The major function of cofilin-1 is the regulation of actin dynamics. Actin is a family of globular proteins that form microfilaments as part of the cell cytoskeleton. Actin can be present either as a free monomer called G-actin or as part of linear polymer microfilament called F-actin (Carlier et al. 1999). At steady state, F-actin grows at one end through ATP-loaded G-actin molecule association. At the opposite end of the filament, monomers undergo dissociation via the hydrolysis of ATP bound to ADP. To enter into a new polymerization cycle, ADP-loaded actin needs to exchange ADP for ATP.
Cofilin-1 controls actin dynamics by binding to both F-actin and G-actin. Studies have suggested that, depending on its concentration, cofilin-1 can promote assembly or disassembly of actin filaments (Bernstein and Banburg 2010). When only a few cofilin-1 molecules are bound to F-actin, it leads to filament breakage. On the other hand, when the concentration of cofilin-1 is high, severing is no longer observed. In this case, there is dissociation from the pointed ends of the microfilament, which is related to cofilin-1’s capacity to enhance Pi release and thus promote transformation of filaments to their ADP-loaded state. Furthermore, very high cofilin-1 concentration leads to increased nucleation and actin assembly promotion (Bernstein and Banburg 2010).
Therefore, since actin dynamics plays a role in the control of cellular morphology, cell migration, cell division, endocytosis, intracellular transport, and neuronal development, cofilin-1 is a fundamental protein in biological systems.
Cofilin-1 has highly complex modes of regulation, which can lead to inactivation and activation, as well as changes in actin-binding affinity. Three mechanisms have been widely described: (i) phosphorylation, (ii) increase of the pH and binding of phosphatidylinositol 4,5-bisphosphate (PIP2), and (iii) oxidation. Evidence indicates that these three factors can also act in concert (Bernstein and Bamburg 2010).
The best characterized mechanism of cofilin-1 regulation is the Ser-3 phosphorylation by LIM (1 and 2) or TES (testicles) kinases (TESK) (Arber et al. 1998) and its dephosphorylation by Slingshot phosphatase isoform 1 (SSH1) (Bernstein and Banburg 2010). Phosphorylation of cofilin-1 results in protein inactivation and subsequent elimination of its actin-binding function (Arber et al. 1998). The phosphorylation of cofilin-1 is also mediated by upstream enzymes that regulate LIMK activity, such as Rho/Rac GTPases-PAK pathways (Yang et al. 1998). On the other hand, cofilin-1 is activated by dephosphorylation (by SSH-1), which is activated by calcineurin, a Ca2+/calmodulin-dependent phosphatase (Wang et al. 2005).
Another regulation mechanism of cofilin-1 is the dissociation of PIP2 binding and the increase of pH levels. A study using yeast cofilin-1 showed that the PIP2 binding site is a large positively charged surface that consists of residues in helix 3 as well as residues in other parts of the cofilin-1 molecule. Moreover, PIP2 binding overlaps F-actin binding sites, which can explain the decrease in actin activity upon PIP2 binding. The pH control of cofilin-1 activity is related to the deprotonation of His133 in the F-actin binding site. In vitro, its activity increased in neutral and alkaline pHs. However, these two mechanisms can interact with each other as when intracellular pH increases via the influx of Na+ and efflux of H+, cofilin-1 is released from its PIP2 inhibitory binding.
Oxidative stress can also mediate the regulation of actin dynamics. A study using human T cells treated with H2O2 demonstrated the formation of an intramolecular disulfide bridge between Cys-39 and Cys-80, causing a conformational change in the molecule. Furthermore, it was found that oxidized (and dephosphorylated) cofilin-1 is unable to regulate actin dynamics. Hence, although dephosphorylated cofilin-1 is able to bind to F-actin, oxidized cofilin-1 leads to its inactivation (Klemke et al. 2008). Studies have established the key role of CFL1 in oxidant-induced apoptosis in tumor cells (Klamt et al. 2009). Mechanistically, once oxidized, cofilin-1 translocates to the mitochondria where it induces swelling and cytochrome c release by mediating the opening of the permeability transition pore (PTP). Knockdown of endogenous CFL1 using targeted siRNA inhibits oxidant-induced apoptosis, which is restored by reexpression of wildtype CFL1 but not by CFL1 containing Cys-to-Ala mutations (a nonoxidizable form of the protein). Thus, this data suggests that oxidized cofilin-1 mediates mitochondrial dysfunction and apoptosis induced by oxidants in tumor cells.
There is a link between impaired synaptic plasticity observed with both age and neurodegenerative diseases and the cofilin-1 pathway. Cofilin-1 is highly concentrated in the growth cone and dendritic spine of neurons. Overexpression of cofilin-1 and its nonphosphorylatable S3A mutant can induce more growth cone-like waves and result in longer axons. On the other hand, overexpression of LIMK-1 disrupts the growth cone structure and axon elongation (Flynn et al. 2009). Thus, cofilin-1 may play a crucial role in synaptic plasticity through the regulation of the growth cone and spine dynamics via phosphorylation/dephosphorylation. Additionally, studies have shown that LIMK-1 knockout causes an abnormal elevation of cofilin-1 activity. This leads to the distortion of spine morphology and may be correlated with William’s syndrome (Frangiskakis et al. 1996). Lastly, hyperphosphorylation of cofilin-1 results in a reduction in dendrite number, leading to the neurodegeneration found in Alzheimer’s disease. Furthermore, cofilin-1 has also been shown to be present beside amyloid plaques in human brain (Heredia et al. 2006).
Recently, a growing body of evidence suggests that actin/cofilin-1 rod formation (aggregates composed primarily of actin and cofilin-1) may be a central initiation step for neurodegeneration. Actin/cofilin-1 rods can be generated by the excessive expression of active cofilin-1 and by cellular stress. Cofilin-1 oxidation may directly facilitate actin/cofilin-1 rod formation by the actin bundling activity of cofilin-1 oligomers or by the impairment of cofilin-1 phosphorylation. In neurodegenerative diseases such as Parkinson’s disease, neuronal cytoplasmic rods accumulate within neurites, where they disrupt synaptic function and are a likely early cause of synaptic loss without neuronal loss (Bamburg and Bernstein 2016; Schönhofen et al. 2014).
Cell motility is the cornerstone event of the invasion and metastasis found in aggressive cancers. It is clear in cancer models that the activation of the motility cycle is required for cell migration and invasion, tumor progression, and metastasis. In this scenario, cofilin-1 arises as a key player in cell migration, contributing in actin polymerization and in the formation of free barbed ends. The ability of cofilin-1 to interact with the actin cytoskeleton molecules suggests that it has a direct role in the processes of cell polarity, migration, and chemotaxis (Wang et al. 2007).
The importance of cofilin-1 in cancer cell motility and migration, plus its role in oxidants-induced apoptosis, suggests that this protein is a marker of an aggressive cancer phenotype (Wang et al. 2007; Müller et al. 2011). Moreover, cofilin-1 has been associated with chemotherapy resistance, especially towards alkylating drugs and as such is a possible target for cancer treatment (Castro et al. 2010; Becker et al. 2014).
Several in vitro and in vivo studies have correlated the expression of cofilin-1 with the potential for tumor cells to migrate and generate metastases. An imbalance of this pathway has been described for different tumors, including breast, lung, ovarian, head and neck, melanoma, gastrointestinal, genitourinary, and central nervous system tumors. Several studies, including metaanalysis of microarray data, cultures of human cancer cell lines, and small clinical retrospective cohorts, have demonstrated that aggressive cancer behavior correlates with high expression levels of cofilin-1, with similar results in the different tumors studied. As such, cofilin-1 expression levels may be a useful tool for discriminating between high and low aggressiveness tumors and possibly between good and bad prognoses (Castro et al. 2010).
A current challenge in the modern era of oncology is the concept of personalized medicine. In this setting, targeted treatments to driver molecules are becoming the focus of therapeutic intervention, and several drugs have already being approved for use in clinical practice. The first study of cofilin-1 as a target for cancer treatment was undertaken in human breast cancer cells and cancer metastasis xenograft models. JG6, a novel marine-derived oligosaccharide, was used to bind to cofilin-1 and inhibit cofilin-actin turnover by disrupting their interaction. JG6 was the first compound to demonstrate a positive effect in the inhibition of cell migration and prevention of cancer cell metastases. It is important to point out that JG6 effects were dependent of high levels of cofilin-1 and did not had an inherent cytotoxic effect (Huang et al. 2014).
Although tumor aggressiveness and its potential to generate metastasis are very important tumor characteristics, chemoresistance is also a major issue in the clinical setting. Two studies in lung cancer and one in an ovarian cell lines attempted to correlate cofilin-1 expression and chemotherapeutic resistance. Analysis of microarray data obtained from six human non–small cell lung cancer cell lines with different degrees of cofilin-1 expression revealed a positive correlation between high levels of CFL1 mRNA and resistance against different anticancer drugs. When these cell lines were exposed to different concentrations of chemotherapy drugs, resistance to alkylating agents (cisplatin and carboplatin) was observed (Castro et al. 2010). This correlation was validated in another study with lung cancer adenocarcinoma cell lines where cofilin-1 was overexpressed and drug sensitivity/resistance was evaluated (Becker et al. 2014). Another study using ovarian cell lines cancer evidenced that taxol-resistant cells had a higher expression level of cofilin-1 showed an upregulation of the protein in the taxol-resistant samples (Li et al. 2013).
Studies have also shown that cofilin-1 expression confers radiation resistance in tumors. Lee and colleagues demonstrated that high cofilin-1 expression enhanced cellular radiosensitivity in H1299 cells (non–small cell lung carcinoma), which is possibly due to reduced capacity to repair double-strand breaks in DNA (Lee et al. 2005). At the same time, Wei et al. reported that cofilin-1, among other proteins, could predict multidrug resistance (MDR) and elevated radioresistance (RDR) (Wei et al. 2012). This study consisted of the irradiation of A549 lung cancer cell cultures with 6 MV photon beams of different doses, following evaluation of upregulated proteins by immunohistochemistry. The evaluation of radiation response of astrocytomas indicated that cofilin-1 might be involved in the radioresistant phenotype. All the data correlating cofilin-1 and cancer clearly demonstrate a positive link between high expression levels of the protein and a more aggressive cancer phenotype. This relationship can be observed in different tumors subtypes, both in microarray data and in culture cell. The major challenge now is to translate all these laboratory data to the clinical setting. It is imperative to further investigate cofilin-1 as a candidate for cancer treatment and also to obtain a better understanding of its role in the mechanisms of chemoresistance and radiation response.
Cofilin-1 is one of the major proteins responsible in cell migration, playing a key role in actin filament dynamics. The regulation mechanisms of this protein are phosphorylation (inactivation)/dephosphorylation (activation), via LIM kinase/TESK and SSH, subcellular localization, pH and oxidation of its internal cysteine residues. Since actin dynamics plays a role in morphology, cell migration, cell division, endocytosis, intracellular transport, and neuronal development, cofilin-1 is a fundamental protein in biological systems. Moreover, imbalance in the physiology of this protein plays a major role in several pathological processes, such as neurodegenerative diseases and cancer.
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