p57(KIP2) control of actin cytoskeleton dynamics is responsible for its mitochondrial pro-apoptotic effect

LIM Kinases, LIMK1 and LIMK2, Are Crucial Node Actors of the Cell Fate: Molecular to Pathological Features

LIM kinase 1 (LIMK1) and LIM kinase 2 (LIMK2) are serine/threonine and tyrosine kinases and the only two members of the LIM kinase family. They play a crucial role in the regulation of cytoskeleton dynamics by controlling actin filaments and microtubule turnover, especially through the phosphorylation of cofilin, an actin depolymerising factor. Thus, they are involved in many biological processes, such as cell cycle, cell migration, and neuronal differentiation. Consequently, they are also part of numerous pathological mechanisms, especially in cancer, where their involvement has been reported for a few years and has led to the development of a wide range of inhibitors. LIMK1 and LIMK2 are known to be part of the Rho family GTPase signal transduction pathways, but many more partners have been discovered over the decades, and both LIMKs are suspected to be part of an extended and various range of regulation pathways. In this review, we propose to consider the different molecular mechanisms involving LIM kinases and their associated signalling pathways, and to offer a better understanding of their variety of actions within the physiology and physiopathology of the cell.

Functional Versatility of the CDK Inhibitor p57Kip2

The cyclin/CDK inhibitor p57^Kip2 belongs to the Cip/Kip family, with p21^Cip1 and p27^Kip1, and is the least studied member of the family. Unlike the other family members, p57^Kip2 has a unique role during embryogenesis and is the only CDK inhibitor required for embryonic development. p57^Kip2 is encoded by the imprinted gene CDKN1C, which is the gene most frequently silenced or mutated in the genetic disorder Beckwith-Wiedemann syndrome (BWS), characterized by multiple developmental anomalies. Although initially identified as a cell cycle inhibitor based on its homology to other Cip/Kip family proteins, multiple novel functions have been ascribed to p57^Kip2 in recent years that participate in the control of various cellular processes, including apoptosis, migration and transcription. Here, we will review our current knowledge on p57^Kip2 structure, regulation, and its diverse functions during development and homeostasis, as well as its potential implication in the development of various pathologies, including cancer.

p57KIP2‑mediated inhibition of human trophoblast apoptosis and promotion of invasion in vitro

Placental hypoxia serves a role in the early stages of normal pregnancy and is involved in the pathophysiology of preeclampsia. Previously, it was suggested that p57kinase inhibitory protein (KIP)2 regulates the cell cycle during embryogenesis and apoptosis. Recent evidence has indicated that p57KIP2 is increased in preeclamptic placentas and absence of p57KIP2 induces preeclampsia‑type symptoms in rats. However, effects of p57KIP2 on apoptosis under hypoxic conditions remain to be elucidated. In the present study, HTR‑8/SVneo trophoblasts were cultured under hypoxic conditions (2% O2). Knockdown using small interfering (si)RNA and overexpression of p57KIP2 were utilized to explore the biological function of p57KIP2 in apoptosis and cell function in vitro. Furthermore, expression of p57KIP2 and apoptosis were evaluated by western blotting, flow cytometry and TUNEL assays, and the response of trophoblasts to hypoxia and the role of p57KIP2 in trophoblast migration and invasion was assessed. The role of p57KIP2 in the JNK signaling pathway in HTR‑8/SVneo trophoblasts was further studies. In vitro, protein expression of p57KIP2 was increased in HTR‑8/SVneo cells exposed to 2% O2. Exogenous p57KIP2 overexpression significantly decreased the expression of pro‑apoptosis proteins, including p53, Bax and cleaved caspase3, under hypoxic conditions for 24 h. In addition, knockdown of p57KIP2 increased the response to apoptosis following hypoxia for 24 h. The present study revealed that overexpression of p57KIP2 decreased the levels of phosphorylated‑JNK. JNK inhibitor treatment combined with the overexpression of p57KIP2 significantly decreased the levels of apoptosis and increased cell invasion and migration. Taken together, p57KIP2 knockdown significantly increased apoptosis in HTR‑8/SVneo cells exposed to 2% O2, whereas overexpression of p57KIP2 had opposite effects, mediated by the JNK/stress activated protein kinase (SAPK) signaling pathway. The results indicated that hypoxia‑induced expression of p57KIP2 promoted trophoblast migration and invasion by mediating the JNK/SAPK signaling pathway, which is crucial during placentation. These results may provide a novel molecular mechanism to understand the involvement of p57KIP2 in the pathogenesis of preeclampsia.

The long non-coding RNA Kcnq1ot1 controls maternal p57 expression in muscle cells by promoting H3K27me3 accumulation to an intragenic MyoD-binding region

BACKGROUND

The cell-cycle inhibitor p57^kip2 plays a critical role in mammalian development by coordinating cell proliferation and differentiation in many cell types. p57^kip2 expression is finely regulated by several epigenetic mechanisms, including paternal imprinting. Kcnq1ot1, a long non-coding RNA (LncRNA), whose gene maps to the p57^Kip2 imprinting domain, is expressed exclusively from the paternal allele and participates in the cis-silencing of the neighboring imprinted genes through chromatin-level regulation. In light of our previous evidence of a functional interaction between myogenic factors and imprinting control elements in the regulation of the maternal p57^Kip2 allele during muscle differentiation, we examined the possibility that also Kcnq1ot1 could play an imprinting-independent role in the control of p57^Kip2 expression in muscle cells.

RESULTS

We found that Kcnq1ot1 depletion by siRNA causes the upregulation of the maternal and functional p57^Kip2 allele during differentiation, suggesting a previously undisclosed role for this LncRNA. Consistently, Chromatin Oligo-affinity Precipitation assays showed that Kcnq1ot1 physically interacts not only with the paternal imprinting control region of the locus, as already known, but also with both maternal and paternal alleles of a novel p57^Kip2 regulatory region, located intragenically and containing two binding sites for the muscle-specific factor MyoD. Moreover, chromatin immunoprecipitation assays after Kcnq1ot1 depletion demonstrated that the LncRNA is required for the accumulation of H3K27me3, a chromatin modification catalyzed by the histone-methyl-transferase EZH2, at the maternal p57^kip2 intragenic region. Finally, upon differentiation, the binding of MyoD to this region and its physical interaction with Kcnq1ot1, analyzed by ChIP and RNA immunoprecipitation assays, correlate with the loss of EZH2 and H3K27me3 from chromatin and with p57^Kip2 de-repression.

CONCLUSIONS

These findings highlight the existence of an imprinting-independent role of Kcnq1ot1, adding new insights into the biology of a still mysterious LncRNA. Moreover, they expand our knowledge about the molecular mechanisms underlying the tight and fine regulation of p57^Kip2 during differentiation and, possibly, its aberrant silencing observed in several pathologic conditions.

Down-regulation of the cyclin-dependent kinase inhibitor p57 is mediated by Jab1/Csn5 in hepatocarcinogenesis

Down-regulation of p57 (KIP2) cyclin-dependent kinase inhibitors accelerates the growth and invasion of hepatocellular carcinoma (HCC), suggesting that p57 may play an important role in liver carcinogenesis. However, the mechanism or oncogenic signal leading to p57 down-regulation in HCC remains to be determined. Herein, we demonstrated that Jab1/Csn5 expression is negatively correlated with p57 levels in HCC tissues. Kaplan-Meier analysis of tumor samples revealed that high Jab1/Csn5 expression with concurrent low p57 expression is associated with poor overall survival. The inverse pattern of Jab1 and p57 expression was also observed during carcinogenesis in a chemically induced rat HCC model. We also found that mechanistically, Jab1-mediated p57 proteolysis in HCC cells is dependent on 26S-proteasome inhibitors. We further demonstrated that direct physical interaction between Jab1 and p57 triggers p57 down-regulation, independently of Skp2 and Akt pathways, in HCC cells. These data suggest that Jab1 is an important upstream negative regulator of p57 and that aberrant expression of Jab1 in HCC could lead to a significant decrease in p57 levels and contribute to tumor cell growth. Furthermore, restoration of p57 levels induced by loss of Jab1 inhibited tumor cell growth and further increased cell apoptosis in HCC cells. Moreover, silencing Jab1 expression further enhanced the antitumor effects of cisplatin-induced apoptosis in HCC cells.

CONCLUSION

Jab1-p57 pathway confers resistance to chemotherapy and may represent a potential target for investigational therapy in HCC.

TNFα driven HIF-1α-hexokinase II axis regulates MHC-I cluster stability through actin cytoskeleton

Hypoxia-inducible Factor-1α (HIF-1α)-regulated expression of Hexokinase-II (HKII) remains a cornerstone in the maintenance of high metabolic demands subserving various pro-tumor functions including immune evasion in gliomas. Since inflammation-induced HIF-1α regulates Major Histocompatibility Complex class I (MHC-I) gene expression, and as cytoskeletal dynamics affect MHC-I membrane clusters, we investigated the involvement of HIF-1α-HKII axis in Tumor Necrosis Factor-α (TNFα)-mediated MHC-I membrane cluster stability in glioma cells and the involvement of actin cytoskeleton in the process. TNFα increased the clustering and colocalization of MHC-I with cortical actin in a HIF-1α dependent manner. siRNA mediated knockdown of HIF-1α as well as enzymatic inhibition of HK II by Lonidamine, delocalized mitochondrially bound HKII. This altered subcellular HKII localization affected TNFα-induced cofilin activation and actin turnover, as pharmacological inhibition of HKII by Lonidamine decreased Actin-related protein 2 (ARP2)/cofilin interaction. Photobleaching studies revealed destabilization of TNFα- induced stable MHC-I membrane clusters in the presence of Lonidamine and ARP2 inhibitor CK666. This work highlights how TNFα triggers a previously unknown function of metabolic protein HKII to influence an immune related outcome. Our study establishes the importance of inflammation induced HIF-1α in integrating two crucial components- the metabolic and immune, through reorganization of cytoskeleton.

Cellular Response upon Stress: p57 Contribution to the Final Outcome

Progression through the cell cycle is one of the most important decisions during the life of a cell and several kinds of stress are able to influence this choice. p57 is a cyclin-dependent kinase inhibitor belonging to the CIP/KIP family and is a well-known regulator of the cell cycle during embryogenesis and tissue differentiation. p57 loss has been reported in a variety of cancers and great effort has been spent during the past years studying the mechanisms of p57 regulation and the effects of p57 reexpression on tumor growth. Recently, growing amount of evidence points out that p57 has a specific function in cell cycle regulation upon cellular stress that is only partially shared by the other CIP/KIP inhibitors p21 and p27. Furthermore, it is nowadays emerging that p57 plays a role in the induction of apoptosis and senescence after cellular stress independently of its cell cycle related functions. This review focuses on the contribution that p57 holds in regulating cell cycle arrest, apoptosis, and senescence after cellular stress with particular attention to the response of cancer cells.

Methylation of KvDMR1 involved in regulating the imprinting of CDKN1C gene in cattle

The CDKN1C gene encodes a cyclin-dependent kinase inhibitor and is one of the key genes involved in the development of Beckwith-Wiedemann syndrome and cancer. In this study, using a direct sequencing approach based on a single nucleotide polymorphism (SNP) at genomic DNA and cDNA levels, we show that CDKN1C exhibits monoallelic expression in all seven studied organs (heart, liver, spleen, lung, kidney, muscle and subcutaneous fat) in cattle. To investigate how methylation regulates imprinting of CDKN1C in cattle, allele-specific methylation patterns in two putative differential methylation regions (DMRs), the CDKN1C DMR and KvDMR1, were analyzed in three tissues (liver, spleen and lung) using bisulfite sequencing PCR. Our results show that in the CDKN1C DMR both parental alleles were unmethylated in all three analyzed tissues. In contrast, KvDMR1 was differentially methylated between the two parental alleles in the same tissues. Statistical analysis showed that there is a significant difference in the methylation level between the two parental alleles (P < 0.01), confirming that this region is the DMR of KvDMR1 and that it may be correlated with CDKN1C imprinting.

Novel roles for actin in mitochondrial fission

Mitochondrial dynamics, including fusion, fission and translocation, are crucial to cellular homeostasis, with roles in cellular polarity, stress response and apoptosis. Mitochondrial fission has received particular attention, owing to links with several neurodegenerative diseases. A central player in fission is the cytoplasmic dynamin-related GTPase Drp1, which oligomerizes at the fission site and hydrolyzes GTP to drive membrane ingression. Drp1 recruitment to the outer mitochondrial membrane (OMM) is a key regulatory event, which appears to require a pre-constriction step in which the endoplasmic reticulum (ER) and mitochondrion interact extensively, a process termed ERMD (ER-associated mitochondrial division). It is unclear how ER-mitochondrial contact generates the force required for pre-constriction or why pre-constriction leads to Drp1 recruitment. Recent results, however, show that ERMD might be an actin-based process in mammals that requires the ER-associated formin INF2 upstream of Drp1, and that myosin II and other actin-binding proteins might be involved. In this Commentary, we present a mechanistic model for mitochondrial fission in which actin and myosin contribute in two ways; firstly, by supplying the force for pre-constriction and secondly, by serving as a coincidence detector for Drp1 binding. In addition, we discuss the possibility that multiple fission mechanisms exist in mammals.

TAp73β-mediated suppression of cell migration requires p57Kip2 control of actin cytoskeleton dynamics

The TP73 gene, a member of the p53 family, due to the use of different promoters and alternative splicing, is transcribed into different isoforms with contrasting attributes and which contribute to its functional diversity. Considerable efforts are made to identify the functional diversity of the p73 splicing variants during tumorigenesis.TAp73α and TAp73β isoforms have been shown to differentially regulate cell cycle progression, differentiation and apoptosis. Interestingly, a particular increase in expression of the TAp73 isoform, in favor of the α splicing variant, has been reported in multiple tumour types. Here, we report a distinctive role for TAp73β isoform in the control of cell migration and invasion. In fact, TAp73β-dependent induction of p57^Kip2 expression accounted for inhibitory effects on the actin cytoskeleton dynamics and thereby cancer cell motility. In contrast, TAp73α is not able to induce p57^Kip2 expression, and exhibits a positive effect on actin cytoskeleton dynamics as well as cell migration and invasion. In conclusion, the inhibitory effect on cell migration and invasion of TAp73β would qualify this distinct p73 isoform as tumor suppressor gene. In contrast, the promoting effect of TAp73α on cell motility and invasion strengthens the potential oncogenic activities of this p73 isoform.

Effects of EHD2 interference on migration of esophageal squamous cell carcinoma

C-Terminal EH domain-containing protein 2 (EHD2) of the EHD family is associated with plasma membrane. We investigated the expression of EHD2 in human esophageal squamous cell carcinoma (ESCC) and the EHD2 expression to study the therapeutic effect of chemotherapy drugs. Western blot and immunohistochemistry were used to measure the expression of EHD2 protein in ESCC and adjacent normal tissue in 98 patients. EHD2 protein level was reduced in ESCC tissues in comparison with adjacent normal tissues. Under-expression of EHD2 increased the motility property of ESCC cell TE1 in vitro by wound-healing assays and transwell migration assays, and it was concurrent with the decreased expression of epithelial marker E-cadherin. Under-expression of EHD2 in TE1 can cause resistance to cisplatin. Our results suggested that EHD2 low expression is involved in the pathogenesis of ESCC, and it might be a favorable independent poor prognostic parameter for ESCC.

TRPM7, the cytoskeleton and neuronal death

Ischemic stroke is one of the leading causes of disability and death in the world. Elucidation of the underlying mechanisms associated with neuronal death during this detrimental process has been of significant interest in the field of research. One principle component vital to the maintenance of cellular integrity is the cytoskeleton. Studies suggest that abnormalities at the level of this fundamental structure are directly linked to adverse effects on cellular well-being, including cell death. In recent years, evidence has also emerged regarding an imperative role for the transient receptor potential (TRP) family member TRPM7 in the mediation of excitotoxic-independent neuronal demise. In this review, we will elaborate on the current knowledge and unique properties associated with the functioning of this structure. In addition, we will deliberate the involvement of distinct mechanistic pathways during TRPM7-dependent cell death, including modifications at the level of the cytoskeleton.