Viral oncogenes accelerate conversion to immortality of cultured conditionally immortal human mammary epithelial cells

Our recent studies on the process of immortalization of cultured human mamm ary epithelial cells (HMEC) have uncovered a previously undescribed, appare ntly epigenetic step, termed conversion. When first isolated, clonally deri ved HMEC lines of indefinite lifespan showed little or no telomerase activi ty or ability to maintain growth in the presence of TGF beta. Cell populati ons whose mean terminal restriction fragment length had declined to <3 kb a lso exhibited slow heterogeneous growth, and contained many non-proliferati ve cells. With continued passage, these conditionally immortal cell populat ions very gradually converted to a fully immortal phenotype of good growth/-TGF beta, expression of high levels of telomerase activity, and stabiliza tion of telomere length. We now show that exposure of the early passage con ditionally immortal 184A1 HMEC line to the viral oncogenes human papillomav irus type 16 (HPV16)-E6, -E7, or SV40T, results in either immediate (E6) or rapid (E7; SV40T) conversion of these telomerase negative, TGF beta sensit ive conditionally immortal cells to the fully immortal phenotype. Unlike co nditional immortal 184A1, the HPV16-E7 and SV40T exposed cells were able to maintain growth in TGF beta prior to expression of high levels of telomera se activity. A mutated HPV16-E6 oncogene, unable to inactivate p53, was sti ll capable of rapidly converting conditional immortal 184A1. Our studies pr ovide further evidence that the transforming potential of these viral oncog enes may involve activities beyond their inactivation of p53 and pRB functi ons. These additional activities may greatly accelerate a step in HMEC immo rtal transformation, conversion, that would be rate-limiting in the absence of viral oncogene exposure.