Cyclic AMP and LDL trigger a rapid enhancement in gap junction assembly through a stimulation of connexin trafficking

Given the rapid turnover of connexin proteins, gap junction (GJ) assembly represents an important means of regulating the extent of GJ communication between cells. This report describes an increase in the level of GJ assembly within one hour following treatment with cAMP-elevating reagents or low density lipoprotein (LDL). Dye transfer methods and freeze-fracture with electron microscopy were used to assay junctional permeability and structure, respectively, subsequent to the dissociation, recovery and reaggregation of Novikoff hepatoma cells. Reaggregating cells in the presence of agents that increase cAMP levels (8-Br-cAMP, forskolin and IBMX) enhanced both dye transfer rates between cells and the extent of GJ formation 2- to 3-fold. These data and studies with the protein kinase A inhibitor, H-89, indicate that cAMP signaling plays a key role in enhanced assembly. The response to LDL parallels that to cAMP and relies on the activity of both adenylyl cyclase and protein kinase A. Immunoblot analysis revealed no change in the level of connexin43 (Cx43) or its phosphorylation states over a period of 2.5 hours. However, three agents (brefeldin A, monensin and nocodazole), that inhibit intracellular membrane trafficking by different mechanisms, all blocked the enhanced assembly of GJs when triggered by either elevated cAMP or exposure to LDL. Related studies, which employed trafficking inhibitors at different stages in GJ assembly, suggested that Cx43 trafficking during enhanced assembly is regulated, in part, by cell contact. Intracellular sources of Cx43 were characterized by colabeling for several markers of cytoplasmic membrane systems. We conclude that an increase in GJ assembly: (i) occurs rapidly in the presence of elevated cAMP or LDL, (ii) does not require an increase in Cx43 levels or major changes in Cx43 phosphorylation and (iii) is dependent upon the trafficking of Cx43 from intracellular storage sites.

Immortalized connexin43 knockout cell lines display a subset of biological properties associated with the transformed phenotype

Immortalized cells from embryonic connexin43 knockout mice (Cx43-/-) and homozygous littermates (Cx43+/+) were cloned and characterized to determine whether the absence of Cx43 function would induce observable phenotypic changes. Cells of the Cx43+/+ clones expressed Cx43 and engaged in gap junctional communication with 10-12 neighboring cells. The Cx43-/- cells were devoid of Cx43 and communicated to less than 1 cell. Electrophysiological analysis indicated that the Cx43-/- cells communicated through Cx45 channels from 8-80-fold less than did the Cx43+/+ subclones, which seemed to communicate through Cx43 and Cx45 channels. The Cx43-/- clones grew at faster rates and to higher saturation densities, had a more spindly morphology, were more refractile, and adhered less well to the substratum than did the Cx43+/+ clones. Reintroducing the Cx43 gene into the Cx43-/- clones resulted in three subclones that communicated to 3-4 cells. Partial restoration of gap junctional communication in the three subclones was accompanied by reduced growth rates and saturation densities (2-fold compared to that of parental Cx43-/- clones) but no reversions in morphology or cell-substratum adhesion. The increased growth rates and saturation densities, altered morphology, and decreased cell adhesion displayed by the Cx43-/- clones reflect a subset of the properties of transformed cells. These studies advance the hypothesis that loss of Cx43 function during development may cause cells to acquire a preneoplastic condition.