Restoration of Functional Gap Junctions through Internal Ribosome Entry Site-Dependent Synthesis of Endogenous Connexins in Density-Inhibited Cancer Cells

Abstract

Gap junctions are composed of connexins and are critical for the maintenance of the differentiated state. Consistently, connexin expression is impaired in most cancer cells, and forced expression of connexins following cDNA transfection reverses the tumor phenotype. We have found that the restoration of density inhibition of human pancreatic cancer cells by the antiproliferative somatostatin receptor 2 (sst2) is due to overexpression of endogenous connexins Cx26 and Cx43 and consequent formation of functional gap junctions. Immunoblotting along with protein metabolic labeling and mRNA monitoring revealed that connexin expression is enhanced at the level of translation but is not sensitive to the inhibition of cap-dependent translation initiation. Furthermore, we identified a new internal ribosome entry site (IRES) in the Cx26 mRNA. The activity of Cx26 IRES and that of the previously described Cx43 IRES are enhanced in density-inhibited cells. These data indicate that the restoration of functional gap junctions is likely a critical event in the antiproliferative action of the sst2 receptor. We further suggest that the existence of IRESes in connexin mRNAs permits connexin expression in density-inhibited or differentiated cells, where cap-dependent translation is generally reduced.

Contribution of HIF-1α in 4E-BP1 Gene Expression

“…As sst2 normally triggers antiproliferative signals (17), its absence leads to uncontrolled cell proliferation. We have however shown that sst2 reexpression conversely limits the proliferation of pancreatic cancer cells, likely via the induction of 4E-BP1 protein and the restoration of contact inhibition (18). Here, we first wish to confirm that sst2-triggered contact inhibition is dependent on 4E-BP1 protein accumulation.…”

Homo sapiens Systemic RNA Interference-defective-1 Transmembrane Family Member 1 (SIDT1) Protein Mediates Contact-dependent Small RNA Transfer and MicroRNA-21-driven Chemoresistance

“…S3) (46,47). Coculture and flow cytometric analysis of Cy3-siRNA transfer was repeated following pretreatment with AGA prior to a 90-min coculture at 37°C, as described above.…”

Thrombospondin-1 is a critical effector of oncosuppressive activity of sst2 somatostatin receptor on pancreatic cancer

“…1 A and B). We have previously demonstrated that the inhibitor of cap-dependent translation 4E-BP1 is activated in BxPC-3/sst2 cells, sst2 increasing both its transcriptional expression, and its activity by repressing PI3K activity (17,18). Furthermore, treatment of BxPC-3 cells with the mTOR inhibitor, rapamycin, which inhibits 4E-BP1 activity, does not affect TSP-1 expression, suggesting a cap-independent process in the control of TSP-1 mRNA translation.…”