High susceptibility of a human breast epithelial cell type with stem cell characteristics to telomerase activation and immortalization

We have recently characterized two types of normal human breast epithelial cells (HBECs) from reduction mammoplasty, Type I cells express estrogen rec eptor, luminal epithelial cell markers, and stem cell characteristics (i.e. , the ability to differentiate into other cell types and to form. budding/d uctal structures on Matrigel), whereas Type II cells show basal epithelial cell phenotypes, In this study, rye have examined whether Type I HBECs are more susceptible to telomerase activation and immortalization after transfe ction with SV40 large T-antigen. The results show that both types of cells acquire extended life span [(EL); i.e., bypassing senescence] at a comparab le frequency. However, they differ significantly in the ability to become i mmortal in continuous culture, ie., 11 of 11 Type I EL clones became immort al compared with I of 10 Type II EL clones, Both parental Type I and Type I I cells as well as their transformed EL clones at early passages [similar t o 30 cumulative population doubling level (cpdl)] showed a low level of tel omerase activity as measured by the telomeric repeat amplification protocol assay. For all II of the Type I EL clones and the single Type II EL clone that became immortal, telomerase activities were invariably activated at mi ddle passages (similar to 60 cpdl) or late passages (similar to 100 cpdl), For the four Type II EL, clones randomly selected from the nine Type II clo nes that did not become immortal, the telomerase activities were found to b e further diminished at mid-passage, before the end of the life span, Thus, normal HBECs do have a low level of telomerase activity, and Type I HBECs with stem cell characteristics are more susceptible to telomerase activatio n and immortalization, a basis on which they may he major target cells for breast carcinogenesis.