Protumorigenic overexpression of Stathmin/Op18 by gain-of-function mutation in p53 in human hepatocarcinogenesis

The microtubule (MI)-destabilizing protein stathmin/Op18 has previously been described to be negatively regulated by p53 and to be highly expressed in several tumor entities. However, little is known about its expression profile, functional or therapeutic relevance, and regulation in human hepatocarcinogenesis. Here we demonstrate cytoplasmic overexpression of stathmin in premalignant lesions (dysplastic nodules; DNs) and hepatocellular carcinomas (HCCs), which significantly correlated with tumor progression, proliferation, and activation of other protumorigenic factors (e.g., nuclear p53). Inhibition of stathmin expression by gene-specific short interfering RNA (siRNA) was associated with a significant reduction of MT-dependent cellular functions such as tumor cell viability, proliferation, migration, and increased apoptosis in HCC cells. Loss of stathmin expression increased responsiveness of tumor cells to the treatment with cytostatic drugs targeting MT-stability (paclitaxel, vinblastine) and to DNA cross-linking agents (cisplatin). Surprisingly, inducible expression of p53(wt) in p53-negative HCC cells as well as a reduction of p53(wt) by siRNA in p53(wt)-positive cells did not alter stathmin expression. However, stathmin was down-regulated after siRNA-based reduction of p53(mu/Y220C) and p53(mut/R213Q) expression in different tumor cell types. Conclusion: Our results demonstrate that overexpression of stathmin is an early protumorigenic event in human hepatocarcinogenesis, and its up-regulation can be mediated by gain-of-fimction mutations in p53. Thus, stathmin represents a potential therapeutic target, for example, by increasing responsiveness of tumor cells to treatment with chemotherapeutic agents after reduction of stathmin bioactivity.