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We have previously demonstrated that CD95-mediated apoptosis of hepatocytes is blocked in a murine model of hepatocarcinogenesis due to the expression of SV40 early sequences encoding the large-T and small-t antigens. In this study, we set out to pinpoint the sequences involved in this apoptosis-resistant phenotype, and tested several mutants of the SV40 early region for their ability to confer protection against CD95-induced apoptosis in transgenic mice. We show that resistance to apoptosis is independent of the transforming character of the mutants and demonstrate that the expression of the small-t antigen alone in transgenic mice is sufficient to confer this resistance. Our data also reveal an increased level of activated Akt kinase in these transgenic mice, and this could account for this hitherto unknown function of the SV40 small-t antigen., (Copyright 2001 Academic Press.)

Little is known about the expression of cell cycle regulatory genes upon terminal differentiation of skeletal muscle cells. In this report, we demonstrate that the expressions of cyclin A, cyclin D1 and p34cdc2 are downregulated upon C2C12 myocytes differentiation and are not inducible in differentiated myotubes. SV40 large T antigen can induce cell cycle entry of myotubes through its induction of these genes' expressions and pRB phosphorylation as well as its suppression of Rb expression. These results provide the first direct evidence that the irreversible downregulation of cyclins and cyclin-dependent kinases is one mechanism for the permanent cell cycle withdrawal of myotubes.

We have cloned two isoforms of rat kidney prostaglandin E2 receptor EP3 subtype (rEP3A and rEP3B), which differ only in their cytosolic carboxyl-terminal tails (30 and 29 amino acids, respectively). The aim is to clarify the functional difference between two rEP3 receptor isoforms by examining formation of adenosine 3',5'-monophosphate (cAMP) and change in cytosolic free calcium ([Ca2+]i) in cultured cells transiently transfected with cloned rEP3A or rEP3B receptor cDNA. In immortalized renal distal tubule cells (TKC2), vasopressin (VP) stimulated cAMP formation, and the cAMP formation was significantly attenuated by a non-peptide VP receptor antagonist, OPC-31260. The VP-induced increase in cAMP formation was also attenuated by over-expression of rEP3A receptor but not that of rEP3B receptor. On the other hand, in COS-7 cells transfected with rEP3B receptor cDNA, PGE2 induced an increase in [Ca2+]i, but no increase in [Ca2+]i was observed in the cells transfected with rEP3A cDNA. In conclusion, rEP3A receptor is suggested to antagonize VP (V2) receptor by inhibiting cAMP formation, whereas rEP3B receptor is linked with Ca2+ messenger system.

CHOP2 cells, a subline of the Chinese hamster ovary (CHO) cell glycosylation mutant Lec2 which expresses polyoma virus large T antigen, was used as the host cell line to select cDNAs conferring resistance to the toxic effects of ricin. Glycoconjugates in CHOP2 cells are deficient in sialic acid and therefore the cells are hypersensitive to ricin, a galactose-binding lectin. CHOP2 cells acquiring cDNA that either corrected the Lec2 mutation or created a Lec2/Lec1 phenotype were expected to show a selective growth advantage in ricin-containing medium. After a single cycle of transfection with a lymphoid cDNA library in pCDM8 followed by ricin selection, the the predominant cDNA recovered from the cells was antisense N-acetyl-glucosaminyltransferase I (GlcNAc-TI) which conferred a Lec2/Lec1 phenotype. cDNA encoding GlcNAc-TI in a sense orientation was also enriched by transfecting a cDNA library into CHOP-1 cells, a mutant deficient in this enzyme, and selecting in medium containing ConA lectin. The results show that cell selection with toxic agents can be used to expression-clone both antisense and sense cDNA sequences from bi-directional cDNA libraries in the pCDM8.