Functional conservation of the telomerase protein Est1p in humans.

Eukaryotic telomerase contains a telomerase reverse transcriptase (TERT) and an RNA template component that are essential for telomerase catalytic activity and several other telomerase-associated factors of which only a few appear to be integral enzyme components [1-3]. The first essential telomerase protein identified was S. cerevisiae Est1p, whose deletion leads to ever-shorter telomeres despite the persistence of telomerase activity [4-6]. Extensive genetic and biochemical data show that Est1p, via its interaction with the telomerase RNA and telomere end DNA binding complex Cdc13p/Stn1p/Ten1p, promotes the ability of telomerase to elongate telomeres in vivo [7-22]. The characterization of Est1p homologs outside of yeast has not been documented. We report the characterization of two putative human homologs of Est1p, hEST1A and hEST1B. Both proteins specifically associated with telomerase activity in human cell extracts and bound hTERT in rabbit reticulocyte lysates independently of the telomerase RNA. Overproduction of hEST1A cooperated with hTERT to lengthen telomeres, an effect that was specific to cells containing telomerase activity. Like Est1p, hEST1A (but not hEST1B) exhibited a single-stranded telomere DNA binding activity. These results suggest that the telomerase-associated factor Est1p is evolutionarily conserved in humans.

Potent activity of soluble B7RP-1-Fc in therapy of murine tumors in syngeneic hosts.

We have characterized a receptor:ligand pair, ICOS:B7RP-1, that is structurally and functionally related to CD28:B7.1/2. We reported previously that B7RP-1 costimulates T cell proliferation and immune responses (Yoshinaga et al., Nature 1999;402:827-32; Guo et al., J Immunol 2001;166:5578-84; Yoshinaga et al., Int Immunol 2000;12:1439-47). We report that B7RP-1-Fc causes rejection or growth inhibition of Meth A, SA-1 and EMT6 tumors in syngeneic mice. Established Meth A tumors were rejected effectively with a single dose of B7RP-1-Fc, however, the treatment was less effective on larger tumors. Mice that rejected Meth A tumors previously by Day 30, also rejected a subsequent Meth A challenge on Day 60, without additional B7RP-1-Fc treatment, indicating a long-lived memory response. Tumor cells believed to be less immunogenic, such as P815 and EL-4 cells, were less responsive to this treatment. The EL-4 responsiveness to the B7RP-1-Fc treatment was enhanced, however, by pre-treatment of the mice with cyclophosphamide. As expected, T cells appeared to be targeted by B7RP-1-Fc treatment. Thus, the administration of soluble B7RP-1-Fc may have therapeutic value in generating or enhancing anti-tumor activity in a clinical setting.