Cloning and biochemical characterization of LIMK-2, a protein kinase containing two LIM domains

Decoding the tumour-modulatory roles of LIMK2

LIM domains kinase 2 (LIMK2) is a 72 kDa protein that regulates actin and cytoskeleton reorganization. Once phosphorylated by its upstream activator (ROCK1), LIMK2 can phosphorylate cofilin to inactivate it. This relieves the levering stress on actin and allows polymerization to occur. Actin rearrangement is essential in regulating cell cycle progression, apoptosis, and migration. Dysregulation of the ROCK1/LIMK2/cofilin pathway has been reported to link to the development of various solid cancers such as breast, lung, and prostate cancer and liquid cancer like leukemia. This review aims to assess the findings from multiple reported in vitro, in vivo, and clinical studies on the potential tumour-regulatory role of LIMK2 in different human cancers. The findings of the selected literature unraveled that activated AKT, EGF, and TGF-β pathways can upregulate the activities of the ROCK1/LIMK2/cofilin pathway. Besides cofilin, LIMK2 can modulate the cellular levels of other proteins, such as TPPP1, to promote microtubule polymerization. The tumour suppressor protein p53 can transactivate LIMK2b, a splice variant of LIMK2, to induce cell cycle arrest and allow DNA repair to occur before the cell enters the next phase of the cell cycle. Additionally, several non-coding RNAs, such as miR-135a and miR-939-5p, could also epigenetically regulate the expression of LIMK2. Since the expression of LIMK2 is dysregulated in several human cancers, measuring the tissue expression of LIMK2 could potentially help diagnose cancer and predict patient prognosis. As LIMK2 could play tumour-promoting and tumour-inhibiting roles in cancer development, more investigation should be conducted to carefully evaluate whether introducing a LIMK2 inhibitor in cancer patients could slow cancer progression without posing clinical harms.

Revisiting Lung Cancer Metastasis: Insight From the Functions of Long Non-coding RNAs

Globally, lung cancer is the most common cause of cancer-related deaths. After diagnosis at all stages, <7% of patients survive for 10 years. Thus, diagnosis at later stages and the lack of effective and personalized drugs reflect a significant need to better understand the mechanisms underpinning lung cancer progression. Metastasis should be responsible for the high lethality and recurrence rates seen in lung cancer. Metastasis depends on multiple crucial steps, including epithelial–mesenchymal transition, vascular remodeling, and colonization. Therefore, in-depth investigations of metastatic molecular mechanisms can provide valuable insights for lung cancer treatment. Recently, long noncoding RNAs (lncRNAs) have attracted considerable attention owing to their complex roles in cancer progression. In lung cancer, multiple lncRNAs have been reported to regulate metastasis. In this review, we highlight the major molecular mechanisms underlying lncRNA-mediated regulation of lung cancer metastasis, including (1) lncRNAs acting as competing endogenous RNAs, (2) lncRNAs regulating the transduction of several signal pathways, and (3) lncRNA coordination with enhancer of zeste homolog 2. Thus, lncRNAs appear to execute their functions on lung cancer metastasis by regulating angiogenesis, autophagy, aerobic glycolysis, and immune escape. However, more comprehensive studies are required to characterize these lncRNA regulatory networks in lung cancer metastasis, which can provide promising and innovative novel therapeutic strategies to combat this disease.

LIMK2-1, a new isoform of human LIMK2, regulates actin cytoskeleton remodeling via a different signaling pathway than that of its two homologs, LIMK2a and LIMK2b

LIMK1 and LIMK2 (LIMKs, LIM kinases) are kinases that play a crucial role in cytoskeleton dynamics by independently regulating both actin filament and microtubule remodeling. LIMK1 and, more recently, LIMK2 have been shown to be involved in cancer development and metastasis, resistance of cancer cells to microtubule-targeted treatments, neurological diseases, and viral infection. LIMKs have thus recently emerged as new therapeutic targets. Databanks describe three isoforms of human LIMK2: LIMK2a, LIMK2b, and LIMK2-1. Evidence suggests that they may not have completely overlapping functions. We biochemically characterized the three isoforms to better delineate their potential roles, focusing on LIMK2-1, which has only been described at the mRNA level in a single study. LIMK2-1 has a protein phosphatase 1 (PP1) inhibitory domain at its C-terminus which its two counterparts do not. We showed that the LIMK2-1 protein is indeed synthesized. LIMK2-1 does not phosphorylate cofilin, the canonical substrate of LIMKs, although it has kinase activity and promotes actin stress fiber formation. Instead, it interacts with PP1 and partially inhibits its activity towards cofilin. Our data suggest that LIMK2-1 regulates actin cytoskeleton dynamics by preventing PP1-mediated cofilin dephosphorylation, rather than by directly phosphorylating cofilin as its two counterparts, LIMK2a and LIMK2b. This specificity may allow for tight regulation of the phospho-cofilin pool, determining the fate of the cell.

Role of lncRNA and EZH2 Interaction/Regulatory Network in Lung Cancer

Lung cancer is the leading cause of cancer-related deaths worldwide. Long non-coding RNAs (lncRNAs) are non-protein-coding transcripts and longer than 200 nucleotides. LncRNAs have been demonstrated to modulate gene expression at transcriptional, post-transcriptional, as well as epigenetic levels in lung cancer. Interestingly, compelling studies have revealed that lncRNAs participated in the EZH2 oncogenic regulatory network. EZH2 plays an important role in the initiation, progression and metastasis of cancer. On one hand, lncRNAs can directly bind to EZH2, recruit EZH2 to the promoter region of genes and repress their expression. On the other hand, lncRNAs can also serve as EZH2 effectors or regulators. In this review, we summarized the types of lncRNA-EZH2 interaction and regulatory network identified till date and discussed their influence on lung cancer. Better understanding regarding the interaction and regulatory network will provide new insights on lncRNA- or EZH2-based therapeutic development in lung cancer.

Long non-coding RNA TUG1 is involved in cell growth and chemoresistance of small cell lung cancer by regulating LIMK2b via EZH2

BACKGROUND

Taurine upregulated gene1 (TUG1) as a 7.1-kb lncRNA, has been shown to play an oncogenic role in various cancers. However, the biological functions of lncRNA TUG1 in small cell lung cancer (SCLC) remain unknown. The aim of this study is to explore the roles of TUG1 in cell growth and chemoresistance of SCLC and its possible molecular mechanism.

METHODS

The expression of TUG1 in thirty-three cases of SCLC tissues and SCLC cell line were examined by quantitative RT-PCR (qRT-PCR). The functional roles of TUG1 in SCLC were demonstrated by CCK8 assay, colony formation assay, wound healing assay and transwell assay, flow cytometry analysis and in vivo study through siRNA or shRNA mediated knockdown. Western blot assays were used to evaluate gene and protein expression in cell lines. Chromatin immunoprecipitation (ChIP) and RNA binding protein immunoprecipitation (RIP) were performed to confirm the molecular mechanism of TUG1 involved in cell growth and chemoresistance of small cell lung cancer.

RESULTS

We found that TUG1 was overexpressed in SCLC tissues, and its expression was correlated with the clinical stage and the shorter survival time of SCLC patients. Moreover, downregulation of TUG1 expression could impair cell proliferation and increased cell sensitivity to anticancer drugs both in vitro and in vivo. We also discovered that TUG1 knockdown significantly promoted cell apoptosis and cell cycle arrest, and inhibited cell migration and invasion in vitro . We further demonstrated that TUG1 can regulate the expression of LIMK2b (a splice variant of LIM-kinase 2) via binding with enhancer of zeste homolog 2 (EZH2), and then promoted cell growth and chemoresistance of SCLC.

CONCLUSIONS

Together, these results suggested that TUG1 mediates cell growth and chemoresistance of SCLC by regulating LIMK2b via EZH2.

LIM kinase regulation of cytoskeletal dynamics is required for salivary gland branching morphogenesis

LIMK regulation of actin and microtubule dynamics is required for epithelial regulation of early- and late-stage cleft stabilization and progression. LIMK stimulates focal adhesion assembly and integrin β1 activation in cleft regions, causing fibronectin fibrillogenesis and promoting cleft progression during salivary gland branching morphogenesis.

Coordinated actin microfilament and microtubule dynamics is required for salivary gland development, although the mechanisms by which they contribute to branching morphogenesis are not defined. Because LIM kinase (LIMK) regulates both actin and microtubule organization, we investigated the role of LIMK signaling in mouse embryonic submandibular salivary glands using ex vivo organ cultures. Both LIMK 1 and 2 were necessary for branching morphogenesis and functioned to promote epithelial early- and late-stage cleft progression through regulation of both microfilaments and microtubules. LIMK-dependent regulation of these cytoskeletal systems was required to control focal adhesion protein–dependent fibronectin assembly and integrin β1 activation, involving the LIMK effectors cofilin and TPPP/p25, for assembly of the actin- and tubulin-based cytoskeletal systems, respectively. We demonstrate that LIMK regulates the early stages of cleft formation—cleft initiation, stabilization, and progression—via establishment of actin stability. Further, we reveal a novel role for the microtubule assembly factor p25 in regulating stabilization and elongation of late-stage progressing clefts. This study demonstrates the existence of multiple actin- and microtubule-dependent stabilization steps that are controlled by LIMK and are required in cleft progression during branching morphogenesis.

LIMK2 mediates resistance to chemotherapeutic drugs in neuroblastoma cells through regulation of drug-induced cell cycle arrest

Drug resistance is a major obstacle for the successful treatment of many malignancies, including neuroblastoma, the most common extracranial solid tumor in childhood. Therefore, current attempts to improve the survival of neuroblastoma patients, as well as those with other cancers, largely depend on strategies to counter cancer cell drug resistance; hence, it is critical to understand the molecular mechanisms that mediate resistance to chemotherapeutics. The levels of LIM-kinase 2 (LIMK2) are increased in neuroblastoma cells selected for their resistance to microtubule-targeted drugs, suggesting that LIMK2 might be a possible target to overcome drug resistance. Here, we report that depletion of LIMK2 sensitizes SHEP neuroblastoma cells to several microtubule-targeted drugs, and that this increased sensitivity correlates with enhanced cell cycle arrest and apoptosis. Furthermore, we show that LIMK2 modulates microtubule acetylation and the levels of tubulin Polymerization Promoting Protein 1 (TPPP1), suggesting that LIMK2 may participate in the mitotic block induced by microtubule-targeted drugs through regulation of the microtubule network. Moreover, LIMK2-depleted cells also show an increased sensitivity to certain DNA-damage agents, suggesting that LIMK2 might act as a general pro-survival factor. Our results highlight the exciting possibility of combining specific LIMK2 inhibitors with anticancer drugs in the treatment of multi-drug resistant cancers.

LIM kinase 1, a key regulator of actin dynamics, is widely expressed in embryonic and adult tissues

The expression of endogenous LIM kinase 1 (LIMK1) protein was investigated in embryonic and adult mice using a rat monoclonal antibody (mAb), which recognizes specifically the PDZ domain of LIMK1 and not LIMK2. Immunoblotting analysis revealed widespread expression of LIMK1 existing as a 70-kDa protein in tissues and in cell lines, with a higher mass form (approximately 75 kDa) present in some tissues and cell lines. Smaller isoforms of approximately 50 kDa were also occasionally evident. Immunofluorescence analysis demonstrated LIMK1 subcellular localization at focal adhesions in fibroblasts as revealed by co-staining with actin, paxillin and vinculin in addition to perinuclear (Golgi) and occasional nuclear localization. Furthermore, an association between LIMK1 and paxillin but not vinculin was identified by co-immunoprecipitation analysis. LIMK1 is enriched in both axonal and dendritic growth cones of E18 rat hippocampal pyramidal neurons where it is found in punctae that extend far out into filopodia, as well as in a perinuclear region identified as Golgi. In situ, we identify LIMK1 protein expression in all embryonic and adult tissues examined, albeit at different levels and in different cell populations. The rat monoclonal LIMK1 antibody recognizes proteins of similar size in cell and tissue extracts from numerous species. Thus, LIMK1 is a widely expressed protein that exists as several isoforms.

Abnormal spine morphology and enhanced LTP in LIMK-1 knockout mice

In vitro studies indicate a role for the LIM kinase family in the regulation of cofilin phosphorylation and actin dynamics. In addition, abnormal expression of LIMK-1 is associated with Williams syndrome, a mental disorder with profound deficits in visuospatial cognition. However, the in vivo function of this family of kinases remains elusive. Using LIMK-1 knockout mice, we demonstrate a significant role for LIMK-1 in vivo in regulating cofilin and the actin cytoskeleton. Furthermore, we show that the knockout mice exhibited significant abnormalities in spine morphology and in synaptic function, including enhanced hippocampal long-term potentiation. The knockout mice also showed altered fear responses and spatial learning. These results indicate that LIMK-1 plays a critical role in dendritic spine morphogenesis and brain function.

Cotransduction of nondividing cells using lentiviral vectors

Diseases such as AIDS and cancers may require the introduction of multiple genes into either stem cells or nondividing cells, among others, for therapeutic purposes. Such genes may act at different points of the disease pathway, or may constitute a regulatory loop to bypass or rectify the defective gene or pathway underpinning the disease. Ideally, the therapeutic genes must be transduced together in diverse combinations, and the introduction should occur without constraints. Since lentiviral vectors can transduce both dividing and nondividing cells, they are ideal vehicles to investigate combinatorial gene transfer into diverse cells. In this study, we demonstrate that by using two independent lentiviral vectors, pseudotyped with the protein g of vesicular stomatitis virus, up to four genes can be introduced simultaneously into single dividing and nondividing cells. Up to 45% and 73% of dividing and nondividing cells, respectively, could be transduced with two lentiviral vectors. The efficiency of cotransducing a single cell was the product of the individual transduction efficiencies and suggested the absence of viral interference. Multiple and combinatorial gene transduction using lentiviral vectors may prove useful in gene therapy.