Figure 6. Combinatorial effects of lapatinib and palbociclib on cell survival and E2F1-driven DNA synthesis.
A) BT-474 cells treated with SERMs in the presence of vehicle or palbociclib. Palbociclib reduced the mitogenic effects of estrogen and sensitized the cells to tamoxifen.
B) Cells were treated with palbociclib and the SRC-3 inhibitor bufalin. Palbociclib and bufalin have additive effects. Asterisks denote significance tested using 2-way ANOVA with Bonferroni posttest, *<0.05, **<0.01, ***<0.001.
C) Palbociclib cooperatively sensitizes cells to lapatinib treatment.
D) Bufalin cooperatively sensitizes cells to lapatinib treatment.
E) E2F1 and cell cycle transcripts are reduced upon treatment with palbociclib.
F) Lapatinib reduces E2F1 and cell cycle transcripts analogous to HER2 knockdown.
G) Schematic representation of HER2/E2F1 signaling axis affects on cell cycle, nucleotide metabolism, and DNA replication gene transcription. DNA replication fork genes significantly downregulated upon HER2 knockdown are depicted.
H) Tritiated thymidine (3H) incorporation (DNA replication) in BT-474 and SKBR-3 cells after 24 hour treatment with lapatinib, palbociclib, or bufalin. Note the sharp decrease in DNA synthesis in both cell lines despite any significant effect on cell size, shape, or viability at this time point.
I) Cell cycle proteins E2F1, CCND1, and CDC6 are affected by 24h lapatinib or palbociclib treatment. Phosphorylation of RB and HER2 serve as markers of drug efficacy.
J) Ectopic expression of E2F1 rescues MCM10, CDC6, and CCND1 protein expression and DNA synthesis in the presence of lapatinib in HER2+ breast cancer cells.