Abstract PR1: NKX2-1/TITF1/TTF-1-induced ROR1 is required to sustain EGFR survival signaling in lung adenocarcinoma

Abstract

Emerging evidence, though currently sparse, suggests that “lineage-specific transcription factors” with developmental roles in normal progenitor cells of particular lineages may also confer dependency for survival to certain types of cancer cells. Accumulated evidence indicates that NKX2-1, a lineage-specific transcription factor also known as TITF1 and TTF-1, which is expressed in a major fraction of lung adenocarcinomas, plays essential roles in peripheral lung development. We previously identified NKX2-1 as a lineage-survival oncogene in lung adenocarcinoma, while other investigators reached similar conclusions through genome-wide searches for focal genomic aberrations. Previous findings including ours thus clearly indicate the requirement of sustained NKX2-1 expression for lung adenocarcinoma survival, while it remains elusive how NKX2-1 mediates survival signals. In the present study, we show that NKX2-1 induces the expression of the receptor tyrosine kinase-like orphan receptor 1 (ROR1), which in turn sustains a favorable balance between prosurvival PI3K-AKT and proapoptotic p38 signaling, in part through ROR1 kinase-dependent c-Src activation. Interestingly, ROR1 was also identified as a receptor tyrosine kinase with a “sustainer role” for the interaction of EGFR-ERBB3 and ERBB3 phosphorylation in a ROR1 kinase-independent manner, consequentially leading to PI3K activation. It was also of note that ROR1 knockdown effectively inhibited lung adenocarcinoma cell lines irrespective of their EGFR status, including those with resistance to the EGFR tyrosine kinase inhibitor gefitinib. Secondary EGFR mutation, MET amplification, and HGF overexpression may arise in lung adenocarcinomas in patients undergoing EGFR-TKI treatment, leading to treatment resistance. The existence of such diverse mechanisms makes it difficult to predict which should be targeted to prevent expansion of resistant clones. From a clinical point of view, it is thus of particular interest that ROR1 inhibition appears to be effective for treatment of gefitinib-resistant lung adenocarcinomas with various resistance mechanisms. Taken together, the present findings identify ROR1 as an “Achilles' heel” in lung adenocarcinomas. Future development of novel therapeutic means including ROR1-specific antibodies and small molecules that inhibit both or either of the two distinct prosurvival signal-sustaining functions is greatly anticipated for attempts to reduce the intolerable death toll from currently “hard-to-cure” lung adenocarcinomas. This proffered talk is also presented as Poster A28.