Human heterochromatin protein 1 isoforms HP1(Hsalpha) and HP1(Hsbeta) interfere with hTERT-telomere interactions and correlate with changes in cell growth and response to ionizing radiation.
Telomeres are associated with the nuclear matrix and are thought to be heterochromatic. We show here that in human cells the overexpression of green fluorescent protein-tagged heterochromatin protein 1 (GFP-HP1) or nontagged HP1 isoforms HP1(Hsalpha) or HP1(Hsbeta), but not HP1(Hsgamma), results in decreased association of a catalytic unit of telomerase (hTERT) with telomeres. However, reduction of the G overhangs and overall telomere sizes was found in cells overexpressing any of these three proteins. Cells overexpressing HP1(Hsalpha) or HP1(Hsbeta) also display a higher frequency of chromosome end-to-end associations and spontaneous chromosomal damage than the parental cells. None of these effects were observed in cells expressing mutants of GFP-DeltaHP1(Hsalpha), GFP-DeltaHP1(Hsbeta), or GFP-DeltaHP1(Hsgamma) that had their chromodomains deleted. An increase in the cell population doubling time and higher sensitivity to cell killing by ionizing radiation (IR) treatment was also observed for cells overexpressing HP1(Hsalpha) or HP1(Hsbeta). In contrast, cells expressing mutant GFP-DeltaHP1(Hsalpha) or GFP-DeltaHP1(Hsbeta) showed a decrease in population doubling time and decreased sensitivity to IR compared to the parental cells. The effects on cell doubling times were paralleled by effects on tumorigenicity in mice: overexpression of HP1(Hsalpha) or HP1(Hsbeta) suppressed tumorigenicity, whereas expression of mutant HP1(Hsalpha) or HP1(Hsbeta) did not. Collectively, the results show that human cells are exquisitely sensitive to the amount of HP1(Hsalpha) or HP1(Hsbeta) present, as their overexpression influences telomere stability, population doubling time, radioresistance, and tumorigenicity in a mouse xenograft model. In addition, the isoform-specific effects on telomeres reinforce the notion that telomeres are in a heterochromatinized state.
Radiation and Cancer Biology Division, Washington University School of Medicine, 4511 Forest Park, St Louis, MO 63108, USA
Girdhar G
GG
Sharma
Kyu-kye
KK
Hwang
Raj K
RK
Pandita
Arun
A
Gupta
Sonu
S
Dhar
Julie
J
Parenteau
Manjula
M
Agarwal
Howard J
HJ
Worman
Raymund J
RJ
Wellinger
Tej K
TK
Pandita
D000818:Animals; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D002455:Cell Division; D002460:Cell Line; D002470:Cell Survival / Q000528:radiation effects; D002471:Cell Transformation, Neoplastic; D000090266:Chromobox Protein Homolog 5; D002868:Chromosomal Proteins, Non-Histone / Q000235:genetics* / Q000378:metabolism*; D004260:DNA Repair; D004268:DNA-Binding Proteins; D049452:Green Fluorescent Proteins; D006570:Heterochromatin / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008164:Luminescent Proteins / Q000235:genetics / Q000378:metabolism; D051379:Mice; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D011836:Radiation Tolerance; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D019098:Telomerase / Q000235:genetics / Q000378:metabolism*; D016615:Telomere / Q000378:metabolism*; D014183:Transplantation, Heterologous
D002352:Carrier Proteins; D002868:Chromosomal Proteins, Non-Histone; D004268:DNA-Binding Proteins; D006570:Heterochromatin; D008164:Luminescent Proteins; D020033:Protein Isoforms; D011993:Recombinant Fusion Proteins; D000090266:Chromobox Protein Homolog 5; D049452:Green Fluorescent Proteins; D019098:Telomerase