The actions of retinoids on cellular growth correlate with their actions on gap junctional communication

Retinoic acid modulates gap junctional permeability: a comparative study of dye spreading and ionic coupling in cultured cells

All-trans retinoic acid (RA), which was recently identified as a morphogen, affects gap junctional permeability in a dose- and time-dependent manner. In five different established mammalian cell lines (FL, BRL, BICR/M1Rk, HEL37, BT5C1) 100 mumol/liter RA reduced Lucifer yellow spreading within 30 min to 20-50% of the control. Ionic coupling, however, remained almost unaffected under the same conditions. Freeze-fractured membranes of untreated and RA-treated cells were similar with regard to frequency and sizes of gap junction plaques. With concentrations of less than 10 mumol/liter RA the dye spreading increased significantly in the human amniotic cell line FL, pointing to a possible modulatory effect of RA on junctional communication.

Gap-junction quantification in biological tissues: freeze-fracture replicas versus thin sections

The relative efficiency of freeze-fracture replicas versus thin sections for the visualization and quantification of gap junctions in biological tissues has been evaluated. Both methods may underestimate gap-junction number--thin sections for reasons of tissue resolution and freeze-fracture replicas due to the mechanics of the fracturing process. Freeze-fracture misses gap junctions in regions of plasma membrane which are highly contoured, such as the overlapping basal cell processes of Drosophila imaginal wing discs and the interdigitating lateral membrane plications of intercalated discs in cardiac tissue. If the missed gap junctions are relatively large, as they are in both of these examples, freeze-fracture significantly underestimates the total gap-junctional area. Thin sections may miss small gap junctions, but in tissues which contain a range of gap-junction sizes the lost junctions constitute a relatively small fraction of the total junctional area. In neoplastic imaginal wing discs, thin sections were as efficient as freeze-fracture replicas in identifying even the smallest gap junctions. Although freeze-fracture may be the better technique for the qualitative and quantitative documentation of small gap junctions in tissues with relatively flat to gently contoured plasma membranes and thin sections may be the superior method for gap-junction quantification in tissues containing a range of gap-junctional sizes and highly contoured cellular processes, the data suggest that a combination of the two approaches should be utilized whenever possible.

Retinoic acid modulates both invasion and plasma membrane ruffling of MCF-7 human mammary carcinoma cells in vitro

The invasiveness of MCF-7 human mammary carcinoma cells was tested in vitro via confronting cultures with embryonic chick heart fragments. Invasive (e.g. MCF-7/6) and non-invasive (e.g. MCF-7/AZ) variants were detected. Automated image analysis of time-lapse video-microscopy recordings showed that the plasma membrane ruffling activity of the invasive MCF-7/6 variant was higher than the ruffling activity of the non-invasive MCF-7/AZ variant. Addition of all-trans-retinoic acid to the culture medium (10(-6) M) inhibited both invasion and ruffling of MCF-7/6 cells, while MCF-7/AZ cells became invasive and acquired an increased ruffling by the same type of treatment. A similar opposite effect on MCF-7 cells was not found after treatment with other ligands of the nuclear steroid/thyroid receptor superfamily. Triiodo-l-thyronine (up to 10(-5) M) and beta-oestradiol (up to 10(-6) M) did not alter the invasiveness of the cells, while dexamethasone (10(-6) M) and the pure anti-oestrogen ICI 164,384 inhibited both invasion and ruffling. Our data show that retinoic acid can modulate invasiveness in opposite directions.

Aberrant expression and function of gap junctions during carcinogenesis

Gap junctional intercellular communication plays a key role in the maintenance of homeostasis in multicellular organisms. Reflecting deranged homeostasis in cancer cells, most transformed or cancerous cells show aberrant gap junctional intercellular communication; they have decreased junctional communication between each other and/or with surrounding normal cells. Studies with in vitro cell transformation and animal carcinogenesis models suggest an involvement of blocked intercellular communication in later stages of carcinogenesis. Analysis of expression of gap junction proteins (connexins) and corresponding mRNA indicates that a number of regulation sites are involved in aberrant function of gap junctions during carcinogenesis. Suppression of transformed phenotypes is often seen when transformed cells are physically in contact with their normal counterparts. Some studies suggest that gap junctional intercellular communication is involved in such tumor suppression.

Differential regulation of communication by retinoic acid in homologous and heterologous junctions between normal and transformed cells

The permeability of junctions between cells of the same type (homologous junctions) is greatly increased by retinoic acid (10(-9)-10(-8) M), a probable morphogen, and this responsiveness is shared by a variety of normal and transformed cell types (Mehta, P.P., J.S. Bertram, and W.R. Loewenstein. 1989. J. Cell Biol. 108:1053-1065). Here we report that the heterologous junctions between the normal and transformed cells respond in the opposite direction; their permeability is reduced by retinoic acid (greater than or equal to 10(-9) M) and its benzoic acid derivative tetrahydrotetramethylnaphthalenylpropenylbenzoic acid (greater than or equal to 10(-11) M). The opposite responses of the two classes of junction are shown to be concurrent; in cocultures of normal 10T1/2 cells and their methylcholanthrene-transformed counterparts, the permeability of the heterologous junctions, which is lower than that of the homologous junctions to start with, falls (within 20 h of retinoid application), at the same time that the permeability of the homologous junctions rises in both cell types. Such a counter-regulation requires a minimum of three degrees of cellular differentiation. A model is proposed in which the differentiations reside in a trio of junctional channel protein. The principle of the model may have wide applications in the regulation of intercellular communication at tissue boundaries, including embryonic ones.

Positive selection of candidate tumor-suppressor genes by subtractive hybridization

A positive selection system designed to identify and recover candidate tumor-suppressor genes is described. The system compares mRNA expression of genes from normal and tumor-derived human mammary epithelial cells grown in a special medium that supports similar growth rates of the two cell types. mRNAs uniquely expressed in normal cells are recovered as cDNAs after subtraction with mRNA from tumor cells. Seven different clones, from 0.6 to 4.8 kilobases in transcript size and including both rare and abundunt transcripts, were recovered in the first 23 clones analyzed. Among the isolated clones were genes encoding the gap-junction protein connexin 26, two different keratins, and glutathione-S-transferase pi, as well as an unknown gene in the S100 family of small calcium-binding proteins. In principle, tumor-suppressor genes include two classes: class I, in which loss of function results from mutation or deletion of DNA and class II, in which loss of function is from a regulatory block to expression. A class II suppressor gene is assumed to be regulated by a different suppressor gene that lost its function by mutation or deletion. Both classes of tumor-suppressor genes may provide valuable proteins with clinical applications in cancer diagnosis or therapy. Class II suppressors may be especially useful because the normal genes are present and their reexpression may be inducible by drugs or other treatments.

Modulation of endothelial cell shape and growth by retinoids

Physiologic concentrations of retinol (1 X 10(-6) M) caused capillary and aortic endothelial cells (EC) to undergo a morphologic change, characterized by a rounder cell body, increased refractility at cell edges, and longer cytoplasmic processes distributed in a bipolar fashion. Computer image analysis of retinoid-treated EC revealed that both retinoic acid and retinol affected cellular area. Twenty-four hours following retinoic acid treatment, EC occupied a greater area than control (P less than 0.03) or retinol-treated EC (P less than 0.02). By Day 7, however, retinoic acid-treated EC occupied equivalent cellular areas as compared to control cells (P = 0.8). In contrast, by Day 7, retinol-treated EC occupied a smaller cellular area than control (P less than 0.002) or retinoic acid-treated EC (P less than 0.001). Proliferation studies revealed that within the first 72 hr of retinol treatment, basal EC growth was inhibited by 33% and the cells exhibited a lowered responsiveness to basic fibroblast growth factor (bFGF). In contrast, EC treated with retinoic acid and pericytes treated with each of the retinoids were not inhibited. The inhibitory effect of the 72 hr retinol treatment was reversible. Following 3 days exposure to retinol, EC given fresh media without retinoid underwent a population doubling in a subsequent 3-day period. However, in the continued presence of retinol, EC were 100% growth-inhibited. After a 3-day pretreatment with retinol, with or without continued retinol treatment, EC were refractile to the mitogenic action of bFGF in a subsequent 3-day period. These results demonstrate that retinol inhibits the basal and growth factor-stimulated growth of EC and causes a significant shape alteration of EC, supporting our hypothesis that vitamin A may be one of the signals that modify the growth and phenotype of EC.

Cell-to-cell communication and the control of growth

Growth of transformed cells is inhibited by cell-to-cell communication with normal cells. This communication is regulated by certain oncogenes that alone or in cooperation can block the communication.

Ligand-receptor and junction-mediated cell-cell interactions: comparison of the two principles

Two major forms of cell-cell interaction--namely, ligand-receptor interactions and junction-mediated interactions--have different functional implications and naturally supplement each other in the most important tissue processes. The former type of interaction may be defined as distant and mainly "intertissue"; as a rule it determines temporal coordination of cell functioning. The latter is local and preferentially "intratissue"; it determines spatial coordination of cell activity.

Defective gap-junctional communication associated with imaginal disc overgrowth and degeneration caused by mutations of the dco gene in Drosophila

The lethal(3)discs overgrown (dco) locus of Drosophila melanogaster, located on the third chromosome at cytogenetic position 100A5,6-100B1,2, is necessary for normal development and growth control in the imaginal discs of the larva. Three recessive lethal alleles (dco2, dco3, and dco18) in heteroallelic combinations and one allele (dco3) when homozygous cause the imaginal discs to continue to grow beyond the normal disc-intrinsic limit during an extended larval period. Some degeneration also occurs in the overgrowing discs. The discs overgrow even when transplanted early in their development into wild-type hosts, whereas normal discs stop growth at about the normal final size under such conditions, indicating that the overgrowth is a disc-autonomous effect of the mutations. During overgrowth the imaginal discs retain their single-layered epithelial structure except near regions of degeneration, and they differentiate into disc-appropriate but abnormal adult structures when transplanted into wild-type larval hosts. When the mutant larvae are reared under certain conditions a small percentage develop to the pharate adult stage, and these animals show a characteristic syndrome of abnormalities including swollen leg segments with many extra bristles, small or missing eyes, duplicated antennae and palpi, and separated vesicles of cuticle. A fourth recessive lethal allele (dcole88), when homozygous or in heteroallelic combination with the overgrowth alleles, causes the imaginal discs to degenerate, producing a "discless" phenotype. Gap junction-mediated communication was assayed by observing the intercellular transfer of injected fluorescein complexon (dye coupling). Dye coupling in the imaginal discs of the dco genotypes that cause overgrowth was dramatically reduced at 4 days after egg laying (AEL) compared with wild-type controls. Coupling was more normal although still significantly reduced at 7-8 and 12-14 days AEL. In c43hs1, another disc overgrowth mutant, the imaginal disc cells also showed very reduced dye coupling at 4 days and incomplete coupling at 9 days. In contrast, discs from wild-type larvae, two other imaginal disc overgrowth mutants, and a cell death mutant showed extensive dye coupling at all stages tested. Electron microscopic morphometry revealed a reduction in gap-junction length per unit lateral plasma membrane length in dco3/dco18 and c43hs1 wing discs, although not in dco2/dco3, compared with wild-type wing discs. The results suggest that gap-junctional cell communication may be involved in the cell interactions that limit cell proliferation in vivo.

Retinoid-enhanced gap junctional communication is achieved by increased levels of connexin 43 mRNA and protein

Natural and synthetic retinoids are potent inhibitors of experimental carcinogenesis in animals and cause reversion of premalignant lesions in humans. In the model C3H 10T1/2 cell system, retinoids enhance postconfluent growth control, reversibly inhibit carcinogen-induced transformation, and enhance gap junctional intercellular communication. These effects are highly correlated. 10T1/2 cells were found to express low levels of connexin 43, a gap junctional protein first found in the heart. After treatment of confluent 10T1/2 cells with the synthetic retinoid tetrahydrotetramethylnapthalenylpropenylbenzoic acid (TTNPB), levels of connexin 43 mRNA and protein increased within 6 h of treatment, while elevation of junctional communication was detected within 12-18 h. The maximally effective concentration of TTNPB (10(-8) M) caused an approximate 10-fold elevation of connexin 43 gene transcripts after 72 h. Indirect immunofluorescence microscopy using a polyclonal antibody to the synthetic C-terminal region of connexin 43 demonstrated that TTNPB induced many fluorescent plaques in regions of cell-cell contact. These results provide a molecular basis for the retinoid-enhanced junctional communication in 10T1/2 cells. It is proposed that one action of retinoids is to modulate the intercellular transfer of signal molecules. These could mediate many of the physiological actions of retinoids on growth control and carcinogenesis.

Growth factors modulate junctional cell-to-cell communication

The epidermal growth factor (EGF) and the platelet-derived growth factor (PDGF) inhibit gap junctional communication in the mammalian cell lines NRK and BalbC 3T3: cell-to-cell transfer of a 400-dalton tracer molecule is reduced and junctional conductance is reduced. The inhibition of cell-to-cell transfer is reversible and dose dependent; half-maximal effects are obtained at 10(-9) and 10(-11) M concentrations of EGF and PDGF, respectively. The response of junctional conductance is detectable within 2 min of EGF application and reaches a maximum within 10 min. It is among the earliest cellular responses to this growth factor and may be significant in the regulation of growth. The response is lacking in EGF receptor-deficient NIH 3T3 cells. The transforming factor beta (TGF beta) enhances junctional communication in BalbC 3T3: cell-to-cell transfer is increased over a period of 8 hr. But in NRK cells, where it upregulates EGF receptors, TGF beta reduces junctional communication synergistically with EGF.