The Roles of 14-3-3σ in Regulating Cell Migration and Invasion of Hepatocellular Carcinoma

14-3-3σ (also known as stratifin, SFN) is a member of 14-3-3 family and its role in regulating tumor progression remains controversial. Results from earlier studies suggest 14-3-3σ plays as a potential tumor suppressor or promoter in various human malignancy including hepatocellular carcinoma (HCC). However, the molecular mechanism by which 14-3-3σ confers HCC cell proliferation, migration, invasion as well as tumor metastasis remains unclear. In this study, we investigated the potential downstream targets of 14-3-3σ in modulating HCC development and the interaction with surrounding tumor associated stromal cells. We found that 14-3-3σ is abundantly expressed in HCC tumors. Both heat shock factor-1α (HSF-1) and heat shock protein 70 (HSP70) are major downstream factors of 14-3-3σ. Stable or transient over-expression of 14-3-3σ induces the expression of HSF-1 and HSP70 in HCC cells. 14-3-3σ mediated cell migration is impaired by siRNA knockdown of HSP70. 14-3-3σ-induced HSF-1α/HSP70 expression and cell migration is also abolished by knockdown of β-catenin or activation of GSK-3β. Moreover, both 14-3-3σ and HSP70 overexpression associated with micro-vascular thrombin in HCC patients. The results suggested that both 14-3-3σ/HSP70 expression are potentially involved in cell migration/invasion. In addition, HCC cells co-cultured with 14-3-3σ-conditioned media (CM) treated stromal cells (H68 fibroblasts, THP-1 and phorbol-12-myristate-13-acetate (PMA)-treated THP-1 (PMA-THP-1) significantly enhanced their invasive ability compared with control-CM treated cells. Incubation with 14-3-3σ-CM induced differential expression profiles of matrix metalloproteinases (MMPs) in fibroblasts (MMP-1, MMP-2, MMP-9, MMP-12 and MMP-14), THP-1 (MMP-1 and MMP-12) and PMA-THP-1 cells (MMP-2, MMP-12 and MMP-14). In contrast, silencing of 14-3-3σ by siRNA significantly abolished 14-3-3σ-CM induced MMPs. Treatment of all stromal cells (HS68, THP-1 and PMA-THP-1) with recombinant human 14-3-3σ (r14-3-3σ) protein exhibits a similar expression profile of MMPs induced by 14-3-3σ-CM. Knockdown of aminopeptidase N (APN) significantly abrogated 14-3-3σ-CM or r14-3-3σ induced expression of MMPs in HS68 fibroblasts. Finally, HCC-secreted 14-3-3 promotes cell invasion was significantly abolished by APN siRNA or treated with GM6001 (an inhibitor of MMPs). Taken together, our findings suggest that 14-3-3σ regulates HCC tumor progression not only by promoting cancer cell migration but also educating stromal cells in the tumor-associated microenvironment. 14-3-3σ is thus a potential biomarker and therapeutic target of HCC.