Expression level is a key determinant of E2F1-mediated cell fate

Purpose: dose response analysis of E2F1 target genes expression in flow-sorted fractions with increasing amounts of fluorescently labled E2F1 Methods:U2OS pTRIPZ-YFP-ER-E2F1 cells were grown in full serum-containing growth medium and treated with 500 ng/ml doxycycline for 48 hours followed by addition of 90 nM OHT for an additional 20 hours. Cells from different YFP fractions were sorted by flow cytometry. mRNA profiles were generated by deep sequencing using Illumina HiSeq 4000. Results: different target genes have different E2F1 activation thresholds. Numerous proliferation-related target genes are induced already by the lowest E2F1-levels. Intermediate E2F1 levels induce cdk inhibitors, which might be responsible for cell cycle arrest. Finally, although some apoptotic E2F1 targets are induced already by low E2F1 levels, many key apoptotic genes require higher E2F1 levels for induction. Conclusions: induction of different cell fates by increasing E2F1 levels might pertain to differential affinities of the targets. Overall design: Methods:U2OS pTRIPZ-YFP-ER-E2F1 cells were grown in full serum-containing growth medium and treated with 500 ng/ml doxycycline for 48 hours followed by addition of 90 nM OHT for an additional 20 hours. Cells from different YFP fractions were sorted by flow cytometry. mRNA profiles were generated by deep sequencing using Illumina HiSeq 4000.