Effects of a tumour promoter, 12-0-tetradecanoyl-phorbol-13-acetate (TPA), on expression of differentiated phenotype in the chick retinal pigmented epithelial cells and on their interactions with the native basement membrane and with artificial substrata

Chick retinal pigmented epithelial (RPE) cells grown in vitro on basement membrane matrices from the Engelbreth-Holm-Swarm tumour (BM-matrigel) do not spread, and they maintain their differentiated phenotype, most notably the heavy pigmentation. Maintenance of the differentiated phenotype by RPE cells on BM-matrigel is promoted not only by the biochemical composition of the gel but also by its mechanical properties, i.e., its low rigidity prevents cell spreading. In this report, RPE cells on BM-matrigel were treated with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) to promote the transformed phenotype and diminish cell traction. In contrast to most cell types TPA treatment induced RPE cells to increase their spread area. TPA promoted RPE cell spreading on BM-matrigel and changed the spatial organization of actin and actin-associated proteins in the cytoskeleton-ECM linkage complexes, uncoupling actin from its extracellular counterpart. TPA did not affect other components of the cytoskeleton in RPE cells. TPA also affected labile adhesions i.e., focal contacts and adherens junctions in statu nascendi, but preformed, stable adherens junctions were resistant to TPA. TPA enhanced proliferation, blocked melanogenesis and thus inhibited differentiation of RPE cells grown on either artificial substrata or their natural basement membrane.     

The inducible blood--brain barrier specific molecule HT7 is a novel immunoglobulin-like cell surface glycoprotein.

The unique properties of brain endothelial cells, which form the blood-brain barrier, are reflected by the expression of specific cell surface molecules. We report here the purification, cloning and expression of one such molecule which is recognized by HT7 monoclonal antibodies. The HT7 antigen is a highly glycosylated 45-52 kd protein localized in brain endothelial cells, kidney epithelial cells and erythroblasts. The protein was purified to homogeneity from plasma membrane proteins isolated from all three sources using immunoaffinity chromatography and reverse phase HPLC. The amino-terminal amino acid sequences of the proteins were found to be identical. Based on amino acid sequence information, specific primers were designed and the polymerase chain reaction was used to obtain a full length cDNA clone. The nucleotide sequence encoded a novel glycoprotein with two C2-like immunoglobulin related domains, one transmembrane domain and a cytoplasmic tail. Expression of the transfected cDNA in COS cells resulted in the appearance of the HT7 antigen on the surface of these cells. On the basis of our results we propose that the protein may be a receptor involved in cell surface recognition at the blood-brain barrier.     

Differential effects of phorbol ester on the in vitro invasiveness of malignant and non-malignant human fibroblast cells

The effect of the phorbol ester tumor promoter 12-O-tetradecanoylphorbol 13-acetate (TPA) on cell invasion was studied using an in vitro assay for cell invasion through a reconstituted basement membrane matrix (Matrigel). TPA inhibited the invasiveness of malignant human fibrosarcoma HT1080 cells. In contrast, WI-38 lung fibroblasts, which show a very low invasive capacity, were stimulated (3-fold) to invade Matrigel after exposure to TPA for 48 hours. The inhibitory or stimulatory effects of TPA on cell invasion were correlated with a decrease or an increase in cell motility and collagenase IV activity, respectively. Synthetic diacylglycerols partially mimicked the inhibitory action of TPA on HT1080 cells but failed to stimulate WI-38 cell invasion. Immunoblots demonstrated that in both cell lines the alpha and beta isoforms of protein kinase C were equally down-regulated after a 5 hour exposure to TPA despite the basal low level of protein kinase C polypeptide in the malignant cells. Thus, whereas in WI-38 cells induction of an invasive behavior could be observed in the absence of protein kinase C, in the malignant cells disappearance of the kinase was associated with a non-invasive phenotype.     

Biosynthesis and secretion of fibronectin in human melanoma cells

The biosynthesis and secretion of cellular fibronectin from human melanoma cells have been investigated by pulse-chase/immunoprecipitation analysis. Melanoma cells synthesize endoglycosidase H (Endo H)-sensitive glycoprotein precursors of fibronectin glycoproteins which chase to an Endo H-resistant monomer with an apparent Mr of 240,000 (240 K). This molecule, which has a significantly higher molecular weight than normal plasma or cellular fibronectin, is rapidly secreted by melanoma cells, resulting in the secretion of 80% of newly synthesized fibronectin in 120 min, following a 10-min biosynthetic pulse. This active secretory process can be inhibited by brief exposure of melanoma cells to sodium monensin (10(-7) M), which also results in a modified fibronectin of lower apparent Mr. Monosaccharide-incorporation studies of melanoma fibronectin reveal that monensin significantly inhibits galactose and fucose incorporation into this glycoprotein, correlating with reported effects of monensin on Golgi apparatus functions. These studies indicate that this tumor-associated and biosynthetically altered cellular fibronectin is a rapidly secreted major N-linked glycoprotein of metastatic human melanoma cells.     

Lipoxygenase products regulate IRGpIIb/IIIa receptor mediated adhesion of tumor cells to endothelial cells, subendothelial matrix and fibronectin

Tumor cell adhesion to endothelial cells, subendothelial matrix, and fibronectin is stimulated by the lipoxygenase metabolite of arachidonic acid, 12(S)-HETE, but not by 12(R)-HETE, 5-HETE or 15-HETE. Adhesion is also stimulated by the phorbol ester TPA, an effect inhibited by lipoxygenase but not cyclooxygenase inhibitors. TPA and 12(S)-HETE mediated adhesion is due, in part, to an integrin receptor (i.e., IRGpIIb/IIIa) related to the platelet glycoprotein IIb/IIIa complex and is inhibited by specific monoclonal and polyclonal antibodies against platelet IIb/IIIa. TPA and 12(S)-HETE stimulated adhesion is also inhibited by a lipoxygenase product of linoleic acid; i.e., 13-HODE. These results suggest bidirectional control of tumor cell adhesion by lipoxygenase products of arachidonic acid (increase) and linoleic acid (decrease).     

Response of cultured rat liver epithelial cell lines to tumour-promoting phorbol esters

The membrane effects of a potent tumour promoter, 12-O-tetradecanoyl phorbol-13-acetate (TPA), were studied in a series of cultured rat liver epithelial cell lines. Treatment with TPA resulted in the formation of strand-like aggregates (ridges) of viable cells over the monolayer of IAR 6-1 cells, but not of three other cell lines tested (IAR 20, IAR 6, IAR 6-7). The morphological response of IAR 6-1 to TPA was investigated by determination of phorbol ester receptors, analysis of cellular fucoproteins, surface galactoproteins and iodinatable surface proteins, and specific immunofluorescence for components of the extracellular matrix (fibronectin, laminin-entactin, procollagen type III). A class of specific, saturable, high-affinity receptors for phorbol esters was demonstrated in all four cell lines employing a conventional [20(-3)H]phorbol-12,13-dibutyrate ([3H]PDBu)-binding assay. The dissociation constants were similar in four lines, but the number of receptors per cell in IAR 6-1 cells was about twice that in other lines. Down-regulation of receptors was demonstrated in IAR 20 and IAR 6-1 cells with similar characteristics. Iodinatable surface proteins and galactose-containing surface glycoproteins did not respond to TPA. The distribution of fibronectin, laminin-entactin and procollagen type III was not affected by TPA. A TPA-responsive cell line, IAR 6-1, contained considerably less laminin-entactin than did the other lines. TPA had no influence on metabolic labelling of [3H]fucose-containing cellular glycoprotein in IAR 6-1 cells. One specific protein, with molecular mass of 78 kD, was more heavily labelled with [3H]fucose in IAR 6-1 cells than in the other cell lines. Taken together, the results of this study show that the responsive cells (IAR 6-1) differed from non-responsive ones in having more phorbol ester receptors, increased fucosylation of a specific glycoprotein and decreased deposition of laminin-entactin in the extracellular matrix. These surface properties of IAR 6-1 cells may contribute to their ability to respond to TPA.     

Differentiation-promoting effect of 1-O (2 methoxy) hexadecyl glycerol in human colon cancer cells

Alkylglycerols are naturally occurring bioactive ether lipids found in great abundance in the livers of many marine species. In this study, we evaluated the differentiation-promoting potential of a methoxy substituted alkylglycerol--1-O (2 methoxy) hexadecyl glycerol (MHG)--to promote a more benign or differentiated phenotype in human colon cancer cells. Three cell lines with different biological and phenotypic properties were used. They were the moderately differentiated and growth factor-responsive Moser, the growth factor-unresponsive and malignant HT29, and the poorly differentiated and growth factor-unresponsive HCT116. Treatment of these cell lines with MHG resulted in a downmodulation of cellular proliferation, a reduced propensity for anchorage-independent growth, and a reduced capacity in cellular invasion. Induction of the colon-associated and differentiation-related molecule carcinoembryonic antigen was also observed in the three cell lines. Induction of the transformation-sensitive and differentiation-related glycoprotein fibronectin was observed in the HT29 cells. It is concluded that MHG was biologically active and promoted a more benign or differentiated phenotype in these colon cancer cells. Since differentiation-inducing agents may possess chemoprevention properties, the use of MHG and the alkylglycerols in inducing differentiation or in chemoprevention of malignant diseases warrants further investigation.     

Cell surface fibronectin and oncogenic transformation

Fibronectin is a large glycoprotein at the cell surface of many different cell types; a related protein is present in plasma. Fibronectin is a dimer of 230,000-dalton subunits and also occurs in larger aggregates; it forms fibrillar networks at the cell surface, between cells and substrata and between adjacent cells, and it is not a typical membrane protein. Cell surface fibronectin is reduced in amount or absent on transformed cells and in many cases its loss correlates with acquisition of tumorigenicity and, in particular, metastatic ability. Exceptions to the correlations with transformation and tumorigenicity exist. Loss of fibronectin and the resulting reduced adhesion appear to be involved in pleiotrpoic alterations in cell behavior and may be responsible for several aspects of the transformed phenotype in vitro. Fibronectin interacts with other macromolecules (collagen/gelatin, fibrin/fibrinogen, proteoglycans) and is apparently connected to microfilaments inside the cell.     

Collagenase production by immortalized human aortic endothelial cells infected with simian virus 40.

Human aortic endothelial cells, isolated at autopsy from a 52-year-old male dying from lung cancer, were treated with simian virus 40 (SV40). One colony was isolated from the infected endothelial cell culture 4 weeks after infection. The cells expressed SV40 large T antigen and p53 protein (p53) in their nuclei but lacted the characteristics of a transformed phenotype. The cells grew well in a monolayer over the 97th passage and exhibited Factor VIII-related antigen, Ulex europaeus 1 agglutinin (UEA-1) as endothelial cell markers, and a well-developed fibronectin network. The amount of prostacyclin synthesized by the cells was less than the amount synthesized by normal aortic or umbilical cord vein endothelial cells. The cells produced relatively large amounts of procollagenase, and 12-o-tetradecanoyl-phorbol-13-acetate (TPA) augmented the ability of the cells to produce this enzyme. These immortalized human aortic endothelial cells, which have some characteristics of normal endothelial cells and, like capillary endothelial cells, have the ability to produce collagenase, will probably prove useful for studies of atherosclerosis and angiogenesis.     

Collagenase production by immortalized human aortic endothelial cells infected with simian virus 40

Human aortic endothelial cells, isolated at autopsy from a 52-year-old male dying from lung cancer, were treated with simian virus 40 (SV40). One colony was isolated from the infected endothelial cell culture 4 weeks after infection. The cells expressed SV40 large T antigen and p53 protein (p53) in their nuclei but lacted the characteristics of a transformed phenotype. The cells grew well in a monolayer over the 97th passage and exhibited Factor Vlll-related antigen, Ulex europaeus 1 agglutinin (UEA-1) as endothelial cell markers, and a well-developed fibronectin network. The amount of prostacyclin synthesized by the cells was less than the amount synthesized by normal aortic or umbilical cord vein endothelial cells. The cells produced relatively large amounts of procollagenase, and 12-o-tetradecanoyl-phorbol-13-acetate (TPA) augmented the ability of the cells to produce this enzyme. These immortalized human aortic endothelial cells, which have some characteristics of normal endothelial cells and, like capillary endothelial cells, have the ability to produce collagenase, will probably prove useful for studies of atherosclerosis and angiogenesis.     

Expression of different regional patterns of fibronectin immunoreactivity during mesoblast formation in the chick blastoderm

The appearance and distribution of the extracellular material glycoprotein, fibronectin, was investigated in gastrulating chick embryos using affinity-purified anti-human plasma fibronectin antibodies. Preservation of tissue structure and immunoreactivity was carried out by ethanol/acetic acid fixation or by formaldehyde/glutaraldehyde fixation. Using the former fixation method, fibronectin immunoreactivity was detected (1) at the ventral surface of the upper layer or epiblast, mainly anterior and lateral to Hensen's node, in regions where middle-layer or mesoblast cells are not yet present, and (2) sparsely in extracellular spaces of the deep layer. Using the latter fixation method, fibronectin immunoreactivity was, moreover, found at the entire ventral surface of the upper layer, i.e., also at the epithelial-mesenchymal interface, where a basement membrane was previously described. At the light microscope level, we could not detect significant immunoreactivity in the middle layer. Treatment of sections of ethanol-fixed blastoderms with testicular hyaluronidase before immunostaining for fibronectin partially demasked the antigenic sites of this glycoprotein at the epithelial-mesenchymal interface. The present report indicates that the different regional patterns of fibronectin immunoreactivity in the basement membrane of the upper layer are spatially and temporally correlated with migration and positioning of mesoblast cells. These regional patterns are probably due to differences in the composition of fibronectin-associated material such as chondroitin sulfate A and/or C proteoglycans, and/or hyaluronate, before and after mesoblast expansion, rather than to differences in the distribution of fibronectin itself. In this respect, it is noteworthy that the chemical composition of the basement membrane of an epithelium changes as mesenchyme cells migrate over it. The results also favor the idea that fibronectin is a structural component of the whole basement membrane which is used as a substrate for migration of mesenchymal cells.     

Transformation of cultured epithelial cells by ethylnitrosourea: Altered expression of type I procollagen chains

Cultured epithelial rodent cells were transformed in vitro using ethylnitrosourea as a carcinogen either alone or in combination with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). The frequency of transformation in the absence of TPA was 5·10^?4 at 10 μg/ml ethylnitrosourea. Growth of ethylnitrosourea-treated cells in TPA-substituted medium increased the transformation frequency 8-fold. Colonies of transformed cells were isolated from soft agar and analyzed for the production of pericellular matrix glycoproteins. The ethylnitrosourea-transformed cells retained pericellular matrix structures, typical of the nontransformed control cells. Parent cells produced into their culture media fibronectin and procollagen types I and III as their major pericellular glycoproteins. The ethylnitrosourea-transformed cells synthesized and secreted altered procollagen polypeptides. The procollagen of ethylnitrosourea-transformed cells apparently consisted mainly of homotrimeric proα1 molecules, with smaller amounts of basement membrane procollagen-like chains. Fibronectin synthesis or secretion was not affected by ethylnitrosourea-induced transformation, but the production of fibronectin was enhanced in the transformed cultures treated with TPA. Also, the deposition of procollagen and fibronectin into the pericellular matrix was not affected by ethylnitrosourea-transformation. Very similar changes had previously been observed in murine sarcoma virus-transformed cells. The change of procollagen type I thus appears to be a correlate of malignant transformation of cultured epithelial cells. The results indicate that ethylnitrosourea can induce malignant transformation of epithelial cells in culture and modify production and deposition of pericellular glycoproteins.     

Collagenase is a major gene product of induced rabbit synovial fibroblasts

We have investigated the effects of the tumor-promoting phorbol diester, 12-O-tetradecanoylphorbol-13-acetate (TPA), on rabbit synovial fibroblasts, and found that this agent induced a major switch in gene expression in these cells that was marked by the specific induction of the neutral proteinase, collagenase, and was always accompanied by alterations in cell morphology. Procollagenase synthesis and secretion was first observed 6-12 h after the addition of TPA. The rate of collagenase production (1-5 U, or approximately 0.2-1 micrograms secreted procollagenase protein per 10(5) cells per 24 h) depended on the TPA concentration (1-400 ng/ml) and time of exposure (1-72 h). Procollagenase was the most prominent protein visible by direct silver staining or by autoradiography after SDS PAGE of [35S]methionine-labeled proteins. The two procollagenase bands of Mr 53,000 and 57,000, which migrated as a family of spots on two-dimensional gels and were immunoprecipitated by antibodies to purified rabbit collagenase, accounted for 23% of the newly synthesized, secreted protein in TPA-treated cells. Cell-free translation of mRNA from TPA-treated cells in rabbit reticulocyte lysate produced a single band of immunoprecipitable preprocollagenase (Mr 55,000) as a major product (5% of total) that migrated as a single spot on two-dimensional gels. Secreted procollagenase, preprocollagenase , and active collagenase (purified to homogeneity; specific activity 1.2 X 10(4) U/mg protein) had related peptide maps. Two other major secreted proteins, a neutral metalloproteinase of Mr 51,000 and a polypeptide of Mr 47,000, were also induced by TPA. In contrast to the induction of these four polypeptides, TPA decreased synthesis and secretion of a number of proteins, including collagen and fibronectin. Thus, collagenase is a convenient marker for major alterations in the pattern of protein synthesis and secretion by rabbit synovial fibroblasts treated with TPA.     

The Epstein-Barr virus encoded membrane protein (LMP) induces phenotypic changes in epithelial cells

The Epstein-Barr virus (EBV)-encoded membrane protein, LMP, is expressed in a proportion of undifferentiated nasopharyngeal carcinomas (NPC). Previous studies have shown that the transfection of the gene encoding LMP into a human keratinocyte line, RHEK-1, induces morphological alterations and a reduced expression of cytokeratins. We have analyzed immunophenotypic changes in the RHEK-1 line following LMP-transfection and compared these changes with the phenotype of NPC biopsies. We demonstrate a downregulation of two epithelial markers, an epithelial glycoprotein (EGP) defined by the monoclonal antibody Ber-EP4 and the epithelial membrane antigen (EMA). Furthermore, a lymphocyte activation-associated antigen, CDw70 antigen, which was absent from the parental line was expressed in virtually all LMP-transfected cells, whereas no similar effect was seen with respect to the CD30 activation antigen. Nine EBV-positive human NPCs, six of which were LMP-positive expressed the EGP and EMA. The CDw70 antigen, which is not normally present in epithelial cells, was expressed in eight biopsies, whereas the CD30 antigen was not detectable. Our findings are in keeping with the notion that LMP expression may contribute to the immunophenotype of human NPCs.     

Apical membrane marker is expressed early in colonic epithelial cells

We have identified and characterized a membrane glycoprotein located at the apical plasma membrane of adult human colon epithelial cells, by the use of the monoclonal antibody technique in combination with immunocytochemical and biochemical methods. Analysis of membranes extracted with Triton X-114 and treated with specific hydrolases indicated that the antigen was an integral membrane glycoprotein. In the colon, the antigen was expressed in differentiated cells and along the entire crypt. It was also expressed at the apical membrane of the crypt cells of the distal ileum. It was not found in the proximal ileum, jejunum, or duodenum. In contrast, the antigen was found in all segments of the intestine of a 24-week-old embryo. Furthermore, the antigen had different apparent molecular weights in the adult ileum (200 kDa), adult colon (200 kDa and 301 kDa), and embryo (170 kDa). Therefore, this antigen should prove to be a useful marker to study the appearance of epithelial cell polarity during embryogenesis.     

Tissue polypeptide antigen liberation and proliferation of MCF-7 cells after synchronization with hydroxytamoxifen and rescue with estradiol

The kinetic of release of the tissular polypeptide antigen (TPA) by MCF-7 cells synchronised by sequential treatment with hydroxytamoxifen (OH-TAM) and 17 beta-estradiol has been studied. The present findings confirm the proliferative effect of estradiol on MCF-7 cells, with a shortening of the doubling time (TD) (22.2 h versus 24.8 h) and an increase in the growing fraction (Fc) (94% versus 81%) when compared with the same parameters measured in cells rescued from OH-TAM but not treated with estradiol. In addition, the action of estradiol was followed by a simultaneous increase in the amount of TPA in the culture medium related with the phases G2/M and G1 of the mitotic cycle. This phenomenon seems to be the reason for the steplike shape of the TPA released curves. The experimental results suggest that in MCF-7 cells the sequential combination of antiestrogenic agents and estrogens leads to a cellular synchronization. Furthermore this synchronization is maintained for at least 3 cycles of cell division.     

De novo expression of T cell markers on Theileria parva-transformed lymphoblasts in cattle

We describe monoclonal antibodies that detect two new membrane antigens on bovine T cells. One molecule is only expressed on activated T cells and has a m.w. of 100,000. The other antigen is a glycoprotein that is precipitated as two bands of m.w. 150,000 and 158,000 and is expressed on a subpopulation of T cells and all myeloid cells. We show that when bovine lymphocytes are transformed by the protozoan parasite Theileria parva, both antigens become expressed on the cell surface. The infected cell acquires the phenotype of a proliferating T cell, irrespective of its precursor cell phenotype.     

Effects of differentiation-inducing agents on maturation of human MCF-7 breast cancer cells

The effects of the differentiation inducing agents (DIAS), sodium butyrate (NaBu), retinoic acid (RA), dimethylformamide (DMF), hexamethylene bisacetamide (HMBA), forskolin, and 12-O-tetradecanoylphorbol-13-acetate (TPA), on the growth, morphology, and estrogen receptor (ER) content and epithelial membrane antigen (EMA) expression on a serumless human breast cancer cell line (MCF-7) were compared. All these agents reversibly caused a concentration-dependent growth inhibition in monolayers and markedly reduced colony-forming efficiency in soft agar. A twofold increase in doubling time was obtained with RA (1 microM), but cell replication ceased with NaBu (1 mM), forskolin (50 microM), DMF (1%), HMBA (5 mM), and TPA (8 nM). Total growth arrest induced by these last compounds was preceded by an accumulation of cells in G0/G1 phase observed at 24 h by flow cytometry and accompanied by a change in cell morphology as seen by light and electronic microscopy. An increase in cell volume and the presence of lipid droplets was noted in treated cells that were spread out, as compared with controls. The acquisition of a more mature phenotype was confirmed by an increased expression of EMA monitored by flow cytometry. A specific reduction in the number of ER without any constant dissociation (Kd) modification was also observed after treatment with the 5 DIAs. No modification of morphological or biochemical characteristics, including EMA expression and ER binding, were observed for RA (1 microM)-treated cells. All these results suggest that induction of a more differentiated phenotype is associated with a block in G1 cell cycle phase, resulting in total growth arrest. Apparently, RA (1 microM)-treated cells did not fulfill these criteria, since only a slight accumulation in G1 and a slowed growth rate were evaluated.     

Regulation of fibronectin biosynthesis by glucocorticoids in human fibrosarcoma cells and normal fibroblasts

When treated with the synthetic glucocorticoid dexamethasone, HT1080 human fibrosarcoma cells show changes in morphology, adhesion, and the extracellular matrix. Dexamethasone treatment results in a tenfold increase in the rate of fibronectin biosynthesis in HT1080 cells and a twofold increase in untransformed, normal human fibroblasts. Maximal induction levels are attained within one cell generation, while decay of the response requires several cell cycles. Pulse-chase studies showed that most of the newly synthesized fibronectin is secreted into the medium. The glucocorticoid antagonist, RU-486, blocks the dexamethasone-induced changes but does not alter the basal rate of fibronectin production. Therefore, fibronectin biosynthesis appears to be controlled by two distinct mechanisms--one, regulating basal rates of fibronectin production, which is transformation-sensitive and glucocorticoid-independent; and another, which is mediated by the glucocorticoid receptor, resulting in elevated rates of fibronectin biosynthesis upon dexamethasone treatment both in normal fibroblasts and in HT1080 cells.     

Induction of polarized apical expression and vectorial release of carcinoembryonic antigen (CEA) during the process of differentiation of HT29-D4 cells

HT29-D4 clonal cells can be induced to differentiate by a simple alteration of the culture medium, that is, by the replacement of glucose by galactose [Fantini, J., et al. (1986) J. Cell Sci., 83:235-249] as reported for the nonclonal HT29 cells [Pinto, M., (1982) Biol. Cell, 44:193-196]. An essential property of the HT29-D4 cell line is the fact that no cell loss occurs after the medium change, so that the differentiated cells can be considered as the true counterpart of the undifferentiated one. This model is particularly suitable to study morphological and biochemical events associated with the progressive establishment of the differentiation state. We report here that carcinoembryonic antigen (CEA), a 180 kDa glycoprotein originally described as a colon tumor associated antigen, is faintly expressed at the surface of undifferentiated HT29-D4 cells. These cells release a small amount of CEA (2.5 ng/10(6) cells/24 hr) in the culture medium. Fourty-eight hours after glucose substitution by galactose, both CEA cell surface expression and release are strongly enhanced as demonstrated by immunofluorescence and immunoprecipitation studies. Ten days after the medium change, the amount of CEA released reaches a maximum value of 130 ng/10(6) cells/24 hr, which remains stable for differentiated HT29-D4 cells cultured in glucose-free, galactose-containing medium (Gal-medium) for several months. HT29-D4 cells grown in Gal-medium in porous-bottom culture dishes generate leakproof epithelial monolayers. We have successfully performed an independent radioiodination of the apical and basolateral domains of these cells, followed by immunoprecipitation. We demonstrate that CEA is expressed exclusively at the apical surface of differentiated HT29-D4 cells, since the 180 kDa polypeptide was immunoprecipitated only when the radioiodination was performed at the apical side of the monolayer. Leakproof HT29-D4 monolayers cultured in permeable chambers were also used to demonstrate that CEA was exclusively released in the medium bathing the apical side of the cells. In conclusion, this study of cell surface CEA expression and CEA release during the process of differentiation of HT29-D4 cells demonstrated that 1) CEA cell surface expression and CEA release are correlated with cell differentiation; 2) CEA is expressed in the apical brush border membrane of differentiated HT29-D4 cells; and 3) CEA release is exclusively oriented toward the apical side of the polarized monolayer.