Fig. 5: Shift to Luminal A sensitizes cells to anti-CDK4/6 treatments.

a Dose–response curve of BT474, SKBR3, HCC1954, BT474-derived lapatinib and trastuzumab resistant (BT474-L^RT^R) and BT474-derived tucatinib and trastuzumab resistant (BT474-Tu^RT^R) cells upon treatment with anti-HER2 drugs for 24 h and subsequent treatment with anti-HER2 plus increasing concentrations of palbociclib for 72 h. Surviving fraction was assessed by staining with Hoechst 33342. Data points represent the mean; error bars represent the standard error of the mean of 3 independent experiments. b Schematic representation of HER2-E breast cancer cells that are sensitive to dual HER2 blockade treatment, which triggers cell growth inhibition but also a shift to a Luminal A molecular phenotype in residual cells. These changes are reversible after treatment discontinuation, while inhibition of HER2 improves sensitivity to CDK4/6 targeted therapies. BT474 resistant to anti-HER2 treatments remain HER2-E and do not respond to inhibition of HER2 and CDK4/6. c Western Blot assessing the phosphorylation of RB and the expression of Cyclin D1 in BT474 cells upon 24 h of treatment with 10 nM TKI (lapatinib, neratinib, tucatinib), with 10 µg ml⁻¹ trastuzumab, with the combination of TKI plus trastuzumab, 10 nM palbociclib or with the combination of palbociclib with anti-HER2 treatments, and in BT474-L^RT^R and BT474-Tu^RT^R treated with 2 µg ml⁻¹ trastuzumab + 2 nM TKI (lapatinib and tucatinib respectively) with or without palbociclib. (d) CCND1 mRNA levels in HER2+/HR+ tumors of the PAMELA trial at baseline and day 14. Each line represents a paired sample. Increases are represented in red and decreases in green. P-value (p) was determined by two-tailed paired t-tests. (e) Schematics of the mechanism by which double-targeting the HER2 and CDK4/6 pathways can prevent RB phosphorylation and arrest cell cycle. Source data are provided as a Source Data file.