HUNK suppresses metastasis of basal type breast cancers by disrupting the interaction between PP2A and cofilin-1

Abstract

Metastasis leads to the death of most cancer patients, and basal breast cancer is the most aggressive breast tumor type. Metastasis involves a complex cell migration process dependent on cytoskeletal remodeling such that targeting such remodeling in tumor cells could be clinically beneficial. Here we show that Hormonally Upregulated Neu-associated Kinase (HUNK) is dramatically downregulated in tumor samples and cell lines derived from basal breast cancers. Reconstitution of HUNK expression in basal breast cancer cell lines blocked actin polymerization and reduced cell motility, resulting in decreased metastases in two in vivo murine cancer models. Mechanistically, HUNK overexpression sustained the constitutive phosphorylation and inactivation of cofilin-1 (CFL-1), thereby blocking the incorporation of new actin monomers into actin filaments. HUNK reconstitution in basal breast cancer cell lines prevented protein phosphatase 2-A (PP2A), a phosphatase putatively acting on CFL-1, from binding to CFL-1. Our investigation of HUNK suggests that the interaction between PP2A and CFL-1 may be a target for antimetastasis therapy, particularly for basal breast cancers.M etastasis is a hallmark of cancer and remains the major cause of cancer-related mortality (90%) (1). Currently, there is no FDA-approved drug that specifically blocks metastasis. Metastasis is a multistep process that requires a cancer cell to leave a primary tumor, intravasate, survive in the blood, extravasate, migrate, invade through basement membranes and connective tissues, and establish a viable tumor in a distant site (2). Cytoskeletal reorganization and cell movement underlie all these events (3), and disruption of these processes could therefore constitute an effective anticancer approach.The major subtypes of breast cancer (luminal A/B/C, HER-2, and basal) can be distinguished by their gene expression profiles (4). The basal subtype is the most aggressive, has the worst prognosis, and shows the greatest extent of metastasis (4, 5). To identify molecules whose expression varies by breast cancer subtype and might be linked to metastasis, we screened the online data of Sorlie and colleagues (4) for candidate promoters and suppressors of metastasis. Our hypothesis was that the mRNA expression of kinases involved in cell migration and invasion should be altered in basal breast cancers relative to the mRNA profiles of other breast cancer subtypes. One molecule emerging from this screen was Hormonally Up-regulated Neuassociated Kinase (HUNK), an 80-kDa protein (6). HUNK was down-regulated almost threefold in basal cancer samples compared to the other subtypes. HUNK contains a 260-aa domain predicted to have serine/threonine kinase activity; however, to date, no clear kinase activity, substrates, interacting proteins, or indeed physiological role for HUNK have been identified (6, 7). In normal murine mammary cells, HUNK levels vary with the hormonal cycle (6). With respect to transformed cells, Wertheim et al. recently reported that HUNK is highly express...

Hunk is required for HER2/neu-induced mammary tumorigenesis

“…In this regard, in contrast to our genetic and biochemical evidence for pro-tumorigenic effects of Hunk in HER2/neuinduced tumorigenesis and pro-metastatic effect of Hunk in cmyc-induced mammary tumorigenesis, it has recently been suggested by Mak and colleagues that Hunk might play an opposing role in basal-like human breast cancers, wherein Hunk overexpression may suppress metastatic potential (87). While the findings of that study are formally consistent with the possibility that the role of Hunk in metastasis may be cell type specific, it is important to note that their study focused primarily on in vitro systems in which Hunk was exogenously overexpressed in cell lines that do not otherwise express Hunk.…”

Hunk negatively regulates c-myc to promote Akt-mediated cell survival and mammary tumorigenesis induced by loss of Pten

“…This observation extends our previous finding that Hunk is required for the development of HER2/neu-induced mammary tumors (3), which exhibit a luminal epithelial phenotype, by demonstrating that Hunk is also required for the development of mammary tumors in Pten mutant mice, which display basal-like characteristics (22). This latter finding is of particular interest in light of a recent report proposing that Hunk functions as a suppressor of metastasis, based on experiments in basal-like human breast cancer cell lines (23). To extend their in vitro findings, these authors suggested that crossing Hunk −/− mice to mouse models for HER2/neu as well as basal-like breast cancers would yield information pertinent to human breast cancer.…”

Targeting cytoskeleton reorganisation as antimetastatic treatment

“…In this case, we believe that the only way to ensure that the fi nal effect of CFL-1 is inhibition of its function, is direct targeting (or targeting through its main regulators LIMK or PP2A). We and others have demonstrated how the mutation of serine 3 to aspartic acid of CFL-1 (causing a conformational change similar to constitutive phosphorylation) exerts a clean cytoskeletal effect in tumour cells [116]. Whereas viability and replication remain unaltered, cancer cells are unable to grow on in adhesion-independent manner, migrate in vitro or produce metastasis in vivo [116].…”

“…We and others have demonstrated how the mutation of serine 3 to aspartic acid of CFL-1 (causing a conformational change similar to constitutive phosphorylation) exerts a clean cytoskeletal effect in tumour cells [116]. Whereas viability and replication remain unaltered, cancer cells are unable to grow on in adhesion-independent manner, migrate in vitro or produce metastasis in vivo [116]. Drugs targeting upstream signalling molecules are justifi ed in the case of activating mutations (i.e., Pi3K, EGFR, etc.…”