Restoration of functional gap junctions through internal ribosome entry site-dependent synthesis of endogenous connexins in density-inhibited cancer cells.
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.
Lahlou
Hicham
H
INSERM U531, Institut Louis Bugnard, CHU Rangueil, TSA 50032, 31059 Toulouse cedex 9, France.
Fanjul
Marjorie
M
Pradayrol
Lucien
L
Susini
Christiane
C
Pyronnet
Stéphane
S
D020121:5' Untranslated Regions / Q000235:genetics; D002450:Cell Communication; D002452:Cell Count; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D000072259:Connexin 26; D018031:Connexin 43 / Q000096:biosynthesis* / Q000235:genetics / Q000378:metabolism; D017630:Connexins / Q000096:biosynthesis* / Q000235:genetics / Q000378:metabolism; D017629:Gap Junctions / Q000502:physiology*; D006801:Humans; D009369:Neoplasms / Q000235:genetics / Q000378:metabolism* / Q000473:pathology*; D014176:Protein Biosynthesis / Q000187:drug effects; D017481:Receptors, Somatostatin / Q000235:genetics / Q000378:metabolism; D012045:Regulatory Sequences, Nucleic Acid / Q000235:genetics; D012270:Ribosomes / Q000378:metabolism*; D020123:Sirolimus / Q000494:pharmacology; D015854:Up-Regulation / Q000187:drug effects
D020121:5' Untranslated Regions; D018031:Connexin 43; D017630:Connexins; C000607042:GJB2 protein, human; D017481:Receptors, Somatostatin; C106140:somatostatin receptor sst2A; D000072259:Connexin 26; D020123:Sirolimus