Evidence for involvement of calpain in c-Myc proteolysis in vivo

Testing calpain inhibition in tumor endothelial cells: novel targetable biomarkers against glioblastoma malignancy

Introduction

Glioblastoma IDH-wildtype (GBM) is the most malignant brain tumor in adults, with a poor prognosis of approximately 15 months after diagnosis. Most patients suffer from a recurrence in <1 year, and this renders GBM a life-threatening challenge. Among molecular mechanisms driving GBM aggressiveness, angiogenesis mediated by GBM endothelial cells (GECs) deserves consideration as a therapeutic turning point. In this scenario, calpains, a family of ubiquitously expressed calcium-dependent cysteine proteases, emerged as promising targets to be investigated as a novel therapeutic strategy and prognostic tissue biomarkers.

Methods

To explore this hypothesis, GECs were isolated from n=10 GBM biopsies and characterized phenotypically by immunofluorescence. The expression levels of calpains were evaluated by qRT-PCR and Western blot, and their association with patients' prognosis was estimated by Pearson correlation and Kaplan-Meier survival analysis. Calpain targeting efficacy was assessed by a time- and dose-dependent proliferation curve, MTT assay for viability, caspase-3/7 activity, migration and angiogenesis in vitro, and gene and protein expression level modification.

Results

Immunofluorescence confirmed the endothelial phenotype of our primary GECs. A significant overexpression was observed for calpain-1/2/3 (CAPN) and calpain-small-subunits-1/2 (CAPNS1), whereas calpastatin gene, the calpain natural inhibitor, was reported to be downregulated. A significant negative correlation was observed between CAPN1/CAPNS1 and patient overall survival. GEC challenging revealed that the inhibition of calpain-1 exerts the strongest proapoptotic efficacy, so GEC mortality reached the 80%, confirmed by the increased activity of caspase-3/7. Functional assays revealed a strong affection of in vitro migration and angiogenesis. Gene and protein expression proved a downregulation of MAPK, VEGF/VEGFRs, and Bcl-2, and an upregulation of caspases and Bax-family mediators.

Conclusion

Overall, the differential expression of calpains and their correlation with patient survival suggest a novel promising target pathway, whose blockade showed encouraging results toward precision medicine strategies.

Exploring Myc puzzle: Insights into cancer, stem cell biology, and PPI networks

"The grand orchestrator," "Universal Amplifier," "double-edged sword," and "Undruggable" are just some of the Myc oncogene so-called names. It has been around 40 years since the discovery of the Myc, and it remains in the mainstream of cancer treatment drugs. Myc is part of basic helix-loop-helix leucine zipper (bHLH-LZ) superfamily proteins, and its dysregulation can be seen in many malignant human tumors. It dysregulates critical pathways in cells that are connected to each other, such as proliferation, growth, cell cycle, and cell adhesion, impacts miRNAs action, intercellular metabolism, DNA replication, differentiation, microenvironment regulation, angiogenesis, and metastasis. Myc, surprisingly, is used in stem cell research too. Its family includes three members, MYC, MYCN, and MYCL, and each dysfunction was observed in different cancer types. This review aims to introduce Myc and its function in the body. Besides, Myc deregulatory mechanisms in cancer cells, their intricate aspects will be discussed. We will look at promising drugs and Myc-based therapies. Finally, Myc and its role in stemness, Myc pathways based on PPI network analysis, and future insights will be explained.

Changes in calpain-2 expression during glioblastoma progression predisposes tumor cells to temozolomide resistance by minimizing DNA damage and p53-dependent apoptosis

BACKGROUND

Glioblastoma multiforme (GBM) is characterized by an unfavorable prognosis for patients affected. During standard-of-care chemotherapy using temozolomide (TMZ), tumors acquire resistance thereby causing tumor recurrence. Thus, deciphering essential molecular pathways causing TMZ resistance are of high therapeutic relevance.

METHODS

Mass spectrometry based proteomics were used to study the GBM proteome. Immunohistochemistry staining of human GBM tissue for either calpain-1 or -2 was performed to locate expression of proteases. In vitro cell based assays were used to measure cell viability and survival of primary patient-derived GBM cells and established GBM cell lines after TMZ ± calpain inhibitor administration. shRNA expression knockdowns of either calpain-1 or calpain-2 were generated to study TMZ sensitivity of the specific subunits. The Comet assay and ɣH2AX signal measurements were performed in order to assess the DNA damage amount and recognition. Finally, quantitative real-time PCR of target proteins was applied to differentiate between transcriptional and post-translational regulation.

RESULTS

Calcium-dependent calpain proteases, in particular calpain-2, are more abundant in glioblastoma compared to normal brain and increased in patient-matched initial and recurrent glioblastomas. On the cellular level, pharmacological calpain inhibition increased the sensitivities of primary glioblastoma cells towards TMZ. A genetic knockdown of calpain-2 in U251 cells led to increased caspase-3 cleavage and sensitivity to neocarzinostatin, which rapidly induces DNA strand breakage. We hypothesize that calpain-2 causes desensitization of tumor cells against TMZ by preventing strong DNA damage and subsequent apoptosis via post-translational TP53 inhibition. Indeed, proteomic comparison of U251 control vs. U251 calpain-2 knockdown cells highlights perturbed levels of numerous proteins involved in DNA damage response and downstream pathways affecting TP53 and NF-κB signaling. TP53 showed increased protein abundance, but no transcriptional regulation.

CONCLUSION

TMZ-induced cell death in the presence of calpain-2 expression appears to favor DNA repair and promote cell survival. We conclude from our experiments that calpain-2 expression represents a proteomic mode that is associated with higher resistance via "priming" GBM cells to TMZ chemotherapy. Thus, calpain-2 could serve as a prognostic factor for GBM outcome.

Biological Role of MYCN in Medulloblastoma: Novel Therapeutic Opportunities and Challenges Ahead

The constitutive and dysregulated expression of the transcription factor MYCN has a central role in the pathogenesis of the paediatric brain tumour medulloblastoma, with an increased expression of this oncogene correlating with a worse prognosis. Consequently, the genomic and functional alterations of MYCN represent a major therapeutic target to attenuate tumour growth in medulloblastoma. This review will provide a comprehensive synopsis of the biological role of MYCN and its family components, their interaction with distinct signalling pathways, and the implications of this network in medulloblastoma development. We will then summarise the current toolbox for targeting MYCN and highlight novel therapeutic avenues that have the potential to results in better-tailored clinical treatments.

MYC-nick promotes cell migration by inducing fascin expression and Cdc42 activation

MYC-nick is a cytoplasmic, transcriptionally inactive member of the MYC oncoprotein family, generated by a proteolytic cleavage of full-length MYC. MYC-nick promotes migration and survival of cells in response to chemotherapeutic agents or withdrawal of glucose. Here we report that MYC-nick is abundant in colonic and intestinal tumors derived from mouse models with mutations in the Wnt, TGF-β, and PI3K pathways. Moreover, MYC-nick is elevated in colon cancer cells deleted for FBWX7, which encodes the major E3 ligase of full-length MYC frequently mutated in colorectal cancers. MYC-nick promotes the migration of colon cancer cells assayed in 3D cultures or grown as xenografts in a zebrafish metastasis model. MYC-nick accelerates migration by activating the Rho GTPase Cdc42 and inducing fascin expression. MYC-nick, fascin, and Cdc42 are frequently up-regulated in cells present at the invasive front of human colorectal tumors, suggesting a coordinated role for these proteins in tumor migration.

The Importance of Ubiquitination and Deubiquitination in Cellular Reprogramming

Ubiquitination of core stem cell transcription factors can directly affect stem cell maintenance and differentiation. Ubiquitination and deubiquitination must occur in a timely and well-coordinated manner to regulate the protein turnover of several stemness related proteins, resulting in optimal embryonic stem cell maintenance and differentiation. There are two switches: an E3 ubiquitin ligase enzyme that tags ubiquitin molecules to the target proteins for proteolysis and a second enzyme, the deubiquitinating enzyme (DUBs), that performs the opposite action, thereby preventing proteolysis. In order to maintain stemness and to allow for efficient differentiation, both ubiquitination and deubiquitination molecular switches must operate properly in a balanced manner. In this review, we have summarized the importance of the ubiquitination of core stem cell transcription factors, such as Oct3/4, c-Myc, Sox2, Klf4, Nanog, and LIN28, during cellular reprogramming. Furthermore, we emphasize the role of DUBs in regulating core stem cell transcriptional factors and their function in stem cell maintenance and differentiation. We also discuss the possibility of using DUBs, along with core transcription factors, to efficiently generate induced pluripotent stem cells. Our review provides a relatively new understanding regarding the importance of ubiquitination/deubiquitination of stem cell transcription factors for efficient cellular reprogramming.

MYC degradation

The MYC oncoprotein is an essential transcription factor that regulates the expression of many genes involved in cell growth, proliferation, and metabolic pathways. Thus, it is important to keep MYC activity in check in normal cells in order to avoid unwanted oncogenic changes. Normal cells have adapted several ways to control MYC levels, and these mechanisms can be disrupted in cancer cells. One of the major ways in which MYC levels are controlled in cells is through targeted degradation by the ubiquitin-proteasome system (UPS). Here, we discuss the role of the UPS in the regulation of MYC protein levels and review some of the many proteins that have been shown to regulate MYC protein stability. In addition, we discuss how this relates to MYC transcriptional activity, human cancers, and therapeutic targeting.

MYC degradation under low O2 tension promotes survival by evading hypoxia-induced cell death

Cells encounter oxygen deprivation (hypoxia) in various physiological and pathological contexts. Adaptation to hypoxic stress occurs in part by suppressing MYC, a key regulator of cellular metabolism, proliferation, and survival. Hypoxia has been reported to inhibit MYC through multiple means, including disruption of MYC transcriptional complexes and decreased MYC protein abundance. Here we identify enhanced proteasomal degradation and cathepsin-mediated proteolysis as important mechanisms for hypoxic MYC inhibition in human colon carcinoma cells. MYC protein levels were similarly reduced in hypoxic primary keratinocytes. Increased MYC turnover at low O2 tension was dependent on the E3 ubiquitin ligases FBXW7 and DDB1, as well as hypoxic induction of cathepsins D and S. Reduced MYC protein levels coincided with hypoxic inhibition of RNA polymerase III-dependent MYC target genes, which MYC regulates independently of its binding partner MAX. Finally, MYC overexpression in hypoxic cells promoted cell cycle progression but also enhanced cell death via increased expression of the proapoptotic genes NOXA and PUMA. Collectively, these results indicate that hypoxic cells promote MYC degradation as an adaptive strategy to reduce proliferation, suppress biosynthetic processes, and promote cell survival under low O2 tension.

Effects of lysophosphatidic acid on calpain-mediated proteolysis of focal adhesion kinase in human prostate cancer cells

BACKGROUND

Calcium-mediated proteolysis plays an important role in cell migration. Lysophosphatidic acid (LPA), a lipid mediator present in serum, enhances migration of carcinoma cells. The effects of LPA on calpain-mediated proteolysis were, therefore, examined in PC-3, a human prostate cancer cell line.

METHODS

Cultured PC-3 cells were used in studies utilizing pharmacologic interventions, immunoblotting, and confocal immunolocalization.

RESULTS

Focal adhesion kinase (FAK), a tyrosine kinase involved in cell adhesion, is rapidly proteolyzed in serum-starved PC-3 cells exposed to the calcium ionophore, ionomycin; Nck, p130CAS, PKCα, and Ras-GAP are also degraded. Thapsigargin, which causes more moderate increases in intracellular calcium, induces partial proteolysis of these proteins. Calpain inhibitors block the proteolytic responses to ionomycin and thapsigargin. Ionomycin does not induce proteolysis in cells maintained in serum, suggesting a protective role for growth factors contained in serum. LPA causes minor FAK proteolysis when added alone, but protects against ionomycin-induced proteolysis in a time-dependent manner. LPA also protects against the cell detachment that eventually follows ionomycin treatment. The response to LPA is blocked by an LPA receptor antagonist. A similar effect of LPA is observed in ionomycin-treated Rat-1 fibroblasts. In PC-3 cells, the protective effects of LPA and serum are correlated with phosphorylation and redistribution of paxillin, suggesting roles for phosphorylation-mediated protein-protein interactions.

CONCLUSIONS

The complex effects of LPA on calpain-mediated proteolysis of FAK and other adhesion proteins are likely to play a role in the ability of LPA to promote attachment, migration, and survival of prostate cancer cells.

Proteolytic control of the oncoprotein transcription factor Myc

The c-Myc oncogene encodes a multifunctional transcription factor that directs the expression of genes required for cell growth and proliferation. Consistent with its potent growth-promoting properties, cells have evolved numerous mechanisms that limit the expression and activity of Myc. One of the most prominent of these mechanisms is proteolysis, which destroys Myc within minutes of its synthesis. The rapid and controlled destruction of Myc keeps its levels low and precisely tied to processes that regulate Myc production. In this review, we discuss how Myc protein stability is regulated and the influence of Myc proteolysis on its function. We describe what is known about how Myc is destroyed by ubiquitin (Ub)-mediated proteolysis, attempt to rationalize the role of different Ub-protein ligases and deubiquitylating enzymes (dUbs) in the regulation of Myc stability, and detail how these processes go awry in cancer. Finally, we discuss how our understanding of Myc regulation by the ubiquitin-proteasome system (UPS) can expose strategies for therapeutic intervention in human malignancies.

Calpain-catalyzed proteolysis of human dUTPase specifically removes the nuclear localization signal peptide

Background

Calpain proteases drive intracellular signal transduction via specific proteolysis of multiple substrates upon Ca²⁺-induced activation. Recently, dUTPase, an enzyme essential to maintain genomic integrity, was identified as a physiological calpain substrate in Drosophila cells. Here we investigate the potential structural/functional significance of calpain-activated proteolysis of human dUTPase.

Methodology/Principal Findings

Limited proteolysis of human dUTPase by mammalian m-calpain was investigated in the presence and absence of cognate ligands of either calpain or dUTPase. Significant proteolysis was observed only in the presence of Ca(II) ions, inducing calpain action. The presence or absence of the dUTP-analogue α,β-imido-dUTP did not show any effect on Ca²⁺-calpain-induced cleavage of human dUTPase. The catalytic rate constant of dUTPase was unaffected by calpain cleavage. Gel electrophoretic analysis showed that Ca²⁺-calpain-induced cleavage of human dUTPase resulted in several distinctly observable dUTPase fragments. Mass spectrometric identification of the calpain-cleaved fragments identified three calpain cleavage sites (between residues ⁴SE⁵; ⁷TP⁸; and ³¹LS³²). The cleavage between the ³¹LS³² peptide bond specifically removes the flexible N-terminal nuclear localization signal, indispensable for cognate localization.

Conclusions/Significance

Results argue for a mechanism where Ca²⁺-calpain may regulate nuclear availability and degradation of dUTPase.

The calpain system and cancer

The calpains are a conserved family of cysteine proteinases that catalyse the controlled proteolysis of many specific substrates. Calpain activity is implicated in several fundamental physiological processes, including cytoskeletal remodelling, cellular signalling, apoptosis and cell survival. Calpain expression is altered during tumorigenesis, and the proteolysis of numerous substrates, such as inhibitors of nuclear factor-κB (IκB), focal adhesion proteins (including, focal adhesion kinase and talin) and proto-oncogenes (for example, MYC), has been implicated in tumour pathogenesis. Recent evidence indicates that the increased expression of certain family members might influence the response to cancer therapies, providing justification for the development of novel calpain inhibitors.

Calpains as potential anti-cancer targets

INTRODUCTION

The intracellular signaling cysteine proteases, calpains (specifically the ubiquitous calpains 1 and 2), are involved in numerous physiological and pathological phenomena. Several works have highlighted the implication of calpains in processes crucial for cancer development and progression. For these reasons, calpains are considered by several authors as potential anti-cancer targets.

AREAS COVERED

How calpains are implicated in cancer formation and development, how these enzymes are deregulated in cancer cells and how these proteases could be targeted by anti-cancer drugs. Studies published in the last 10 years are focused on.

EXPERT OPINION

Targeting calpain activity with specific inhibitors could be a novel approach to limiting development of primary tumors and formation of metastases, by inhibiting tumor cell migration and invasion, which allows dissemination as well as tumor neovascularization, which in turn allows expansion. However, such drugs could interfere with anti-cancer treatments, as ubiquitous calpains play crucial roles in chemotherapy-induced apoptosis. For these reasons, drugs targeting calpains would have to be used selectively to avoid interference with other treatments and physiological processes. Further studies will be required concerning the other members of the calpain family and their potential implication in cancer development before considering treatments targeting their activity.

Post-translational control of Myc function during differentiation

Myc proteins are deeply involved in multiple biological processes including cell proliferation, growth, metabolism, apoptosis, differentiation, and tumorigenesis. Paradoxically, Myc proteins have been found to be capable of both inhibiting and facilitating differentiation depending on the biological context. Recently we identified a new mode of Myc regulation in differentiating muscle cells in which c-Myc protein is proteolytically cleaved by calcium-dependent calpains in the cytoplasm. This cleavage serves two purposes. First, it inactivates the transcriptional function of Myc by removing its C-terminus, a region responsible for the interaction of Myc with Max and DNA. Second, it alters cytoskeletal architecture and accelerates muscle differentiation through the activity of the remaining N-terminal cleavage product (termed Myc-nick). Here we discuss the roles and regulation of full-length Myc and Myc-nick in terminal differentiation and propose a model in which calpain-mediated cleavage of Myc operates as a functional switch.

Myc-nick: a cytoplasmic cleavage product of Myc that promotes alpha-tubulin acetylation and cell differentiation

The Myc oncoprotein family comprises transcription factors that control multiple cellular functions and are widely involved in oncogenesis. Here we report the identification of Myc-nick, a cytoplasmic form of Myc generated by calpain-dependent proteolysis at lysine 298 of full-length Myc. Myc-nick retains conserved Myc box regions but lacks nuclear localization signals and the bHLHZ domain essential for heterodimerization with Max and DNA binding. Myc-nick induces alpha-tubulin acetylation and altered cell morphology by recruiting histone acetyltransferase GCN5 to microtubules. During muscle differentiation, while the levels of full-length Myc diminish, Myc-nick and acetylated alpha-tubulin levels are increased. Ectopic expression of Myc-nick accelerates myoblast fusion, triggers the expression of myogenic markers, and permits Myc-deficient fibroblasts to transdifferentiate in response to MyoD. We propose that the cleavage of Myc by calpain abrogates the transcriptional inhibition of differentiation by full-length Myc and generates Myc-nick, a driver of cytoplasmic reorganization and differentiation.

Rexinoids modulate steroid and xenobiotic receptor activity by increasing its protein turnover in a calpain-dependent manner

The steroid and xenobiotic receptor SXR (human pregnane X receptor) is a nuclear receptor that plays a key role in the body's detoxification response by regulating genes involved in drug metabolism and transport. SXR ligands include a wide range of compounds, which induce transcription of SXR target genes via activation of a heterodimeric transcription factor consisting of SXR and the related nuclear receptor retinoid X receptor (RXR). We investigated the effect of RXR-selective ligands, rexinoids, on SXR/RXR activity. In agreement with previous reports, we found that rexinoids are weak activators of SXR, but we also found that they can antagonize SXR activation by the potent SXR agonist rifampicin. This antagonism included suppression of rifampicin-induced expression of SXR target genes, as well as reduced binding of SXR/RXR to SXR response elements both in vivo and in vitro. Interestingly, two rexinoids, bexarotene (LGD1069/Targretin) and LG100268, caused a rapid and sustained decrease in the protein levels of both SXR and RXR. The decrease in SXR level was due to an enhanced rate of protein degradation and was dependent on calpain activity, as opposed to rexinoid-induced RXR degradation, which is mediated via the proteasome. Thus, we have demonstrated a novel, rexinoid-modulated mechanism regulating SXR protein stability, which may explain why rexinoids are only weak activators of SXR/RXR-mediated transcription, despite reports that they bind to SXR with high affinity. We suggest that the ability of rexinoids to induce degradation of both SXR and RXR, in combination with competition for binding to SXR, can also explain why rexinoids antagonize the activation of SXR by drugs like rifampicin.

The Kaposi's sarcoma-associated herpesvirus LANA protein stabilizes and activates c-Myc

The Kaposi's sarcoma-associated herpesvirus (KSHV) latency-associated nuclear antigen (LANA) protein is functionally pleiotropic. LANA contributes to KSHV-associated pathogenesis, in part, by increasing entry of cells into S phase through a process that is driven by LANA interaction with the serine-threonine kinase glycogen synthase kinase 3 (GSK-3) and stabilization of beta-catenin. We now show that LANA affects the activity of another protein involved in cell cycle regulation, c-Myc. Sequencing of c-Myc coding sequences revealed that c-Myc in KSHV-positive primary effusion lymphoma (PEL) cell lines is wild type in the N-terminal region that regulates c-Myc protein stability. Despite this, c-Myc in PEL cells is stabilized. In LANA-expressing cells, inactivation of nuclear GSK-3 reduced phosphorylation of c-Myc at Thr58 and contributed to c-Myc stabilization by decreasing c-Myc ubiquitination. Phosphorylation of c-Myc on Ser62 also affects c-Myc stability and function. We now show that LANA increases the level of phosphorylated extracellular signal-regulated kinase 1 (ERK1) and increases ERK phosphorylation of c-Myc on Ser62. LANA also interacted with c-Myc, and c-Myc amino acids 147 to 220 were required for this interaction. LANA (L1006P) retained the ability to bind to c-Myc and activate ERK1, indicating that these events did not require LANA interaction with GSK-3. Thus, LANA stabilizes c-Myc; prevents the phosphorylation of c-Myc at Thr58, an event that promotes Myc-induced apoptosis; and independently stimulates phosphorylation of c-Myc at Ser62, an event that transcriptionally activates c-Myc. LANA-mediated manipulation of c-Myc function is likely to contribute to KSHV-associated tumorigenesis through the induction of c-Myc regulated cellular genes, as well as by the stimulation of cell cycle progression.

Role of post-translational modifications in regulating c-Myc proteolysis, transcriptional activity and biological function

The Myc proteins play a central role in cellular proliferation, differentiation, apoptosis and tumorigenesis. Although it is clear that multiple molecular mechanisms mediate these functions, it is unclear how individual mechanisms contribute and if different mechanisms work in concert or separately in mediating the diverse biological functions of c-Myc. Similarly, the role of post-translational modifications in regulating c-Myc molecular and biological properties has remained uncertain, despite over 20 years of research. In particular, phosphorylation of the N-terminal transcriptional regulatory domain has been shown to have a variety of consequences ranging from dramatic effects on apoptosis, tumorigenesis and c-Myc proteolysis to negligible effects on cellular transformation and transcriptional activity. This review attempts to provide a comprehensive and critical evaluation of the accumulated evidence to address the complex and controversial issues surrounding the role of post-translational modifications in c-Myc function, focusing on phosphorylation and ubiquitination of the N-terminal transcriptional regulatory domain. An overall model emerges that suggests phosphorylation and ubiquitination play critical roles in cell cycle progression, cell growth, apoptosis and tumorigenesis that are mediated by phosphorylation-dependent transcriptional activation of distinct sets of target genes and synchronized proteolysis.

Proteasome inhibitors induce a p38 mitogen-activated protein kinase (MAPK)-dependent anti-apoptotic program involving MAPK phosphatase-1 and Akt in models of breast cancer

Proteasome inhibitors represent a novel class of anti-tumor agents that have clinical efficacy against hematologic malignancies, but single-agent activity against solid tumors such as breast cancer has been disappointing, perhaps due to activation of anti-apoptotic survival signals. To evaluate a possible role for the p38 mitogen-activated protein kinase (MAPK), A1N4-myc human mammary epithelial, and BT-474 and MDA-MB-231 breast carcinoma cells, were studied. Exposure of these lines to pharmacologic p38 blockade enhanced proteasome inhibitor-mediated apoptosis, as did overexpression of dominant negative (DN)-p38-alpha and -beta-MAPK isoforms. Inhibition of p38 resulted in suppression of induction of anti-apoptotic MAPK phosphatase (MKP)-1, in association with enhanced activation of the pro-apoptotic c-Jun-N-terminal kinase (JNK). Moreover, infection of cells treated with a proteasome inhibitor/p38 inhibitor combination with Adenovirus (Ad) inducing over-expression of MKP-1 suppressed apoptosis compared with controls. Further targets of p38 MAPK were also studied, and proteasome inhibition activated phosphorylation of MAPK-activated protein kinase-2, heat shock protein (HSP)-27, and the AKT8 virus oncogene cellular homolog (Akt). Inhibition of p38 MAPK resulted in decreased phospho-HSP-27 and phospho-Akt, while down-regulation of HSP-27 with a small interfering RNA decreased phosphorylation of Akt, directly linking activation of p38 to Akt. Finally, inhibition of Akt with phosphatidylinositol-3-kinase inhibitors increased apoptosis, as did over-expression of DN-Akt. These studies support the hypothesis that proteasome inhibitors activate an anti-apoptotic survival program through p38 MAPK that involves MKP-1 and Akt. Further, they suggest that strategies targeting MKP-1 and Akt could enhance the anti-tumor efficacy of proteasome inhibitors against breast cancer.

Interferon beta increases c-Myc proteolysis in mouse monocyte/macrophage leukemia cells

Growth inhibitory activity of interferons (IFNs) has been attributed to several events. These include rapid induction of cyclin-dependent kinase inhibitors, such as those in the Cip/Kip and Ink 4 families and down-regulation of c-myc mRNA and c-Myc transcriptional activity. Here, we report an additional mechanism, involving regulation of Myc protein levels, through which type 1 IFN may halt proliferation of cells. This was discovered using a cell line which constitutively expresses c-myc from a retrovirus vector and which was reported to have undergone deletion of genes encoding the Ink 4 tumor suppressors p15 and p16. IFNbeta caused a reduction in the steady state level of c-Myc protein by increasing degradation through the 26S proteasome. Our data, as well as that of others, indicate that multiple levels of c-Myc expression can be affected by IFN treatment and this contributes to rapid growth arrest in the G1 phase of the cell cycle.

Nuclear calpain regulates Ca2+-dependent signaling via proteolysis of nuclear Ca2+/calmodulin-dependent protein kinase type IV in cultured neurons

Accumulating evidence indicates that calpains can reside in or translocate to the cell nucleus, but their functions in this compartment remain poorly understood. Dissociated cultures of cerebellar granule cells (GCs) demonstrate improved long-term survival when their growth medium is supplemented with depolarizing agents that stimulate Ca(2+) influx and activate calmodulin-dependent signaling cascades, notably 20 mm KCl. We previously observed Ca(2+)-dependent down-regulation of Ca(2+)/calmodulin-dependent protein kinase (CaMK) type IV, which was attenuated by calpain inhibitors, in GCs supplemented with 20 mm KCl (Tremper-Wells, B., Mathur, A., Beaman-Hall, C. M., and Vallano, M. L. (2002) J. Neurochem. 81, 314-324). CaMKIV is highly enriched in the nucleus and thought to be critical for improved survival. Here, we demonstrate by immunolocalization/confocal microscopy and subcellular fractionation that the regulatory and catalytic subunits of m-calpain are enriched in GC nuclei, including GCs grown in medium containing 5 mm KCl. Calpain-mediated proteolysis of CaMKIV is selective, as several other nuclear and non-nuclear calpain substrates were not degraded under chronic depolarizing culture conditions. Depolarization and Ca(2+)-dependent down-regulation of CaMKIV were associated with significant alterations in other components of the Ca(2+)-CaMKIV signaling cascade: the ratio of phosphorylated to total cAMP response element-binding protein (a downstream CaMKIV substrate) was reduced by approximately 10-fold, and the amount of CaMK kinase (an upstream activator of CaMKIV) protein and mRNA was significantly reduced. We hypothesize that calpain-mediated CaMKIV proteolysis is an autoregulatory feedback response to sustained activation of a Ca(2+)-CaMKIV signaling pathway, resulting from growth of cultures in medium containing 25 mm KCl. This study establishes nuclear m-calpain as a regulator of CaMKIV and associated signaling molecules under conditions of sustained Ca(2+) influx.

Calpain activity is generally elevated during transformation but has oncogene-specific biological functions

Several oncogene and tumor-suppressor gene products are known substrates for the calpain family of cysteine proteases, and calpain is required for transformation by v-src and tumor invasion. Thus, we have now addressed whether calpain is generally associated with transformation and how calpain contributes to oncogene function. Our results demonstrate that calpain activity is enhanced upon transformation induced by the v-Src, v-Jun, v-Myc, k-Ras, and v-Fos oncoproteins. Furthermore, elevated calpain activity commonly promotes focal adhesion remodelling, disruption of actin cytoskeleton, morphological transformation, and cell migration, although proteolysis of target substrates (such as focal adhesion kinase, talin, and spectrin) is differently specified by individual oncoproteins. Interestingly, v-Fos differs from other common oncoproteins in not requiring calpain activity for actin/adhesion remodelling or migration of v-Fos transformed cells. However, anchorage-independent growth of all transformed cells is sensitive to calpain inhibition. In addition, elevated calpain activity contributes to oncogene-induced apoptosis associated with transformation by v-Myc. Taken together, these studies demonstrate that calpain activity is necessary for full cellular transformation induced by common oncoproteins, but has distinct roles in oncogenic events induced by individual transforming proteins. Thus, targeting calpain activity may represent a useful general strategy for interfering with activated proto-oncogenes in cancer cells.

SRC-mediated phosphorylation of focal adhesion kinase couples actin and adhesion dynamics to survival signaling

Integrin-associated focal adhesions not only provide adhesive links between cellular actin and extracellular matrix but also are sites of signal transmission into the cell interior. Many cell responses signal through focal adhesion kinase (FAK), often by integrin-induced autophosphorylation of FAK or phosphorylation by Src family kinases. Here, we used an interfering FAK mutant (4-9F-FAK) to show that Src-dependent FAK phosphorylation is required for focal adhesion turnover and cell migration, by controlling assembly of a calpain 2/FAK/Src/p42ERK complex, calpain activation, and proteolysis of FAK. Expression of 4-9F-FAK in FAK-deficient fibroblasts also disrupts F-actin assembly associated with normal adhesion and spreading. In addition, we found that FAK's ability to regulate both assembly and disassembly of the actin and adhesion networks may be linked to regulation of the protease calpain. Surprisingly, we also found that the same interfering 4-9F-FAK mutant protein causes apoptosis of serum-deprived, transformed cells and suppresses anchorage-independent growth. These data show that Src-mediated phosphorylation of FAK acts as a pivotal regulator of both actin and adhesion dynamics and survival signaling, which, in turn, control apparently distinct processes such as cell migration and anchorage-independent growth. This also highlights that dynamic regulation of actin and adhesions (which include the integrin matrix receptors) is critical to signaling output and biological responses.

m-Calpain implication in cell cycle during muscle precursor cell activation

Milli-calpain, a member of the ubiquitous cysteine protease family, is known to control late events of cell-cell fusion in skeletal muscle tissue through its involvement in cell membrane and cytoskeleton component reorganization. In this report, we describe the characterization of m-calpain compartmentalization and activation during the initial steps of muscle precursor cell recruitment and differentiation. By immunofluorescence analysis, we show that m-calpain is present throughout the cell cycle in the nucleus of proliferating myoblast C2 cells. However, when myoblasts enter a quiescent/G0 stage, m-calpain staining is detected only in the cytoplasm. Moreover, comparison of healthy and injured muscle shows distinct m-calpain localization in satellite stem cells. Indeed, m-calpain is not found in quiescent satellite cells, but following muscle injury, when satellite cells start to proliferate, m-calpain appears in the nucleus. To determine the implication of m-calpain during the cell cycle progression, quiescent myoblasts were forced to re-enter the cell cycle in the presence or not of the specific calpain inhibitor MDL 28170. We demonstrate that this calpain inhibitor blocks the cell cycle, prevents accumulation of MyoD in the G1 phase and enhances Myf5 expression. These data support an important new role for m-calpain in the control of muscle precursor cell activation and thus suggest its possible implication during the initial events of muscle regeneration.

A novel role for FAK as a protease-targeting adaptor protein: regulation by p42 ERK and Src

Cell migration on extracellular matrix requires the turnover of integrin-dependent adhesions. The nonreceptor tyrosine kinases Src and FAK regulate focal-adhesion turnover by poorly understood mechanisms. ERK/MAP kinase-mediated activation of the protease Calpain 2 also promotes focal-adhesion turnover; however, it is not known if this is linked to the activities of Src and FAK. Calpain 2 has previously been demonstrated to colocalize with focal-adhesion structures and can cleave several focal-adhesion complex components, including FAK. Studies utilizing Calpain inhibitors or Calpain-deficient cells confirm that Calpain's role in regulating focal-adhesion turnover is necessary for cell migration. We have identified a novel and kinase-independent function for FAK as an adaptor molecule that mediates the assembly of a complex consisting of at least Calpain 2 and p42ERK. Mutation of proline residues (Pro2) in the amino-terminal region of FAK blocks direct binding with Calpain 2 and also prevents formation of the Calpain 2/p42ERK complex in cells. We show that both complex formation and MEK/ERK activity are associated with Calpain-mediated proteolysis of FAK and focal adhesion turnover during transformation and migration. Furthermore, FAK is necessary for recruiting both Calpain 2 and p42ERK/MAPK to peripheral adhesion sites facilitating maximal Calpain activity.