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

Tissue Fabrication: Reconstitution and Remodeling in Vitro

Two approaches to the reconstitution of tissues and organs are reviewed. The first consists of imitating the architecture of actual tissues and organs by combining cultured specialized cells with extracellular matrix components to produce a connective tissue substrate on or in which epithelial, mesothelial or endothelial cells can be plated or seeded and subsequently differentiate into mono or multilayered tissues and other structures. The second consists of providing an acellular framework of extracellular matrix constituents that can be occupied by adjacent host tissues after implantation in vivo and be remodeled by them to resemble the host tissues it is designed to replace. A paradigm for events in vivo, designed to study the process of remodeling of acellular matrices in vitro has been developed. The living skin equivalent (LSE), an example of a product fabricated using the first approach to tissue engineering, has been adapted to study events of extracellular matrix remodeling, relevent to the second approach to tissue engineering. After creating a disc shaped wound bed in an LSE, the wound is filled with a collagen matrix with or without added supplements and the process of epidermal wound closure and associated events in the dermis are followed. It is shown that fibroblast conditioned medium or a simple molecule such as ascorbic acid, added with no additional growth factors to the collagen matrix used to fill the wound bed, strongly stimulates the process of repair. Dermal fibroblasts from the adjacent tissue invade the collagen lattice that forms in the wound bed, and keratinocytes recruited from the wound edge overgrow the new dermal tissue. The applicability of the paradigm to the repair of vascular and other tissues will be discussed and approaches to optimizing the composition of acellular constructs considered.

Lipid synthesis and secretion by primary cultures of rat mammary epithelial cells

Lipid synthesis and secretion was measured in primary rat mammary epithelial cells cultured on basement matrix in medium supplemented with lactogenic hormones. The cells grew and differentiated to form alveolar-like structures reminiscent of lactating mammary gland. They synthesized abundant triacylglycerol, containing fatty acids characteristic of rat milk (C10:0-C14:0), using 14C-glucose, 14C-oleic acid or 14C-glycerol as precursors. Basal levels of triacylglycerol secretion were measured using 14C-oleic acid labeling; 1.3 +/- 0.3% of the labeled cellular triacylglycerol was secreted into the medium in 24 hours. Secreted lipid droplets were surrounded by a bilayer membrane with an electron-dense inner coat characteristic of fat globules secreted by the mammary gland. The rate of triglycerol secretion was increased to 998 +/- 98% of control (P < 0.01) by the addition of phorbol 12-myristate 13-acetate (PMA) in combination with staurosporine, a protein kinase inhibitor. Several other protein kinase inhibitors, when combined with PMA, also markedly stimulated secretion. Effective protein kinase inhibitors included sphingosine (has diverse cellular effects including the inhibition of protein kinase C; 13-fold increase in secretion), and KT5823 (a cGMP dependent protein kinase inhibitor; 5-fold increase). KT5720 (a cAMP-dependent protein kinase inhibitor) did not alter secretion. Kinase inhibitors were effective only in the presence of a phorbol ester. 4 alpha-phorbol-12,13-didecanoate, a phorbol ester which does not activate protein kinase C (PKC), could substitute for PMA. Lipid release was not mediated by disruption of cell-cell tight junctions, as EGTA did not release lipid. Based on these observations we suggest that two signals are needed to enable or stimulate lipid secretion in cultured rat mammary epithelial cells: 1) inhibition of a protein kinase and 2) a PKC-independent effect of phorbol ester. We have, for the first time, characterized a cell culture model suitable for studying lipid synthesis and secretion by mammary epithelial cells.

Modulation of milk protein synthesis through alteration of the cytoskeleton in mouse mammary epithelial cells cultured on a reconstituted basement membrane

Recent studies indicate that the cytoskeleton may be involved in modulating tissue-specific gene expression in mammalian cells. We have studied the role of the cytoskeleton in regulating milk protein synthesis and secretion by primary mouse mammary epithelial cells cultured on a reconstituted basement membrane that promotes differentiation. After 8 days in culture, cells were treated with cytochalasin D (CD) (0.5-1 micrograms/ml) to alter actin filaments or acrylamide (Ac) (5 mM) to alter intermediate filaments (cytokeratins). CD inhibited synthesis of most proteins in a concentration-dependent manner, with beta-casein being the first affected. In contrast, Ac increased protein synthesis and secretion by 17-31% after a 12 hr treatment. Polyacrylamide gel electrophoresis of total secreted proteins indicates that synthetic rates of most proteins were increased equally by Ac treatment. This increase is apparently controlled at the level of translation, because control and Ac-treated cells contained the same amount of poly-A+ RNA, and neither CD nor Ac altered mRNA levels for beta-casein. There was also no indication that either CD or Ac can induce the expression of milk proteins in quiescent cells cultured on a plastic substratum. In conjunction with the biochemical studies, changes in cytoskeletal morphology caused by the drug treatments were analyzed by immunofluorescence microscopy. As has been observed in other cell types, low concentrations of CD caused cells to round up by disrupting actin filaments. Ac treatment slightly decreased the intensity of actin staining, but no changes in microfilament organization were observed. Ac-treated cells showed slight disorganization of the cytokeratin filaments, with some peripheral interfibrillar bundling, but the cytokeratin network did not collapse and no retraction of cell extensions or breakdown of cell-cell contacts was observed. These results confirm previous reports that the actin cytoskeleton may play a role in regulating tissue-specific protein synthesis. How Ac stimulates protein synthesis is unknown, but it is unlikely that this effect is directly mediated through intermediate filaments.

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.

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.