title: Cell density related gene expression: SV40 large T antigen levels in immortalized astrocyte lines.
abstract: BACKGROUND
  Gene expression is affected by population density. Cell density is a potent negative regulator of cell cycle time during exponential growth. Here, we asked whether SV40 large T antigen (Tag) levels, driven by two different promoters, changed in a predictable and regular manner during exponential growth in clonal astrocyte cell lines, immortalized and dependent on Tag.
  
  
  RESULTS
  Expression and cell cycle phase fractions were measured and correlated using flow cytometry. T antigen levels did not change or increased during exponential growth as a function of the G1 fraction and increasing cell density when Tag was transcribed from the Moloney Murine Leukemia virus (MoMuLV) long terminal repeat (LTR). When an Rb-binding mutant T antigen transcribed from the LTR was tested, levels decreased. When transcribed from the herpes thymidine kinase promoter, Tag levels decreased. The directions of change and the rates of change in Tag expression were unrelated to the average T antigen levels (i.e., the expression potential).
  
  
  CONCLUSIONS
  These data show that Tag expression potential in these lines varies depending on the vector and clonal variation, but that the observed level depends on cell density and cell cycle transit time. The hypothetical terms, expression at zero cell density and expression at minimum G1 phase fraction, were introduced to simplify measures of expression potential.
authors.affiliation: Comprehensive Cancer Center, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 4106-4944, USA. jwj@po.cwru.edu
authors.forename: Phyllis S
authors.initials: PS
authors.lastname: Frisa
authors.forename: James W
authors.initials: JW
authors.lastname: Jacobberger
mesh_terms: D000818:Animals; D000952:Antigens, Polyomavirus Transforming / Q000235:genetics* / Q000378:metabolism; D001253:Astrocytes / Q000166:cytology / Q000378:metabolism*; D015153:Blotting, Western; D002452:Cell Count; D002460:Cell Line; D002472:Cell Transformation, Viral; D005434:Flow Cytometry; D016193:G1 Phase; D015870:Gene Expression; D051379:Mice
chemical_list: D000952:Antigens, Polyomavirus Transforming