Inhibition of Karyopherin-β1-Mediated Nuclear Import of Lineage-Defining Transcription Factors in Small Cell Lung Cancer

Small cell lung cancer (SCLC) is an aggressive neuroendocrine tumor that accounts for approximately 16% of lung cancer diagnoses. Recent genomic studies support the classification of this disease into four different subtypes based on the expression of the lineage-defining transcription factors ASCL1 (SCLC-A), NEUROD1 (SCLC-N), POU2F3 (SCLC-P), and YAP1 (SCLC-Y). Together, the SCLC-A and SCLC-N subtypes account for a majority of SCLC. ASCL1 and NEUROD1 are Class II bHLH transcription factors that command the expression of SCLC oncogenes and are known to drive distinct transcriptional programs, conferring SCLC-A and SCLC-N with different molecular and physiological features. ASCL1 has also been shown to be required for tumor formation in SCLC mouse models, and, where tested, both ASCL1 and NEUROD1 play key roles in maintaining growth of SCLC-A and SCLC-N cell lines. Together, these findings suggest that strategies to inhibit ASCL1 and NEUROD1 activity may represent an attractive SCLC therapy, and provide new insight into the underlying plasticity of the major SCLC subtypes. ASCL1 and NEUROD1 are translated in the cytoplasm and must be transported into the nucleus in order to regulate gene expression. Interestingly, it has been shown that NEUROD1 is selectively imported into the nucleus via the nuclear transport receptor Karyopherin-β1, or KPNB1 (also known as Importin-β1) in several mouse and human cell lines. Additionally, a muscle-specific bHLH transcription factor, MYOD, has also been shown to be imported into the nucleus by KPNB1, suggesting a common nuclear import mechanism for Class II bHLH transcription factors. However, whether ASCL1 is also imported into the nucleus via the same mechanism, and whether the nuclear import of NEUROD1 is mediated by KPNB1 in the setting of SCLC, had not been tested. Here, I 1. identified KPNB1 as a nuclear import receptor for ASCL1 and NEUROD1 in SCLC, 2. showed that KPNB1 inhibition disrupts SCLC- A patient derived xenograft (PDX) growth in vivo, and preferentially disrupts the growth of ASCL1/NEUROD1+ SCLC in vitro, and 3. compared the changes in gene expression following the inhibition of ASCL1 or KPNB1 activity across in vitro and in vivo models of SCLC.