Human Heterochromatin Protein 1 Isoforms HP1 super(Hs alpha ) and HP1 super(Hs beta ) 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 super(Hs alpha ) or HP1 super(Hs beta ), but not HP1 super(Hs gamma ), 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 super(Hs alpha ) or HP1 super(Hs beta ) 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- [Delta] HP1 super(Hs alpha ), GFP-[Delta] HP1 super(Hs beta ), or GFP-[Delta] HP1 super(Hs gamma ) 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 super(Hs alpha ) or HP1 super(Hs beta ). In contrast, cells expressing mutant GFP- [Delta] HP1 super(Hs alpha ) or GFP-[Delta] HP1 super(Hs beta ) 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 super(Hs alpha ) or HP1 super(Hs beta ) suppressed tumorigenicity, whereas expression of mutant HP1 super(Hs alpha ) or HP1 super(Hs beta ) did not. Collectively, the results show that human cells are exquisitely sensitive to the amount of HP1 super(Hs alpha ) or HP1 super(Hs beta ) 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.