GRADUAL PHENOTYPIC CONVERSION ASSOCIATED WITH IMMORTALIZATION OF CULTURED HUMAN MAMMARY EPITHELIAL-CELLS

Examination of the process of immortal transformation in early passage s of two human mammary epithelial cell (HMEC) lines suggests the invol vement of an epigenetic step. These lines, 184A1 and 184B5, arose afte r in vitro exposure of finite lifespan 184 HMEC to a chemical carcinog en, and both are clonally derived. Although early-passage mass culture s of 184A1 and 184B5 maintained continuous slow growth, most individua l cells lost proliferative ability. Uniform good growth did not occur until 20-30 passages after the lines first appeared. Early-passage cul tures expressed little or no telomerase activity and telomeres continu ed to shorten with increasing passage. Telomerase activity was first d etected when the telomeres became critically short, and activity level s gradually increased thereafter. Early-passage cultures had little or no ability to maintain growth in transforming growth factor-beta (TGF beta); however, both mass cultures and clonal isolates showed a very gradual increase in the number of cells displaying progressively incre ased ability to maintain growth in TGF beta. A strong correlation betw een capacity to maintain growth in the presence of TGF beta and expres sion of telomerase activity was observed. We have used the term ''conv ersion'' to describe this process of gradual acquisition of increased growth capacity in the absence or presence of TGF beta and reactivatio n of telomerase. We speculate that the development of extremely short telomeres may result in gradual, epigenetic-based changes in gene expr ession. Understanding the underlying mechanisms of HMEC conversion in vitro may provide new insight into the process of carcinogenic progres sion in vivo and offer novel modes for therapeutic intervention.