Phosphorylated ribosomal S6 (p-rpS6) as a post-treatment indicator of HER2 signalling targeted drug resistance

<i>Objective</i>: To identify clinically relevant predictive biomarkers of trastuzumab resistance.<i>Material and methods</i>: MTT, FACS assays, immunoblotting and immunocytochemistry were used to phenotypically characterize drug responses of two cell models BT474R and SKBR3R. Student's <i>t</i>-test and Spearman's correlation were applied for statistic analysis.<i>Results</i>: The activity of a downstream effector of the HER2 pathway phosphorylated ribosomal protein S6 (p-rpS6), was suppressed by trastuzumab in the parental cell lines yet remained unchanged in the resistant cells following treatment. The level of p-rpS6 was inversely correlated to the drug induced growth inhibition of trastuzumab-resistant cells when they are treated with selected HER2 targeting drugs.<i>Conclusion</i>: p-rpS6 is a robust post-treatment indicator of HER2 pathway-targeted therapy resistance.