Ras Transformation Overrides a Proliferation Defect Induced by Tpm3.1 Knockout

Correlative cryo-ET identifies actin/tropomyosin filaments that mediate cell-substrate adhesion in cancer cells and mechanosensitivity of cell proliferation

The actin cytoskeleton is the primary driver of cellular adhesion and mechanosensing due to its ability to generate force and sense the stiffness of the environment. At the cell's leading edge, severing of the protruding Arp2/3 actin network generates a specific actin/tropomyosin (Tpm) filament population that controls lamellipodial persistence. The interaction between these filaments and adhesion to the environment is unknown. Using cellular cryo-electron tomography we resolve the ultrastructure of the Tpm/actin copolymers and show that they specifically anchor to nascent adhesions and are essential for focal adhesion assembly. Re-expression of Tpm1.8/1.9 in transformed and cancer cells is sufficient to restore cell-substrate adhesions. We demonstrate that knock-out of Tpm1.8/1.9 disrupts the formation of dorsal actin bundles, hindering the recruitment of α-actinin and non-muscle myosin IIa, critical mechanosensors. This loss causes a force-generation and proliferation defect that is notably reversed when cells are grown on soft surfaces. We conclude that Tpm1.8/1.9 suppress the metastatic phenotype, which may explain why transformed cells naturally downregulate this Tpm subset during malignant transformation.

Nicotine promotes the development of oral leukoplakia via regulating peroxiredoxin 1 and its binding proteins

Tobacco can induce reactive oxygen species (ROS) production extensively in cells, which is a major risk factor for oral leukoplakia (OLK) development. Peroxiredoxin 1 (Prx1) is a key antioxidant protein, upregulated in a variety of malignant tumors. We previously found that nicotine, the main ingredient of tobacco, promotes oral carcinogenesis via regulating Prx1. The aim of the present study was to screen and identify the Prx1 interacting proteins and investigate the mechanisms of nicotine on the development of OLK. Through liquid chromatography-tandem mass spectrometry combined with bioinformatics analysis, the candidate Prx1 interacting proteins of cofilin-1 (CFL1), tropomyosin alpha-3 chain (TPM3), and serine/threonine-protein phosphatase 2A 65 kDa regulatory subunit A alpha isoform (PPP2R1A) were screened in human dysplastic oral keratinocyte cells treated with nicotine. CFL1, TPM3, and PPP2R1A were highly expressed in human OLK tissues. The expression of CFL1 increased and the expression of PPP2R1A decreased in OLK of smokers compared to that in OLK of non-smokers. Nicotine upregulated CFL1 and downregulated PPP2R1A in 4-nitro-quinoline-1-oxide (4NQO)-induced OLK tissues in mice in part dependent on Prx1. Furthermore, the in-situ interaction of CFL1, TPM3, and PPP2R1A with Prx1 were validated in human OLK tissues. Our results suggested that tobacco might promote the development of OLK via regulating Prx1 and its interacting proteins CFL1 and PPP2R1A.

Targeting the cytoskeleton against metastatic dissemination

Cancer is a pathology characterized by a loss or a perturbation of a number of typical features of normal cell behaviour. Indeed, the acquisition of an inappropriate migratory and invasive phenotype has been reported to be one of the hallmarks of cancer. The cytoskeleton is a complex dynamic network of highly ordered interlinking filaments playing a key role in the control of fundamental cellular processes, like cell shape maintenance, motility, division and intracellular transport. Moreover, deregulation of this complex machinery contributes to cancer progression and malignancy, enabling cells to acquire an invasive and metastatic phenotype. Metastasis accounts for 90% of death from patients affected by solid tumours, while an efficient prevention and suppression of metastatic disease still remains elusive. This results in the lack of effective therapeutic options currently available for patients with advanced disease. In this context, the cytoskeleton with its regulatory and structural proteins emerges as a novel and highly effective target to be exploited for a substantial therapeutic effort toward the development of specific anti-metastatic drugs. Here we provide an overview of the role of cytoskeleton components and interacting proteins in cancer metastasis with a special focus on small molecule compounds interfering with the actin cytoskeleton organization and function. The emerging involvement of microtubules and intermediate filaments in cancer metastasis is also reviewed.

Stopping transformed cancer cell growth by rigidity sensing

A common feature of cancer cells is the alteration of kinases and biochemical signalling pathways enabling transformed growth on soft matrices, whereas cytoskeletal protein alterations are thought to be a secondary issue. However, we report here that cancer cells from different tissues can be toggled between transformed and rigidity-dependent growth states by the absence or presence of mechanosensory modules, respectively. In various cancer lines from different tissues, cells had over tenfold fewer rigidity-sensing contractions compared with normal cells from the same tissues. Restoring normal levels of cytoskeletal proteins, including tropomyosins, restored rigidity sensing and rigidity-dependent growth. Further depletion of other rigidity sensor proteins, including myosin IIA, restored transformed growth and blocked sensing. In addition, restoration of rigidity sensing to cancer cells inhibited tumour formation and changed expression patterns. Thus, the depletion of rigidity-sensing modules through alterations in cytoskeletal protein levels enables cancer cell growth on soft surfaces, which is an enabling factor for cancer progression.

Chemical biology approaches targeting the actin cytoskeleton through phenotypic screening

The actin cytoskeleton is dysregulated in cancer, yet this critical cellular machinery has not translated as a druggable clinical target due to cardio-toxic side-effects. Many actin regulators are also considered undruggable, being structural proteins lacking clear functional sites suitable for targeted drug design. In this review, we discuss opportunities and challenges associated with drugging the actin cytoskeleton through its structural regulators, taking tropomyosins as a target example. In particular, we highlight emerging data acquisition and analysis trends driving phenotypic, imaging-based compound screening. Finally, we consider how the confluence of these trends is now bringing functionally integral machineries such as the actin cytoskeleton, and associated structural regulatory proteins, into an expanded repertoire of druggable targets with previously unexploited clinical potential.

Circulating Microvesicles from Pancreatic Cancer Accelerate the Migration and Proliferation of PANC-1 Cells

Circulating microvesicles are able to mediate long-distance cell-cell communications. It is essential to understand how microvesicles from pancreatic cancer act on other cells in the body. In this work, serum-derived microvesicles were isolated from 10 patients with locally advanced pancreatic cancer and healthy controls. Using Cell Transwell and WST-1 reagents, we found that microvesicles from pancreatic cancer accelerated migration and proliferation of PANC-1 cells. Meanwhile, the proliferation of these cancer-microvesicle-treated cells (CMTCs) was affected less by 10 μM of gemcitabine relative to healthy microvesicle-treated cells (HMTCs). Next, we optimized the filter-aided sample preparation method to increase the recovery of protein samples and then applied it to the quantification of the proteome of CMTCs and HMTCs. The peptides were labeled and analyzed by liquid chromatography-tandem mass spectrometry. In total, 4102 proteins were identified, where 35 proteins were up-regulated with 27 down-regulated in CMTCs. We verified the quantitative results of three key proteins CD44, PPP2R1A, and TP53 by Western blot. The Ingenuity Pathway Analysis revealed pathways that cancer microvesicles might participate in to promote cell migration and proliferation. These findings may provide novel clues of treatment for tumorigenesis and metastasis.

Plasma-derived extracellular vesicle proteins as a source of biomarkers for lung adenocarcinoma

Exosomes and other extracellular vesicles (EVs) have been implicated as mediators of intercellular communication. Their release into the circulation has the potential to inform about tumor status. In-depth proteomic characterization of plasma-derived EVs has been limited by challenges in isolating EVs from protein-abundant biological fluids. We implemented a novel single-step density gradient flotation workflow for efficient and rapid isolation of highly enriched circulating EVs from plasma. Mass-spectrometry analysis of plasma EVs from subjects with lung adenocarcinoma and matched controls resulted in the identification of 640 proteins. A total of 108 proteins exhibited significant (p<0.05) differential expression in vesicle preparations derived from lung adenocarcinoma case plasmas compared to controls, of which 43 were also identified in EVs from lung adenocarcinoma cell lines. Four top performing EV-associated proteins that distinguished adenocarcinoma cases from controls, SRGN, TPM3, THBS1 and HUWE1, yielded a combined area under the receiver operating characteristic curve (AUC) of 0.90 (95% CI = 0.76-1). Our findings support the potential of EV derived proteins as a source of biomarkers that complement other approaches for tumor assessment.

Tropomyosin Promotes Lamellipodial Persistence by Collaborating with Arp2/3 at the Leading Edge

At the leading edge of migrating cells, protrusion of the lamellipodium is driven by Arp2/3-mediated polymerization of actin filaments [1]. This dense, branched actin network is promoted and stabilized by cortactin [2, 3]. In order to drive filament turnover, Arp2/3 networks are remodeled by proteins such as GMF, which blocks the actin-Arp2/3 interaction [4, 5], and coronin 1B, which acts by directing SSH1L to the lamellipodium where it activates the actin-severing protein cofilin [6, 7]. It has been shown in vitro that cofilin-mediated severing of Arp2/3 actin networks results in the generation of new pointed ends to which the actin-stabilizing protein tropomyosin (Tpm) can bind [8]. The presence of Tpm in lamellipodia, however, is disputed in the literature [9-19]. Here, we report that the Tpm isoforms 1.8/9 are enriched in the lamellipodium of fibroblasts as detected with a novel isoform-specific monoclonal antibody. RNAi-mediated silencing of Tpm1.8/9 led to an increase of Arp2/3 accumulation at the cell periphery and a decrease in the persistence of lamellipodia and cell motility, a phenotype consistent with cortactin- and coronin 1B-deficient cells [2, 7]. In the absence of coronin 1B or cofilin, Tpm1.8/9 protein levels are reduced while, conversely, inhibition of Arp2/3 with CK666 leads to an increase in Tpm1.8/9 protein. These findings establish a novel regulatory mechanism within the lamellipodium whereby Tpm collaborates with Arp2/3 to promote lamellipodial-based cell migration.