial cells, but it augmented the inductive capacity of laminin-1. These data suggest that nidogen-1 can cooperate with laminin-1 to regulate (beta)-casein expression. Addition of full-length nidogen-1 to the mixed cultures had no effect on (beta)-casein gene expression; however, a nidogen-1 fragment containing the laminin-1 binding domain, but lacking the type IV collagen-binding domain, had a dominant negative effect on (beta)-casein expression. These data point to a physiological role for nidogen-1 in the basement membrane-induced gene expression by epithelial cells.

C/EBPbeta, but not C/EBPalpha, is essential for ductal morphogenesis, lobuloalveolar proliferation, and functional differentiation in the mouse mammary gland.

The CCAAT/enhancer binding proteins (C/EBPs) are differentially expressed throughout mammary gland development and interact with binding sites within the promoter of a milk protein gene, beta-casein. The specific roles of C/EBPbeta and C/EBPalpha in mouse mammary gland development and differentiation have been investigated in mice that carry targeted deletions of these genes. C/EBPbeta-/- virgin mice exhibited cystic, enlarged mammary ducts with decreased secondary branching. Transplantation of C/EBPbeta-/- mammary epithelium into the cleared mammary fat pads of nude mice confirmed that this defect in ductal morphogenesis was intrinsic to the epithelium. When treated with estrogen/progesterone (E+P) to simulate pregnancy, C/EBPbeta-/- mammary glands displayed only limited lobuloalveolar development and ductal side branching. Primary mammary epithelial cells obtained from E+P-treated C/EBPbeta-/- mice that were cultured on extracellular matrix gels did not functionally differentiate in response to lactogenic hormones despite their organization into three-dimensional structures. Expression of beta-casein protein was inhibited 85%-100% and whey acidic protein (WAP) was undetectable. In contrast, no detectable alterations in mammary development or beta-casein expression were observed in mammary outgrowths derived from newborn C/EBPalpha-/- mammary epithelium transplanted into the cleared mammary fat pads of syngeneic hosts. These results demonstrate that C/EBPbeta, but not C/EBPalpha, is required for ductal morphogenesis, lobuloalveolar development, and functional differentiation of mammary epithelial cells.