Background: LIM kinase 1 (LIMK1) is expressed in both cytoplasmic and nuclear compartments, and is a key regulator of cytoskeletal organization involved in cell migration and proliferation. LIMK1 levels are increased in several human cancers, with LIMK1 over-expression in prostate and breast cancer cells leading to tumor progression. While it has been presumed that the mechanism by which LIMK1 promotes cancer progression is via its cytoplasmic effects, the role of nuclear vs cytoplasmic LIMK1 in the tumorigenic process has not been examined.
Results: To determine if cytoplasmic or nuclear LIMK1 expression correlated with breast cancer, we performed immunohistochemical (IHC) analysis of breast tissue microarrays (TMAs), The IHC analysis of breast TMAs revealed that 76% of malignant breast tissue samples strongly expressed LIMK1 in the cytoplasm, with 52% of these specimens also expressing nuclear LIMK1. Only 48% of benign breast samples displayed strong cytoplasmic LIMK1 expression and 27% of these expressed nuclear LIMK1. To investigate the respective roles of cytoplasmic and nuclear LIMK1 in breast cancer progression, we targeted GFP-LIMK1 to cytoplasmic and nuclear subcellular compartments by fusing nuclear export signals (NESs) or nuclear localization sequences (NLS), respectively, to the amino-terminus of GFP-LIMK1. Stable pools of MDA-MB-231 cells were generated by retroviral transduction, and fluorescence microscopy revealed that GFP alone (control) and GFP-LIMK1 were each expressed in both the cytoplasm and nucleus of MDA-MB-231 cells, whereas NLS-GFP-LIMK1 was expressed in the nucleus and NES-GFP-LIMK1 was expressed in the cytoplasm. Western blot analyses revealed equal expression of GFP-LIMK1 and NES-GFP-LIMK1, with NLS-GFP-LIMK1 expression being less but equal to endogenous LIMK1. Also, Western blotting revealed increased levels of phospho-cofilin, phospho-FAK, phospho-paxillin, phospho-Src, phospho-AKT, and phospho-Erk1/2 in cells expressing all GFP-LIMK1 fusions, compared to GFP alone. Invasion assays revealed that all GFP-LIMK1 fusions increased MDA-MB-231 cell invasion ~1.5-fold, compared to GFP-only control cells. Tumor xenograft studies in nude mice revealed that MDA-MB-231 cells stably expressing GFP-LIMK, NLS-GFP-LIMK1 and NES-GFP-LIMK1 enhanced tumor growth 2.5-, 1.6- and 4.7-fold, respectively, compared to GFP-alone.
Conclusion: Taken together, these data demonstrate that LIMK1 activity in both the cytoplasmic and nuclear compartments promotes breast cancer progression, underscoring that nuclear LIMK1 contributes to the transforming function of LIMK1.