Telomerase is a specialized type of reverse transcriptase which catalyzes the synthesis and extension of telomeric DNA (for review, see ref.1). This enzyme is highly active in most cancer cells, but is inactive in most somatic cells. This striking observation led to the suggestion that telomerase might be important for the continued growth or progression of cancer cells. However, little is known about the molecular mechanism of telomerase activation in cancer cells. Human telomerase reverse transcriptase (hTRT) has recently been identified as a putative human telomerase catalytic subunit. We transfected the gene encoding hTRT into telomerase-negative human normal fibroblast cells and demonstrated that expression of wild-type hTRT induces telomerase activity, whereas hTRT mutants containing mutations in regions conserved among other reverse transcriptases did not. Hepatocellular carcinoma (20 samples) and non-cancerous liver tissues (19 samples) were examined for telomerase activity and expression of hTRT, the human telomerase RNA component (hTR; encoded by TERC) and the human telomerase-associated protein (hTLP1; encoded by TEP1). A significant correlation between hTRT expression and telomerase activity was observed. These results indicate that the hTRT protein is the catalytic subunit of human telomerase, and that it plays a key role in the activation of telomerase in cancer cells.

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Balloon-occluded retrograde transvenous obliteration followed by any hemostatic procedure might be effective for both prophylaxis of rebleeding and eradication of gastric fundal varices, even in urgent cases.

Background-Constriction of the ductus arteriosus (DA) is initiated at birth by inhibition of O 2 -sensitive K ϩ channels in DA smooth muscle cells. Subsequent membrane depolarization and calcium influx through L-type calcium channels initiates functional closure. We hypothesize that Rho-kinase activation is an additional mechanism that sustains DA constriction. Methods and Results-The effect of increased PO 2 on the activity and expression of Rho-kinase was assessed in DAs from neonates with hypoplastic left-heart syndrome (nϭ15) and rabbits (339 term and 99 preterm rabbits). Rho-kinase inhibitors (Y-27632 and fasudil) prevent and reverse O 2 constriction. Heterogeneity exists in the sensitivity of constrictors (PO 2 ϭendothelinϭphenylephrineϾKCl) and of fetal vessels (DAϭpulmonary arteryϾaorta) to Rho-kinase inhibition. Inhibition of L-type calcium channels (nifedipine) or removal of extracellular calcium inhibits approximately two thirds of O 2 constriction. Residual DA constriction reflects calcium sensitization, which persists after removal of extracellular calcium and blocking of sarcoplasmic reticulum Ca 2ϩ -ATPase. In term DA, an increase in PO 2 activates Rho-kinase and thereby increases RhoB and ROCK-1 expression. Activation of Rho-kinase in DA smooth muscle cells is initiated by a PO 2 -dependent, rotenone-sensitive increase in mitochondrion-derived reactive O 2 species. O 2 effects on Rho-kinase are mimicked by exogenous H 2 O 2 . In preterm DAs, immaturity of mitochondrial reactive oxygen species generation is associated with reduced and delayed O 2 constriction and lack of PO 2 -dependent upregulation of Rho-kinase expression. Conclusions-O 2 activates Rho-kinase and increases Rho-kinase expression in term DA smooth muscle cells by a redox-regulated, positive-feedback mechanism that promotes sustained vasoconstriction. Conversely, Rho-kinase inhibitors may be useful in maintaining DA patency, as a bridge to congenital heart surgery.