The actin-bundling protein palladin is an Akt1-specific substrate that regulates breast cancer cell migration

SGK3 mediates INPP4B-dependent PI3K signaling in breast cancer

Oncogenic mutations in PIK3CA, the gene encoding the catalytic subunit of phosphoinositide 3-kinase (PI3K), occur with high frequency in breast cancer. The protein kinase Akt is considered to be the primary effector of PIK3CA, although mechanisms by which PI3K mediates Akt-independent tumorigenic signals remain obscure. We show that serum and glucocorticoid-regulated kinase 3 (SGK3) is amplified in breast cancer and activated downstream of PIK3CA in a manner dependent on the phosphoinositide phosphatase INPP4B. Expression of INPP4B leads to enhanced SGK3 activation and suppression of Akt phosphorylation. Activation of SGK3 downstream of PIK3CA and INPP4B is required for 3D proliferation, invasive migration, and tumorigenesis in vivo. We further show that SGK3 targets the metastasis suppressor NDRG1 for degradation by Fbw7. We propose a model in which breast cancers harboring oncogenic PIK3CA activate SGK3 signaling while suppressing Akt, indicative of oncogenic functions for both INPP4B and SGK3 in these tumors.

Actin cross-linking protein palladin and spermatogenesis

In the seminiferous epithelium of the mammalian testis, the most distinctive ultrastructure is the extensive bundles of actin filaments that lie near the Sertoli-spermatid interface and the Sertoli-Sertoli cell interface known as the apical ectoplasmic specialization (apical ES) and the basal ES, respectively. These actin filament bundles not only confer strong adhesion at these sites, they are uniquely found in the testis. Recent studies have shown that ES also confers spermatid and Sertoli cell polarity in the seminiferous epithelium during the epithelial cycle. While these junctions were first described in the 1970s, there are few functional studies in the literature to examine the regulation of these actin filament bundles. It is conceivable that these actin filament bundles at the ES undergo extensive re-organization to accommodate changes in location of developing spermatids during spermiogenesis as spermatids are transported across the seminiferous epithelium. Additionally, these actin filaments are rapidly reorganized during BTB restructuring to accommodate the transit of preleptotene spermatocytes across the barrier at stage VIII of the epithelial cycle. Thus, actin binding and regulatory proteins are likely involved in these events to confer changes in F-actin organization at these sites. Interestingly, there are no reports in the field to study these regulatory proteins until recently. Herein, we summarize some of the latest findings in the field regarding a novel actin cross-linker and actin-bundling protein called palladin. We also discuss in this opinion article the likely role of palladin in regulating actin filament bundles at the ES during spermatogenesis, highlighting the significant of palladin and how this protein is plausibly working in concert with other actin-binding/regulatory proteins and components of polarity proteins to regulate the cyclic events of actin organization and re-organization during the epithelial cycle of spermatogenesis. We also propose a hypothetic model by which palladin regulates ES restructuring during the epithelial cycle of spermatogenesis.

rpS6 regulates blood-testis barrier dynamics through Akt-mediated effects on MMP-9

Mammalian target of rapamycin complex 1 (mTORC1) is an emerging regulator of blood-tissue barriers that utilizes ribosomal protein S6 (rpS6) as the downstream signaling molecule. To explore the role of rpS6 in blood-testis barrier (BTB) function, a constitutively active quadruple rpS6 phosphomimetic mutant was constructed in mammalian expression vector and overexpressed in Sertoli cells cultured in vitro that mimicked the BTB in vivo. Using this quadruple phosphomimetic mutant, phosphorylated (p)-rpS6 was shown to disrupt IGF-1/insulin signaling, thereby abolishing Akt phosphorylation, which led to an induction of MMP-9. This increase in MMP-9 secretion perturbed the Sertoli cell tight junction permeability barrier by proteolysis-mediated downregulation of tight junction proteins at the BTB. These findings were confirmed by the use of a specific MMP-9 inhibitor that blocked the disruption of the tight junction permeability barrier by the rpS6 mutant. Additionally, RNA interference (RNAi)-mediated Akt silencing was able to mimic the results of rpS6 mutant overexpression in Sertoli cells, further confirming this p-rpS6-Akt-MMP-9 signaling pathway. In conclusion, these data support a new concept of mTORC1-mediated BTB regulation, that is possibly also applicable to other blood-tissue barriers.

Profilin-1 suppresses tumorigenicity in pancreatic cancer through regulation of the SIRT3-HIF1α axis

BACKGROUND

Tumor cells exhibit abnormal actin remodeling profiles, which involve the altered expressions of several important actin-binding proteins. Profilin1 (Pfn1), originally identified as an actin-associated protein, has been linked to several human malignancies. Our recent studies suggested that Pfn1 facilitates apoptosis in pancreatic cancer cells. Here, we investigated the exact role of Profilin1 (Pfn1) in pancreatic adenocarcinoma (PDAC) and the underlying mechanisms.

METHODS

Pfn1 protein expression in PDAC specimens was analyzed by immunohistochemistry using a tissue microarray (TMA) containing PDAC tumor tissue and corresponding normal tissue samples from 72 patients. The effect of Pfn1 expression on cancer proliferation was assessed in cells by up- and down-regulation of Pfn1 in vitro and in vivo. Immunoprecipitation and mass spectrometry were used to identify the Pfn1-associated proteins and potential pathways.

RESULTS

Pfn1 was downregulated in clinical pancreatic adenocarcinoma specimens compared with the surrounding benign tissues. Univariate survival analysis of the PDAC cohorts showed that low expression of Pfn1 was significantly correlated with shortened patient survival (mean 14.2 months versus 20.9 months, P < 0.05). Restoration of Pfn1 in pancreatic cancer cells with low endogenous Pfn1 expression resulted in a nontumorigenic phenotype, suggesting that Pfn1 may be a negative regulator of pancreatic cancer progression. Overexpression of Pfn1 in vivo decreased the tumor volume in orthotopic xenograft nude mice models. Pfn1 upregulated the expression of SIRT3, leading to HIF1α destabilization. This data revealed that aberrant Pfn1 expression contributes to pancreatic cancer progression.

CONCLUSIONS

Our data suggest that Pfn1 is a tumor suppressor in pancreatic cancer that acts via a novel mechanism of regulating the SIRT3-HIF1α axis, independently of its cytoskeleton-related activity.

Role of lifeguard β-isoform in the development of breast cancer

In the last century there has been great progress in the treatment of breast cancer by improving drug and radiation therapy as well as surgical techniques. Despite this development, breast cancer remains a major cause of death among women in Europe and the US. The cause of breast cancer at the cellular level is still not fully understood. In the present study, we investigated the expression of the Lifeguard β-isoform in breast cancer tissues. In contrast to Lifeguard, the β-isoform has one transmembrane domain less, which is the last of seven (99 bp), and due to this we suspect that the Lifeguard β-isoform exhibits a different function. We determined the expression and function of the β-isoform of Lifeguard in breast cancer cell lines (MCF-7 and MDA-MB-231), a human breast epithelial cell line (MCF10A) and in breast tumour tissue sections. Western blotting, PCR arrays and immunofluorescence were used to investigate the expression of Lifeguard and its β-isoform. Moreover, we investigated the ability of Lifeguard β-isoform expression to inhibit apoptosis induced by Fas. Our results indicated that Lifeguard β-isoform is strongly expressed in breast tumour tissues. More notably, we demonstrated that Fas sensitivity was reduced in the MCF10A breast cells expressing the Lifeguard β-isoform. Taken together, our findings indicate the role of the Lifeguard β-isoform as an anti-apoptotic protein and provide further evidence of the potential of the Lifeguard β-isoform as a target for the development of novel therapeutic strategies.

Actin-associated protein palladin promotes tumor cell invasion by linking extracellular matrix degradation to cell cytoskeleton

Basal-like breast carcinomas, characterized by unfavorable prognosis and frequent metastases, are associated with epithelial-to-mesenchymal transition. During this process, cancer cells undergo cytoskeletal reorganization and up-regulate membrane-type 1 matrix metalloproteinase (MT1-MMP; MMP14), which functions in actin-based pseudopods to drive invasion by extracellular matrix degradation. However, the mechanisms that couple matrix proteolysis to the actin cytoskeleton in cell invasion have remained unclear. On the basis of a yeast two-hybrid screen for the MT1-MMP cytoplasmic tail-binding proteins, we identify here a novel Src-regulated protein interaction between the dynamic cytoskeletal scaffold protein palladin and MT1-MMP. These proteins were coexpressed in invasive human basal-like breast carcinomas and corresponding cell lines, where they were associated in the same matrix contacting and degrading membrane complexes. The silencing and overexpression of the 90-kDa palladin isoform revealed the functional importance of the interaction with MT1-MMP in pericellular matrix degradation and mesenchymal tumor cell invasion, whereas in MT1-MMP-negative cells, palladin overexpression was insufficient for invasion. Moreover, this invasion was inhibited in a dominant-negative manner by an immunoglobulin domain-containing palladin fragment lacking the dynamic scaffold and Src-binding domains. These results identify a novel protein interaction that links matrix degradation to cytoskeletal dynamics and migration signaling in mesenchymal cell invasion.

Induction of cell cycle arrest and apoptosis in caspase-3 deficient MCF-7 cells by Dillenia suffruticosa root extract via multiple signalling pathways

BACKGROUND

Dillenia suffruticosa root dichloromethane extract (DCM-DS) has been reported to exhibit strong cytotoxicity towards breast cancer cells. The present study was designed to investigate the cell cycle profile, mode of cell death and signalling pathways of DCM-DS-treated human caspase-3 deficient MCF-7 breast cancer cells.

METHODS

Dillenia suffruticosa root was extracted by sequential solvent extraction. The anti-proliferative activity of DCM-DS was determined by using MTT assay. The mode of cell death was evaluated by using inverted light microscope and Annexin-V/PI-flow cytometry analysis. Cell cycle analysis and measurement of intracellular reactive oxygen species (ROS) were performed by using flow cytometry. MCF-7 cells were co-treated with antioxidants α-tocopherol and ascorbic acid to evaluate whether the cell death was mainly due to oxidative stress. GeXP-based multiplex system was employed to investigate the expression of apoptotic, growth and survival genes in MCF-7 cells. Western blot analysis was performed to confirm the expression of the genes.

RESULTS

DCM-DS was cytotoxic to the MCF-7 cells in a time-and dose-dependent manner. The IC50 values of DCM-DS at 24, 48 and 72 hours were 20.3 ± 2.8, 17.8 ± 1.5 and 15.5 ± 0.5 μg/mL, respectively. Cell cycle analysis revealed that DCM-DS induced G0/G1 and G2/M phase cell cycle arrest in MCF-7 cells at low concentration (12.5 and 25 μg/mL) and high concentration (50 μg/mL), respectively. Although Annexin-V/PI-flow cytometry analysis has confirmed that DCM-DS induced apoptosis in MCF-7 cells, the distinct characteristics of apoptosis such as membrane blebbing, chromatin condensation, nuclear fragmentation and formation of apoptotic bodies were not observed under microscope. DCM-DS induced formation of ROS in MCF-7 cells. Nevertheless, co-treatment with antioxidants did not attenuate the cell death at low concentration of DCM-DS. The pro-apoptotic gene JNK was up-regulated whereby anti-apoptotic genes AKT1 and ERK1/2 were down-regulated in a dose-dependent manner. Western blot analysis has confirmed that DCM-DS significantly up-regulated the expression of pro-apoptotic JNK1, pJNK and down-regulated anti-apoptotic AKT1, ERK1 in MCF-7 cells.

CONCLUSION

DCM-DS induced cell cycle arrest and apoptosis in MCF-7 cells via multiple signalling pathways. It shows the potential of DCM-DS to be developed to target the cancer cells with mutant caspase-3.

Akt-ivation of RNA splicing

Cells must tightly control alternative splicing of RNA to maintain homeostasis; in this issue of Molecular Cell, Sanidas et al. (2014) provide new insights into the regulation of RNA splicing by Akt isoforms through phosphorylation of histone modification machinery.

Signaling specificity in the Akt pathway in biology and disease

Akt/PKB is a key master regulator of a wide range of physiological functions including metabolism, proliferation, survival, growth, angiogenesis and migration and invasion. The Akt protein kinase family comprises three highly related isoforms encoded by different genes. The initial observation that the Akt isoforms share upstream activators as well as several downstream effectors, together with the high sequence homology suggested that their functions were mostly redundant. By contrast, an increasing body of evidence has recently uncovered the concept of Akt isoform signaling specificity, supported by distinct phenotypes displayed by animal strains genetically modified for each of the three genes, as well as by the identification of isoform-specific substrates and association with discrete subcellular locations. Given that Akt is regarded as a promising therapeutic target in a number of pathologies, it is essential to dissect the relative contributions of each isoform, as well as the degree of compensation in pathophysiological function. Here we summarize our view of how Akt selectivity is achieved in the context of subcellular localization, isoform-specific substrate phosphorylation and context-dependent functions in normal and pathophysiological settings.

Actin binding proteins, spermatid transport and spermiation

The transport of germ cells across the seminiferous epithelium is composed of a series of cellular events during the epithelial cycle essential to the completion of spermatogenesis. Without the timely transport of spermatids during spermiogenesis, spermatozoa that are transformed from step 19 spermatids in the rat testis fail to reach the luminal edge of the apical compartment and enter the tubule lumen at spermiation, thereby arriving the epididymis for further maturation. Step 19 spermatids and/or sperms that remain in the epithelium beyond stage VIII of the epithelial cycle will be removed by the Sertoli cell via phagocytosis to form phagosomes and be degraded by lysosomes, leading to subfertility and/or infertility. However, the biology of spermatid transport, in particular the final events that lead to spermiation remain elusive. Based on recent data in the field, we critically evaluate the biology of spermiation herein by focusing on the actin binding proteins (ABPs) that regulate the organization of actin microfilaments at the Sertoli-spermatid interface, which is crucial for spermatid transport during this event. The hypothesis we put forth herein also highlights some specific areas of research that can be pursued by investigators in the years to come.

PIK3CA mutations define favorable prognostic biomarkers in operable breast cancer: a systematic review and meta-analysis

Background

Mutations of the p110α catalytic subunit of phosphatidylinositol 3-kinase (PIK3CA) are among the most common genetic aberrations in human breast cancer. At present, controversy exists concerning the prognostic value of the mutations.

Methods

We performed a systematic review and meta-analysis to clarify the association between PIK3CA mutations and survival outcomes. A comprehensive, computerized literature search of PubMed, Web of Science databases, the Chinese Biomedical Literature Database, and Wangfang Data until August 27, 2013 was carried out. Eligible studies were included according to specific inclusion criteria. Pooled hazard ratio was estimated by using the fixed effects model or random effects model according to heterogeneity between studies.

Results

Eight eligible studies were included in the analysis, all of which were retrospective cohort studies. The overall meta-analysis demonstrated that the PIK3CA mutations were associated with better clinical outcomes (hazard ratio 0.72; 95% confidence interval: 0.57–0.91; P=0.006). None of the single studies materially altered the original results and no evidence of publication bias was found. Further subgroup analysis of mutations in exons 9 and 20 did not show statistical significance.

Conclusion

PIK3CA mutations in operable primary breast cancer indicate a good prognosis. Further studies should be conducted to investigate the effect of PIK3CA mutations on clinical outcomes in different histologic types, different molecular subtypes of breast cancer, and different exons of PIK3CA.

A putative role of the Sup35p C-terminal domain in the cytoskeleton organization during yeast mitosis

Based on structural analysis of several effectors and partners, Sup35pC is proposed to serve as actin modulator during mitosis.

Palladin promotes assembly of non-contractile dorsal stress fibers through VASP recruitment

Stress fibers are major contractile actin structures in non-muscle cells, where they have an important role in adhesion, morphogenesis and mechanotransduction. Palladin is a multidomain protein, which associates with stress fibers in a variety of cell-types. However, the exact role of palladin in stress fiber assembly and maintenance has remained obscure, and whether it functions as an actin filament cross-linker or scaffolding protein was unknown. We demonstrate that palladin is specifically required for assembly of non-contractile dorsal stress fibers, and is consequently essential for generation of stress fiber networks and regulation of cell morphogenesis in osteosarcoma cells migrating in three-dimensional collagen matrix. Importantly, we reveal that palladin is necessary for the recruitment of vasodilator stimulated phosphoprotein (VASP) to dorsal stress fibers, and that it promotes stress fiber assembly through VASP. Both palladin and VASP display similar rapid dynamics at dorsal stress fibers, suggesting that they associate with stress fibers as a complex. Thus, palladin functions as a dynamic scaffolding protein, which promotes the assembly of dorsal stress fibers by recruiting VASP to these structures.

Targeting Akt3 signaling in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is currently the only major breast tumor subtype without effective targeted therapy and, as a consequence, in general has a poor outcome. To identify new therapeutic targets in TNBC, we performed a short hairpin RNA (shRNA) screen for protein kinases commonly amplified and overexpressed in breast cancer. Using this approach, we identified AKT3 as a gene preferentially required for the growth of TNBCs. Downregulation of Akt3 significantly inhibits the growth of TNBC lines in three-dimensional (3D) spheroid cultures and in mouse xenograft models, whereas loss of Akt1 or Akt2 have more modest effects. Akt3 silencing markedly upregulates the p27 cell-cycle inhibitor and this is critical for the ability of Akt3 to inhibit spheroid growth. In contrast with Akt1, Akt3 silencing results in only a minor enhancement of migration and does not promote invasion. Depletion of Akt3 in TNBC sensitizes cells to the pan-Akt inhibitor GSK690693. These results imply that Akt3 has a specific function in TNBCs; thus, its therapeutic targeting may provide a new treatment option for this tumor subtype.

Distinct roles of Akt1 in regulating proliferation, migration and invasion in HepG2 and HCT 116 cells

Elucidating the effects of genes involved in tumors may improve therapeutic strategies for human cancer. Recently, several studies discovered that Akt1 plays a dual role in mediating cell proliferation, migration and invasion, depending on the cell type. However, the pathophysiological role of Akt1 in hepatocellular carcinoma (HCC) and colorectal carcinoma cells remains poorly understood. In the present study, we transfected the Akt1-expressing plasmids into the tumor cells that expressed only low levels of Akt1. The migration and invasion abilities were analyzed in 24-well Boyden chambers. The expression of proteins was detected using western blot analysis. Our results demonstrated that overexpression of Akt1 significantly enhanced the proliferation rates and promoted the colony formation in both HepG2 and HCT 116 cells. When treated with wortmannin, the ability to form colonies was significantly attenuated in both cell lines. Of note, enforced expression of Akt1 induced HepG2 cell migration and invasion; by contrast, upregulation of Akt1 expression suppressed the migration and invasion of HCT 116 cells. Subsequent mechanistic investigations revealed that upregulation of Akt1 markedly induced the expression of Bcl-2 and NF-κB in both types of tumor cells. Notably, we observed a similar increase of MMP2, MMP9, HIF1α and VEGF in HCC cells, whereas Akt1 significantly suppressed the expression of these molecules in colorectal carcinoma cells. These data suggest a dual role for Akt1 in tumor cell migration and invasion and highlight the cell type-specific actions of Akt1 kinases in the regulation of cell motility.

The adherens junction protein afadin is an AKT substrate that regulates breast cancer cell migration

The PI3K-AKT signaling pathway regulates all phenotypes that contribute to progression of human cancers, including breast cancer. AKT mediates signal relay by phosphorylating numerous substrates, which are causally implicated in biologic responses such as cell growth, survival, metabolic reprogramming, migration, and invasion. Here a new AKT substrate is identified, the adherens junction protein Afadin, which is phosphorylated by AKT at Ser1718. Importantly, under conditions of physiologic IGF-1 signaling and oncogenic PI3K and AKT, Afadin is phosphorylated by all AKT isoforms, and this phosphorylation elicits a relocalization of Afadin from adherens junctions to the nucleus. Also, phosphorylation of Afadin increased breast cancer cell migration that was dependent on Ser1718 phosphorylation. Finally, nuclear localization of Afadin was observed in clinical breast cancer specimens, indicating that regulation of Afadin by the PI3K-AKT pathway has pathophysiologic significance.

IMPLICATIONS

Phosphorylation of the adhesion protein Afadin by AKT downstream of the PI3K pathway, leads to redistribution of Afadin and controls cancer cell migration.

Sin1 phosphorylation impairs mTORC2 complex integrity and inhibits downstream Akt signalling to suppress tumorigenesis

The mechanistic target of rapamycin (mTOR) functions as a critical regulator of cellular growth and metabolism by forming multi-component, yet functionally distinct complexes mTORC1 and mTORC2. Although mTORC2 has been implicated in mTORC1 activation, little is known about how mTORC2 is regulated. Here we report that phosphorylation of Sin1 at Thr 86 and Thr 398 suppresses mTORC2 kinase activity by dissociating Sin1 from mTORC2. Importantly, Sin1 phosphorylation, triggered by S6K or Akt, in a cellular context-dependent manner, inhibits not only insulin- or IGF-1-mediated, but also PDGF- or EGF-induced Akt phosphorylation by mTORC2, demonstrating a negative regulation of mTORC2 independent of IRS-1 and Grb10. Finally, a cancer-patient-derived Sin1-R81T mutation impairs Sin1 phosphorylation, leading to hyper-activation of mTORC2 by bypassing this negative regulation. Together, our results reveal a Sin1-phosphorylation-dependent mTORC2 regulation, providing a potential molecular mechanism by which mutations in the mTORC1-S6K-Sin1 signalling axis might cause aberrant hyper-activation of the mTORC2-Akt pathway, which facilitates tumorigenesis.

The switch-associated protein 70 (SWAP-70) bundles actin filaments and contributes to the regulation of F-actin dynamics

Coordinated assembly and disassembly of actin into filaments and higher order structures such as stress fibers and lamellipodia are fundamental for cell migration and adhesion. However, the precise spatiotemporal regulation of F-actin structures is not completely understood. SWAP-70, a phosphatidylinositol 3,4,5-trisphosphate-interacting, F-actin-binding protein, participates in actin rearrangements through yet unknown mechanisms. Here, we show that SWAP-70 is an F-actin-bundling protein that oligomerizes through a Gln/Glu-rich stretch within a coiled-coil region. SWAP-70 bundles filaments in parallel and anti-parallel fashion through its C-terminal F-actin binding domain and delays dilution-induced F-actin depolymerization. We further demonstrate that SWAP-70 co-localizes and directly interacts with cofilin, an F-actin severing and depolymerization factor, and contributes to the regulation of cofilin activity in vivo. In line with these activities, upon stem cell factor stimulation, murine bone marrow-derived mast cells lacking SWAP-70 display aberrant regulation of F-actin and actin free barbed ends dynamics. Moreover, proper stem cell factor-dependent cofilin activation via dephosphorylation and subcellular redistribution into a detergent-resistant cytoskeletal compartment also require SWAP-70. Together, these findings reveal an important role of SWAP-70 in the dynamic spatiotemporal regulation of F-actin networks.

Coordinate phosphorylation of multiple residues on single AKT1 and AKT2 molecules

Aberrant AKT activation is prevalent across multiple human cancer lineages providing an important new target for therapy. Twenty-two independent phosphorylation sites have been identified on specific AKT isoforms likely contributing to differential isoform regulation. However, the mechanisms regulating phosphorylation of individual AKT isoform molecules have not been elucidated because of the lack of robust approaches able to assess phosphorylation of multiple sites on a single AKT molecule. Using a nanofluidic proteomic immunoassay (NIA), consisting of isoelectric focusing followed by sensitive chemiluminescence detection, we demonstrate that under basal and ligand-induced conditions that the pattern of phosphorylation events is markedly different between AKT1 and AKT2. Indeed, there are at least 12 AKT1 peaks and at least 5 AKT2 peaks consistent with complex combinations of phosphorylation of different sites on individual AKT molecules. Following insulin stimulation, AKT1 was phosphorylated at Thr308 in the T-loop and Ser473 in the hydrophobic domain. In contrast, AKT2 was only phosphorylated at the equivalent sites (Thr309 and Ser474) at low levels. Further, Thr308 and Ser473 phosphorylation occurred predominantly on the same AKT1 molecules, whereas Thr309 and Ser474 were phosphorylated primarily on different AKT2 molecules. Although basal AKT2 phosphorylation was sensitive to inhibition of phosphatidylinositol 3-kinase (PI3K), basal AKT1 phosphorylation was essentially resistant. PI3K inhibition decreased pThr451 on AKT2 but not pThr450 on AKT1. Thus, NIA technology provides an ability to characterize coordinate phosphorylation of individual AKT molecules providing important information about AKT isoform-specific phosphorylation, which is required for optimal development and implementation of drugs targeting aberrant AKT activation.

Regulation of APC(Cdh1) E3 ligase activity by the Fbw7/cyclin E signaling axis contributes to the tumor suppressor function of Fbw7

Fbw7 and Cdh1 are substrate-recognition subunits of the SCF- and APC-type E3 ubiquitin ligases, respectively. There is emerging evidence suggesting that both Fbw7 and Cdh1 function as tumor suppressors by targeting oncoproteins for destruction. Loss of Fbw7, but not Cdh1, is frequently observed in various human tumors. However, it remains largely unknown how Fbw7 mechanistically functions as a tumor suppressor and whether there is a signaling crosstalk between Fbw7 and Cdh1. Here, we report that Fbw7-deficient cells not only display elevated expression levels of SCF(Fbw7) substrates, including cyclin E, but also have increased expression of various APC(Cdh1) substrates. We further defined cyclin E as the critical signaling link by which Fbw7 governs APC(Cdh1) activity, as depletion of cyclin E in Fbw7-deficient cells results in decreased expression of APC(Cdh1) substrates to levels comparable to those in wild-type (WT) cells. Conversely, ectopic expression of cyclin E recapitulates the aberrant APC(Cdh1) substrate expression observed in Fbw7-deficient cells. More importantly, 4A-Cdh1 that is resistant to Cdk2/cyclin E-mediated phosphorylation, but not WT-Cdh1, reversed the elevated expression of various APC(Cdh1) substrates in Fbw7-deficient cells. Overexpression of 4A-Cdh1 also resulted in retarded cell growth and decreased anchorage-independent colony formation. Altogether, we have identified a novel regulatory mechanism by which Fbw7 governs Cdh1 activity in a cyclin E-dependent manner. As a result, loss of Fbw7 can lead to aberrant increase in the expression of both SCF(Fbw7) and APC(Cdh1) substrates. Our study provides a better understanding of the tumor suppressor function of Fbw7, and suggests that Cdk2/cyclin E inhibitors could serve as effective therapeutic agents for treating Fbw7-deficient tumors.

Aggregation of Human Eyelid Adipose-derived Stem Cells by Human Body Fluids

Fetal bovine serum (FBS) is the most frequently used serum for the cultivation of mammalian cells. However, since animal-derived materials might not be appropriate due to safety issues, allogeneic human serum (HS) has been used to replace FBS, particularly for the culture of human cells. While there has been a debate about the advantages of HS, its precise effect on human adult stem cells have not been clarified. The present study aimed to investigate the effect of HS on the human eyelid adipose stem cells (HEACs) in vitro. When HEACs were cultivated in a medium containing 10% HS, many cells moved into several spots and aggregated there. The phenomenon was observed as early as 9 days following 10% HS treatment, and 12 days following 5% HS plus 5% FBS treatment. However, the aggregation was never observed when the same cells were cultivated with 10% FBS or bovine serum albumin. To examine whether cell density might affect the aggregation, cells were seeded with different densities on 12-well dish. Until the beginning of aggregation, cells seeded at low densities exhibited the longest culture period of 16 days whereas cells seeded at high densities showed the shortest period of 9 days to form aggregation. The number of cells was 15.1±0.2×10⁴ as the least for the low density group, and 29.3±2.8×10⁴ as the greatest for the high density group. When human cord blood serum or normal bovine serum was examined for the same effect on HEACs, interestingly, cord blood serum induced the aggregation of cells whereas bovine serum treatment has never induced. When cells were cultivated with 10% HS for 9 days, they were obtained and analyzed by RT-PCR. Compared to FBS-cultivated HEACs, HS-cultivated HEACs did not express VIM, and less expressed GATA4, PALLD. On the other hand, HS-cultivated HEACs expressed MAP2 more than FBS-cultivated HEACs. In conclusion, human adult stem cells could move and form aggregates by the treatment with human body fluids.

Protein kinase B/AKT isoform 2 drives migration of human mesenchymal stem cells

This study was designed to investigate the migratory behavior of adult human mesenchymal stem cells (MSC) and the underlying mechanism. Cell migration was assessed by transwell, wound healing and time-lapse in vivo motility assays. Pharmacological inhibitors were used to determine the potential mechanism responsible for cell migration and invasion. The tests that were implemented revealed that MSC were fairly migratory. Protein kinase B (AKT) was strongly activated at the basal level. Through our analyses we demonstrated that pharmacological inactivation of AKT2 but not AKT1 significantly decreased cell migration and invasion. Although preliminary, collectively our results indicate that AKT2 activation plays a critical role in enabling MSC migration.

Defining Akt actions in muscle differentiation

Muscle development in childhood and muscle regeneration in adults are highly regulated processes that are necessary for reaching and maintaining optimal muscle mass and strength throughout life. Muscle repair after injury relies on stem cells, termed satellite cells, whose activity is controlled by complex signals mediated by cell-cell contact, by growth factors, and by hormones, which interact with genetic programs controlled by myogenic transcription factors. Insulin-like growth factors (IGFs) play key roles in muscle development and help coordinate muscle repair after injury, primarily by stimulating the phosphatidylinositol 3-kinase-Akt signaling pathway, and both in vitro and in vivo studies have shown that Akt kinase activity is critical for optimal muscle growth and regeneration. Here we find that of the two Akts expressed in muscle, Akt1 is essential for initiation of differentiation in culture and is required for normal myoblast motility, while Akt2 is dispensable. Although Akt2 deficiency did lead to diminished myotube maturation, as assessed by a decline in myofiber area and in fusion index, either Akt1 or Akt2 could restore these processes toward normal. Thus levels of Akt expression rather than distinct actions of individual Akt species are critical for normal myofiber development during the later stages of muscle differentiation.

Biomarkers of response to Akt inhibitor MK-2206 in breast cancer

PURPOSE

We tested the hypothesis that allosteric Akt inhibitor MK-2206 inhibits tumor growth, and that PTEN/PIK3CA mutations confer MK-2206 sensitivity.

EXPERIMENTAL DESIGN

MK-2206 effects on cell signaling were assessed in vitro and in vivo. Its antitumor efficacy was assessed in vitro in a panel of cancer cell lines with differing PIK3CA and PTEN status. Its in vivo efficacy was tested as a single agent and in combination with paclitaxel.

RESULTS

MK-2206 inhibited Akt signaling and cell-cycle progression, and increased apoptosis in a dose-dependent manner in breast cancer cell lines. Cell lines with PTEN or PIK3CA mutations were significantly more sensitive to MK-2206; however, several lines with PTEN/PIK3CA mutations were MK-2206 resistant. siRNA knockdown of PTEN in breast cancer cells increased Akt phosphorylation concordant with increased MK-2206 sensitivity. Stable transfection of PIK3CA E545K or H1047R mutant plasmids into normal-like MCF10A breast cells enhanced MK-2206 sensitivity. Cell lines that were less sensitive to MK-2206 had lower ratios of Akt1/Akt2 and had less growth inhibition with Akt siRNA knockdown. In PTEN-mutant ZR75-1 breast cancer xenografts, MK-2206 treatment inhibited Akt signaling, cell proliferation, and tumor growth. In vitro, MK-2206 showed a synergistic interaction with paclitaxel in MK-2206-sensitive cell lines, and this combination had significantly greater antitumor efficacy than either agent alone in vivo.

CONCLUSIONS

MK-2206 has antitumor activity alone and in combination with chemotherapy. This activity may be greater in tumors with PTEN loss or PIK3CA mutation, providing a strategy for patient enrichment in clinical trials.

Distinct roles of AKT isoforms in regulating β1-integrin activity, migration, and invasion in prostate cancer

On the basis of an RNA interference screen, we identify AKT1 and AKT2 as inhibitors of β1-integrin activity and invasion in prostate cancer. AKT1 siRNA induces β-integrin activity and up-regulation of RTKs known to function in cooperation with integrins. In contrast, AKT2 siRNA up-regulates microRNA-200, which increases integrin activity.

AKT1 and AKT2 kinases have been shown to play opposite roles in breast cancer migration and invasion. In this study, an RNA interference screen for integrin activity inhibitors identified AKT1 as an inhibitor of β1-integrin activity in prostate cancer. Validation experiments investigating all three AKT isoforms demonstrated that, unlike in breast cancer, both AKT1 and AKT2 function as negative regulators of cell migration and invasion in PC3 prostate cancer cells. Down-regulation of AKT1 and AKT2, but not AKT3, induced activation of cell surface β1-integrins and enhanced adhesion, migration, and invasion. Silencing of AKT1 and AKT2 also resulted in increased focal adhesion size. Importantly, the mechanisms involved in integrin activity regulation were distinct for the two AKT isoforms. Silencing of AKT1 relieved feedback suppression of the expression and activity of several receptor tyrosine kinases, including EGFR and MET, with established cross-talk with β1-integrins. Silencing of AKT2, on the other hand, induced up-regulation of the microRNA-200 (miR-200) family, and overexpression of miR-200 was sufficient to induce integrin activity and cell migration in PC3 cells. Taken together, these data define an inhibitory role for both AKT1 and AKT2 in prostate cancer migration and invasion and highlight the cell type–specific actions of AKT kinases in the regulation of cell motility.

Pleiotropic effects of miR-183~96~182 converge to regulate cell survival, proliferation and migration in medulloblastoma

Medulloblastomas are the most common malignant brain tumors in children. Several large-scale genomic studies have detailed their heterogeneity, defining multiple subtypes with unique molecular profiles and clinical behavior. Increased expression of the miR-183~96~182 cluster of microRNAs has been noted in several subgroups, including the most clinically aggressive subgroup associated with genetic amplification of MYC. To understand the contribution of miR-183~96~182 to the pathogenesis of this aggressive subtype of medulloblastoma, we analyzed global gene expression and proteomic changes that occur upon modulation of miRNAs in this cluster individually and as a group in MYC-amplified medulloblastoma cells. Knockdown of the full miR-183~96~182 cluster results in enrichment of genes associated with apoptosis and dysregulation of the PI3K/AKT/mTOR signaling axis. Conversely, there is a relative enrichment of pathways associated with migration, metastasis and epithelial to mesenchymal transition, as well as pathways associated with dysfunction of DNA repair in cells with preserved miR-183 cluster expression. Immunocytochemistry and FACS analysis confirm induction of apoptosis upon knockdown of the miR-183 cluster. Importantly, cell-based migration and invasion assays verify the positive regulation of cell motility/migration by the miR-183 cluster, which is largely mediated by miR-182. We show that the effects on cell migration induced by the miR-183 cluster are coupled to the PI3K/AKT/mTOR pathway through differential regulation of AKT1 and AKT2 isoforms. Furthermore, we show that rapamycin inhibits cell motility/migration in medulloblastoma cells and phenocopies miR-183 cluster knockdown. Thus, the miR-183 cluster regulates multiple biological programs that converge to support the maintenance and metastatic potential of medulloblastoma.

N-cadherin regulates mammary tumor cell migration through Akt3 suppression

N-cadherin is a cell-cell adhesion molecule that plays a role in breast cancer metastasis. Here, we show that in vivo expression of N-cadherin in the PyMT mouse model, which enhances mammary tumor metastasis, results in selective inhibition of Akt3 expression and phosphorylation. Similarly, exogenous expression of N-cadherin in PyMT or MCF-7 mammary tumor cells enhanced cell motility and caused a dramatic reduction in Akt3 expression and phosphorylation. Moreover, knockdown of Akt3 in PyMT tumor cells increased cell motility and disrupted mammary morphogenesis, but had no effect on cell proliferation. Conversely, overexpression of wild-type Akt3 in PyMT-N-cadherin cells inhibited cell motility promoted by N-cadherin. Taken altogether, these findings demonstrate that N-cadherin suppresses Akt3 to promote cell motility and highlight the intricate regulation of Akt isoforms by N-cadherin during metastasis.

The protein kinase Akt1 regulates the interferon response through phosphorylation of the transcriptional repressor EMSY

The protein kinases Akt1, Akt2, and Akt3 possess nonredundant signaling properties, few of which have been investigated. Here, we present evidence for an Akt1-dependent pathway that controls interferon (IFN)-regulated gene expression and antiviral immunity. The target of this pathway is EMSY, an oncogenic interacting partner of BRCA2 that functions as a transcriptional repressor. Overexpression of EMSY in hTERT-immortalized mammary epithelial cells, and in breast and ovarian carcinoma cell lines, represses IFN-stimulated genes (ISGs) in a BRCA2-dependent manner, whereas its knockdown has the opposite effect. EMSY binds to the promoters of ISGs, suggesting that EMSY functions as a direct transcriptional repressor. Akt1, but not Akt2, phosphorylates EMSY at Ser209, relieving EMSY-mediated ISG repression. The Akt1/EMSY/ISG pathway is activated by both viral infection and IFN, and it inhibits the replication of HSV-1 and vesicular stomatitis virus (VSV). Collectively, these data define an Akt1-dependent pathway that contributes to the full activation of ISGs by relieving their repression by EMSY and BRCA2.

Arousal of cancer-associated stroma: overexpression of palladin activates fibroblasts to promote tumor invasion

Background

Cancer-associated fibroblasts, comprised of activated fibroblasts or myofibroblasts, are found in the stroma surrounding solid tumors. These myofibroblasts promote invasion and metastasis of cancer cells. Mechanisms regulating the activation of the fibroblasts and the initiation of invasive tumorigenesis are of great interest. Upregulation of the cytoskeletal protein, palladin, has been detected in the stromal myofibroblasts surrounding many solid cancers and in expression screens for genes involved in invasion. Using a pancreatic cancer model, we investigated the functional consequence of overexpression of exogenous palladin in normal fibroblasts in vitro and its effect on the early stages of tumor invasion.

Principal Findings

Palladin expression in stromal fibroblasts occurs very early in tumorigenesis. In vivo, concordant expression of palladin and the myofibroblast marker, alpha smooth muscle actin (α-SMA), occurs early at the dysplastic stages in peri-tumoral stroma and progressively increases in pancreatic tumorigenesis. In vitro introduction of exogenous 90 kD palladin into normal human dermal fibroblasts (HDFs) induces activation of stromal fibroblasts into myofibroblasts as marked by induction of α-SMA and vimentin, and through the physical change of cell morphology. Moreover, palladin expression in the fibroblasts enhances cellular migration, invasion through the extracellular matrix, and creation of tunnels through which cancer cells can follow. The fibroblast invasion and creation of tunnels results from the development of invadopodia-like cellular protrusions which express invadopodia proteins and proteolytic enzymes. Palladin expression in fibroblasts is triggered by the co-culture of normal fibroblasts with k-ras-expressing epithelial cells.

Conclusions

Overall, palladin expression can impart myofibroblast properties, in turn promoting the invasive potential of these peri-tumoral cells with invadopodia-driven degradation of extracellular matrix. Palladin expression in fibroblasts can be triggered by k-ras expression in adjacent epithelial cells. This data supports a model whereby palladin-activated fibroblasts facilitate stromal-dependent metastasis and outgrowth of tumorigenic epithelium.

Role of palladin phosphorylation by extracellular signal-regulated kinase in cell migration

Phosphorylation of actin-binding proteins plays a pivotal role in the remodeling of the actin cytoskeleton to regulate cell migration. Palladin is an actin-binding protein that is phosphorylated by growth factor stimulation; however, the identity of the involved protein kinases remains elusive. In this study, we report that palladin is a novel substrate of extracellular signal-regulated kinase (ERK). Suppression of ERK activation by a chemical inhibitor reduced palladin phosphorylation, and expression of active MEK alone was sufficient for phosphorylation. In addition, an in vitro kinase assay demonstrated direct palladin phosphorylation by ERK. We found that Ser77 and Ser197 are essential residues for phosphorylation. Although the phosphorylation of these residues was not required for actin cytoskeletal organization, we found that expression of non-phosphorylated palladin enhanced cell migration. Finally, we show that phosphorylation inhibits the palladin association with Abl tyrosine kinase. Taken together, our results indicate that palladin phosphorylation by ERK has an anti-migratory function, possibly by modulating interactions with molecules that regulate cell migration.

Selective signaling by Akt1 controls osteoblast differentiation and osteoblast-mediated osteoclast development

Maintaining optimal bone integrity, mass, and strength throughout adult life requires ongoing bone remodeling, which involves coordinated activity between actions of bone-resorbing osteoclasts and bone forming-osteoblasts. Osteoporosis is a disorder of remodeling in which bone resorption outstrips deposition, leading to diminished bone mass and an increased risk of fractures. Here we identify Akt1 as a unique signaling intermediate in osteoblasts that can control both osteoblast and osteoclast differentiation. Targeted knockdown of Akt1 in mouse primary bone marrow stromal cells or in a mesenchymal stem cell line or genetic knockout of Akt1 stimulated osteoblast differentiation secondary to increased expression of the osteogenic transcription factor Runx2. Despite enhanced osteoblast differentiation, coupled osteoclastogenesis in Akt1 deficiency was markedly inhibited, with reduced accumulation of specific osteoclast mRNAs and proteins and impaired fusion to form multinucleated osteoclasts, defects secondary to diminished production of receptor activator of NF-κB ligand (RANKL) and macrophage colony-stimulating factor (m-CSF), critical osteoblast-derived osteoclast differentiation factors. Delivery of recombinant lentiviruses encoding Akt1 but not Akt2 to Akt1-deficient osteoblast progenitors reversed the increased osteoblast differentiation and, by boosting accumulation of RANKL and m-CSF, restored normal osteoclastogenesis, as did the addition of recombinant RANKL to conditioned culture medium from Akt1-deficient osteoblasts. Our results support the idea that targeted inhibition of Akt1 could lead to therapeutically useful net bone acquisition, and they indicate that closely related Akt1 and Akt2 exert distinct effects on cellular differentiation pathways.

Phosphotyrosine mediated protein interactions of the discoidin domain receptor 1

The receptor tyrosine kinase DDR1 has been implicated in multiple human cancers and fibrosis and is targeted by the leukemia drug Gleevec. This suggests that DDR1 might be a new therapeutic target. However, further insight into the DDR1 signaling pathway is required in order to support its further development. Here, we investigated DDR1 proximal signaling by the analysis of protein-protein interactions using proteomic approaches. All known interactors of DDR1 were identified and localized to specific phosphotyrosine residues on the receptor. In addition, we identified numerous signaling proteins as new putative phosphotyrosine mediated interactors including RasGAP, SHIP1, SHIP2, STATs, PI3K and the SRC family kinases. Most of the new proteins contain SH2 and PTB domains and for all interactors we could directly point the site of interaction to specific phosphotyrosine residues on the receptor. The identified proteins have roles in the early steps of the signaling cascade, propagating the signal from the DDR1 receptor into the cell. The map of phosphotyrosine mediated interactors of DDR1 created in this study will serve as a starting point for functional investigations which will enhance our knowledge on the role of the DDR1 receptor in health and disease. This article is part of a Special Section entitled: Understanding genome regulation and genetic diversity by mass spectrometry.

Oxidation of Akt2 kinase promotes cell migration and regulates G1-S transition in the cell cycle

Phosphorylation has long been recognized as the key mediator of protein signaling. New modes of signaling regulation are emerging with the development of specific chemical probes and application of high-throughput mass spectrometry technologies. Using biotin-tagged chemical probes for protein oxidation, mass spectrometry and functional assays, our group has recently reported isoform-specific oxidation of Akt2 in response to PDGF signaling. The studies included here investigate the functional consequence of oxidation on Akt2-mediated cell migration and cell cycle. Akt2-KO MEFs transduced with WT and Cys124Ser Akt2 were used as the model system for these studies. The implications of these findings on disease pathology are discussed.

Small-molecule inhibitors of the PI3K signaling network

The phosphoinositide 3-kinase (PI3K) signaling pathway controls a wide variety of cellular processes including cell death and survival, cell migration, protein synthesis and metabolism. Aberrant PI3K-dependent signaling, mediated by Akt kinase, has been implicated in many human diseases including cancer, inflammation, cardiovascular disease and metabolic diseases, making this pathway a principle target for drug development. In this article we will summarize the PI3K signaling network and discuss current strategies for pathway inhibition. We will also explore the importance and emerging relevance of Akt-independent PI3K signaling pathways and discuss attempts being made to harness these pathways by inhibiting the binding of a product of PI3K, phosphatidylinositol-(3,4,5)-trisphosphate, to effector pleckstrin homology domains.

The effect Akt2 deletion on tumor development in Pten(+/-) mice

The serine/threonine kinase Akt is frequently activated in human cancers and is considered an attractive therapeutic target. However, the relative contributions of the different Akt isoforms to tumorigenesis, and the effect of their deficiencies on cancer development are not well understood. We had previously shown that Akt1 deficiency is sufficient to markedly reduce the incidence of tumors in Pten^+/− mice. Particularly, Akt1 deficiency inhibits endometrial carcinoma and prostate neoplasia in Pten^+/− mice. Here, we analyzed the effect of Akt2 deficiency on the incidence of tumors in Pten^+/− mice. Relative to Akt1, Akt2 deficiency had little-to-no effect on the incidence of prostate neoplasia, endometrial carcinoma, intestinal polyps and adrenal lesions in Pten^+/− mice. However, Akt2 deficiency significantly decreased the incidence of thyroid tumors in Pten^+/−, which correlates with the relatively high level of Akt2 expression in the thyroid. Thus, unlike Akt1 deletion, Akt2 deletion is not sufficient to markedly inhibit tumorigenesis in Pten^+/− mice in most tested tissues. The relatively small effect of Akt2 deletion on the inhibition of tumorigenesis in Pten^+/− mice could be explained, in part, by an insufficient decrease in total Akt activity, due to the relatively lower Akt2 versus Akt1 expression, and relatively high blood insulin levels in Pten^+/−Akt2^−/− mice. The relatively high blood insulin levels in Pten^+/−Akt2^−/− mice may elevate the activity of Akt1, and possibly Akt3, thus, limiting the reduction of total Akt activity and preventing this activity from dropping to a threshold level required to inhibit tumorigenesis.

Promiscuous affairs of PKB/AKT isoforms in metabolism

The protein kinase B (PKB) family encompasses three isoforms; PKBα (AKT1), PKBβ (AKT2) and PKBγ (AKT3). PKBα and PKBβ but not PKBγ, are prominently expressed in classical insulin-sensitive tissues like liver, muscle and fat. Transgenic mice deficient for PKBα, PKBβ or PKBγ have been analysed to study the roles of PKB isoforms in metabolic regulation. Until recently, only loss of PKBβ was reported to result in metabolic disorders, especially insulin resistance, in humans and mice. However, a new study has shown that PKBα-deficient mice can show enhanced glucose tolerance accompanied by improved β-cell function and higher insulin sensitivity in adipocytes. These findings prompted us to review the relevant literature on the regulation of glucose metabolism by PKB isoforms in liver, skeletal muscle, adipocytes and pancreas.

Akt isoform-specific signaling in breast cancer: uncovering an anti-migratory role for palladin

Numerous studies have shown that Akt isoforms promote tumorigenesis by enhancing cancer cell survival and growth, and it is well established that signaling through the Akt upstream regulator PI 3-K enhances cancer cell migration. Therefore, it is conventionally accepted that PI 3-K/Akt pathway promotes tumor formation and metastasis. A few years ago, studies from several laboratories added a new layer to the pleiotropic effects of Akt function by showing that the Akt1 isoform inhibits breast cancer cell migration and invasion, whereas Akt2 promotes these phenotypes. These studies challenged the dogma and identified non-redundant functions of Akt isoforms in cancer progression. The identification of palladin as an Akt1-specific substrate in our recently published work has exemplified distinct Akt isoform-specific signaling in breast cancer. Here, we review these findings and discuss the implications for the understanding of the mechanistic basis for designing more effective anti-cancer therapeutics targeting the Akt pathway.

The actin associated protein palladin in smooth muscle and in the development of diseases of the cardiovasculature and in cancer

Palladin is an actin associated protein serving as a cytoskeleton scaffold, and actin cross linker, localizing at stress fibers, focal adhesions, and other actin based structures. Recent studies showed that palladin plays a critical role in smooth muscle differentiation, migration, contraction, and more importantly contributes to embryonic development. This review will focus on the functions and possible mechanisms of palladin in smooth muscle and in pathological conditions such as cardiovascular diseases and cancers.

Metastasis: tumor cells becoming MENAcing

During breast cancer metastasis cells emigrate from the primary tumor to the bloodstream, and this carries them to distant sites where they infiltrate and sometimes form metastases within target organs. These cells must penetrate the dense extracellular matrix comprising the basement membrane of the mammary duct/acinus and migrate toward blood and lymphatic vessels, processes that mammary tumor cells execute primarily using epidermal growth factor (EGF)-dependent protrusive and migratory activity. Here, we focus on how the actin regulatory protein Mena affects EGF-elicited movement, invasion and metastasis. Recent findings indicate that, in invasive migratory tumor cells, Mena isoforms that endow heightened sensitivity to EGF and increased protrusive and migratory abilities are upregulated, whereas other isoforms are selectively downregulated. This change in Mena isoform expression enables tumor cells to invade in response to otherwise benign EGF stimulus levels and could offer an opportunity to identify metastatic risk in patients.

Akt2 regulates expression of the actin-bundling protein palladin

The phosphatidylinositol 3-kinase/Akt pathway is responsible for key aspects of tumor progression, and is frequently hyperactivated in cancer. We have recently identified palladin, an actin-bundling protein that functions to control the actin cytoskeleton, as an Akt1-specific substrate that inhibits breast cancer cell migration. Here we have identified a role for Akt isoforms in the regulation of palladin expression. Akt2, but not Akt1, enhances palladin expression by maintaining protein stability and upregulating transcription. These data reveal that Akt signaling regulates the stability of palladin, and further supports the notion that Akt isoforms have distinct and specific roles in tumorigenesis.