DNA synthesis inhibition and reduced functional differentiation of midpregnant mouse mammary epithelia on collagen gels

Mammary epithelial cells were examined for a link between DNA synthesis and subsequent synthesis and secretion of casein. Cells isolated from mice midway through pregnancy and cultured on collagen gels spread to form monolayers (spreading phase). Release of monolayer/gels to float in surrounding culture medium induces synthesis and secretion of casein (secretory phase). DNA synthesis was blocked during the spreading phase with cytosine arabinofuranoside (ARA C). Culture medium was assayed for casein by direct quantification of protein from SDS-PAGE fluorographs, and by immunoblotting. When induced to become secretory, cells exposed to ARA C during the spreading phase showed a marked reduction of secretion of casein as compared to control cultures (72% reduction). In contrast, cells exposed to ARA C during the secretory phase (after monolayer formation was complete) showed no significant reduction in secretion of casein. Measurement of intracellular casein in secretory phase cells showed that reduced secretion of casein by cultures blocked during the spreading phase occurs as a consequence of reduced levels of casein synthesized, and not because of an inability to secrete intracellular accumulations. The inhibitor effect was specific; there was no significant reduction in levels of total intracellular protein synthesis, and neither cell spreading nor monolayer formation was impaired by treatments. These data support the notion that DNA synthesis is a prerequisite to functional differentiation of midpregnant mouse mammary epithelia maintained on floating collagen gels.

Purification, characterization, and localization of 70 kDa calcium-sensitive protein (calelectrin) from mammary glands

Mammary glands contain a group of calcium-sensitive proteins that bind to membranes in a calcium-dependent manner. Using the calcium-dependent binding to hydrophobic surfaces in combination with conventional techniques, we have purified the 70 kDa mammary calcium-binding protein (70 kDa M-CBP) to homogeneity. Antisera prepared to the 70 kDa M-CBP or to bovine liver 67 kDa calelectrin reacted in immunoblot analysis with the 70 kDa M-CBP antigen and with several additional mammary CBP species in crude tissue homogenates. Limited proteolysis of the 70 kDa M-CBP produced smaller immunoreactive species; extensive proteolysis resulted in more complete degradation of the protein. Identical data were obtained with digestion of 67 kDa calelectrin. The pl for the 70 kDa M-CBP was determined to be approximately 5.8; the same value reported for 67 kDa calelectrin. Phosphorylation of 70 kDa M-CBP was not detected in epithelial cell culture metabolic labeling. Immunohistochemical localization showed the protein to be located in ductal epithelia of virgin mouse mammary glands with a pattern of increased staining of the basal portions of the cells. Some stromal cells were also reactive. Apparently, the 70 kDa M-CBP and 67 kDa calelectrin are the same protein. Furthermore, like the 32.5 calelectrin (endonexin) and calpactin I/p36/lipocortin II, the 70 kDa protein appears to be a ductal epithelial cell associated protein in the mammary gland.

Casein accumulation in distended rough endoplasmic reticulum of collagen gel-cultivated mouse mammary epithelia

Mouse mammary epithelial cells cultivated on collagen gels synthesize and secrete casein in a hormone-dependent manner. Fine-structure electron microscopy of secretory cultures revealed numerous cytoplasmic structures surrounded by membrane that is studded with ribosomes. The structures appear to be distended rough endoplasmic reticulum (RER). Electron microscope protein A-colloidal gold immunolocalization showed casein antiserum-specific deposition of gold particles over the RER cytoplasmic vesicles in cells provided insulin, prolactin, and hydrocortisone (IPF). Nonimmune antiserum showed no gold particle deposition over these cytoplasmic structures. Epithelia provided only insulin showed no such cytoplasmic vesicles nor any specific deposition of gold particles. Immunoblot analysis of cell lysate and culture medium showed casein only in IPF-treated cultures. It appears that the casein secretory pathway in collagen gel cultured mammary epithelia is blocked at the step that fuses RER vesicles to Golgi membrane. The data raise questions regarding the processing and maturation of casein and the mechanism of casein secretion in these cultures.

Synthesis of calelectrins and calpactin I during cytochalasin mediated cell spreading inhibition

Mammary epithelial cell spreading on collagen gels has previously been shown to be correlated with the synthesis of a group of calcium-binding proteins (CBPs) which we have identified as the calcium-binding proteins termed calelectrins and calpactin I monomer/p36. To determine whether cell spreading per se is required for CBP synthesis, we examined the effect of cytochalasin D on these two events. Concentrations of cytochalasin D that did not reduce total protein synthesis, caused inhibition of cell spreading in a dose-dependent manner, but did not cause inhibition of CBP synthesis. Synthesis of collagen also continued during cytochalasin inhibition of cell spreading. Removal of the inhibitor from the cultures initiated cell spreading and CBP synthesis continued. Membrane-cytoskeleton complexes from control and CD treated cells were identical in regard to binding CBPs in a calcium-dependent manner. Colchicine, which inhibited cell spreading, was shown to be toxic to general protein synthesis at 75 nM. The data clearly indicate that mere inhibition of epithelial cell spreading does not automatically suppress CBP synthesis.

Developmental regulation of calcium-binding proteins (calelectrins and calpactin I) in mammary glands

We recently showed that mammary glands contain a novel class of calcium-binding proteins (CBPs) that bind to membranes in a calcium-dependent manner. We have also established that these mammary CBPs are equivalent to the calelectrins and calpactin I/p36. Since it has been suggested that these proteins might be involved in exocytosis, we examined mammary glands for these CBPs during secretory differentiation. Immunohistochemical examination showed glands from virgin animals to be rich in calelectrins and calpactin I/p36, while glands from lactating animals contained little immunoreactive material. In addition, silver-staining and immunoblot estimation of the CBPs in lysates from collagenase harvested secretory epithelia showed these proteins to be significantly reduced compared to nonsecretory epithelia. Close examination of the CBP immunoreactive cells of the mammary gland shows that ductal cells are prominent in their staining and that the immunoreactive material is associated with the cell surface. Also, in juvenile glands the myoepithelial stem cells (cap cells) of the elongating end bud are devoid of the CBPs. In contrast to the in vivo data, epithelia cultivated on collagen gels demonstrate comparable levels of the CBPs in both nonsecretory and secretory monolayers. The in vivo data indicate that the CBPs are developmentally regulated during mammary gland differentiation such that secretory epithelia are essentially devoid of these novel proteins. Furthermore, a role for calelectrin and calpactin I/p36 in exocytotic casein secretion is questioned.

Mammary gland Ca2+-binding (-dependent) proteins: identification as calelectrins and calpactin I/p36

Calcium-binding (-dependent) proteins (CBPs) associated with the spreading of mammary epithelial cell cultures have been identified as various calelectrins and calpactins (p36). In immunoblot analysis, the CBPs of 30-36 kD and 68-70 kD variously react with different calelectrin and calpactin I monomer/p36 antisera. The same immunoreactive proteins were shown to be present in virgin mammary glands and collagen gel mouse mammary epithelial cell cultures. The mammary CBPs show extensive immunochemical relatedness; however, they fail to show cross-reaction with antiserum to calpactin II (lipocortin) antiserum. These immunoreactive CBPs comigrate in electrophoresis with 35S-methionine-labeled CBPs isolated from mammary epithelial cell cultures. Unlike calmodulin, the mammary CBPs that correspond to calelectrins and calpactin I monomer/p36 are not stable to thermal denaturation. The mammary CBPs bind to epithelial cell membranes in a Ca2+-dependent manner and are differentially released from ruptured cells, compared with calmodulin, suggesting subcellular localization. Phenothiazine-agarose and phenylagarose are equivalent in their ability to bind the mammary CBPs. Thus, mammary gland CBPs of 30-36 kD and 68-70 kD have been shown to be related or equivalent to the calelectrins and to calpactin I monomer/p36. Since these proteins are known to bind Ca2+, we conclude that the mammary gland CBPs are also Ca2+-binding proteins. The mammary gland CBPs are immunologically related and probably represent members of a larger family of related proteins.

Casein secretion by mammary gland epithelia from collagen gel cultures and lactating glands

Amino acid incorporation experiments show that epithelial cells from lactating mouse mammary glands and from collagen gel culture both synthesize and secrete four principal phosphocaseins (p45, p40, p27, and p23 kD). In both cases, however, the casein production is largely dominated by the p27 species. The average percentage distribution of the above casein species in medium from cultured epithelia is approximately 13%, 6%, 68%, and 14%, respectively; for milk the distribution is approximately 23%, 7%, 54%, and 16%. The predominance of the p27 species is not a consequence of extensive extracellular differential degradation of the secreted caseins since no significant casein degradation was observed in culture medium, either in contact or isolated from epithelial cell monolayers. Synthesis and secretion of all the caseins by cultured epithelia is dependent upon insulin, prolactin, and hydrocortisone. Presumably some intracellular events result in the secretion of p27 as the principal casein in mouse milk. Apparently, some selection factor(s) operate to make p27 a major nitrogenous nutritional component for a newborn mouse. In addition, on a quantitative basis, the relative levels of various caseins secreted by epithelia from lactating mammary glands is essentially duplicated by epithelia in collagen gel culture.

Basal lamina inhibition suppresses synthesis of calcium-dependent proteins associated with mammary epithelial cell spreading

Spreading of mouse mammary epithelial cells on collagen gels is closely correlated with the synthesis of a group of putative calcium-binding proteins (CBP) (Braslau et al., Exp cell res 155 (1984) 213). Collagen synthesis was shown to occur during cell spreading, while omission of serum prevented cell spreading and the synthesis of collagen. The proline analogues cis-hydroxyproline and L-azetidine-2-carboxylic acid were shown to inhibit epithelial cell spreading and to suppress the collagen synthesis that occurs during serum-supported cell spreading. Inhibition of collagen synthesis resulted in the inhibition of CBP synthesis associated with cell spreading. In contrast, the collagen cross-linking inhibitor B-aminopropionitrile did not inhibit cell spreading nor did it suppress collagen synthesis; CBP synthesis was also normal during treatment with this inhibitor. Thus, mammary epithelial cell spreading on collagen gels and CBP synthesis can both be suppressed by inhibition of collagen synthesis indicating that they may be integrated in some manner. It is suggested that inhibition of cell spreading during inhibition of collagen synthesis results from failure to assemble a normal basal lamina; this may in turn signal suppression of CBP synthesis.

Collagen synthesis and deposition during mammary epithelial cell spreading on collagen gels

Mouse mammary epithelial cultivated on collagen gels demonstrate active spreading as the cells form monolayers. In this novel system, initiation of cell spreading is preceded by de novo synthesis of type IV collagen. The newly synthesized collagen is partitioned such that after 48 hr, approximately 24% is found in the culture medium, 35% is intracellular, and 41% is deposited in the extracellular matrix of the developing epithelium. Cultures deprived of serum failed to spread and to synthesize collagen. Proline analogues were shown to inhibit cell spreading and to suppress collagen synthesis in a dose-dependent manner. Cytochalasin D inhibition of F-actin elongation was shown to prevent cell spreading but not to suppress total collagen synthesis. During cytochalasin D treatment, inhibition of cell spreading was shown to result from failure to deposit or to maintain deposited collagen in the epithelium extracellular matrix. The data indicate that synthesis and extracellular deposition of a major basal lamina component (viz. type IV collagen) must precede and then accompany epithelial cell spreading in collagen gel culture. It is suggested that the microfilament apparatus, through some hypothetical integral membrane protein, can anchor extracellular type IV collagen, which then provides a necessary condition for cell spreading.

Casein production during differentiation of mammary epithelial cells in collagen gel culture

Mouse mammary epithelial cells cultivated on floating collagen gels secrete, as judged by immunoblotting, the full array of caseins found in mouse milk. The secreted caseins are all phosphorylated and have estimated minimum molecular weights (MWs) of 45, 40, 27, and 23 kD in SDS-PAGE. Intracellular caseins of epithelia from collagen gel cultivation or from lactating mammary glands are a combination of mature caseins identical with the secreted molecules and novel caseins whose apparent size in SDS-PAGE is different from the secreted molecules. The novel caseins were shown to be non-phosphorylated species apparently insufficiently mature for secretion. Our data indicate that, with regard to casein expression, cultivation of mouse mammary epithelia on collagen gels essentially duplicates their behavior in the lactating mouse mammary glands.

Synthesis of novel calcium-dependent proteins associated with mammary epithelial cell migration and differentiation

A class of proteins from mouse mammary epithelial cells has been isolated which, like the calcium-binding protein calmodulin (CaM), binds to phenothiazine in a calcium-dependent manner. These proteins do not bind to phenothiazine through binding to CaM; we infer that they are calcium-binding proteins, and that they may be related to the similarly isolated 'calcimedins' of Moore, P D & Dedman, J, J biol chem 257 (1982) 9663 [8]. In primary cultures of mouse mammary cells on collagen gels, synthesis of certain of these proteins is associated with the spreading of cells to form monolayers; failure of cells to spread and differentiate, through omission of serum from culture medium, results in the inhibition of calcium-binding protein synthesis, with the exception of CaM and a 15 kD species. The CaM/15 kD pair are prominent during all phases of culture, and are secreted during the secretory differentiation phase of culture (floating gels). We propose that these calcium-binding proteins play a specific role in the motility of mammary epithelial cells and that they may also be involved in mammary secretory differentiation.

Xanthine oxidase, an indicator of secretory differentiation in mammary cells

Elevated levels of xanthine oxidase were found in (1) lactating mouse mammary glands, compared with virgin and midpregnant glands; and (2) primary mouse mammary cells cultured on floating collagen gels, compared with non-secretory cells on attached gels. In primary culture, increase in xanthine oxidase activity above a basal level coincided with secretory activity as measured by casein production; intracellular levels of casein and xanthine oxidase showed a high degree of correspondence. It is suggested that xanthine oxidase levels can be used as an indicator of in vivo and in vitro secretory differentiation in mammary epithelial cells.

Interspecies hybrid tryptophan synthase-modified beta 2 protein formed from separate folding regions of the beta monomer

Escherichia coli and Serratia marcescens tryptophan synthase beta 2 protein (EC 4.2.1.20) was subjected to mild trypsin proteolysis. Two separate folding regions (domains) of the E. coli (EF1 and EF2) and the S. marcescens (SF1 and SF2) enzyme were shown to form interspecies hybrid reconstituted molecules [(EF1-SF2)2 and (SF1-EF2)2] and intraspecies reconstituted molecules [(EF1-EF2)2 and (SF1-SF2)2] with equal efficiency. The data suggest that structural regions, associated with beta monomer assembly, exist somewhere on the domain fragments and that these regions are conserved.

Hybrid tryptophan synthase beta 2 proteins: apparent conservation of the beta-beta binding region of the beta monomer among enteric bacteria

Purified borohydride-reduced tryptophan synthase beta 2 protein (EC 4.2.1.20) from Escherichia coli and purified native beta 2 protein from Serratia marcescens were mixed and dissociated in urea. Removal of the urea resulted in random reassociation of the reduced and native beta monomers, forming interspecies hybrid beta 2 molecules. Interspecies hybrid beta 2 protein molecules of the reciprocal composition were also formed. Interspecies hybrid reconstituted molecules were formed with approximately the same efficiency as intraspecies reconstituted molecules (reduced and native monomers from the same species) indicating no particular preference for reassembly. The data provide evidence that the structural region of interaction between the beta monomers necessary for dimerization is highly conserved in the enzymes from the two organisms examined.

Conservation of primary structure of the pyridoxyl peptide of Escherichia coli and Serratia marcescens tryptophan synthase beta2 protein

Two labeled peptides were recovered from tryptic digests of the NaB3H4-reduced, performic acid-oxidized beta2 protein of Serratia marcescens tryptophan synthase. These two pyridoxyl peptides were identical except for the presence or absence of an NH2-terminal arginyl residue. Tryptic digestion of nonreduced, performic acid-oxidized protein allowed isolation of the peptides that comprise the two halves of the pyridoxyl peptide. The partial primary structure for this region of the protein was shown to be Arg-Glx-Asx-Ler-Leu-His(Gly,Gly,Ala,His)Lys(Pxy)-Thr-Asx-Glx-Val(Leu,Gly,Glx,Ala,Leu,Leu,Ala)Lys. All the data available indicate that the sequence is identical with the homologous region from the Escherichia coli enzyme.

Trypsin peptide patterns of tryptophan synthase beta2 protein among four species of the Enterobacteriaceae

The tryptophan synthase beta 2 protein (EC 4.2.1.20) of Escherichia coli, Enterobacter aerogenes, Serratia marcescens, and Erwinia carotovora was purified and compared. Two-dimensional total peptide patterns for each of the four beta2 proteins obtained after digestion with trypsin showed that approximately three quarters of the total peptides are common to all four peptides. Examination of only arginine-containing peptides showed that approximately half of these peptides are common. From a comparative standpoint, the data provide evidence that the primary structure of beta 2 proteins is relatively similar, indicating that the trpB cistron is evolutionarily conserved in the enteric bacteria group.

Purification and partial characterization of the B subunit of Serratia marcescens tryptophan synthetase

A trpE mutant of Serratia marcescens (E-7) was isolated, and the multimeric enzyme tryptophan synthetase (EC 4.2.1.20) was purified to homogeneity from derepressed cells. The A and B subunits were resolved, and the B subunit was partially characterized and compared with the Escherichia coli B subunit as part of a comparative evolution study of the trpB cistron of the trp operon in the Enterobacteriaceae. The S. marcescens B subunit is a dimer (beta(2)), and its molecular weight was estimated to be 89,000. The separate subunits (beta monomers) had molecular weights of approximately 43,000. The B subunit required pyridoxal phosphate for catalytic activity and had an apparent K(m) of 9 x 10(-6) M. The N terminus of the B subunit was unavailable for reaction with terminal amine reagents (blocked), whereas carboxypeptidase digestion released a C-terminal isoleucine. Using S. marcescens B antiserum in agar immunodiffusion gave an almost complete reaction of identity between the B subunits of S. marcescens and E. coli. The antiserum was used in microcomplement fixation, allowing for a comparison of the overall antigenic surface structure of the two B subunits. The index of dissimilarity for the heterologous E. coli enzyme compared with the homologous S. marcescens enzyme was 2.4, indicating extensive similarity of the two proteins at their surfaces. Comparative antiserum neutralization of B-subunit enzyme activity showed the E. coli enzyme to cross-react 85% as well as the S. marcescens enzyme. With regard to the biochemical and immunochemical parameters used in this study, the S. marcescens and E. coli B subunits were either identical or very similar. These findings support the idea that the trpB cistron of the trp operon is a relatively conserved gene in the Enterobacteriaceae.

Immunochemical comparison of phosphoribosylanthranilate isomerase-indoleglycerol phosphate synthetase among the Enterobacteriaceae

The bifunctional enzyme of the tryptophan operon, phosphoribosylanthranilate isomerase-indoleglycerol phosphate synthetase (PRAI-InGPS;EC 4.1.1.48), was characterized by an immunochemical study of six representative members of the Enterobacteriaceae: Escherichia coli, Salmonella typhimurium, Enterobacter aerogenes, Serratia marcescens, Erwinia carotovora, and Proteus vulgaris. PRAI-InGPS was purified from E. coli, and antisera were prepared in rabbits. These antisera were utilized in quantitative microcomplement fixation allowing for a comparison of the overall antigenic surface structure of the various homologous enzymes. These data showed E. coli PRAI-InGPS and S. marcescens and E. carotovora PRAI-InGPS (taken as a group) to have an index of dissimilarity of approximately 10, whereas the other organisms had values intermediate. In addition, antiserum to E. coli tryptophan synthetase beta2 subunit was used in microcomplement fixation to extend the previous comparison of this subunit (Rocha, Crawford, and Mills, 1972) to E. carotovora and P. vulgaris. Indexes of dissimilarity for E. coli compared to P. vulgaris of E. carotovora were 1.0 and 1.7, respectively. Agar immunodiffusion using PRAI-Ingps antisera showed significant cross-reaction among E. coli, E. aerogenes, S. typhimurium, and P. vulgaris whereas the enzymes from S. marcescens and E. carotovora cross-reacted to a lesser extent, with the latter reaction being quite weak. Comparative enzyme neutralization using E. coli PRAI-InGPS antisera showed significant cross-reactions among the enzymes in that all were neutralized at least 25%. The data taken together indicate that the trpC gene products in the Enterobacteriaceae are a homologous group of proteins, that the genetic divergene of the trpC gene is basically the same as the trpA gene, and that both are less conserved than the trpB gene. Furthermore, the PRAI-InGPS, enzyme active site appears to represent a more evolutionarily conserved region of the protein. These findings indicate that, with respect to PRAI-InGPS, similarity to E. coli among the organisms examined is in the following order: (E. aerogenes, S. typhimurium, P. vulgaris) greater than (S. marcescens, E. carotovora).

Comparative immunological and enzymatic study of the tryptophan synthetase beta 2 subunit in the Enterobacteriaceae

The beta(2) subunits of tryptophan synthetase, formula alpha(2)beta(2), from Escherichia coli, Shigella dysenteriae, Enterobacter aerogenes, Salmonella typhimurium, and Serratia marcescens were compared by three criteria. (i) alphabeta association constants for the various beta(2) subunits and E. coli alpha subunit varied between 3.6 x 10(8)m(-1) for E. coli and 0.33 x 10(8)m(-1) for S. marcescens; values for the other organisms were intermediate. (ii) Antiserum neutralization of the beta(2) subunit enzyme activity using anti-E. coli beta(2) serum showed significant cross-reaction among the organisms (E. coli, 1.0; S. dysenteriae, 0.98; S. typhimurium, 0.67; E. aerogenes, 0.61; S. marcescens, 0.42). (iii) Quantitative microcomplement fixation showed E. coli beta(2) and S. marcescens beta(2) subunits to have an index of dissimilarity of 1.8 while the other organisms had intermediate indexes. Similar complement fixation data were obtained with antisera from separate rabbits and from first course and boost sera. These findings suggest that the general surface structure and the respective alpha subunit binding site of the beta(2) subunits from these Enterobacteriaceae have been strongly conserved.

Tissue distribution of microbody, mitochondrial, and soluble malate dehydrogenase isoenzymes

Images