CDK5 is a major regulator of the tumor suppressor DLC1

PI3K/Akt inhibition promotes AR activity and prostate cancer cell proliferation through p35-CDK5 modulation

Aberrant PI3K/Akt activation is linked to prostate cancer (PCa) malignancy, while androgen receptor (AR) is critical in early-stage PCa development. Investigating the interaction between these pathways is crucial for PCa malignancy. Our previous study demonstrated that p35-CDK5 mediates post-translational modifications of AR, STAT3, and p21CIP1, eventually promoting PCa cell growth. This study revealed the role of p35-CDK5 in between PI3K/Akt and AR by utilizing LNCaP and 22Rv1 cells. Through the TCGA database analysis, we observed a positive correlation between PTEN and p35 expression, implying a potential negative correlation between PI3K/Akt activation and p35-CDK5. Inhibiting PI3K/Akt with LY294002, Capivasertib (AZD5363), or using an inactive Akt mutant significantly increased p35 expression and subsequently enhanced AR stability and activation in PCa cells. On the other hand, CDK5-knockdown reversed these effects. The involvement of the β-catenin/Egr1-axis was observed in regulating PI3K/Akt inhibition and p35-CDK5 activation, implying a possible mechanistic connection. Importantly, CDK5 knockdown further reduced PI3K/Akt-inhibition-induced AR and cell viability maintenance, suggesting a compensatory role for CDK5-AR in maintaining cell viability under Akt inhibition. In conclusion, PI3K/Akt inhibition could trigger p35-CDK5-dependent AR activation and cell viability, highlighting p35-CDK5 as a critical link connecting PI3K/Akt inhibition to AR activation and pivotal in PCa cell resistance to PI3K/Akt blockade.

Regulation and functions of the DLC family of RhoGAP proteins: Implications for development and cancer

The DLC (Deleted in Liver Cancer) family of RhoGAP (Rho GTPase-activating) proteins has been extensively studied since the identification of the first family member nearly 30 years ago. Rho GTPase signaling is essential for various cellular processes, including cytoskeletal dynamics, cell migration, and proliferation. Members of the DLC family are key regulators of this signaling pathway, with well-established roles in development and carcinogenesis. Here, we provide a comprehensive review of research into DLC regulation and cellular functions over the last three decades. In particular, we summarize control mechanisms of DLC gene expression at both the transcriptional and post-transcriptional level. Additionally, recent advances in understanding the post-translational regulation of DLC proteins that allow for tuning of protein activity and localization are highlighted. This detailed overview will serve as resource for future studies aimed at further elucidating the complex regulatory mechanisms of DLC family proteins and exploring their potential as targets for therapeutic applications.

The pro-oncogenic noncanonical activity of a RAS•GTP:RanGAP1 complex facilitates nuclear protein export

Canonical RAS signaling, including PI3K/AKT- and RAF/MEK-dependent activities, results mainly from RAS•GTP interaction with its effectors at the plasma membrane. Here, we identified a fundamental, oncogenic, noncanonical RAS•GTP activity that increases XPO1-dependent export of nuclear protein cargo into the cytoplasm and is independent of PI3K/AKT and RAF/MEK signaling. This RAS-dependent step acts downstream from XPO1 binding to nuclear protein cargo and is mediated by a perinuclear protein complex between RAS•GTP and RanGAP1 that facilitates hydrolysis of Ran•GTP to Ran•GDP, which promotes release of nuclear protein cargo into the cytoplasm. The export of nuclear EZH2, which promotes cytoplasmic degradation of the DLC1 tumor suppressor protein, is a biologically important component of this pro-oncogenic activity. Conversely, preventing nuclear protein export contributes to the antitumor activity of KRAS inhibition, which can be further augmented by reactivating the tumor suppressor activity of DLC1 or potentially combining RAS inhibitors with other cancer treatments.

DLC1 promotes mechanotransductive feedback for YAP via RhoGAP-mediated focal adhesion turnover

Angiogenesis is a tightly controlled dynamic process demanding a delicate equilibrium between pro-angiogenic signals and factors that promote vascular stability. The spatiotemporal activation of the transcriptional co-factors YAP (herein referring to YAP1) and TAZ (also known WWTR1), collectively denoted YAP/TAZ, is crucial to allow for efficient collective endothelial migration in angiogenesis. The focal adhesion protein deleted-in-liver-cancer-1 (DLC1) was recently described as a transcriptional downstream target of YAP/TAZ in endothelial cells. In this study, we uncover a negative feedback loop between DLC1 expression and YAP activity during collective migration and sprouting angiogenesis. In particular, our study demonstrates that signaling via the RhoGAP domain of DLC1 reduces nuclear localization of YAP and its transcriptional activity. Moreover, the RhoGAP activity of DLC1 is essential for YAP-mediated cellular processes, including the regulation of focal adhesion turnover, traction forces, and sprouting angiogenesis. We show that DLC1 restricts intracellular cytoskeletal tension by inhibiting Rho signaling at the basal adhesion plane, consequently reducing nuclear YAP localization. Collectively, these findings underscore the significance of DLC1 expression levels and its function in mitigating intracellular tension as a pivotal mechanotransductive feedback mechanism that finely tunes YAP activity throughout the process of sprouting angiogenesis.

CDK5: an oncogene or an anti-oncogene: location location location

Recent studies have uncovered various physiological functions of CDK5 in many nonneuronal tissues. Upregulation of CDK5 and/or its activator p35 in neurons promotes healthy neuronal functions, but their overexpression in nonneuronal tissues is causally linked to cancer of many origins. This review focuses on the molecular mechanisms by which CDK5 recruits diverse tissue-specific substrates to elicit distinct phenotypes in sixteen different human cancers. The emerging theme suggests that CDK5's role as an oncogene or anti-oncogene depends upon its subcellular localization. CDK5 mostly acts as an oncogene, but in gastric cancer, it is a tumor suppressor due to its unique nuclear localization. This indicates that CDK5's access to certain nuclear substrates converts it into an anti-oncogenic kinase. While acting as a bonafide oncogene, CDK5 also activates a few cancer-suppressive pathways in some cancers, presumably due to the mislocalization of nuclear substrates in the cytoplasm. Therefore, directing CDK5 to the nucleus or exporting tumor-suppressive nuclear substrates to the cytoplasm may be promising approaches to combat CDK5-induced oncogenicity, analogous to neurotoxicity triggered by nuclear CDK5. Furthermore, while p35 overexpression is oncogenic, hyperactivation of CDK5 by inducing p25 formation results in apoptosis, which could be exploited to selectively kill cancer cells by dialing up CDK5 activity, instead of inhibiting it. CDK5 thus acts as a molecular rheostat, with different activity levels eliciting distinct functional outcomes. Finally, as CDK5's role is defined by its substrates, targeting them individually or in conjunction with CDK5 should create potentially valuable new clinical opportunities.

CDK5 promotes apoptosis and attenuates chemoresistance in gastric cancer via E2F1 signaling

BACKGROUND

Chemoresistance is a major clinical challenge that leads to tumor metastasis and poor clinical outcome. The mechanisms underlying gastric cancer resistance to chemotherapy are still unclear.

METHODS

We conducted bioinformatics analyses of publicly available patient datasets to establish an apoptotic phenotype and determine the key pathways and clinical significance. In vitro cell models, in vivo mouse models, and numerous molecular assays, including western blotting, qRT-PCR, immunohistochemical staining, and coimmunoprecipitation assays were used to clarify the role of factors related to apoptosis in gastric cancer in this study. Differences between datasets were analyzed using the Student's t-test and two-way ANOVA; survival rates were estimated based on Kaplan-Meier analysis; and univariate and multivariate Cox proportional hazards models were used to evaluate prognostic factors.

RESULTS

Bulk transcriptomic analysis of gastric cancer samples established an apoptotic phenotype. Proapoptotic tumors were enriched for DNA repair and immune inflammatory signaling and associated with improved prognosis and chemotherapeutic benefits. Functionally, cyclin-dependent kinase 5 (CDK5) promoted apoptosis of gastric cancer cells and sensitized cells and mice to oxaliplatin. Mechanistically, we demonstrate that CDK5 stabilizes DP1 through direct binding to DP1 and subsequent activation of E2F1 signaling. Clinicopathological analysis indicated that CDK5 depletion correlated with poor prognosis and chemoresistance in human gastric tumors.

CONCLUSION

Our findings reveal that CDK5 promotes cell apoptosis by stabilizing DP1 and activating E2F1 signaling, suggesting its potential role in the prognosis and therapeutic decisions for patients with gastric cancer.

Proteasomal turnover of the RhoGAP tumor suppressor DLC1 is regulated by HECTD1 and USP7

The Rho GTPase activating protein Deleted in Liver Cancer 1 (DLC1) is frequently downregulated through genetic and epigenetic mechanisms in various malignancies, leading to aberrant Rho GTPase signaling and thus facilitating cancer progression. Here we show that in breast cancer cells, dysregulation of DLC1 expression occurs at the protein level through rapid degradation via the ubiquitin–proteasome system. Using mass spectrometry, we identify two novel DLC1 interaction partners, the ubiquitin-ligase HECTD1 and the deubiquitinating enzyme ubiquitin-specific-processing protease 7 (USP7). While DLC1 protein expression was rapidly downregulated upon pharmacological inhibition of USP7, siRNA-mediated knockdown of HECTD1 increased DLC1 protein levels and impaired its degradation. Immunofluorescence microscopy analyses revealed that the modulation of HECTD1 levels and USP7 activity altered DLC1 abundance at focal adhesions, its primary site of action. Thus, we propose opposing regulatory mechanisms of DLC1 protein homeostasis by USP7 and HECTD1, which could open up strategies to counteract downregulation and restore DLC1 expression in cancer.

ROPGAP-dependent interaction between brassinosteroid and ROP2-GTPase signaling controls pavement cell shape in Arabidopsis

The epidermal pavement cell shape in Arabidopsis is driven by chemical and mechanical cues that direct partitioning mechanisms required for the establishment of the lobe- and indentation-defining polar sites. Brassinosteroid (BR) hormones regulate pavement cell morphogenesis, but the underlying mechanism remains unclear. Here, we identified two PLECKSTRIN HOMOLOGY GTPase-ACTIVATING proteins (PHGAPs) as substrates of the GSK3-like kinase BR-INSENSITIVE2 (BIN2). The phgap1phgap2 mutant displayed severe epidermal cell shape phenotypes, and the PHGAPs were markedly enriched in the anticlinal face of the pavement cell indenting regions. BIN2 phosphorylation of PHGAPs was required for their stability and polarization. BIN2 inhibition activated ROP2-GTPase signaling specifically in the lobes because of PHGAP degradation, while the PHGAPs restrained ROP2 activity in the indentations. Hence, we connect BR and ROP2-GTPase signaling pathways via the regulation of PHGAPs and put forward the importance of spatiotemporal control of BR signaling for pavement cell interdigitation.

Inhibition of cytoplasmic EZH2 induces antitumor activity through stabilization of the DLC1 tumor suppressor protein

mRNA expression of the DLC1 tumor suppressor gene is downregulated in many lung cancers and their derived cell lines, with DLC1 protein levels being low or absent. Although the role of increased EZH2 methyltransferase in cancer is usually attributed to its histone methylation, we unexpectedly observed that post-translational destabilization of DLC1 protein is common and attributable to its methylation by cytoplasmic EZH2, leading to CUL-4A ubiquitin-dependent proteasomal degradation of DLC1. Furthermore, siRNA knockdown of KRAS in several lines increases DLC1 protein, associated with a drastic reduction in cytoplasmic EZH2. Pharmacologic inhibition of EZH2, CUL-4A, or the proteasome can increase the steady-state level of DLC1 protein, whose tumor suppressor activity is further increased by AKT and/or SRC kinase inhibitors, which reverse the direct phosphorylation of DLC1 by these kinases. These rational drug combinations induce potent tumor growth inhibition, with markers of apoptosis and senescence, that is highly dependent on DLC1 protein.

Three decades of Cdk5

Cdk5 is a proline-directed serine/threonine protein kinase that governs a variety of cellular processes in neurons, the dysregulation of which compromises normal brain function. The mechanisms underlying the modulation of Cdk5, its modes of action, and its effects on the nervous system have been a great focus in the field for nearly three decades. In this review, we provide an overview of the discovery and regulation of Cdk5, highlighting recent findings revealing its role in neuronal/synaptic functions, circadian clocks, DNA damage, cell cycle reentry, mitochondrial dysfunction, as well as its non-neuronal functions under physiological and pathological conditions. Moreover, we discuss evidence underscoring aberrant Cdk5 activity as a common theme observed in many neurodegenerative diseases.

Dynamic interplay of two molecular switches enabled by the MEK1/2-ERK1/2 and IL-6-STAT3 signaling axes controls epithelial cell migration in response to growth factors

Cell migration is an essential physiological process, and aberrant migration of epithelial cells underlies many pathological conditions. However, the molecular mechanisms governing cell migration are not fully understood. We report here that growth factor–induced epithelial cell migration is critically dependent on the crosstalk of two molecular switches, namely phosphorylation switch (P-switch) and transcriptional switch (T-switch). P-switch refers to dynamic interactions of deleted in liver cancer 1 (DLC1) and PI3K with tensin-3 (TNS3), phosphatase and tensin homolog (PTEN), C-terminal tension, and vav guanine nucleotide exchange factor 2 (VAV2) that are dictated by mitogen-activated protein kinase kinase 1/2–extracellular signal–regulated protein kinase 1/2–dependent phosphorylation of TNS3, PTEN, and VAV2. Phosphorylation of TNS3 and PTEN on specific Thr residues led to the switch of DLC1–TNS3 and PI3K–PTEN complexes to DLC1–PTEN and PI3K–TNS3 complexes, whereas Ser phosphorylation of VAV2 promotes the transition of the PI3K–TNS3/PTEN complexes to PI3K–VAV2 complex. T-switch denotes an increase in C-terminal tension transcription/expression regulated by both extracellular signal–regulated protein kinase 1/2 and signal transducer and activator of transcription 3 (STAT3) via interleukin-6–Janus kinase–STAT3 signaling pathway. We have found that, the P-switch is indispensable for both the initiation and continuation of cell migration induced by growth factors, whereas the T-switch is only required to sustain cell migration. The interplay of the two switches facilitated by the interleukin-6–Janus kinase–STAT3 pathway governs a sequence of dynamic protein–protein interactions for sustained cell migration. That a similar mechanism is employed by both normal and tumorigenic epithelial cells to drive their respective migration suggests that the P-switch and T-switch are general regulators of epithelial cell migration and potential therapeutic targets.

Knockdown of CDK5 down-regulates PD-L1 via the ubiquitination-proteasome pathway and improves antitumor immunity in lung adenocarcinoma

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Knockdown of CDK5 down-regulates PD-L1 in lung adenocarcinoma and improves tumor immunity.

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Interference of CDK5 leads to ubiquitination and degradation of PD-L1 protein.

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TRIM21 mediates the ubiquitination and degradation process of PD-L1.

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Combination of CDK5 disruption and anti-PD-L1 therapy has a stronger effect on inhibiting tumor formation, compared with CDK5 knockdown alone.

Although immunotherapy (anti-PD-1/PD-L1 antibodies) has been approved for clinical treatment of lung cancer, only a small proportion of patients respond to monotherapy. Hence, understanding the regulatory mechanism of PD-L1 is particularly important to identify optimal combinations. In this study, we found that inhibition of CDK5 induced by shRNA or CDK5 inhibitor leads to reduced expression of PD-L1 protein in human lung adenocarcinoma cells, while the mRNA level is not substantially altered. The PD-L1 protein degradation is mediated by E3 ligase TRIM21 via ubiquitination-proteasome pathway. Subsequently, we studied the function of CDK5/PD-L1 axis in LUAD. In vitro, the absence of CDK5 in mouse Lewis lung cancer cell (LLC) has no effect on cell proliferation. However, the attenuation of CDK5 or combined with anti-PD-L1 greatly suppresses tumor growth in LLC implanted mouse models in vivo. Disruption of CDK5 elicits a higher level of CD3⁺, CD4⁺ and CD8⁺ T cells in spleens and lower PD-1 expression in CD4⁺ and CD8⁺ T cells. Our findings highlight a role for CDK5 in promoting antitumor immunity, which provide a potential therapeutic target for combined immunotherapy in LUAD.

Tumor suppressor gene DLC1: Its modifications, interactive molecules, and potential prospects for clinical cancer application

Deleted in liver cancer 1 (DLC1) is a recognized tumor suppressor gene that negatively regulates Rho family proteins by hydrolyzing the active GTP-bound state to its inactive GDP-bound state. Active Rho proteins play a positive role in tumorigenesis. Numerous in vitro and in vivo experiments have shown that DLC1 is downregulated or inactivated in various solid tumors, which may be due to the following five reasons: genomic deletion, epigenetic modification and ubiquitin-dependent proteasomal degradation may cause DLC1 underexpression; phosphorylation at the post-translation level may cause DLC1 inactivation; and failure to localize at focal adhesions (FAs) may prevent DLC1 from exerting full activity. All of the causes could be attributed to molecular binding. Experimental evidence suggests that direct or indirect targeting of DLC1 is feasible for cancer treatment. Therefore, elucidating the interaction of DLC1 with its binding partners might provide novel targeted therapies for cancer. In this review, we summarized the binding partners of DLC1 at both the gene and protein levels and expounded a variety of anticancer drugs targeting DLC1 to provide information about DLC1 as a cancer diagnostic indicator or therapeutic target.

Tensins - emerging insights into their domain functions, biological roles and disease relevance

Tensins are a family of focal adhesion proteins consisting of four members in mammals (TNS1, TNS2, TNS3 and TNS4). Their multiple domains and activities contribute to the molecular linkage between the extracellular matrix and cytoskeletal networks, as well as mediating signal transduction pathways, leading to a variety of physiological processes, including cell proliferation, attachment, migration and mechanical sensing in a cell. Tensins are required for maintaining normal tissue structures and functions, especially in the kidney and heart, as well as in muscle regeneration, in animals. This Review discusses our current understanding of the domain functions and biological roles of tensins in cells and mice, as well as highlighting their relevance to human diseases.

The role of tensins in malignant neoplasms

Tensins belong to the family of adhesion proteins which form focal adhesions serving as a bridge between the extracellular matrix and intracellular actin skeleton. The tensin family consists of four members (tensin-1 to -4) which are widely expressed in normal and cancerous tissues. The presence of Src homology 2 and phosphotyrosine binding domains is a unique feature of tensins which enables them to interact with tyrosine-phosphorylated proteins in PI3K/Akt and β-integrin/FAK signaling pathways. The tensin-mediated signaling pathway regulates physiological processes including cell motility and cytoskeleton integrity. The expression of tensins varies among cancers. Several papers report tensins as tumor suppressive proteins, whereas tensins may promote epithelial to mesenchymal transition and cancer cell metastasis. Recent findings and further research on tensins as therapeutic targets in cancers may contribute to identifying effective anti-cancer therapy. In this review we focus on the role of tensins in normal and cancer cells. We discuss potential mechanism(s) involved in carcinogenesis.

Cancer-Associated Point Mutations in the DLC1 Tumor Suppressor and Other Rho-GAPs Occur Frequently and Are Associated with Decreased Function

In advanced cancer, the RHOA GTPase is often active together with reduced expression of genes encoding Rho-specific GTPase-accelerating proteins (Rho-GAP), which negatively regulate RHOA and related GTPases. Here we used the The Cancer Genome Atlas dataset to examine 12 tumor types (including colon, breast, prostate, pancreas, lung adenocarcinoma, and squamous cell carcinoma) for the frequency of codon mutations of 10 Rho-GAP and experimentally tested biochemical and biological consequences for cancer-associated mutants that arose in the DLC1 tumor suppressor gene. DLC1 was the Rho-GAP gene mutated most frequently, with 5%-8% of tumors in five of the tumor types evaluated having DLC1 missense mutations. Furthermore, 20%-26% of the tumors in four of these five tumor types harbored missense mutations in at least one of the 10 Rho-GAPs. Experimental analysis of the DLC1 mutants indicated 7 of 9 mutants whose lesions were located in the Rho-GAP domain were deficient for Rho-GAP activity and for suppressing cell migration and anchorage-independent growth. Analysis of a DLC1 linker region mutant and a START domain mutant showed each was deficient for suppressing migration and growth in agar, but their Rho-GAP activity was similar to that of wild-type DLC1. Compared with the wild-type, the linker region mutant bound 14-3-3 proteins less efficiently, while the START domain mutant displayed reduced binding to Caveolin-1. Thus, mutation of Rho-GAP genes occurs frequently in some cancer types and the majority of cancer-associated DLC1 mutants evaluated were deficient biologically, with various mechanisms contributing to their reduced activity. SIGNIFICANCE: These findings indicate that point mutation of Rho-GAP genes is unexpectedly frequent in several cancer types, with DLC1 mutants exhibiting reduced function by various mechanisms.

Molecular Regulation of the RhoGAP GRAF3 and Its Capacity to Limit Blood Pressure In Vivo

Anti-hypertensive therapies are usually prescribed empirically and are often ineffective. Given the prevalence and deleterious outcomes of hypertension (HTN), improved strategies are needed. We reported that the Rho-GAP GRAF3 is selectively expressed in smooth muscle cells (SMC) and controls blood pressure (BP) by limiting the RhoA-dependent contractility of resistance arterioles. Importantly, genetic variants at the GRAF3 locus controls BP in patients. The goal of this study was to validate GRAF3 as a druggable candidate for future anti-HTN therapies. Importantly, using a novel mouse model, we found that modest induction of GRAF3 in SMC significantly decreased basal and vasoconstrictor-induced BP. Moreover, we found that GRAF3 protein toggles between inactive and active states by processes controlled by the mechano-sensing kinase, focal adhesion kinase (FAK). Using resonance energy transfer methods, we showed that agonist-induced FAK-dependent phosphorylation at ^Y376GRAF3 reverses an auto-inhibitory interaction between the GAP and BAR-PH domains. Y376 is located in a linker between the PH and GAP domains and is invariant in GRAF3 homologues and a phosphomimetic ^E376GRAF3 variant exhibited elevated GAP activity. Collectively, these data provide strong support for the future identification of allosteric activators of GRAF3 for targeted anti-hypertensive therapies.

Targeting the cyclin-dependent kinase 5 in metastatic melanoma

The cyclin-dependent kinase 5 (CDK5), originally described as a neuronal-specific kinase, is also frequently activated in human cancers. Using conditional CDK5 knockout mice and a mouse model of highly metastatic melanoma, we found that CDK5 is dispensable for the growth of primary tumors. However, we observed that ablation of CDK5 completely abrogated the metastasis, revealing that CDK5 is essential for the metastatic spread. In mouse and human melanoma cells CDK5 promotes cell invasiveness by directly phosphorylating an intermediate filament protein, vimentin, thereby inhibiting assembly of vimentin filaments. Chemical inhibition of CDK5 blocks the metastatic spread of patient-derived melanomas in patient-derived xenograft (PDX) mouse models. Hence, inhibition of CDK5 might represent a very potent therapeutic strategy to impede the metastatic dissemination of malignant cells.

A kinase of many talents: non-neuronal functions of CDK5 in development and disease

The cyclin-dependent kinase 5 (CDK5) represents an unusual member of the family of cyclin-dependent kinases, which is activated upon binding to non-cyclin p35 and p39 proteins. The role of CDK5 in the nervous system has been very well established. In addition, there is growing evidence that CDK5 is also active in non-neuronal tissues, where it has been postulated to affect a variety of functions such as the immune response, angiogenesis, myogenesis, melanogenesis and regulation of insulin levels. Moreover, high levels of CDK5 have been observed in different tumour types, and CDK5 was proposed to play various roles in the tumorigenic process. In this review, we discuss these various CDK5 functions in normal physiology and disease, and highlight the therapeutic potential of targeting CDK5.

Mechanistic insight of cyclin-dependent kinase 5 in modulating lung cancer growth

Lung harbors the growth of primary and secondary tumors. Even though numerous factors regulate the complex signal transduction and cytoskeletal remodeling toward the progression of lung cancer, cyclin-dependent kinase 5 (Cdk5), a previously known kinase in the central nervous system, has raised much attention in the recent years. Patients with aberrant Cdk5 expression also lead to poor survival. Cdk5 has already been employed in various cellular processes which shape the fate of cancer. In lung cancer, Cdk5 mainly regulates tumor suppressor genes, carcinogenesis, cytoskeletal remodeling, and immune checkpoints. Inhibiting Cdk5 by using drugs, siRNA or CRISP-Cas9 system has rendered crucial therapeutic advantage in the combat against lung cancer. Thus, the relation of Cdk5 to lung cancer needs to be addressed in detail. In this review, we will discuss various cellular events modulated by Cdk5 and we will go further into their underlying mechanism in lung cancer.

DLC1 SAM domain-binding peptides inhibit cancer cell growth and migration by inactivating RhoA

Deleted-in-liver cancer 1 (DLC1) exerts its tumor suppressive function mainly through the Rho-GTPase-activating protein (RhoGAP) domain. When activated, the domain promotes the hydrolysis of RhoA-GTP, leading to reduced cell migration. DLC1 is kept in an inactive state by an intramolecular interaction between its RhoGAP domain and the DLC1 sterile α motif (SAM) domain. We have shown previously that this autoinhibited state of DLC1 may be alleviated by tensin-3 (TNS3) or PTEN. We show here that the TNS3/PTEN-DLC1 interactions are mediated by the C2 domains of the former and the SAM domain of the latter. Intriguingly, the DLC1 SAM domain was capable of binding to specific peptide motifs within the C2 domains. Indeed, peptides containing the binding motifs were highly effective in blocking the C2-SAM domain-domain interaction. Importantly, when fused to the tat protein-transduction sequence and subsequently introduced into cells, the C2 peptides potently promoted the RhoGAP function in DLC1, leading to decreased RhoA activation and reduced tumor cell growth in soft agar and migration in response to growth factor stimulation. To facilitate the development of the C2 peptides as potential therapeutic agents, we created a cyclic version of the TNS3 C2 domain-derived peptide and showed that this peptide readily entered the MDA-MB-231 breast cancer cells and effectively inhibited their migration. Our work shows, for the first time, that the SAM domain is a peptide-binding module and establishes the framework on which to explore DLC1 SAM domain-binding peptides as potential therapeutic agents for cancer treatment.

Brajendra Tripathi: Keeping an eye out for translational research

Tripathi investigates how the tumor suppressor DLC1 is regulated by oncogenic kinases.

Future Aspects of CDK5 in Prostate Cancer: From Pathogenesis to Therapeutic Implications

Cyclin-dependent kinase 5 (CDK5) is a unique member of the cyclin-dependent kinase family. CDK5 is activated by binding with its regulatory proteins, mainly p35, and its activation is essential in the development of the central nervous system (CNS) and neurodegeneration. Recently, it has been reported that CDK5 plays important roles in regulating various biological and pathological processes, including cancer progression. Concerning prostate cancer, the androgen receptor (AR) is majorly involved in tumorigenesis, while CDK5 can phosphorylate AR and promotes the proliferation of prostate cancer cells. Clinical evidence has also shown that the level of CDK5 is associated with the progression of prostate cancer. Interestingly, inhibition of CDK5 prevents prostate cancer cell growth, while drug-triggered CDK5 hyperactivation leads to apoptosis. The blocking of CDK5 activity by its small interfering RNAs (siRNA) or Roscovitine, a pan-CDK inhibitor, reduces the cellular AR protein level and triggers the death of prostate cancer cells. Thus, CDK5 plays a crucial role in the growth of prostate cancer cells, and AR regulation is one of the important pathways. In this review paper, we summarize the significant studies on CDK5-mediated regulation of prostate cancer cells. We propose that the CDK5–p35 complex might be an outstanding candidate as a diagnostic marker and potential target for prostate cancer treatment in the near future.

SRC and ERK cooperatively phosphorylate DLC1 and attenuate its Rho-GAP and tumor suppressor functions

DLC1 controls focal adhesion dynamics and other processes that suppress tumorigenesis; therefore, it is unclear why some cancers maintain high levels of DLC1. Tripathi et al. show that phosphorylation of DLC1 by SRC and ERK mitigates DLC1’s tumor suppressor activities but these can be reactivated by kinase inhibition as a potential cancer treatment.

SRC and ERK kinases control many cell biological processes that promote tumorigenesis by altering the activity of oncogenic and tumor suppressor proteins. We identify here a physiological interaction between DLC1, a focal adhesion protein and tumor suppressor, with SRC and ERK. The tumor suppressor function of DLC1 is attenuated by phosphorylation of tyrosines Y451 and Y701 by SRC, which down-regulates DLC1’s tensin-binding and Rho-GAP activities. ERK1/2 phosphorylate DLC1 on serine S129, which increases both the binding of SRC to DLC1 and SRC-dependent phosphorylation of DLC1. SRC inhibitors exhibit potent antitumor activity in a DLC1-positive transgenic cancer model and a DLC1-positive tumor xenograft model, due to reactivation of the tumor suppressor activities of DLC1. Combined treatment of DLC1-positive tumors with SRC plus AKT inhibitors has even greater antitumor activity. Together, these findings indicate cooperation between the SRC, ERK1/2, and AKT kinases to reduce DLC1 Rho-GAP and tumor suppressor activities in cancer cells, which can be reactivated by the kinase inhibitors.

Effects of DLC1 Deficiency on Endothelial Cell Contact Growth Inhibition and Angiosarcoma Progression

Background

Deleted in Liver Cancer 1 (DLC1) is a tumor suppressor gene frequently deleted in cancer. However, DLC1 is not known to be deleted in angiosarcoma, an aggressive malignancy of endothelial cell derivation. Additionally, the physiologic functions of DLC1 protein in endothelial cells are poorly defined.

Methods

We investigated the effects of shRNA-induced DLC1 depletion in endothelial cells. Cell growth was measured by 3H thymidine incorporation, IncuCyte imaging, and population doublings; cell death by cell cycle analysis; gene expression by Affimetrix arrays and quantitative polymerase chain reaction; NF-κB activity by reporter assays; and protein levels by immunoblotting and immunofluorescence staining. We tested Tanespimycin/17-AAG and Fasudil treatment in groups of nine to 10 mice bearing ISOS-1 angiosarcoma. All statistical tests were two-sided.

Results

We discovered that DLC1 is a critical regulator of cell contact inhibition of proliferation in endothelial cells, promoting statistically significant (P < .001) cell death when cells are confluent (mean [SD] % viability: control DLC1 = 15.6 [19.3]; shDLC1 = 73.4 [13.1]). This prosurvival phenotype of DLC1-depleted confluent endothelial cells is attributable to a statistically significant and sustained increase of NF-κB activity (day 5, P = .001; day 8, P = .03) associated with increased tumor necrosis factor alpha-induced protein 3 (TNFAIP3/A20) signaling. Consistently, we found that DLC1 is statistically significantly reduced (P < .001 in 5 of 6) and TNFAIP3/A20 is statistically significantly increased (P < .001 in 2 of 3 and P = 0.02 in 1 of 3) in human angiosarcoma compared with normal adjacent endothelium. Treatment with the NF-κB inhibitor Tanespimycin/17-AAG statistically significantly reduced angiosarcoma tumor growth in mice (treatment tumor weight vs control, 0.50 [0.19] g vs 0.91 [0.21] g, P = .001 experiment 1; 0.66 [0.26] g vs 1.10 [0.31] g, P = .01 experiment 2).

Conclusions

These results identify DLC1 as a previously unrecognized regulator of endothelial cell contact inhibition of proliferation that is depleted in angiosarcoma and support NF-κB targeting for the treatment of angiosarcoma where DLC1 is lost.

Ginsenoside Rg3 inhibits the migration and invasion of liver cancer cells by increasing the protein expression of ARHGAP9

Ginsenoside Rg3, a naturally occurring phytochemical, serves an important role in the prevention and treatment of cancer. In the present study, with the aim to reveal the molecular mechanism of Rg3 in liver cancer cell metastasis, the anti-migration and anti-invasion effects of Rg3 on liver cancer cells were investigated. It was demonstrated that Rg3 caused marked inhibition of cell migration and invasion of human liver cancer cells, HepG2 and MHCC-97L, in vitro, and the growth of HepG2 and MHCC-97L tumors in BABL/c nude mice. The protein expression of Rho GTPase activating protein 9 (ARHGAP9) was increased both in HepG2 and MHCC-97L cells. Following ARHGAP9 knockdown, the results of Transwell and tumorigenesis assays revealed that the anti-migration, anti-invasion and anti-tumor growth effects of Rg3 were impaired significantly. The increased expression of ARHGAP9 protein induced by Rg3 was remarkably suppressed. All results suggested that ARHGAP9 protein may be a vital regulator in the anti-metastatic role of Rg3. To the best of our knowledge, the present study is the first to report that Rg3 effectively suppressed the migration and invasion of liver cancer cells by upregulating the protein expression of ARHGAP9, indicating a novel natural therapeutic agent and a therapeutic target for the treatment of liver cancer.

Mutations in six nephrosis genes delineate a pathogenic pathway amenable to treatment

No efficient treatment exists for nephrotic syndrome (NS), a frequent cause of chronic kidney disease. Here we show mutations in six different genes (MAGI2, TNS2, DLC1, CDK20, ITSN1, ITSN2) as causing NS in 17 families with partially treatment-sensitive NS (pTSNS). These proteins interact and we delineate their roles in Rho-like small GTPase (RLSG) activity, and demonstrate deficiency for mutants of pTSNS patients. We find that CDK20 regulates DLC1. Knockdown of MAGI2, DLC1, or CDK20 in cultured podocytes reduces migration rate. Treatment with dexamethasone abolishes RhoA activation by knockdown of DLC1 or CDK20 indicating that steroid treatment in patients with pTSNS and mutations in these genes is mediated by this RLSG module. Furthermore, we discover ITSN1 and ITSN2 as podocytic guanine nucleotide exchange factors for Cdc42. We generate Itsn2-L knockout mice that recapitulate the mild NS phenotype. We, thus, define a functional network of RhoA regulation, thereby revealing potential therapeutic targets.

Functional analysis of the Drosophila RhoGAP Cv-c protein and its equivalence to the human DLC3 and DLC1 proteins

RhoGAP proteins control the precise regulation of the ubiquitous small RhoGTPases. The Drosophila Crossveinless-c (Cv-c) RhoGAP is homologous to the human tumour suppressor proteins Deleted in Liver Cancer 1-3 (DLC1-3) sharing an identical arrangement of SAM, GAP and START protein domains. Here we analyse in Drosophila the requirement of each Cv-c domain to its function and cellular localization. We show that the basolateral membrane association of Cv-c is key for its epithelial function and find that the GAP domain targeted to the membrane can perform its RhoGAP activity independently of the rest of the protein, implying the SAM and START domains perform regulatory roles. We propose the SAM domain has a repressor effect over the GAP domain that is counteracted by the START domain, while the basolateral localization is mediated by a central, non-conserved Cv-c region. We find that DLC3 and Cv-c expression in the Drosophila ectoderm cause identical effects. In contrast, DLC1 is inactive but becomes functional if the central non-conserved DLC1 domain is substituted for that of Cv-c. Thus, these RhoGAP proteins are functionally equivalent, opening up the use of Drosophila as an in vivo model to analyse pharmacologically and genetically the human DLC proteins.

Phosphorylation of ARHGAP19 by CDK1 and ROCK regulates its subcellular localization and function during mitosis

ARHGAP19 is a hematopoietic-specific Rho GTPase-activating protein (RhoGAP) that acts through the RhoA/ROCK pathway to critically regulate cell elongation and cytokinesis during lymphocyte mitosis. We report here that, during mitosis progression, ARHGAP19 is sequentially phosphorylated by the RhoA-activated kinases ROCK1 and ROCK2 (hereafter ROCK) on serine residue 422, and by CDK1 on threonine residues 404 and 476. The phosphorylation of ARHGAP19 by ROCK occurs before mitosis onset and generates a binding site for 14-3-3 family proteins. ARHGAP19 is then phosphorylated by CDK1 in prometaphase. The docking of 14-3-3 proteins to phosphorylated S422 protects ARHGAP19 from dephosphorylation of the threonine sites and prevents ARHGAP19 from relocating to the plasma membrane during prophase and metaphase, thus allowing RhoA to become activated. Disruption of these phosphorylation sites results in premature localization of ARHGAP19 at the cell membrane and in its enrichment to the equatorial cortex in anaphase leading to cytokinesis failure and cell multinucleation.

ARHGAP24 inhibits cell cycle progression, induces apoptosis and suppresses invasion in renal cell carcinoma

Renal cell carcinoma (RCC) is the major cause of kidney malignancy-related deaths. Rho GTPases are key regulators in cancer cell metastasis. ARHGAP24, a Rac-specific member of the Rho GTPase-activating protein family, acts as a functional target of cancer cell migration and invasion. In the present study, we identified ARHGAP24 expression is downregulated in renal cancer tissues and is highly correlated with long-term survival in RCC patients. Therefore, we investigated the biological functions of ARHGAP24 in renal cancer cells. Ectopic expression of ARHGAP24 resulted in inhibited cell proliferation and arrested cell cycle in two renal cancer cell lines (786-0 and Caki-2); the results were confirmed by ARHGAP24 knocking down. In addition, ARHGAP24 significantly reduced the cell invasion ability and induced apoptosis in renal cancer cells. In addition, overexpressing ARHGAP24 impaired tumor formation in vivo. In summary, our results illustrated that ARHGAP24 plays a unique role in RCC progression as a tumor repressor.

The Emerging Role of Cdk5 in Cancer

Cdk5 is an atypical cyclin-dependent kinase that is well characterized for its role in the central nervous system rather than in the cell cycle. However Cdk5 has been recently implicated in the development and progression of a variety of cancers including breast, lung, colon, pancreatic, melanoma, thyroid and brain tumors. This broad pro-tumorigenic role makes Cdk5 a promising drug target for the development of new cancer therapies. Here we review the contribution of Cdk5 to molecular mechanisms that confer upon tumors the ability to grow, proliferate and disseminate to secondary organs, as well as resistance to chemotherapies. We subsequently discuss existing and new strategies for targeting Cdk5 and its downstream mechanisms as anti-cancer treatments.

Receptor tyrosine kinase activation of RhoA is mediated by AKT phosphorylation of DLC1

A new common mechanism for increasing RhoA-GTP is identified in Tripathi et al. The increased RhoA-GTP results from signaling mechanisms that phosphorylate and attenuate the DLC1 tumor suppressor, which encodes RhoGAP. The potentially reversible nature of this attenuation may have therapeutic relevance in cancer.

We report several receptor tyrosine kinase (RTK) ligands increase RhoA–guanosine triphosphate (GTP) in untransformed and transformed cell lines and determine this phenomenon depends on the RTKs activating the AKT serine/threonine kinase. The increased RhoA-GTP results from AKT phosphorylating three serines (S298, S329, and S567) in the DLC1 tumor suppressor, a Rho GTPase-activating protein (RhoGAP) associated with focal adhesions. Phosphorylation of the serines, located N-terminal to the DLC1 RhoGAP domain, induces strong binding of that N-terminal region to the RhoGAP domain, converting DLC1 from an open, active dimer to a closed, inactive monomer. That binding, which interferes with the interaction of RhoA-GTP with the RhoGAP domain, reduces the hydrolysis of RhoA-GTP, the binding of other DLC1 ligands, and the colocalization of DLC1 with focal adhesions and attenuates tumor suppressor activity. DLC1 is a critical AKT target in DLC1-positive cancer because AKT inhibition has potent antitumor activity in the DLC1-positive transgenic cancer model and in a DLC1-positive cancer cell line but not in an isogenic DLC1-negative cell line.

CDK5 in oncology: recent advances and future prospects

Selective abrogation of cyclin-dependent kinases (CDK) activity is a highly promising strategy in cancer treatment. The atypical CDK, CDK5 has long been known for its role in neurodegenerative diseases, and is becoming an attractive drug target for cancer therapy. Myriads of recent studies have uncovered that aberrant expression of CDK5 contributes to the oncogenic initiation and progression of multiple solid and hematological malignancies. CDK5 is also implicated in the regulation of cancer stem cell biology. In this review, we present the current state of knowledge of CDK5 as a druggable target for cancer treatment. We also provide a detailed outlook of designing selective and potent inhibitors of this enzyme.

Down-regulation of DLC1 in endothelial cells compromises the angiogenesis process

DLC1 is a RhoGAP-containing tumor suppressor that inhibits angiogenesis by repressing VEGF production in epithelial cells. Here we report the roles of DLC1 in endothelial cells. Silencing of DLC1 (siDLC1) enhances cell migration but reduces tube formation activities of human umbilical vein endothelial cells (HUVECs). Biochemically, RhoA activity and paxillin protein level are markedly increased in siDLC1 HUVECs. Although further silencing of RhoA restores the cell migration phenotype, the tube formation defect and up-regulated paxillin level remain unchanged. On the other hand, paxillin knockdown rescues tube formation and migration phenotypes but not the up-regulated RhoA activity. These results indicate that DLC1 regulates endothelial cell migration through RhoA and paxillin independently and controls tube formation mainly via paxillin. To further determine endothelial DLC1's function, we have generated endothelial specific knockout mice (DLC1-Tek). DLC1-Tek mice appear to be normal and healthy but their angiogenesis processes are compromised as shown in gel plug and aortic ring sprouting assays. Analysis of endothelial cells isolated from DLC1-Tek mice has further affirmed the cellular and biochemical phenotypes established in siDLC1 HUVECs. Our studies have demonstrated a positive regulatory role of endothelial DLC1 in angiogenesis.

Deciphering the Molecular and Functional Basis of RHOGAP Family Proteins: A SYSTEMATIC APPROACH TOWARD SELECTIVE INACTIVATION OF RHO FAMILY PROTEINS

RHO GTPase-activating proteins (RHOGAPs) are one of the major classes of regulators of the RHO-related protein family that are crucial in many cellular processes, motility, contractility, growth, differentiation, and development. Using database searches, we extracted 66 distinct human RHOGAPs, from which 57 have a common catalytic domain capable of terminating RHO protein signaling by stimulating the slow intrinsic GTP hydrolysis (GTPase) reaction. The specificity of the majority of the members of RHOGAP family is largely uncharacterized. Here, we comprehensively investigated the sequence-structure-function relationship between RHOGAPs and RHO proteins by combining our in vitro data with in silico data. The activity of 14 representatives of the RHOGAP family toward 12 RHO family proteins was determined in real time. We identified and structurally verified hot spots in the interface between RHOGAPs and RHO proteins as critical determinants for binding and catalysis. We have found that the RHOGAP domain itself is nonselective and in some cases rather inefficient under cell-free conditions. Thus, we propose that other domains of RHOGAPs confer substrate specificity and fine-tune their catalytic efficiency in cells.

Regulating Rho GTPases and their regulators

Rho GTPases regulate cytoskeletal and cell adhesion dynamics and thereby coordinate a wide range of cellular processes, including cell migration, cell polarity and cell cycle progression. Most Rho GTPases cycle between a GTP-bound active conformation and a GDP-bound inactive conformation to regulate their ability to activate effector proteins and to elicit cellular responses. However, it has become apparent that Rho GTPases are regulated by post-translational modifications and the formation of specific protein complexes, in addition to GTP-GDP cycling. The canonical regulators of Rho GTPases - guanine nucleotide exchange factors, GTPase-activating proteins and guanine nucleotide dissociation inhibitors - are regulated similarly, creating a complex network of interactions to determine the precise spatiotemporal activation of Rho GTPases.

Cyclic AMP and Polyamines Overcome Inhibition by Myelin-Associated Glycoprotein through eIF5A-Mediated Increases in p35 Expression and Activation of Cdk5

Inhibitory molecules associated with CNS myelin, such as myelin-associated glycoprotein (MAG), represent major obstacles to axonal regeneration following CNS injury. Our laboratory has shown that elevating levels of intracellular cAMP, via application of the nonhydrolyzable analog dibutyryl cAMP (dbcAMP), can block the inhibitory effects of MAG and myelin. We have also shown that elevation of cAMP results in upregulation of arginase I and increased polyamine synthesis. Treatment with putrescine or spermidine blocks myelin-mediated inhibition of neurite outgrowth, but the mechanism underlying this effect has not yet been elucidated. Here we show that cyclin-dependent kinase 5 (Cdk5) is required for dbcAMP and putrescine to overcome MAG-mediated inhibition. The ability of dbcAMP and putrescine to overcome inhibition by MAG is abolished in the presence of roscovitine, a Cdk inhibitor that has greater selectivity for Cdk5, and expression of dominant negative Cdk5 abolishes the ability of dbcAMP or putrescine to enhance neurite outgrowth in the presence of MAG. Importantly, dbcAMP and putrescine increase expression of p35, the neuron-specific activator of Cdk5, and rat DRG neurons transduced with HSV overexpressing p35 can overcome inhibition by MAG. The upregulation of p35 by putrescine is also reflected in increased localization of p35 to neurites and growth cones. Last, we show that putrescine upregulates p35 expression by serving as a substrate for hypusine modification of eIF5A, and that this hypusination is necessary for putrescine's ability to overcome inhibition by MAG. Our findings reveal a previously unknown mechanism by which polyamines may encourage regeneration after CNS injury.

An immunohistochemical study of cyclin-dependent kinase 5 (CDK5) expression in non-small cell lung cancer (NSCLC) and small cell lung cancer (SCLC): a possible prognostic biomarker

Background

Cyclin-dependent kinase 5 (CDK5) is an atypical CDK which plays a vital role in several cancers via regulating migration and motility of cancer cells. However, the clinicopathological impact and function of CDK5 in lung cancer remain poorly understood. The present study was aimed at exploring expression and clinicopathological significance of CDK5 in lung cancer.

Methods

There were 395 samples of lung tissue including 365 lung tumors (339 non-small cell lung cancers and 26 small cell lung cancers) and 30 samples of normal lung. CDK5 expression was detected by immunohistochemistry on lung tissue microarrays.

Results

Over expression was detected in lung cancer compared with normal lung tissues (P = 0.001). Furthermore, area under curve (AUC) of receiver operating characteristic (ROC) of CDK5 was 0.685 (95 % CI 0.564~0.751, P = 0.004). In lung cancer, we also discovered close correlations between CDK5 and pathological grading (r = 0.310, P < 0.001), TNM stage (r = 0.155, P = 0.003), and lymph node metastasis (r = 0.279, P < 0.001) by using Spearman analysis. In two subgroups of non-small cell lung cancer (NSCLC) and small cell lung cancer (SCLC), the expression of CDK5 was also higher than that of normal lung tissue, respectively (P = 0.001 and P = 0.004). Moreover, in NSCLCs, Spearman analysis revealed that expression of CDK5 was correlated with TNM stages (r = 0.129, P = 0.017), lymph node metastasis (r = 0.365, P < 0.001), and pathological grading (r = 0.307, P < 0.001), respectively. The significant correlation was also found between CDK5 expression and TNM stages (r = 0.415, P = 0.049) and lymphatic metastasis (r = 0.469, P = 0.024) in SCLCs.

Conclusions

The results of this present study suggest that the CDK5 expression is associated with several clinicopathological factors linked with poorer prognosis.

Tensin1 positively regulates RhoA activity through its interaction with DLC1

DLC1 is a RhoGAP-containing tumor suppressor and many of DLC1's functions are absolutely dependent on its RhoGAP activity. Through its RhoGAP domain, DLC1 inhibits the activity of RhoA GTPase, which regulates actin cytoskeleton networks and dis/assembly of focal adhesions. Tensin1 (TNS1) is a focal adhesion molecule that links the actin cytoskeleton to integrins and forms signaling complexes through its multiple binding domains. Here, we report that TNS1 enhances RhoA activity in a DLC1-dependent manner. This is accomplished by binding to DLC1 through TNS1's C2, SH2, and PTB domains. Point mutations at these three sites disrupt TNS1's interaction with DLC1 as well as its effect on RhoA activity. The biological relevance of this TNS1-DLC1-RhoA signaling axis is investigated in TNS1 knockout (KO) cells and mice. Endothelial cells isolated from TNS1 KO mice or those silenced with TNS1 siRNA show significant reduction in proliferation, migration, and tube formation activities. Concomitantly, the RhoA activity is down-regulated in TNS1 KO cells and this reduction is restored by further silencing of DLC1. Furthermore, the angiogenic process is compromised in TNS1 KO mice. These studies demonstrate that TNS1 binds to DLC1 and fine-tunes its RhoGAP activity toward RhoA and that the TNS1-DLC1-RhoA signaling axis is critical in regulating cellular functions that lead to angiogenesis.

Cyclin-Dependent Kinase 5 (CDK5) Controls Melanoma Cell Motility, Invasiveness, and Metastatic Spread-Identification of a Promising Novel therapeutic target

Despite considerable progress in recent years, the overall prognosis of metastatic malignant melanoma remains poor, and curative therapeutic options are lacking. Therefore, better understanding of molecular mechanisms underlying melanoma progression and metastasis, as well as identification of novel therapeutic targets that allow inhibition of metastatic spread, are urgently required. The current study provides evidence for aberrant cyclin-dependent kinase 5 (CDK5) activation in primary and metastatic melanoma lesions by overexpression of its activator protein CDK5R1/p35. Moreover, using melanoma in vitro model systems, shRNA-mediated inducible knockdown of CDK5 was found to cause marked inhibition of cell motility, invasiveness, and anchorage-independent growth, while at the same time net cell growth was not affected. In vivo, CDK5 knockdown inhibited growth of orthotopic xenografts as well as formation of lung and liver colonies in xenogenic injection models mimicking systemic metastases. Inhibition of lung metastasis was further validated in a syngenic murine melanoma model. CDK5 knockdown was accompanied by dephosphorylation and overexpression of caldesmon, and concomitant caldesmon knockdown rescued cell motility and proinvasive phenotype. Finally, it was found that pharmacological inhibition of CDK5 activity by means of roscovitine as well as by a novel small molecule CDK5-inhibitor, N-(5-isopropylthiazol-2-yl)-3-phenylpropanamide, similarly caused marked inhibition of invasion/migration, colony formation, and anchorage-independent growth of melanoma cells. Thus, experimental data presented here provide strong evidence for a crucial role of aberrantly activated CDK5 in melanoma progression and metastasis and establish CDK5 as promising target for therapeutic intervention.

Insights into the clinical value of cyclin-dependent kinase 5 in glioma: a retrospective study

Background

Previous studies suggested that expression of cyclin-dependent kinase 5 (CDK5) may promote the migration and invasion of human glioma cells. In this study, we aimed to evaluate the clinical value of CDK5 in different grades of glioma in relation to Ki-67 labeling index (LI).

Methods

We firstly assessed by immunohistochemistry the expression of CDK5 in 152 glioma tissues and 16 normal brain tissues and further explored the relationship between CDK5 expression and other clinical features.

Results

The positive ratio of CDK5 in gliomas (57.2 %) was higher than that in normal brain tissues (12.5 %, P = 0.001). Difference of CDK5 expression among four World Health Organization (WHO) grades was statistically significant (P = 0.001). The significant differences of CDK5 expression were also observed between WHO I glioma (34.8 %) and WHO III glioma (62.5 %), as well as WHO IV glioma (82.8 %; P = 0.026, P < 0.001, respectively). Furthermore, Spearman’s rank correlation confirmed that CDK5 was positively correlated with the pathological grade of glioma (r = 0.831, P < 0.001). The CDK5 expression was also positively correlated with Ki-67 LI (r = 0.347, P < 0.001).

Conclusions

The current result suggests that CDK5 may play an essential role in the tumorigenesis and aggressiveness of gliomas.

A phosphorylation switch controls the spatiotemporal activation of Rho GTPases in directional cell migration

Although cell migration plays a central role in development and disease, the underlying molecular mechanism is not fully understood. Here we report that a phosphorylation-mediated molecular switch comprising deleted in liver cancer 1 (DLC1), tensin-3 (TNS3), phosphatase and tensin homologue (PTEN) and phosphoinositide-3-kinase (PI3K) controls the spatiotemporal activation of the small GTPases, Rac1 and RhoA, thereby initiating directional cell migration induced by growth factors. On epidermal growth factor (EGF) or platelet-derived growth factor (PDGF) stimulation, TNS3 and PTEN are phosphorylated at specific Thr residues, which trigger the rearrangement of the TNS3–DLC1 and PTEN–PI3K complexes into the TNS3–PI3K and PTEN–DLC1 complexes. Subsequently, the TNS3–PI3K complex translocates to the leading edge of a migrating cell to promote Rac1 activation, whereas PTEN–DLC1 translocates to the posterior for localized RhoA activation. Our work identifies a core signalling mechanism by which an external motility stimulus is coupled to the spatiotemporal activation of Rac1 and RhoA to drive directional cell migration.

Directed cell migration requires spatially regulated activity of GTPases Rac1 and RhoA. Here Cao et al. show that growth factor stimulation promotes phosphorylation of tensin-3 and phosphatase and tensin homologue (PTEN) and their association with PI 3-kinase and deleted in liver cancer 1 (DLC1) to regulate GTPase activity.

Rho regulation: DLC proteins in space and time

Rho GTPases function as molecular switches that connect changes of the external environment to intracellular signaling pathways. They are active at various subcellular sites and require fast and tight regulation to fulfill their role as transducers of extracellular stimuli. New imaging technologies visualizing the active states of Rho proteins in living cells elucidated the necessity of precise spatiotemporal activation of the GTPases. The local regulation of Rho proteins is coordinated by the interaction with different guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) that turn on and off GTPase signaling to downstream effectors. GEFs and GAPs thus serve as critical signaling nodes that specify the amplitude and duration of a particular Rho signaling pathway. Despite their importance in Rho regulation, the molecular aspects underlying the spatiotemporal control of the regulators themselves are still largely elusive. In this review we will focus on the Deleted in Liver Cancer (DLC) family of RhoGAP proteins and summarize the evidence gathered over the past years revealing their different subcellular localizations that might account for isoform-specific functions. We will also highlight the importance of their tightly controlled expression in the context of neoplastic transformation.