Overexpression of the cellular src gene in NIH 3T3 cells causes reduction of cell-to-cell transmission of molecules in the 400- to 700-dalton range. This down-regulation of gap junctional communication correlates with the activity of the gene product, the protein tyrosine kinase pp60c-src. The down-regulation was enhanced by point mutation of Tyr527 (a site that is phosphorylated in pp60c-src and that inhibits kinase activity) or by substitution of the viral-src for the cellular-src carboxyl-terminal coding region. Mutation of Tyr416 (a site phosphorylated upon Tyr527 mutation) suppresses both the down-regulation of communication by Tyr527 mutation and that by gene overexpression. The regulation of communication by src may be important in the control of embryonic development and cellular growth.

To study changes of junctional membrane permeability associated with transformation, the junctions and the nonjunctional membranes of quail embryo-, chick embryo- and mouse-3T3 cell cultures, infected with temperature-sensitive mutant Rous sarcoma virus, were probed with fluorescent-labelled glutamate. Junctional permeability fell in the transformed state. In the quail cells, the fall was detectable within 25 min of shifting the temperature down to the level (permissive) at which tyrosine-phosphorylation by the viral src gene product is expressed. This reduction of junctional permeability is one of the earliest manifestations of viral transformation. Normal permeability was restored within 30 min of raising the temperature to the nonpermissive level, a reversibility that could be displayed several times during the span of a cell generation. The reversal seems to reflect a reopening of cell-to-cell channels rather than a synthesis of new ones; it is not blocked by protein-synthesis inhibition. Treatments with cyclic AMP and phosphodiesterase inhibitor or with forskolin, which stimulate serine and threonine phosphorylation--the type of phosphorylation on which normal junctional permeability depends (Wiener& Loewenstein, 1983, Nature 305:433)--did not abolish, in general, the junctional effect of the virus; src tyrosine-phosphorylation apparently overrides the junctional upregulation mediated by cyclic AMP. Nonjunctional membrane permeability was not sensibly affected by the virus. It was affected, however, by temperature: lowering the temperature from the nonpermissive to the permissive level caused the nonjunctional permeability to fall, and vice versa. This change was unrelated to transformation. Its secondary effect on junctional transfer is in the opposite direction to that produced by the temperature-activated viral transformation.

Human Lesch-Nyhan cells, which are coupling and have gap junctions, were fused with mouse cl-ID cells, which are noncoupling and lack gap junctions. The resulting hybrid cells were coupling and had gap junctions while they contained the nearly complete complement of parent chromosomes. As the hybrid cells lost human chromosomes, clones appeared among the segregants, which had reverted to the noncoupling and junction-deficient trait of the mouse parent cell. The human cell appears to contribute a genetic factor to the hybrids that corrects the junctional deficiency of the mouse cell.

The cyclic nucleotide effect on junction was studied in C1-1D cells, a mouse cancer cell type that fails to make permeable junctions in ordinary confluent culture. Upon administration of cyclic AMP, dibutyryl cyclic AMP, dibutyryl cyclic AMP plus caffeine (db-cAMP-caffeine), or cholera toxin (an adenylate cyclase activator), the cells acquired permeable junctions; they became electrically coupled and transferred fluorescent tracer molecules among each other - a transfer exhibiting the molecular size limit of permeation of normal cell-to-cell channels. The effect took several hours to develop. With the db-cAMP-caffeine treatment, junctional permeability emerged within two hours in one-fifth of the cell population, and within the next few hours in the entire population. This development was not prevented by the cytokinesis inhibitor cytochalasin B. Permeable junctions formed also in two other conditions where the cell-endogenous cyclic AMP level may be expected to increase: serum starvation and low cell density. After three weeks of starving, the cells of serum, a junctional permeability arose in confluent cultures, which on feeding with serum disappeared within two to three days. At low cell density, namely below confluency, the cells made permeable junctions, unstarved. In cultures of rather uniform density, the frequency of permeable junctions was inversely related to the average density, over the subconfluent range; at densities of about 1 X 10(4) cells/cm2, where the cells had few mutual contacts, 80% of the pairs presumed to be in contact were electrically coupled. In cultures with adjoining territories of high (confluent) and low cell density, there was coupling only in the last, and in this low-density state the cells were also capable of coupling with other mammalian cell types (mouse 3T3-BalbC and human Lesch-Nyhan cells).

Mouse 3T3-L1 fibroblast cells, also known as preadipocytes, differentiate in vitro into adipocytes when treated with promoting agents and acquire numerous properties characteristic of mature fat cells. We studied junctional cell-to-cell communication by measuring the incidence of electrical coupling and transfer of carboxy-fluorescein among these cells. When 3T3-L1 cells were induced to differentiate into adipocytes, they lost virtually all cellcell communication. Preadipocytes that remained nondifferentiated after the treatment maintained normal communication. Loss of communication in the adipocytes invariably coincided with appearance of lipid droplets and not with other phenotypic changes. In the differentiating cells, loss of cell-to-cell communication and lipid accumulation was prevented if dibutyryl cyclic AMP and caffeine were present in the culture medium. Addition of dibutyryl cyclic AMP and caffeine to already differentiated adipocytes resulted in loss of lipid and simultaneously improved junctional permeability. The results demonstrate that in the in vitro 3T3-L1 cell system, (a) cell-to-cell communication and lipid synthesis are intimately related during the adipose conversion and (b) cAMP affects the expression of the two phenotypes.