We and others previously identified NKX2-1, also known as TITF1 and TTF-1, as a lineage-survival oncogene in lung adenocarcinomas. Here 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 pro-apoptotic p38 signaling, in part through ROR1 kinase-dependent c-Src activation, as well as kinase activity-independent sustainment of the EGFR-ERBB3 association, ERBB3 phosphorylation, and consequential PI3K activation. Notably, ROR1 knockdown effectively inhibited lung adenocarcinoma cell lines, irrespective of their EGFR status, including those with resistance to the EGFR tyrosine kinase inhibitor gefitinib. Our findings thus identify ROR1 as an "Achilles' heel" in lung adenocarcinoma, warranting future development of therapeutic strategies for this devastating cancer.

We previously identified PSMD2, a subunit of the 19S regulatory complex of proteasomes, as a constituent of a signature associated with the acquisition of metastatic phenotype and poor prognosis in lung cancers. In the present study, we found that knockdown of PSMD2 decreased proteasome activity, and induced growth inhibition and apoptosis in lung cancer cell lines. These effects of siRNA-mediated PSMD2 inhibition were associated with changes in the balance between phosphorylated AKT and p38, as well as with induction of p21. In addition, patients with higher PSMD2 expression had poorer prognosis and a small fraction of lung cancer specimens carried increased copies of PSMD2. Notably, our findings clearly illustrate that lung adenocarcinomas can be divided into two groups; those with and without general upregulation of proteasome pathway genes including PSMD2. This general upregulation was significantly more prevalent in the non-terminal respiratory unit (non-TRU)-type, a recently proposed genetically and clinicopathologically relevant expression profile-defined classification of adenocarcinomas (P < 0.001 by Fisher's exact test). Patients with adenocarcinomas with general upregulation had significantly shorter survival after potentially curative resection (P = 0.0001 by log-rank test) independent of disease stage, as shown by multivariate Cox regression analysis. Our results suggest that PSMD2 may be a good molecular target candidate and that other co-regulated proteasome pathway genes and/or their common regulator(s) might also be potential targets, warranting future study including elucidation of the underlying common regulatory mechanism.

No 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.

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. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-17. doi:1538-7445.AM2012-LB-17