How the neighborhood coherence principle (NCP) can give rise to tissue homeostasis: a cellular automaton approach

Cellular sublocalization of Cx43 and the establishment of functional coupling in IMR-32 neuroblastoma cells

Neuroblastoma (NB) is the most common solid pediatric tumor. IMR-32 cells are a highly malignant human NB cell line with uncontrolled proliferation but with the potential to be differentiated under specific conditions. Preliminary research indicated that connexin 43 (Cx43), the most widespread of the Cx family, is aberrantly located in IMR-32 cells, which renders these cells incapable of gap junction (GJ) intercellular communication. Functioning GJ intercellular communication has been strongly associated with growth control and a decrease in tumorigenicity. 8-br-cAMP, known to initiate the differentiation process in cancer cells, was used to examine changes in Cx43 localization and expression via immunocytochemistry, Western blot analysis, and flow cytometry. Exposure of IMR-32 cells to 8-br-cAMP decreased cell proliferation, restored the abnormally localized Cx43 from around the nucleus to the cell membrane, increased de novo Cx43 protein expression, and appeared to phosphorylate Cx43 on serine (Ser) 255 and Ser262. Forskolin, an activator of cAMP dependent protein kinase (PKA), produced identical results to 8-br-cAMP demonstrating the effect that was not unique to a cAMP analog. The use of a PKA inhibitor further confirmed the specificity of 8-br-cAMP and forskolin's effect on Cx43. The cellular relocation of Cx43, combined with the increased protein expression, established first ever GJ intercellular communication between IMR-32 cells as revealed by scrape loading. These results suggest that the GJ-mediated return of growth control, as a prerequisite for further differentiation, offers a new therapeutic avenue in the treatment of NB.

Gap junctional intercellular communication is not a major mediator in the bystander effect in photodynamic treatment of MDCK II cells

Photodynamic treatment (PDT) of confluent MDCK II cells resulted in a noticeable clustering of dead cells, consistent with a significant bystander effect. Likewise, PDT of cells in microcolonies resulted in an overabundance of microcolonies that had responded to the treatment as a single unit, that is, in which either all or no cells were dead. Confluent MDCK II cells appeared to communicate via gap junction channels, while cells in microcolonies did not. Monte Carlo simulation models were fitted to the distributions of dead cells in confluent monolayers and in microcolonies. The simulations showed that the degree of the bystander effect was higher in microcolonies than in confluent cells, suggesting that gap junction communication may be involved in the bystander effect. However, when the gap junction hypothesis was tested by treatment of microcolonies with 30 microM dieldrin, an inhibitor of gap junctional intercellular communication, there was no reduction of the bystander effect, indicating that this effect was not mediated by gap junctional intercellular communication. PDT influenced phosphorylation of tyrosine residues in several proteins in the cells. Protein phosphorylation is important in cellular signaling pathways and may be involved in the bystander effect, for example by influencing the mode of cell death.

Cell population dynamics (apoptosis, mitosis, and cell-cell communication) during disruption of homeostasis

The sequence of events involved in maintenance of homeostasis must encompass mechanisms within single cells as well as interactions between cells within a population. To investigate the interaction among these inter- and intracellular mechanisms, disruption of homeostasis by serum deprivation was performed in WB-F344, a normal diploid epithelial cell line. Changes in cell-cell communication (gap junction function) at the population level and in individual cells were monitored using the scrape load/dye transfer and fluorescence redistribution after photobleaching assays. Apoptosis and mitosis were measured using internucleosomal DNA ladder assays and fluorescence-activated cell sorting. The results indicate that a common element in early apoptosis and early mitosis is sustained gap junction function. As cell life (mitosis) and cell death (apoptosis) progressed, a common process of change in gap junction function occurred. A transient stimulation of mitosis concomitant with increased apoptosis was also observed during serum deprivation. Gap junctions may play a regulatory role during initiation of these opposite yet equally important mechanisms of maintaining homeostasis. This model system is useful for further studies on the relationships among inter- and intracellular mechanisms of homeostasis.