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Abstract

Exchange of macromolecules between the nucleo- and cytoplasm is provided by the nuclear transport system (NTS). Karyopherins represent essential components of NTS) by serving as nuclear transport receptors/adaptor proteins. Dysregulation of karyopherins in (hepato ) carcinogenesis, including the pivotal nuclear import factor karyopherin-α2 (KPNA2), has been previously reported. However, the functional and regulatory role of KPNA2 in hepatocellular carcinoma (HCC) remains incompletely understood. To further characterize KPNA2 in this context, mass spectrometry-based proteomics was combined with functional/mechanistic cell-based approaches and data derived from murine and human HCC samples. Quantitative mass spectrometry upon siRNA-mediated KPNA2 knockdown in HCC cells revealed the microtubule-related oncoprotein stathmin among the most downregulated proteins. KPNA2 depletion resulted in impaired colony formation and tumor cell migration in HCC cells, which could be recapitulated by direct knockdown of stathmin. Having identified stathmin as functional relevant “downstream” target of KPNA2 the underlying molecular mechanism of KPNA2-dependent stathmin regulation was dissected. Out of several candidates the transcription factors E2F1 and TFDP1 were identified as transport substrates of KPNA2 and to be retained in the cytoplasm upon KPNA2 ablation followed by reduced STMN1 expression. Finally, significant correlations of STMN1 with E2F1/TFDP1 and KPNA2 expression were found based on data derived from murine HCC models and human HCC cohorts with high KPNA2 and STMN1 expression being associated with poorer patient outcome. Taken together, these data suggest that KPNA2 regulates STMN1 by mediating nuclear import of E2F1/TFDP1 and thereby provide a functionally relevant link between the NTS and microtubule-interacting proteins in HCC. Though further studies are required, interfering with nuclear import factors such as KPNA2 could represent a promising therapeutic approach in liver cancer.