ROR1 regulates chemoresistance in Breast Cancer via modulation of drug efflux pump ABCB1.

Chemoresistance is one of the leading causes of mortality in breast cancer (BC). Understanding the molecules regulating chemoresistance is critical in order to combat chemoresistant BC. Drug efflux pump ABCB1 is overexpressed in chemoresistant neoplasms where it effluxes various chemotherapeutic agents from cells. Because it is expressed in normal and cancerous cells alike, attempts at targeting ABCB1 directly have failed due to low specificity and disruption of normal tissue. A proposed method to inhibit ABCB1 is to target its cancer-specific, upstream regulators, mitigating damage to normal tissue. Few such cancer-specific upstream regulators have been described. Here we characterize ROR1 as an upstream regulator of ABCB1. ROR1 is highly expressed during development but not expressed in normal adult tissue. It is however highly expressed in several cancers. ROR1 is overexpressed in chemoresistant BC where it correlates with poor therapy response and tumor recurrence. Our data suggests, ROR1 inhibition sensitizes BC cells to chemo drugs. We also show ROR1 regulates ABCB1 stability and transcription via MAPK/ERK and p53. Validating our overall findings, inhibition of ROR1 directly correlated with decreased efflux of chemo-drugs from cells. Overall, our results highlight ROR1's potential as a therapeutic target for multidrug resistant malignancies.