Telomerase Is Controlled by Protein Kinase Cα in Human Breast Cancer Cells

Abstract

Telomerase, a specialized RNA-directed DNA polymerase that extends telomeres of eukaryotic chromosomes, is repressed in human somatic tissues and becomes activated during tumor progression in most human cancers. To date, little is known about how telomerase is activated and controlled in cancer, although activation is thought to be involved in cancer cell immortalization. Here, we report that human telomerase-associated protein 1 (hTEP1) and the telomerase catalytic subunit (human telomerase reverse transcriptase (hTERT)) are phosphoproteins and that their phosphorylation is a prerequisite for the activation of telomerase in intact human breast cancer cells. Identified by hTEP1 peptide affinity chromatography, protein kinase Cα mediates the phosphorylation of hTEP1 and hTERT and induces a marked increase in telomerase activity. Thus, phosphorylation of hTEP1 and hTERT by protein kinase Cα represents an essential step in the generation of a functional telomerase complex in the initiation and maintenance of telomerase activity in human cancer. Telomerase, a specialized RNA-directed DNA polymerase that extends telomeres of eukaryotic chromosomes, is repressed in human somatic tissues and becomes activated during tumor progression in most human cancers. To date, little is known about how telomerase is activated and controlled in cancer, although activation is thought to be involved in cancer cell immortalization. Here, we report that human telomerase-associated protein 1 (hTEP1) and the telomerase catalytic subunit (human telomerase reverse transcriptase (hTERT)) are phosphoproteins and that their phosphorylation is a prerequisite for the activation of telomerase in intact human breast cancer cells. Identified by hTEP1 peptide affinity chromatography, protein kinase Cα mediates the phosphorylation of hTEP1 and hTERT and induces a marked increase in telomerase activity. Thus, phosphorylation of hTEP1 and hTERT by protein kinase Cα represents an essential step in the generation of a functional telomerase complex in the initiation and maintenance of telomerase activity in human cancer. In most eukaryotes, telomerase is responsible for the synthesis and maintenance of telomeric DNA that together with proteins forms a protective cap known as telomeres for eukaryotic linear chromosomes (1Blackburn E.H. Annu. Rev. Biochem. 1992; 61: 113-129Crossref PubMed Scopus (594) Google Scholar, 2de Lange T. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 2882-2885Crossref PubMed Scopus (295) Google Scholar, 3Kim N.W. Piatyszek M.A. Prowse K.R. Harley C.B. West M.D. Ho P.L. Coviello G.M. Wright W.E. Weinrich S.L. Shay J.W. Science. 1994; 266: 2011-2015Crossref PubMed Scopus (6584) Google Scholar, 4Harley C.B. Kim N.W. Prowse K.R. Weinrich S.L. Hirsch K. West M.D. Bacchetti S. Hirte H.W. Counter C.M. Greider C.W. Wright W.E. Shay J.M. Cold Spring Harbor Symp. Quant. Biol. 1995; 59: 307-315Crossref Scopus (409) Google Scholar, 5Zakian V.A. Science. 1995; 270: 1601-1607Crossref PubMed Scopus (795) Google Scholar, 6Greider C.W. Annu. Rev. Biochem. 1996; 65: 337-365Crossref PubMed Scopus (908) Google Scholar, 7Shay J.W. Bacchetti S. Eur. J. Cancer. 1997; 33: 787-791Abstract Full Text PDF PubMed Scopus (2418) Google Scholar, 8Nakamura T.M. Morin G.B. Chapman K.B. Weinrich S.L. Andrews W.H. Lingner J. Harley C.B. Cech T.R. Science. 1997; 277: 955-959Crossref PubMed Scopus (2075) Google Scholar, 9Meyerson M. Counter C.M. Eaton E.N. Ellisen L.W. Steiner P. Caddle S.D. Ziaugra L. Beijersbergen R.L. Davidoff M.J. Liu Q. Bacchetti S. Haber D.A. Weinberg R.A. Cell. 1997; 90: 785-795Abstract Full Text Full Text PDF PubMed Scopus (1666) Google Scholar, 10Bodnar A.G. Ouellette M. Frolkis M. Holt S.E. Chiu C.-P. Morin G.B. Harley C.B. Shay J.W. Lichtsteiner S. Wright W.E. Science. 1998; 279: 349-352Crossref PubMed Scopus (4147) Google Scholar). The integrity of telomeres is essential for genome stability and chromosome paring and separation during cell division (11Kirk K.E. Harmon B.P. Reichardt I.K. Sedat J.W. Blackburn E.H. Science. 1997; 275: 1478-1481Crossref PubMed Scopus (189) Google Scholar, 12de Lange T. Nature. 1998; 392: 753-754Crossref PubMed Scopus (56) Google Scholar). Damage of telomeres, if not repaired, causes cell cycle arrest, chromosome fusion, or degradation. Telomeres shorten, however, as a function of each cell division in normal human somatic cells, where telomerase activity is below detectable levels, contributing to the mechanisms underlying cell senescence. In most human cancers, telomeres stop shortening because of the de novo synthesis of telomeric DNA by activated telomerase. Telomerase may thus play a pivotal role in regulating the structure and function of telomeres during human cell immortalization and tumor progression. Telomerase is a ribonucleoprotein reverse transcriptase. Telomerase RNA subunit from a variety of different organisms has been characterized (13Greider C.W. Blackburn E.H. Nature. 1989; 337: 331-337Crossref PubMed Scopus (1316) Google Scholar, 14Shippen-Lentz D. Blackburn E.H. Science. 1990; 247: 546-552Crossref PubMed Scopus (324) Google Scholar, 15Feng J. Funk W.D. Wang S.S. Weinrich S.L. Avilion A.A. Chiu C.-P. Adams R.R. Chang E. Allsopp R.C. Yu J. Le S. West M.D. Harley C.B. Andrews W.H. Greider C.W. Villeponteau B. Science. 1995; 269: 1236-1241Crossref PubMed Scopus (2087) Google Scholar, 16Blasco M.A. Rizen M. Greider C.W. Hanahan D. Nat. Genet. 1996; 12: 200-204Crossref PubMed Scopus (289) Google Scholar). The RNA component contains the template for telomeric DNA addition and is essential for telomerase activity. Telomerase protein subunits have also been isolated from the ciliates Tetrahymena thermophila and Euplotes aediculatus (17Collins K. Kobayashi R. Greider C.W. Cell. 1995; 81: 677-686Abstract Full Text PDF PubMed Scopus (243) Google Scholar, 18Lingner J. Hughes T.R. Shevchenko A. Mann M. Lundblad V. Cech T.R. Science. 1997; 276: 561-567Crossref PubMed Scopus (1044) Google Scholar) and from yeast and mammals (8Nakamura T.M. Morin G.B. Chapman K.B. Weinrich S.L. Andrews W.H. Lingner J. Harley C.B. Cech T.R. Science. 1997; 277: 955-959Crossref PubMed Scopus (2075) Google Scholar, 9Meyerson M. Counter C.M. Eaton E.N. Ellisen L.W. Steiner P. Caddle S.D. Ziaugra L. Beijersbergen R.L. Davidoff M.J. Liu Q. Bacchetti S. Haber D.A. Weinberg R.A. Cell. 1997; 90: 785-795Abstract Full Text Full Text PDF PubMed Scopus (1666) Google Scholar, 19Harrington L. McPhail T. Mar V. Zhou W. Oulton R. Bass M.B. Arruda I. Robinson M.O. Science. 1997; 275: 973-977Crossref PubMed Scopus (631) Google Scholar, 20Nakayama J. Saito M. Nakamura H. Matsuura A. Ishikawa F. Cell. 1997; 88: 875-884Abstract Full Text Full Text PDF PubMed Scopus (373) Google Scholar). In Tetrahymena, two telomerase protein subunits have been purified with the p80 protein binding to the RNA component and p95 binding to single-stranded telomeric DNA (17Collins K. Kobayashi R. Greider C.W. Cell. 1995; 81: 677-686Abstract Full Text PDF PubMed Scopus (243) Google Scholar). Based on amino acid similarity toTetrahymena p80, mammalian telomerase-associated protein 1 (TEP1) 1The abbreviations used are: TEP1, telomerase-associated protein 1; hTEP1, human telomerase-associated protein 1; hTERT, human telomerase reverse transcriptase; PP2A, protein phosphatase 2A; PKC, protein kinase C; PMA, phorbol 12-myristate 13-acetate; PAGE, polyacrylamide gel electrophoresis. has been identified, which is an ∼300-kDa polypeptide containing theTetrahymena telomerase homology domain in the N-terminal region, a nucleotide-binding domain in the middle region, and multiple WD-40 repeats in the C-terminal portion (19Harrington L. McPhail T. Mar V. Zhou W. Oulton R. Bass M.B. Arruda I. Robinson M.O. Science. 1997; 275: 973-977Crossref PubMed Scopus (631) Google Scholar, 20Nakayama J. Saito M. Nakamura H. Matsuura A. Ishikawa F. Cell. 1997; 88: 875-884Abstract Full Text Full Text PDF PubMed Scopus (373) Google Scholar). Binding to telomerase RNA (19Harrington L. McPhail T. Mar V. Zhou W. Oulton R. Bass M.B. Arruda I. Robinson M.O. Science. 1997; 275: 973-977Crossref PubMed Scopus (631) Google Scholar), TEP1 probably serves not only as a structural protein, but also as a regulatory subunit in mediating telomerase interaction with other molecules. The catalytic subunit of telomerase was initially purified from the ciliate E. aediculatus, called p123, which is homologous to the Est2p protein essential for telomere maintenance and telomerase activity in Saccharomyces cerevisiae (18Lingner J. Hughes T.R. Shevchenko A. Mann M. Lundblad V. Cech T.R. Science. 1997; 276: 561-567Crossref PubMed Scopus (1044) Google Scholar, 21Lingner J. Cech T.R. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 10712-10717Crossref PubMed Scopus (229) Google Scholar,22Counter C.M. Meyerson M. Eaton E.N. Weinberg R.A. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 9202-9207Crossref PubMed Scopus (206) Google Scholar). The human telomerase catalytic subunit or telomerase reverse transcriptase (hTERT) has recently been identified to be an ∼130-kDa protein that is concomitantly expressed with the activation of telomerase during cellular immortalization and tumor progression (8Nakamura T.M. Morin G.B. Chapman K.B. Weinrich S.L. Andrews W.H. Lingner J. Harley C.B. Cech T.R. Science. 1997; 277: 955-959Crossref PubMed Scopus (2075) Google Scholar, 9Meyerson M. Counter C.M. Eaton E.N. Ellisen L.W. Steiner P. Caddle S.D. Ziaugra L. Beijersbergen R.L. Davidoff M.J. Liu Q. Bacchetti S. Haber D.A. Weinberg R.A. Cell. 1997; 90: 785-795Abstract Full Text Full Text PDF PubMed Scopus (1666) Google Scholar,23Kilian A. Bowtell D.D. Abud H.E. Hime G.R. Venter D.J. Keese P.K. Duncan E.L. Reddel R.R. Jefferson R.A. Hum. Mol. Genet. 1997; 6: 2011-2019Crossref PubMed Scopus (530) Google Scholar, 24Harrington L. Zhou W. McPhail T. Oulton R. Yeung D.S. Mar V. Bass M.B. Robinson M.O. Genes Dev. 1997; 11: 3109-3115Crossref PubMed Scopus (410) Google Scholar). Although de novo expression of hTERT is essential for telomerase activity (8Nakamura T.M. Morin G.B. Chapman K.B. Weinrich S.L. Andrews W.H. Lingner J. Harley C.B. Cech T.R. Science. 1997; 277: 955-959Crossref PubMed Scopus (2075) Google Scholar, 9Meyerson M. Counter C.M. Eaton E.N. Ellisen L.W. Steiner P. Caddle S.D. Ziaugra L. Beijersbergen R.L. Davidoff M.J. Liu Q. Bacchetti S. Haber D.A. Weinberg R.A. Cell. 1997; 90: 785-795Abstract Full Text Full Text PDF PubMed Scopus (1666) Google Scholar), little is known about how telomerase is activated and maintained at the enzyme levels in human cancers. One possible mechanism is that the tertiary and quaternary structures of the large telomerase complex are modulated by protein phosphorylation in such a way that the enzyme is activated. This hypothesis is based on our previous finding that protein phosphatase 2A (PP2A) specifically inhibits telomerase activity in the nuclear lysates of human breast cancer cells (25Li H. Zhao L.L. Funder J.W. Liu J.-P. J. Biol. Chem. 1997; 272: 16729-16732Abstract Full Text Full Text PDF PubMed Scopus (192) Google Scholar). To establish if telomerase is directly regulated by protein phosphorylation and which protein kinase(s) are involved in the process, we have made use of affinity chromatography with a synthetic peptide corresponding to the N-terminal sequence of hTEP1 and produced polyclonal antibodies against hTEP1 and hTERT to isolate telomerase and its potentially responsible protein kinase(s) involved in the regulation of telomerase activity. Our data show that protein kinase Cα (PKCα) is selectively eluted from the hTEP1 peptide affinity column and mediates phosphorylation of hTEP1 and hTERT in intact human breast cancer cells. Phosphorylation of partially purified telomerase results in a marked increase in telomerase activity in vitro. Thus, activation of telomerase and subsequent maintenance of its activity may require a prerequisite phosphorylation of telomerase proteins by PKCα in human breast cancer cells. PKC isoforms, the catalytic subunit of protein kinase A, PP2A, and phorbol 12-myristate 13-acetate (PMA) were from BIOMOL Research Laboratories Inc. (Plymouth Meeting, PA). Alkaline phosphatase was from New England Biolabs Inc. (Beverly, MA). DNA-dependent protein kinase was from Promega (Madison, WI). Cdc2 protein kinase was from Upstate Biotechnology, Inc. (Lake Placid, New York). ATP, Taq DNA polymerase, dNTP, and T4 gene 32 protein were from Boehringer Mannheim Australia Pty. Ltd. (New South Wales, Australia). 32P, [α-32P]ATP, and [γ-32P]ATP were from Amersham Australia Pty. Ltd. (New South Wales). Gel electrophoresis reagents were from Bio-Rad. All other chemicals were from Sigma Chemical. The telomerase peptides were synthesized by Chiron Technologies Pty. Ltd. (Victoria, Australia). Several peptides were made to produce telomerase peptide affinity column and hTEP1-specific antibodies, and the peptides WGVTEEETRRNRQLEVC and DSEPTPHLKTRQRR were finally selected for making an affinity purification column used in this study and producing anti-hTEP1 antibodies, respectively. For anti-hTERT antibody, the peptide CAREKPQGSVAAPEEEDTD was used. Anti-hTEP1 and anti-hTERT antibodies were raised in rabbits by multiple immunization with the synthetic peptides. The antibodies were then against the peptides as R.A. H. Liu T.M. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). antibodies and kinase were from Biotechnology, Inc. kinase and protein kinase were from Laboratories was from Upstate Biotechnology, Inc. The proteins were with Australia Pty. The hTEP1 peptide affinity column was for purification of telomerase regulatory protein kinase and phosphatase by a Biochem. 1989; PubMed Scopus Google Scholar). In the peptide affinity chromatography, of proteins of the nuclear telomerase from human breast cancer cells were the column of at a of the column was eluted with and then a The were to by antibodies and the telomerase activity N.W. Piatyszek M.A. Prowse K.R. Harley C.B. West M.D. Ho P.L. Coviello G.M. Wright W.E. Weinrich S.L. Shay J.W. Science. 1994; 266: 2011-2015Crossref PubMed Scopus (6584) Google Scholar). of proteins in the isolated telomerase were to in phosphorylation containing containing of and and the was to at for by the addition of J.-P. Robinson Science. 1994; PubMed Scopus Google Scholar). For protein the proteins were to by of the in the of an of protein phosphatase as in a at for to phosphorylation (25Li H. Zhao L.L. Funder J.W. Liu J.-P. J. Biol. Chem. 1997; 272: 16729-16732Abstract Full Text Full Text PDF PubMed Scopus (192) Google Scholar, J.-P. Robinson Science. 1994; PubMed Scopus Google Scholar). the was by of and at the were then to protein phosphorylation with the of the to phosphorylation by (25Li H. Zhao L.L. Funder J.W. Liu J.-P. J. Biol. Chem. 1997; 272: 16729-16732Abstract Full Text Full Text PDF PubMed Scopus (192) Google J.-P. Robinson Science. 1994; PubMed Scopus Google Scholar). For protein and purified PKCα were and for PKC was in the or of the PKC and The proteins were by by For telomerase protein phosphorylation in intact human breast cancer cells, the cells were with or the PKC for and during the was in the to the and thus was that not in the levels of in cells. the cells were and the lysates were to The were to by or to with or purified PKC from J.-P. Robinson Science. 1994; PubMed Scopus Google Scholar). To phosphorylation and of telomerase proteins in intact human breast cancer cells, hTERT and hTEP1 were from cell lysates or partially purified telomerase antibodies against hTERT and hTEP1 peptides and a as a as J.-P. Robinson Science. 1994; PubMed Scopus Google Scholar). To of telomerase activity was in the by the and in a telomerase with the or not with The of telomerase activity was with the as N.W. Piatyszek M.A. Prowse K.R. Harley C.B. West M.D. Ho P.L. Coviello G.M. Wright W.E. Weinrich S.L. Shay J.W. Science. 1994; 266: 2011-2015Crossref PubMed Scopus (6584) Google Scholar). For telomerase activity in the the telomerase was by the at for with in a (25Li H. Zhao L.L. Funder J.W. Liu J.-P. J. Biol. Chem. 1997; 272: 16729-16732Abstract Full Text Full Text PDF PubMed Scopus (192) Google Scholar), which was then by to the or to the de telomeres for polymerase To the of protein and on telomerase partially purified telomerase was with or and of the with the were to the telomerase activity or to of protein phosphorylation at for by the telomerase activity (25Li H. Zhao L.L. Funder J.W. Liu J.-P. J. Biol. Chem. 1997; 272: 16729-16732Abstract Full Text Full Text PDF PubMed Scopus (192) Google Scholar). The protein the DNA-dependent protein Cdc2 protein and the catalytic subunit of protein kinase telomerase activity is with a variety of human N.W. Piatyszek M.A. Prowse K.R. Harley C.B. West M.D. Ho P.L. Coviello G.M. Wright W.E. Weinrich S.L. Shay J.W. Science. 1994; 266: 2011-2015Crossref PubMed Scopus (6584) Google Scholar). Our previous study that the activity of telomerase be at the enzyme by protein in the of human breast cancer cells (25Li H. Zhao L.L. Funder J.W. Liu J.-P. J. Biol. Chem. 1997; 272: 16729-16732Abstract Full Text Full Text PDF PubMed Scopus (192) Google Scholar). To the mechanisms of regulation of telomerase activity by protein telomerase was partially purified by hTEP1 peptide affinity chromatography, and protein kinase(s) potentially with the hTEP1 peptide were by in A, although telomerase the peptide affinity the column a of telomerase activity be eluted from the column in with the corresponding to the of In a of telomerase activity was eluted in from a column with a peptide not that a functional telomerase complex is selectively in the hTEP1 peptide affinity chromatography probably complex multiple the telomerase complex and the hTEP1 This is by the that a of hTEP1 was with telomerase activity from the column not and that hTEP1 multiple not To protein kinase(s) involved in regulating column were with antibodies against different protein although for the of Cdc2 kinase protein or was not levels of PKCα were by in from the 1 a for PKCα was eluted from the column in which were with a peptide 1 data that PKCα with the hTEP1 peptide with a affinity and a possible of the kinase in the structure and function of telomerase. the structure of hTEP1 is to that of a for activated PKC D. H. J. Proc. Natl. Acad. Sci. U. S. A. 88: PubMed Scopus Google Scholar), in a WD-40 is to that the WD-40 repeats in hTEP1 are also involved in mediating PKC To telomerase proteins are by PKC, hTEP1 was from human breast cancer cells with levels of telomerase activity were in the and the activity was by with previous that hTEP1 with telomerase activity (19Harrington L. McPhail T. Mar V. Zhou W. Oulton R. Bass M.B. Arruda I. Robinson M.O. Science. 1997; 275: 973-977Crossref PubMed Scopus (631) Google J. Saito M. Nakamura H. Matsuura A. Ishikawa F. Cell. 1997; 88: 875-884Abstract Full Text Full Text PDF PubMed Scopus (373) Google Scholar) and that phosphorylation of telomerase protein is for enzyme activity. hTEP1 a protein with a of not of hTEP1, however, that hTEP1 is a in intact cells of cells with the PKC the phosphorylation of hTEP1 with PKC mediating the phosphorylation of hTEP1 in human breast cancer cells. To if hTEP1 is a of PKC and in the were then to and proteins in the of purified were in the of purified PKCα of PKCα or of the PKC to not data that hTEP1 is a directly controlled by PKC in in intact human breast cancer cells. the by PKCα was not in is possible that other protein kinase(s) are potentially involved as mediates telomerase protein phosphorylation in in intact cells. A, of telomerase activity with anti-hTEP1 from human breast cancer cells and role of protein cell lysates of cells were to with or anti-hTEP1 The and were then for telomerase activity. were to with or at for by of the with the telomerase activity hTEP1 is by PKCα and by breast cancer cells were with the PKC or for and the was with by of the in a the cells were for with or anti-hTEP1 The were then to with and or at for by of the with the was the in was to with PKCα in phosphorylation at for which was then by and the in All were then to by and and of hTERT and of hTERT from different of nuclear The of proteins in each are of hTERT from with the against hTERT and or hTEP1 of hTERT and phosphorylation of hTERT in intact cells. is an phosphorylation of proteins by with 1 or the against hTERT and were with for in the or of the protein kinase and the cell lysates were then to The were with and or purified for at to by and of hTERT phosphorylation in cells and in cells with with or in vitro. are the S.D. of in with with a and was also by PKCα and by in the hTEP1 telomerase activity with hTEP1 from cells and the protein was of the is possible that the is the catalytic subunit of telomerase (hTERT) with To if hTEP1 and hTERT nuclear lysates were to with anti-hTERT or anti-hTEP1 in hTERT was detectable by in the nuclear in hTERT activity be with to hTERT or hTEP1 from cells, that the telomerase catalytic subunit with hTEP1 in the cells. To if hTERT is in intact cells, hTERT was from cells. with the hTERT the with levels of which were by the PKC and that PKC also mediates hTERT phosphorylation in human breast cancer cells. of hTERT with in of hTERT phosphorylation and and that hTERT may also be controlled by PKCα and in intact cells. The in the of hTERT by be to the binding of anti-hTERT or an of other The of the phosphoproteins at the of the gel and with a of are although the phosphoproteins at the of the gel be data also that the phosphorylation and of hTERT and hTEP1 may the function of the telomerase complex in human breast cancer cells. To for phosphorylation of telomerase the activity of partially purified telomerase was with different protein PKC isoforms, DNA-dependent protein Cdc2 and the catalytic subunit of protein kinase A. Although DNA-dependent protein Cdc2 and the catalytic subunit of protein kinase not of the partially purified telomerase activity with PKCα in marked in telomerase activity. increase in telomerase activity was at a PKCα as as A, and the were in a with at a of This was not for PKC A, or for PKCα in the of its not that the phosphorylation activity of PKCα is for the activation of telomerase. In addition to the activity of partially purified PKCα also the of telomerase activity by the of and partially purified a also with the phosphatase The telomerase by however, be by PKCα a of with a at Thus, the phosphorylation and of hTERT and hTEP1 as were by the increase and in telomerase activity by PKCα and PP2A, respectively. The finding that telomerase be by PKCα and by that the telomerase ribonucleoprotein complex in two structural and that are with or telomerase and forms of telomerase that be to and by PKCα and in telomerase activity are by expression of the telomerase catalytic subunit hTERT (8Nakamura T.M. Morin G.B. Chapman K.B. Weinrich S.L. Andrews W.H. Lingner J. Harley C.B. Cech T.R. Science. 1997; 277: 955-959Crossref PubMed Scopus (2075) Google Scholar, 9Meyerson M. Counter C.M. Eaton E.N. Ellisen L.W. Steiner P. Caddle S.D. Ziaugra L. Beijersbergen R.L. Davidoff M.J. Liu Q. Bacchetti S. Haber D.A. Weinberg R.A. Cell. 1997; 90: 785-795Abstract Full Text Full Text PDF PubMed Scopus (1666) Google Scholar), in addition to the phosphorylation as is thus during the of telomerase activation in human breast cancer cells, the of a enzyme complex with produced hTERT and other telomerase phosphorylation of hTERT and Although the phosphorylation on hTERT and hTEP1 have to be the PKCα and telomerase proteins to be by binding of the kinase to the telomerase complex other the and of as by other PKC for their on telomerase activity. the and and the telomerase activity This although PKC the to telomerase only PKCα is with partially purified telomerase in the of human breast cancer cells, for a regulatory role for PKCα in telomerase activity in cells. with the for PKC at an of telomerase activity a complex interaction telomerase and and domain and contains a domain F. D. S. Mol. Cell. 1989; 65: PubMed Scopus Google Scholar, J.-P. Mol. Cell. 1996; PubMed Scopus Google Scholar), data also that intact and are not essential for regulating telomerase activity. Thus, PKC and the telomerase proteins to other the and of previous have that PKC telomerase activity during cell activation A.G. Kim N.W. Chiu C.-P. 1996; PubMed Scopus Google Scholar) and in cancer cells Wang Biochem. 1997; PubMed Scopus Google Scholar) and that the acid telomerase activity in human breast cancer cells (25Li H. Zhao L.L. Funder J.W. Liu J.-P. J. Biol. Chem. 1997; 272: 16729-16732Abstract Full Text Full Text PDF PubMed Scopus (192) Google Scholar), is possible that telomerase activity is maintained by a of the PKCα and phosphorylation and of the telomerase proteins hTERT and data establish a telomerase is activated in human cancer cells and PKCα as a pivotal role in telomerase activity in human breast cancer cells. that expression of PKCα is in the of human breast cancer cells, with a increase in PKCα expression and a increase in kinase activity J.W. 1992; Google Scholar, C.W. D. S. R.L. J. Biol. Chem. Full Text PDF PubMed Google Scholar, Rev. 1994; PubMed Scopus Google Scholar), the PKCα and telomerase a possible mechanism PKCα may be involved in tumor Nature. Scopus Google Scholar). In the regulation of telomerase by PKCα and is also with the hypothesis that a activation of PKC Rev. 1994; PubMed Scopus Google Scholar, Nature. Scopus Google Scholar) and of Rev. PubMed Scopus Google Scholar) an in a for of telomerase in intact cancer cells and a for telomerase activity in S. for protein kinase and R. M. A. F. and J. for