Spontaneous senescence in the MDA-MB-231 cell line.

Normal human somatic cells have a limited division potential when they grow in vitro. It is believed that shortening of telomeres, specialized structures at the chromosome ends, controls cell growth. When one telomere achieves critical minimal length, the cell cycle control mechanism recognizes it as a DNA damage and cause exit from cell cycle in G1-phase. Due to the fact that it is not possible to extend the telomeres in normal cells, this non-dividing state is prolonged indefinitely, and it is known as cellular senescence. Immortal cell line MDA-MB-231 has an active telomerase, which stops shortening of their telomeres and allows them permanent divisions. However, there is a fraction of cells that do not divide during several days in culture as it was documented for some other tumor cell lines. The combination of few methods made it possible to isolate these non-growing cells and compare them with the fraction of fast growing cells from the same culture. Although non-growing fraction contains a significant percentage of typical senescent cells, both fractions have an equal telomerase activity and telomere length. In this paper we discuss possible mechanisms that cause appearance of observed non-growing fraction of cells in immortal MDA-MB-231 cell line which point to stress and genome instability rather than variation in telomerase activity or telomere shortening among individual cells.