Retinoic acid enhances adhesiveness, laminin and integrin beta 1 synthesis, and retinoic acid receptor expression in F9 teratocarcinoma cells

Looking into laminin receptor: critical discussion regarding the non-integrin 37/67-kDa laminin receptor/RPSA protein

The 37/67-kDa laminin receptor (LAMR/RPSA) was originally identified as a 67-kDa binding protein for laminin, an extracellular matrix glycoprotein that provides cellular adhesion to the basement membrane. LAMR has evolutionary origins, however, as a 37-kDa RPS2 family ribosomal component. Expressed in all domains of life, RPS2 proteins have been shown to have remarkably diverse physiological roles that vary across species. Contributing to laminin binding, ribosome biogenesis, cytoskeletal organization, and nuclear functions, this protein governs critical cellular processes including growth, survival, migration, protein synthesis, development, and differentiation. Unsurprisingly given its purview, LAMR has been associated with metastatic cancer, neurodegenerative disease and developmental abnormalities. Functioning in a receptor capacity, this protein also confers susceptibility to bacterial and viral infection. LAMR is clearly a molecule of consequence in human disease, directly mediating pathological events that make it a prime target for therapeutic interventions. Despite decades of research, there are still a large number of open questions regarding the cellular biology of LAMR, the nature of its ability to bind laminin, the function of its intrinsically disordered C-terminal region and its conversion from 37 to 67 kDa. This review attempts to convey an in-depth description of the complexity surrounding this multifaceted protein across functional, structural and pathological aspects.

A screen of suitable inducers for germline differentiation of chicken embryonic stem cells

Differentiation of germ cells from embryonic stem cells in vitro could have great application for treating infertility and provide an excellent model for uncovering molecular mechanisms of germline generation. In this study, we aim to screen the suitable inducers that may prove the efficiency of driving chicken embryonic stem cells (ES cells) toward germ cells. The male ES cells were separeted into different groups: single retinoic acid (RA) treatment, co-cultured with sertoli cell feeder with RA induction, cultured on matrix proteins (fibronectin, laminin and collagen) with RA treatment, cultured on fibronectin with sertoli cell feeder and RA induction, and single bone morphogenetic protein 4 (BMP4) treatment. Quantitative RT-PCR and immunoourescence were performed to characterize the ES cells differentiation process. The results showed that spermatogonial stem cells (SSCs)-like were not detected in single RA and RA with collagen groups, but were observed in the other groups. The expression of ES specific genes (Nanog and Sox2) was decreased while SSCs marker genes (Dazl, Stra8, integrin α6, integrinβ1 and C-kit) was remarkably increased. The multiple comparsion results showed that the expression of SSCs marker genes in RA with sertoli cells group was significantly higher than the other groups(P<0.05). Collectively, our results suggested that chicken ES cells possess the potency to differentiate into SSCs-like cells in vitro through RA, matrix proteins, sertoli cells and BMP4 induction, of which co-cultured with sertoli cell feeder with RA induction was proved to be the best.

New therapeutic target for CNS injury? The role of retinoic acid signaling after nerve lesions

Experiments with sciatic nerve lesions and spinal cord contusion injury demonstrate that the retinoic acid (RA) signaling cascade is activated by these traumatic events. In both cases the RA-synthesizing enzyme is RALDH-2. In the PNS, lesions cause RA-induced gene transcription, intracellular translocation of retinoid receptors, and increased transcription of CRBP-I, CRABP-II, and retinoid receptors. The activation of RARbeta appears to be responsible for neurotrophic and neuritogenic effects of RA on dorsal root ganglia and embryonic spinal cord. While the physiological role of RA in the injured nervous system is still under investigation three domains of functions are suggested: (1) neuroprotection and support of axonal growth, (2) modulation of the inflammatory reaction by microglia/macrophages, and (3) regulation of glial differentiation. Few studies have been performed to support nerve regeneration with RA signals in vivo, but a large number of experiments with neuronal and glial cell cultures and spinal cord explants point to beneficial effects of RA, so that future therapeutic approaches will likely focus on the activation of RA signaling.

Retinoic acid signaling in the nervous system of adult vertebrates

The majority of the functions of vitamin A are carried out by its metabolite, retinoic acid (RA), a potent transcriptional activator acting through members of the nuclear receptor family of transcription factors. In the CNS, RA was first recognized to be essential for the control of patterning and differentiation in the developing embryo. It has recently come to light, however, that many of the same functions that RA directs in the embryo are involved in the regulation of plasticity and regeneration in the adult brain. The same intricate metabolic control system of synthetic and catabolic enzymes, combined with cytoplasmic binding proteins, is used in both embryo and adult to create regions of high and low RA to modulate gene transcription. This review summarizes some of the discoveries in the new field of retinoid neurobiology including its functions in neural plasticity and LTP in the hippocampus; its possible role in motor disorders such as Parkinson's disease, motoneuron disease, and Huntington's disease; its role in regeneration after sciatic nerve and spinal cord injury; and its possible involvement in psychiatric diseases such as depression.

Exposure of normal and transformed cells to nevirapine, a reverse transcriptase inhibitor, reduces cell growth and promotes differentiation

Endogenous, nontelomeric reverse transcriptase (RT) is encoded by two classes of repeated elements: retrotransposons and endogenous retroviruses. Expression of RT-coding genes is generally repressed in differentiated nonpathological tissues, yet is active in the mammalian germ line, embryonic tissues and tumor cells. Nevirapine is a non-nucleoside RT inhibitor with a well-characterized inhibitory activity on RT enzymes of retroviral origin. Here, we show that nevirapine is also an effective inhibitor of the endogenous RT in murine and human cell lines. In addition, progenitor and transformed cells undergo a significant reduction in the rate of cell growth upon exposure to nevirapine. This is accompanied by the onset of differentiation, as depicted in F9 and C2C7 progenitor cells cultures in which nevirapine triggers the expression of differentiation-specific markers. Consistent with this, an extensive reprogramming of cell cycle gene expression was depicted in nevirapine-treated F9 cultures. Furthermore, nevirapine exposure rescued the differentiation block present in acute myeloid leukemia (AML) cell lines and primary blasts from two AML patients, as indicated by morphological, functional and immunophenotypic assays. The finding that an RT inhibitor can modulate cell proliferation and differentiation suggests that RT may represent a novel target in the development of therapeutical approaches to neoplasia.

Differentially expressed nucleolar TGF-beta1 target (DENTT) in mouse development

Differentially expressed nucleolar TGF-beta1 target (DENTT) is a recently identified gene whose mRNA is differentially affected by TGF-beta1 in TGF-beta1-responsive human lung cancer cells and who is a new member of the TSPY/TSPY-like/SET/NAP-1 (TTSN) protein superfamily. Here, we report that mouse DENTT mRNA contains a 2031-bp open reading frame that encodes a predicted polypeptide of 677-amino acids with a relative molecular mass of 77,671 Da. The mouse and human DENTT sequences show 77% and 78% homology at the nucleotide and amino acid level, respectively. Mouse DENTT is predicted to be a nuclear protein with two nuclear localization signals (NLS), two coiled-coil regions, and a domain that shows significant identity to a region that defines the TTSN superfamily. Green fluorescent protein (GFP)-tagged full-length mouse DENTT transfected into COS-7 cells showed localization predominantly in the nucleolus. Reverse transcription-polymerase chain reaction amplification, Northern hybridization, and Western blot analyses showed expression of mouse DENTT mRNA and protein throughout mouse embryogenesis. Immunohistochemical staining analysis showed that DENTT is expressed in multiple tissues in a defined spatiotemporal pattern during mouse embryogenesis. The heart and primitive brain were the first organs of the embryo that showed immunoreactivity for the DENTT antibody by day 8 of development (E8). In the developing mouse brain, the choroid plexus was intensely stained for DENTT in all stages of development. The spinal cord and dorsal root ganglia were also positive for DENTT staining beginning in the 11-day-old embryo (E11), where homogeneous immunostaining was observed throughout the developing neurons. By day 16 of development (E16), only a small subset of the neuronal population in the spinal cord and dorsal root ganglia was positively stained for DENTT. DENTT immunoreactivity increased steadily with maturation as the differentiation of cartilage and osteoblasts proceeded and reached a maximum in the growth plate during endochondral ossification. DENTT expression was also detected in multiple rodent cell types in vitro, including mouse F9 embryonal carcinoma (EC) cells. Addition of retinoic acid or sodium butyrate to F9 EC cells showed a rapid decrease in expression of DENTT protein occurring by 1 hr that continued to decrease to almost undetectable levels after 24 hr. Cotransfection of full-length mouse DENTT expression plasmid with 3TPLux or COL7A1Luc Luciferase reporter plasmids into F9 EC cells significantly increased the level of 3TPLux reporter transcription while decreasing the level of COL7A1Luc reporter transcription, suggesting that DENTT may play multiple roles in modulating transcriptional responses. These findings suggest new roles for the TTSN superfamily during embryogenesis and differentiation.

A newly characterized human endometrial adenocarcinoma cell line (CAC-1) differentiates in response to retinoic acid treatment

A new cell line of poorly differentiated human endometrial adenocarcinoma cells termed "CAC-1" cells has been established. These cells are epithelial, as indicated by positive cytokeratin and negative vimentin staining. They are rounded and possess a high nuclear-to-cytoplasmic ratio, desmosomes, surface microvilli, intercelular lumens, and pleomorphic nuclei containing multiple nucleoli. These cells have been in long-term culture for 2 years. Our previous studies demonstrated that moderately differentiated (RL95-2) cells differentiated in response to retinoic acid treatment, illustrated by their reorganization of actin filaments and cell enlargement (Carter et al., 1996; Anticancer Res. 16, 17-24). CAC-1 cells exhibited a similar response because they also organized actin filaments and enlarged in response to retinoic acid treatment. Concurrently, retinoic acid treatment caused a 40% decrease in cell detachment in an in vitro detachment assay compared to controls. A slight lag in cell growth was observed when CAC-1 cells were treated with 1 microM 13-cis or all-trans retinoic acid during a 12-day growth curve. In addition, we examined the effects of retinoic acid on protein kinase C-alpha (PKC-alpha) and myristoylated alanine-rich C-kinase substrate (MARCKS). Treatment with retinoic acid caused cytoplasmic PKC-alpha to increase concomitant with a decrease in PKC-alpha in the membrane. In contrast, MARCKS increased in the membrane in response to retinoic acid treatment. These data indicate that retinoid treatment causes inactivation of PKC-alpha, allowing MARCKS to relocalize to the membrane, where it can cross-link actin filaments. CAC-1 cells represent an ideal model for investigating the effects of retinoids on differentiation induction concomitant with actin reorganization.

Transcriptional regulation of laminin gene expression

Laminins are the most abundant structural non-collagenous glycoproteins ubiquitously present in basement membranes. They are multidomain molecules consisting of of alpha, beta, and gamma chains. Although the precise functional differences between the laminin variants are not well understood, the diversity of laminin isoforms may reflect the formation of distinct basement membranes. The laminins display a remarkable restricted expression profile, suggesting a fine regulation of their genes. In this review, we focus on the most recent developments of laminin biology, centering on transcriptional and posttranscriptional controls. We discuss only those laminin chains whose gene organization and promoter elements have been characterized and proved to be functional. When possible, we correlate the effects of growth factors, cytokines, retinoids, and transcription factors on laminin gene expression with the identity of cis-acting elements in their genomic control regions.

Retinoic acid upregulates beta(1)-integrin in vascular smooth muscle cells and alters adhesion to fibronectin

Retinoic acid has an established physiological role in differentiation, development, and cellular growth. This study investigated the action of all-trans retinoic acid (ATRA) on vascular integrins, cell-surface receptors that control growth and remodeling of blood vessels. The beta(1)-integrin subunit mRNA and protein was induced after treatment with ATRA in two different rat vascular smooth muscle cell lines. To relate this result to the in vivo state, the aortas from adult rats fed with therapeutic doses of ATRA were examined for beta(1)-integrin protein. A significant upregulation of the integrin subunit was observed in vivo. To assess if this increase contributed to physiological changes in cellular function, cells treated with ATRA were tested for alterations in adhesion to extracellular matrix proteins. The cells exposed to the retinoid were seen to adhere more strongly to fibronectin, via the beta(1)-integrin. These results showed that modulation of vascular integrins by ATRA in adult rats contributes to functional changes that can cause remodeling of blood vessels.

Interaction of GRASP, a protein encoded by a novel retinoic acid-induced gene, with members of the cytohesin family of guanine nucleotide exchange factors

A novel, retinoic acid-induced gene, GRP1-associated scaffold protein (GRASP), was isolated from P19 embryonal carcinoma cells using a subtractive screening strategy. GRASP was found to be highly expressed in brain and exhibited lower levels of expression in lung, heart, embryo, kidney, and ovary. The predicted amino acid sequence of GRASP is characterized by several putative protein-protein interaction motifs, suggesting that GRASP may be a component of a larger protein complex in the cell. Although GRASP does not harbor a predicted membrane spanning domain(s), the protein was observed to be associated with the plasma membrane of transiently transfected mammalian cells. Yeast two-hybrid screening revealed that GRASP interacted strongly with the General Receptor for Phosphoinositides 1 (GRP1), a brefeldin A-insensitive guanine nucleotide exchange factor for the ADP-ribosylation factor family of proteins. GRASP. GRP1 interactions were also demonstrated in vitro and in mammalian cells in which GRASP was shown to enhance GRP1 association with the plasma membrane. Furthermore, GRASP colocalized with endogenous ADP-ribosylation factors at the plasma membrane in transfected cells, suggesting that GRASP may modulate signaling by this family of small GTPases.

Retinoic acid affects the EGF-R signaling pathway during differentiation induction of human endometrial adenocarcinoma cells

We have shown that moderately differentiated endometrial adenocarcinoma (RL95-2) cells differentiate in response to retinoic acid treatment, illustrated by their reorganization of actin filaments and cell enlargement (Carter et al., Anticancer Res. 16, 17-24, 1996). Tyrphostin, an inhibitor of epidermal growth factor receptor (EGF-R)-associated protein tyrosine kinases, caused a dramatic reorganization of actin filaments in RL95-2 cells, similar to retinoic-acid-treated cells (Carter and Bellido, J. Cell. Physiol. 178, 320-332, 1999). We evaluated the possibility that the differentiating effects of retinoids are due to retinoic-acid-induced decreases in phosphorylation of EGF-R and changes in downstream effector proteins. Retinoic acid caused a decrease in tyrosine phosphorylation of EGF-R. Retinoic acid treatment induced a dramatic actin filament reorganization and cell enlargement. Treatment with EGF reversed this effect, because cells treated with retinoic acid followed by EGF only possessed disrupted actin aggregates and appeared small, thus resembling medium controls. Retinoic acid induced a relocalization and decrease in the amount of Shc protein, another actin-binding protein which is an adaptor protein for EGF-R signaling. In addition, retinoic acid induced a relocalization of gelsolin from the plasma membrane to the cytoplasm. Retinoic acid decreased cell detachment in detachment assays; one-half as many retinoic-acid-treated cells detached as in controls. These results are consistent with the idea that retinoic acid induces differentiation of RL95-2 cells by interfering with the EGF-R signaling pathway.

Vitamin A status modulates intestinal adaptation after partial small bowel resection

BACKGROUND

Intestinal adaptation after loss of functional small bowel surface area is characterized by cellular hyperplasia and increased absorptive function. Interventions to enhance the adaptive response are needed to decrease the morbidity and mortality associated with short bowel syndrome. Retinoic acid was shown to stimulate crypt cell proliferation in the adapting remnant rat ileum by 6 hours after resection. Thus, vitamin A, which is required for normal epithelial cell proliferation and differentiation and which can modulate programmed cell death, may play an important role in the adapting intestine. On the basis of these observations, the effects of vitamin A deficiency on intestinal morphology, epithelial cell proliferation, and apoptosis in the adapting intestine after resection were investigated.

METHODS

Weanling male Sprague-Dawley rats fed either a vitamin A-deficient or -sufficient diet for 58 days underwent 70% proximal small bowel resection. The deficient rats were divided into cohorts that were either maintained on the experimental diet after surgery or replenished with vitamin A 20 hours before surgery and switched to the control diet after surgery.

RESULTS

Ten days after resection, vitamin A-deficient rats exhibited a markedly blunted adaptive response. The adaptive increase in villus height and crypt depth was absent in the deficient rats. However, adaptive increases in crypt cell proliferation were not attenuated by vitamin A deficiency, and there were no differences in apoptotic indices.

CONCLUSIONS

Vitamin A deficiency inhibits the adaptive response to partial small bowel resection, supporting a role for vitamin A in the adaptive process. Changes in cellular proliferation or programmed cell death are not sufficient to account for this inhibition. This model system will be useful for examining the role of other mechanisms, such as changes in cell-cell and cell-extracellular matrix interactions, and rates of epithelial cell migration and cell extrusion.

Retinoic acid increases BDNF-dependent regeneration of chick retinal ganglion cells in vitro

Brain-derived neurotrophic factor (BDNF) supports survival and regeneration of retinal ganglion cells (RGC). Since the expression of its receptor TrkB can be induced by the transcriptional activator retinoic acid (RA), we have investigated the possibility that RA promotes axonal regeneration of differentiated chick RGC synergistically with BDNF. After injection of all-trans RA onto the chorio-allantoic membrane of stage E16 chick embryos, axonal regeneration was monitored in organ cultures supplemented with BDNF. RA enhanced neurite outgrowth of retinal ganglion cells 2- to 3-fold. The dose-dependent effect was observed only after application of RA in ovo and subsequent use of the neurotrophin, not with RA alone.

Acquisition of cell adhesion and induction of focal adhesion kinase of human colon cancer Colo 201 cells by retinoic acid-induced differentiation

The human colon adenocarcinoma cell lines Colo 201 and Colo 205 lose adhevise capacity to the extracellular matrix (ECM) and take on a round and floating cell shape. Treatment of these cells with all-trans-retinoic acid (RA) results in inhibition of growth and in a marked increase in the production of carcinoembryonic antigen, thereby indicating that the cells undergo differentiation. This RA-induced differentiation was accompanied by a large increase in the degree of cell adhesion with localization of E-cadherin molecules at cell-cell contact sites. We examined several adhesion molecules involved in cell-cell and cell-ECM interaction by immunoblotting, but no change in E-cadherin, intercellular adhesion molecule-1, or CD44 was observed in RA-treated Colo 201 cells. Although the adhesion of Colo 201 cells to ECM depends on the Arg-Gly-Asp sequence, levels of integrins, alpha 2, alpha 3, alpha 5, alpha V, and beta 1 in differentiated adherent cells were similar to those in untreated cells. In contrast to equivalent amounts of cell surface adhesion molecules before and after differentiation, intracellular focal adhesion kinase (FAK) was markedly induced during RA treatment, and the increase in FAK resulted in elevation of tyrosine-phosphorylated FAK. These findings suggest a role for FAK in activation of cell adhesion of RA-induced differentiation of these colon cancer cells. This may serve as an appropriate model to examine the mode of activation of the adhesive capacity of cancer cells.

Regulation of N-acetylglucosaminyltransferase V and Asn-linked oligosaccharide beta(1,6) branching by a growth factor signaling pathway and effects on cell adhesion and metastatic potential

Recent evidence demonstrates that the changes in the size of N-linked oligosaccharides that correlate with cell transformation and tumorigenicity are due at least in part to the regulation of expression of a glycosyltransferase involved in the branching of N-linked structures, N-acetylglucosaminyltransferase V or GlcNAc-T V. Studies have shown that the increases in GlcNAc-T V expression after oncogenic transformation are most likely caused by direct effects on the GlcNAc-T V promoter by the Ets family of transcriptional activators, which are up-regulated by a cellular proliferation signaling pathway. This pathway begins with growth factor receptors that activate tyrosine kinases at the cell surface and proceeds through src, ras, and raf. Additional evidence for the association between cellular proliferation and GlcNAc-T V expression will be presented, as well as a discussion of the effects of beta(1,6) branching on several of the phenotypes of oncogenically transformed cells, including metastatic potential.

Modulation of alpha 6/beta 1 integrin expression during differentiation of F9 murine embryonal carcinoma cells to parietal endoderm

In vitro differentiation of the murine embryonal carcinoma (EC) cell line F9 parallels that of the early blastocyst, where visceral (VE) and parietal endoderm (PE) diverge from a common precursor, the primitive endoderm. This differentiation pathway is induced by retinoic acid (RA) and dibutyryl cyclicAMP (dcAMP) and is accompanied by progressive and dramatic changes in cell morphology and functions. Within 7 days of treatment the cells organize their cytoskeleton and synthesize large amounts of extracellular matrix proteins, becoming fully differentiated migratory cells; all these changes are likely to involve integrins expression and organization. We have investigated the changes in beta 1 integrin expression, its maturation, and organization on the cell surface in association with alpha 6, during the transition from undifferentiated F9 stem cells to migrating PE cells. By Western blotting and immunoprecipitation we showed a gradual decrease in the amount of the beta 1 subunit on the cell surface and a parallel progressive accumulation of immature protein, indicating that the control of beta 1 expression during F9 cells differentiation occurs first at post-translational level and then at the level of transcription. Moreover, the induction of differentiation produces a marked decrease of alpha 6B and its association to a high molecular weight protein, while alpha 6A level increases. By immunofluorescence we found that upon differentiation there is a relocation of the beta 1 and alpha 6B integrin subunits from cell-cell contacts to focal contacts where they colocalize with vinculin. On the contrary alpha 6A, weakly present in F9 stem cells, is present in the focal contacts of PE cells and along the stress fibers. We suggest different roles for the two alpha 6 isoforms.

Expression of the beta 4 integrin subunit in the mouse heart during embryonic development: retinoic acid advances beta 4 expression

Using immunohistochemical techniques as well as in situ hybridization we were able to elicit the expression pattern of the beta 4 integrin subunit in the murine heart during development. We show that beta 4 is not expressed in the heart before E13 and is afterwards restricted to the endocardium of the atrioventricular canal, the outflow tract, and the venous valves in the right atrium. As these are all sites of high shear stress in the heart, we propose a role for alpha 6 beta 4 in the tight adhesion of the endocardial cells to their basement membranes in these segments. Moreover, mouse embryos were treated with all-trans retinoic acid, which was previously shown to induce congenital malformations, among which malformations of the heart. We show an advanced expression without ectopic localization of cardiac beta 4 after the administration of retinoic acid. This advanced appearance of beta 4 was also shown in extracardiac tissue like migrating neural crest cells. Several hypotheses on the mechanism of beta 4 up-regulation and a possible role for alpha 6 beta 4 in the development of heart malformations after the administration of retinoic acid are discussed.

The role of cell adhesion in retinoic acid-induced modulation of chondrocyte phenotype

Retinoic acid (RA) treatment of a suspension of quail chondrocytes inhibits the expression of cartilage collagens and induces cell adhesion along with fibronectin expression. We asked whether the RA-induced modulation of the chondrocyte phenotype was dependent on cell adhesion. Prevention of cell adhesion blocks cell growth and many of the effects associated with RA, such as collagen II inhibition, collagen I activation and fibronectin induction. The activity of the bone/tendon promoter of the alpha 2(I) collagen gene was determined by measuring the transient expression of COL1A2-CAT, a chimaeric gene bearing 3500 bp from upstream of the transcription start site of the human alpha 2(I) gene fused to the chloramphenicol acetyltransferase (CAT) gene. This promoter is activated only in permissive conditions for cell adhesion. The attachment activities of chondrocytes on protein substrates was studied by an in vitro cell adhesion assay. Untreated cells or cells maintained in suspension while undergoing RA treatment do not attach when replated on protein substrates. Chondrocytes treated with RA in permissive conditions for cell adhesion rapidly attach and spread instead on collagen-coated wells. Altogether the results suggest that cell adhesion plays a major role in RA-induced modulation of the chondrocyte phenotype.

Chromosomal locations and modes of action of genes of the retinoid (vitamin A) system support their involvement in the etiology of schizophrenia

Vitamin A (retinoid), an essential nutrient for fetal and subsequent mammalian development, is involved in gene expression, cell differentiation, proliferation, migration, and death. Retinoic acid (RA) the morphogenic derivative of vitamin A is highly teratogenic. In humans retinoid excess or deficit can result in brain anomalies and psychosis. This review discusses chromosomal loci of genes that control the retinoid cascade in relation to some candidate genes in schizophrenia. The paper relates the knowledge about the transport, delivery, and action of retinoids to what is presently known about the pathology of schizophrenia, with particular reference to the dopamine hypothesis, neurotransmitters, the glutamate hypothesis, retinitis pigmentosa, dermatologic disorders, and craniofacial anomalies.

Quantitative switch in integrin expression accompanies differentiation of F9 cells treated with retinoic acid

F9 embryonal carcinoma cells resemble epithelial cells when in monolayer culture. After treatment with retinoic acid these cells differentiate into fibroblastic-like cells in a sequence that has been modeled as the mammalian equivalent of the differentiation from stem cells of the inner cell mass to parietal endoderm. This study examined the changes in integrin subtypes that accompany retinoic acid-induced differentiation of F9 cells. Although several integrins were found to be present on the surface of F9 cells and retinoic acid-induced (RA) cells, the two dominant integrins were alpha 3 beta 1 and alpha 5 beta 1. Differentiation of F9 cells resulted in about 10- to 25-fold increase in the amount of alpha 3 beta 1 integrin protein as measured by immunoprecipitation of cell surface labeled material. There was a corresponding several-fold reduction of alpha 5 beta 1 protein. The concentration of alpha 3 mRNA was about the same in F9 and RA cells while the concentration of alpha 5 mRNA dropped several-fold after retinoic acid treatment. Thus alpha 3 regulation appeared to be largely posttranscriptional while the drop in alpha 5 protein may have been a result of transcriptional down-regulation. Quantitative measurement of adhesion suggested that most of the F9 and RA cell-substrate adhesion to fibronectin or laminin is mediated by these integrins. They are the dominant integrins present, and antibodies to either these integrins or to the substrate blocked the adhesion. Despite the large switch in integrin subtype protein expression there was little difference between the two cell types in initial cell interactions when adhesive affinities were measured quantitatively. Also there was no difference between the two phenotypes in rate of initial adhesive strengthening. The phenotypic difference was first observed with later events in the attachment and spreading of the RA-treated cells to the substrate. These results show that retinoic acid treatment alters the amounts of alpha 5 and alpha 3 integrin subunits during the F9 to RA phenotypic switch. The data show that these integrins are important in the cell-substrate adhesion to fibronectin and laminin. They show, however, that the phenotypic changes observed with differentiation are not associated with the initial preferential adhesions to the substrate, but rather with consequences that alter the cytoskeletal architecture of the cell.

Vitamin A, differentiation and cancer

Retinoids, which are derivatives of vitamin A, have a variety of effects on normal cellular differentiation and on the process of carcinogenesis. A number of novel endogenous retinol metabolites have been identified recently. The response of many cell types to retinoid treatment is mediated by retinoid receptors, and involves changes in gene expression, cell growth and cell differentiation. The gene encoding one of the retinoic acid receptors is disrupted by the chromosome translocations associated with acute promyelocytic leukemia, and the expression of another is altered in epithelial tumors; both of these findings have important implications for the use of retinoids as anti-carcinogenic agents. It has been demonstrated recently that certain homeobox genes are regulated by retinoids; these genes may also prove to be useful agents for anti-carcinogenic therapies.