Protein kinase D is a positive regulator of Bit1 apoptotic function

Bit1 is involved in regulation between integrin and TGFβ signaling in lens epithelial cells

Bit1, as an integrin-specific effector, is specifically expressed in lens epithelial cells (LECs) and may be essential to maintain the normal function of LECs. The present study investigated the function of Bit1 and its regulatory mechanism in LECs. Knockdown of Bit1 was mediated by a lentivirus with a specific short-hairpin RNA against Bit1 in SRA01/04 cells. Cell proliferation ability was measured by CCK-8 assay. Cell migration was examined by transwell and wound-healing assays. The effect of Bit1 knockdown on genome-wide expression patterns was studied via a GeneChip® PrimeView™ Human Gene Expression Array. Based on the ingenuity pathway analysis (IPA), Bit1's regulation of target pathways and genes was verified by real-time qPCR and Western blotting. Bit1 knockdown inhibited proliferation, migration, and regulated cell cycle and apoptosis of LECs. Microarray gene expression analysis and IPA assays revealed that integrin and TGFβ signaling pathways were remarkably impacted by Bit1 expression. FAK, PAK2, ITGA5, and ITGB1 were identified as core node molecules under the control of Bit1. Bit1 participates in integrin and TGFβ signaling via regulating downstream FAK and PAK2 and subsequently affecting EMT-related gene expression including ITGA5, ITGB1, and αSMA. In conclusion, Bit1 plays as an important role in the regulation between integrin and TGFβ signaling, which affects cell survival, migration, and EMT of LECs.

PTRH2: an adhesion regulated molecular switch at the nexus of life, death, and differentiation

Peptidyl-tRNA hydrolase 2 (PTRH2; Bit-1; Bit1) is an underappreciated regulator of adhesion signals and Bcl2 expression. Its key roles in muscle differentiation and integrin-mediated signaling are central to the pathology of a recently identified patient syndrome caused by a cluster of Ptrh2 gene mutations. These loss-of-function mutations were identified in patients presenting with severe deleterious phenotypes of the skeletal muscle, endocrine, and nervous systems resulting in a syndrome called Infantile-onset Multisystem Nervous, Endocrine, and Pancreatic Disease (IMNEPD). In contrast, in cancer PTRH2 is a potential oncogene that promotes malignancy and metastasis. PTRH2 modulates PI3K/AKT and ERK signaling in addition to Bcl2 expression and thereby regulates key cellular processes in response to adhesion including cell survival, growth, and differentiation. In this Review, we discuss the state of the science on this important cell survival, anoikis and differentiation regulator, and opportunities for further investigation and translation. We begin with a brief overview of the structure, regulation, and subcellular localization of PTRH2. We discuss the cluster of gene mutations thus far identified which cause developmental delays and multisystem disease. We then discuss the role of PTRH2 and adhesion in breast, lung, and esophageal cancers focusing on signaling pathways involved in cell survival, cell growth, and cell differentiation.

Downregulation of Bit1 expression promotes growth, anoikis resistance, and transformation of immortalized human bronchial epithelial cells via Erk activation-dependent suppression of E-cadherin

The mitochondrial Bit1 protein exerts tumor-suppressive function in NSCLC through induction of anoikis and inhibition of EMT. Having this dual tumor suppressive effect, its downregulation in the established human lung adenocarcinoma A549 cell line resulted in potentiation of tumorigenicity and metastasis in vivo. However, the exact role of Bit1 in regulating malignant growth and transformation of human lung epithelial cells, which are origin of most forms of human lung cancers, has not been examined. To this end, we have downregulated the endogenous Bit1 expression in the immortalized non-tumorigenic human bronchial epithelial BEAS-2B cells. Knockdown of Bit1 enhanced the growth and anoikis insensitivity of BEAS-2B cells. In line with their acquired anoikis resistance, the Bit1 knockdown BEAS-2B cells exhibited enhanced anchorage-independent growth in vitro but failed to form tumors in vivo. The loss of Bit1-induced transformed phenotypes was in part attributable to the repression of E-cadherin expression since forced exogenous E-cadherin expression attenuated the malignant phenotypes of the Bit1 knockdown cells. Importantly, we show that the loss of Bit1 expression in BEAS-2B cells resulted in increased Erk activation, which functions upstream to promote TLE1-mediated transcriptional repression of E-cadherin. These collective findings indicate that loss of Bit1 expression contributes to the acquisition of malignant phenotype of human lung epithelial cells via Erk activation-induced suppression of E-cadherin expression.

Anoikis and EMT: Lethal "Liaisons" during Cancer Progression

Anoikis is a unique mode of apoptotic cell death that occurs consequentially to insufficient cell-matrix interactions. Resistance to anoikis is a critical contributor to tumor invasion and metastasis. The phenomenon is regulated by integrins, which upon engagement with components of the extracellular matrix (ECM) form adhesion complexes and the actin cytoskeleton drives the formation of cell protrusions used to adhere to ECM, directing cell migration. The epithelial-mesenchymal transition (EMT) confers stem cell properties and leads to acquisition of a migratory and invasive phenotype by causing adherens junction breakdown and circumventing anoikis in the tumor microenvironment. The investigation of drug discovery platforms for apoptosis-driven therapeutics identified several novel agents with antitumor action via reversing resistance to anoikis, inhibiting survival pathways and impacting the EMT landscape in human cancer. In this review, we discuss current evidence on the contribution of the anoikis phenomenon functionally linked to EMT to cancer metastasis and the therapeutic value of antitumor drugs that selectively reverse anoikis resistance and/or EMT to impair tumor progression toward the development/optimization of apoptosis-driven therapeutic targeting of metastatic disease.

Bit1 Regulates Cell Migration and Survival in Oral Squamous Cell Carcinoma

PURPOSE

Increasing evidence has shown that Bcl-2 inhibitor of transcription 1 (Bit1) involves a variety of biological processes in the process of tumor development and progression. We hypothesized that Bit1 would be overexpressed in oral squamous cell carcinoma (OSCC); therefore, we examined Bit1 gene expression and protein production, as well as explored the effect of elevated Bit1 levels on OSCC cells.

MATERIALS AND METHODS

We investigated the use of quantitative real-time reverse transcription-polymerase chain reaction and immunohistochemistry analysis for Bit1 messenger RNA and protein levels. We used 75 OSCC specimens, 25 tumor-adjacent dysplasia specimens, and 25 normal oral tissue samples that matched OSCC specimens in this study. We also transfected Bit1 complementary DNA into human oral cancer cells (Tca8113) to further investigate the potential role of Bit1 in OSCC.

RESULTS

We found that Bit1 levels in OSCC tissues were significantly higher than those in tumor-adjacent dysplasia specimens and normal oral tissue (P < .05). We also confirmed that Bit1 overexpression in the cytosol of Tca8113 cells induced apoptosis.

CONCLUSIONS

Our findings suggest Bit1 overexpression may contribute to oral cancer cell survival and dissemination. In the future, Bit1 may be an important diagnostic and therapeutic target.

The Anoikis Effector Bit1 Inhibits EMT through Attenuation of TLE1-Mediated Repression of E-Cadherin in Lung Cancer Cells

The mitochondrial Bcl-2 inhibitor of transcription 1 (Bit1) protein is part of an anoikis-regulating pathway that is selectively dependent on integrins. We previously demonstrated that the caspase-independent apoptotic effector Bit1 exerts tumor suppressive function in lung cancer in part by inhibiting anoikis resistance and anchorage-independent growth in vitro and tumorigenicity in vivo. Herein we show a novel function of Bit1 as an inhibitor cell migration and epithelial–mesenchymal transition (EMT) in the human lung adenocarcinoma A549 cell line. Suppression of endogenous Bit1 expression via siRNA and shRNA strategies promoted mesenchymal phenotypes, including enhanced fibroblastoid morphology and cell migratory potential with concomitant downregulation of the epithelial marker E-cadherin expression. Conversely, ectopic Bit1 expression in A549 cells promoted epithelial transition characterized by cuboidal-like epithelial cell phenotype, reduced cell motility, and upregulated E-cadherin expression. Specific downregulation of E-cadherin in Bit1-transfected cells was sufficient to block Bit1-mediated inhibition of cell motility while forced expression of E-cadherin alone attenuated the enhanced migration of Bit1 knockdown cells, indicating that E-cadherin is a downstream target of Bit1 in regulating cell motility. Furthermore, quantitative real-time PCR and reporter analyses revealed that Bit1 upregulates E-cadherin expression at the transcriptional level through the transcriptional regulator Amino-terminal Enhancer of Split (AES) protein. Importantly, the Bit1/AES pathway induction of E-cadherin expression involves inhibition of the TLE1-mediated repression of E-cadherin, by decreasing TLE1 corepressor occupancy at the E-cadherin promoter as revealed by chromatin immunoprecipitation assays. Consistent with its EMT inhibitory function, exogenous Bit1 expression significantly suppressed the formation of lung metastases of A549 cells in an in vivo experimental metastasis model. Taken together, our studies indicate Bit1 is an inhibitor of EMT and metastasis in lung cancer and hence can serve as a molecular target in curbing lung cancer aggressiveness.

Filifactor alocis collagenase can modulate apoptosis of normal oral keratinocytes

To successfully colonize host cells, pathogenic bacteria must circumvent the host's structural barrier such as the collagen-rich extracellular matrix (ECM), as a preliminary step to invasion and colonization of the periodontal tissue. Filifactor alocis possesses a putative Peptidase U32 family protein (HMPREF0389_00504) with collagenase activity that may play a significant role in colonization of host tissue during periodontitis by breaking down collagen into peptides and disruption of the host cell. Domain architecture of the HMPREF0389_00504 protein predicted the presence of a characteristic PrtC-like collagenase domain, and a peptidase domain. Our study demonstrated that the recombinant F. alocis peptidase U32 protein (designated PrtFAC) can interact with, and degrade, type I collagen, heat-denatured collagen and gelatin in a calcium-dependent manner. PrtFAC decreased viability and induced apoptosis of normal oral keratinocytes (NOKs) in a time and dose-dependent manner. Transcriptome analysis of NOK cells treated with PrtFAC showed an upregulation of the genes encoding human pro-apoptotic proteins: Apoptotic peptidase activating factor 1 (Apaf1) cytochrome C, as well as caspase 3 and caspase 9, suggesting the involvement of the mitochondrial apoptotic pathway. There was a significant increase in caspase 3/7 activity in NOK cells treated with PrtFAC. Taken together, these findings suggest that F. alocis PrtFAC protein may play a role in the virulence and pathogenesis of F. alocis.

Bit1-a potential positive regulator of epithelial-mesenchymal transition in lens epithelial cells

PURPOSE

Posterior capsule opacification (PCO) is a common postoperative complication of cataract surgery. Epithelial-mesenchymal transition (EMT) of lens epithelial cells (LECs) is an important initial step of PCO pathogenesis. We have previously shown that Bit1 expresses in rat LECs. In this study, we aim to investigate the role of Bit1 in the EMT of human LECs.

METHODS

The expression of Bit1 was firstly detected in human PCO-attached LECs and human lens cell line SRA01/04 by real-time PCR and immunofluorescence staining. The proliferation and migration of Bit1 knockdown SRA01/04 cells were evaluated by cell counting, wound-healing assay, and transwell migration assay. The EMT of LECs was triggered by TGF-β2, and then the effect of Bit1 on EMT with a key biomarker of α-smooth muscle actin (α-SMA) was analyzed by siRNA knockdown assay, and the reversal of EMT was validated by real-time PCR and western blot.

RESULTS

Bit1 was obviously augmented in LECs derived from PCO tissues and Bit1 expressed with high levels in the cytoplasm of cultured SRA01/04 cells. Cell proliferation, invasion, and migration, as well as α-SMA expression, were significantly decreased in Bit1 knockdown SRA01/04 cells. While TGF-β2 elevated Bit1 and α-SMA expression levels in a dose-dependent manner, with peak levels at 10 ng/ml of TGF-β2 treatment, suppression of Bit1 in SRA01/04 cells reversed the EMT process. TGF-β2 induced elevation of α-SMA expression level, as well as migration, and invasion abilities were all suppressed by Bit1 deficiency.

CONCLUSIONS

These findings reveal that Bit1 promotes TGF-β2 induced α-SMA expression and acts as an positive regulator of EMT. Suppressing Bit1 inhibits the proliferation, migration, and EMT of LECs. Bit1 may be a potential novel therapeutic target for the prevention and treatment of PCO.

βA3/A1-crystallin and persistent fetal vasculature (PFV) disease of the eye

BACKGROUND

Persistent fetal vasculature (PFV) is a human disease in which the fetal vasculature of the eye fails to regress normally. The fetal, or hyaloid, vasculature nourishes the lens and retina during ocular development, subsequently regressing after formation of the retinal vessels. PFV causes serious congenital pathologies and is responsible for as much as 5% of blindness in the United States.

SCOPE OF REVIEW

The causes of PFV are poorly understood, however there are a number of animal models in which aspects of the disease are present. One such model results from mutation or elimination of the gene (Cryba1) encoding βA3/A1-crystallin. In this review we focus on the possible mechanisms whereby loss of functional βA3/A1-crystallin might lead to PFV.

MAJOR CONCLUSIONS

Cryba1 is abundantly expressed in the lens, but is also expressed in certain other ocular cells, including astrocytes. In animal models lacking βA3/A1-crystallin, astrocyte numbers are increased and they migrate abnormally from the retina to ensheath the persistent hyaloid artery. Evidence is presented that the absence of functional βA3/A1-crystallin causes failure of the normal acidification of endolysosomal compartments in the astrocytes, leading to impairment of certain critical signaling pathways, including mTOR and Notch/STAT3.

GENERAL SIGNIFICANCE

The findings suggest that impaired endolysosomal signaling in ocular astrocytes can cause PFV disease, by adversely affecting the vascular remodeling processes essential to ocular development, including regression of the fetal vasculature. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Bit-1 is an essential regulator of myogenic differentiation

Muscle differentiation requires a complex signaling cascade that leads to the production of multinucleated myofibers. Genes regulating the intrinsic mitochondrial apoptotic pathway also function in controlling cell differentiation. How such signaling pathways are regulated during differentiation is not fully understood. Bit-1 (also known as PTRH2) mutations in humans cause infantile-onset multisystem disease with muscle weakness. We demonstrate here that Bit-1 controls skeletal myogenesis through a caspase-mediated signaling pathway. Bit-1-null mice exhibit a myopathy with hypotrophic myofibers. Bit-1-null myoblasts prematurely express muscle-specific proteins. Similarly, knockdown of Bit-1 expression in C2C12 myoblasts promotes early differentiation, whereas overexpression delays differentiation. In wild-type mice, Bit-1 levels increase during differentiation. Bit-1-null myoblasts exhibited increased levels of caspase 9 and caspase 3 without increased apoptosis. Bit-1 re-expression partially rescued differentiation. In Bit-1-null muscle, Bcl-2 levels are reduced, suggesting that Bcl-2-mediated inhibition of caspase 9 and caspase 3 is decreased. Bcl-2 re-expression rescued Bit-1-mediated early differentiation in Bit-1-null myoblasts and C2C12 cells with knockdown of Bit-1 expression. These results support an unanticipated yet essential role for Bit-1 in controlling myogenesis through regulation of Bcl-2.

Mutations in PTRH2 cause novel infantile-onset multisystem disease with intellectual disability, microcephaly, progressive ataxia, and muscle weakness

OBJECTIVE

To identify the cause of a so-far unreported phenotype of infantile-onset multisystem neurologic, endocrine, and pancreatic disease (IMNEPD).

METHODS

We characterized a consanguineous family of Yazidian-Turkish descent with IMNEPD. The two affected children suffer from intellectual disability, postnatal microcephaly, growth retardation, progressive ataxia, distal muscle weakness, peripheral demyelinating sensorimotor neuropathy, sensorineural deafness, exocrine pancreas insufficiency, hypothyroidism, and show signs of liver fibrosis. We performed whole-exome sequencing followed by bioinformatic analysis and Sanger sequencing on affected and unaffected family members. The effect of mutations in the candidate gene was studied in wild-type and mutant mice and in patient and control fibroblasts.

RESULTS

In a consanguineous family with two individuals with IMNEPD, we identified a homozygous frameshift mutation in the previously not disease-associated peptidyl-tRNA hydrolase 2 (PTRH2) gene. PTRH2 encodes a primarily mitochondrial protein involved in integrin-mediated cell survival and apoptosis signaling. We show that PTRH2 is highly expressed in the developing brain and is a key determinant in maintaining cell survival during human tissue development. Moreover, we link PTRH2 to the mTOR pathway and thus the control of cell size. The pathology suggested by the human phenotype and neuroimaging studies is supported by analysis of mutant mice and patient fibroblasts.

INTERPRETATION

We report a novel disease phenotype, show that the genetic cause is a homozygous mutation in the PTRH2 gene, and demonstrate functional effects in mouse and human tissues. Mutations in PTRH2 should be considered in patients with undiagnosed multisystem neurologic, endocrine, and pancreatic disease.

The anoikis effector Bit1 displays tumor suppressive function in lung cancer cells

The mitochondrial Bit1 (Bcl-2 inhibitor of transcription 1) protein is a part of an apoptotic pathway that is uniquely regulated by integrin-mediated attachment. As an anoikis effector, Bit1 is released into the cytoplasm following loss of cell attachment and induces a caspase-independent form of apoptosis. Considering that anoikis resistance is a critical determinant of transformation, we hypothesized that cancer cells may circumvent the Bit1 apoptotic pathway to attain anchorage-independence and tumorigenic potential. Here, we provide the first evidence of the tumor suppressive effect of Bit1 through a mechanism involving anoikis induction in human lung adenocarcinoma derived A549 cells. Restitution of Bit1 in anoikis resistant A549 cells is sufficient to induce detachment induced-apoptosis despite defect in caspase activation and impairs their anchorage-independent growth. Conversely, stable downregulation of Bit1 in these cells significantly enhances their anoikis resistance and anchorage-independent growth. The Bit1 knockdown cells exhibit significantly enhanced tumorigenecity in vivo. It has been previously shown that the nuclear TLE1 corepressor is a putative oncogene in lung cancer, and we show here that TLE1 blocks Bit1 mediated anoikis in part by sequestering the pro-apoptotic partner of Bit1, the Amino-terminal Enhancer of Split (AES) protein, in the nucleus. Taken together, these findings suggest a tumor suppressive role of the caspase-independent anoikis effector Bit1 in lung cancer. Consistent with its role as a tumor suppressor, we have found that Bit1 is downregulated in human non-small cell lung cancer (NSCLC) tissues.

Uncovering a key to the process of metastasis in human cancers: a review of critical regulators of anoikis

PURPOSE

Anoikis ('homelessness' in Greek) is a form of apoptosis following the detachment of cells from the appropriate extracellular matrix (Chiarugi and Giannoni in Biochem Pharmacol 76:1352-1364, 2008). Resistance to anoikis is a critical mediator of metastasis in cancer by enabling cancer cells to survive during invasion and transport in the blood and lymph. Numerous regulators and mechanisms of anoikis in human cancer have been proposed to date. Consequently, the identification of key regulators of anoikis that can be targeted to at least partially restore anoikis sensitivity in cancer cells is important in the development of therapies to treat metastatic cancer.

METHODS

A literature search focusing on the regulators of anoikis in human cancer was performed on the Medline, Embase and Scopus databases.

RESULTS

Mcl-1, Cav-1, Bcl-(xL), cFLIP, 14-3-3ζ and Bit1 appear to regulate anoikis in human cancer by participating in the intrinsic apoptotic pathway, extrinsic apoptotic pathway or caspase-independent pathways. Mcl-1, Cav-1, Bcl-(xL), cFLIP and 14-3-3ζ are suppressors of anoikis, and their upregulation confers anoikis resistance to cancer cells. Bit1 is a promoter of anoikis and is downregulated to confer anoikis resistance in metastatic cancer.

CONCLUSION

Anoikis is a complex process involving the crosstalk between different signalling pathways. The dysregulated expression of key regulators of anoikis that participate in these signalling pathways promotes anoikis resistance in human cancer. These regulators of anoikis might therefore be the targets for developing therapies to overcome anoikis resistance in metastatic cancer.

Bit1 in anoikis resistance and tumor metastasis

Epithelial cells and most adherent normal cells rely on adhesion-dependent, integrin-mediated survival signals from the extracellular matrix (ECM) to survive. When these cells are deprived of adhesion to the ECM, they undergo a specific form of apoptosis termed "anoikis." In contrast, malignant cells have attained mechanisms to enable them to survive in the absence of adhesion and are considered anchorage-independent. This review will focus on the biological function of the Bcl2-inhibitor of transcription (Bit1) protein in the anoikis process, the underlying molecular mechanism of Bit1 apoptotic function, and its role in tumor metastasis.

Application of a proapoptotic peptide to intratumorally spreading cancer therapy

Bit1 is a proapoptotic mitochondrial protein associated with anoikis. Upon cell detachment, Bit1 is released into the cytoplasm and triggers caspase-independent cell death. Bit1 consists of 179 amino acids; for the C-terminal, two thirds of the molecule functions as a peptidyl-tRNA hydrolase, whereas the N-terminus contains a mitochondrial localization signal. Here, we localize the cell death domain (CDD) to the N-terminal 62 amino acids of Bit1 by transfecting cells with truncated Bit1 cDNA constructs. CDD was more potent in killing cells than the full-length Bit1 protein when equivalent amounts of cDNA were transfected. To develop Bit1 CDD into a cancer therapeutic, we engineered a recombinant protein consisting of the CDD fused to iRGD, which is a tumor-specific peptide with unique tumor-penetrating and cell-internalizing properties. iRGD-CDD internalized into cultured tumor cells through a neuropilin-1-activated pathway and triggered cell death. Importantly, iRGD-CDD spread extensively within the tumor when injected intratumorally into orthotopically implanted breast tumors in mice. Repeated treatment with iRGD-CDD strongly inhibited tumor growth, resulting in an average reduction of 77% in tumor volume and eradication of some tumors. The caspase independence of Bit1-induced cell death makes CDD a potentially attractive anticancer agent, because tumor resistance to the main mechanisms of apoptosis is circumvented. Using iRGD to facilitate the spreading of a therapeutic agent throughout the tumor mass may be a useful adjunct to local therapy for tumors that are surgically inoperable or difficult to treat systemically.

TLE1 is an anoikis regulator and is downregulated by Bit1 in breast cancer cells

TLE1 is a Groucho-related transcriptional repressor protein that exerts survival and antiapoptotic function in several cellular systems and has been implicated in the pathogenesis of cancer. In the present study, we found that TLE1 is a regulator of anoikis in normal mammary epithelial and breast carcinoma cells. The induction of apoptosis following loss of cell attachment to the extracellular matrix (anoikis) in untransformed mammary epithelial MCF10A cells was associated with significant downregulation of TLE1 expression. Forced expression of exogenous TLE1 in these cells promoted resistance to anoikis. In breast cancer cells, TLE1 expression was significantly upregulated following detachment from the extracellular matrix. Genetic manipulation of TLE1 expression via overexpression and downregulation approaches indicated that TLE1 promotes the anoikis resistance and anchorage-independent growth of breast carcinoma cells. Mechanistically, we show that TLE1 inhibits the Bit1 anoikis pathway by reducing the formation of the proapoptotic Bit1-AES complex in part through sequestration of AES in the nucleus. The mitochondrial release of Bit1 during anoikis as well as exogenous expression of the cytoplasmic localized Bit1 or its cell death domain induced cytoplasmic translocation and degradation of nuclear TLE1 protein. These findings indicate a novel role for TLE1 in the maintenance of anoikis resistance in breast cancer cells. This conclusion is supported by an immunohistochemical analysis of a breast cancer tissue array illustrating that TLE1 is selectively upregulated in invasive breast tumors relative to noninvasive ductal carcinoma in situ and normal mammary epithelial tissues.

Protein kinase D regulates RhoA activity via rhotekin phosphorylation

The members of the protein kinase D (PKD) family of serine/threonine kinases are major targets for tumor-promoting phorbol esters, G protein-coupled receptors, and activated protein kinase C isoforms (PKCs). The expanding list of cellular processes in which PKDs exert their function via phosphorylation of various substrates include proliferation, apoptosis, migration, angiogenesis, and vesicle trafficking. Therefore, identification of novel PKD substrates is necessary to understand the profound role of this kinase family in signal transduction. Here, we show that rhotekin, an effector of RhoA GTPase, is a novel substrate of PKD. We identified Ser-435 in rhotekin as the potential site targeted by PKD in vivo. Expression of a phosphomimetic S435E rhotekin mutant resulted in an increase of endogenous active RhoA GTPase levels. Phosphorylation of rhotekin by PKD2 modulates the anchoring of the RhoA in the plasma membrane. Consequently, the S435E rhotekin mutant displayed enhanced stress fiber formation when expressed in serum-starved fibroblasts. Our data thus identify a novel role of PKD as a regulator of RhoA activity and actin stress fiber formation through phosphorylation of rhotekin.

Regulation of protein kinase D1 activity

Protein kinase D1 (PKD1) is a stress-activated serine/threonine kinase that plays a vital role in various physiologically important biological processes, including cell growth, apoptosis, adhesion, motility, and angiogenesis. Dysregulated PKD1 expression also contributes to the pathogenesis of certain cancers and cardiovascular disorders. Studies to date have focused primarily on the canonical membrane-delimited pathway for PKD1 activation by G protein-coupled receptors or peptide growth factors. Here, agonist-dependent increases in diacylglycerol accumulation lead to the activation of protein kinase C (PKC) and PKC-dependent phosphorylation of PKD1 at two highly conserved serine residues in the activation loop; this modification increases PKD1 catalytic activity, as assessed by PKD1 autophosphorylation at a consensus phosphorylation motif at the extreme C terminus. However, recent studies expose additional controls and consequences for PKD1 activation loop and C-terminal phosphorylation as well as additional autophosphorylation reactions and trans-phosphorylations (by PKC and other cellular enzymes) that contribute to the spatiotemporal control of PKD1 signaling in cells. This review focuses on the multisite phosphorylations that are known or predicted to influence PKD1 catalytic activity and may also influence docking interactions with cellular scaffolds and trafficking to signaling microdomains in various subcellular compartments. These modifications represent novel targets for the development of PKD1-directed pharmaceuticals for the treatment of cancers and cardiovascular disorders.

βA3/A1-Crystallin controls anoikis-mediated cell death in astrocytes by modulating PI3K/AKT/mTOR and ERK survival pathways through the PKD/Bit1-signaling axis

During eye development, apoptosis is vital to the maturation of highly specialized structures such as the lens and retina. Several forms of apoptosis have been described, including anoikis, a form of apoptosis triggered by inadequate or inappropriate cell–matrix contacts. The anoikis regulators, Bit1 (Bcl-2 inhibitor of transcription-1) and protein kinase-D (PKD), are expressed in developing lens when the organelles are present in lens fibers, but are downregulated as active denucleation is initiated. We have previously shown that in rats with a spontaneous mutation in the Cryba1 gene, coding for βA3/A1-crystallin, normal denucleation of lens fibers is inhibited. In rats with this mutation (Nuc1), both Bit1 and PKD remain abnormally high in lens fiber cells. To determine whether βA3/A1-crystallin has a role in anoikis, we induced anoikis in vitro and conducted mechanistic studies on astrocytes, cells known to express βA3/A1-crystallin. The expression pattern of Bit1 in retina correlates temporally with the development of astrocytes. Our data also indicate that loss of βA3/A1-crystallin in astrocytes results in a failure of Bit1 to be trafficked to the Golgi, thereby suppressing anoikis. This loss of βA3/A1-crystallin also induces insulin-like growth factor-II, which increases cell survival and growth by modulating the phosphatidylinositol-3-kinase (PI3K)/AKT/mTOR and extracellular signal-regulated kinase pathways. We propose that βA3/A1-crystallin is a novel regulator of both life and death decisions in ocular astrocytes.

Metastasis of tumor cells is enhanced by downregulation of Bit1

Background

Resistance to anoikis, which is defined as apoptosis induced by loss of integrin-mediated cell attachment to the extracellular matrix, is a determinant of tumor progression and metastasis. We have previously identified the mitochondrial Bit1 (Bcl-2 inhibitor of transcription) protein as a novel anoikis effector whose apoptotic function is independent from caspases and is uniquely controlled by integrins. In this report, we examined the possibility that Bit1 is suppressed during tumor progression and that Bit1 downregulation may play a role in tumor metastasis.

Methodology/Principal Findings

Using a human breast tumor tissue array, we found that Bit1 expression is suppressed in a significant fraction of advanced stages of breast cancer. Targeted disruption of Bit1 via shRNA technology in lowly aggressive MCF7 cells conferred enhanced anoikis resistance, adhesive and migratory potential, which correlated with an increase in active Extracellular kinase regulated (Erk) levels and a decrease in Erk-directed phosphatase activity. These pro-metastasis phenotypes were also observed following downregulation of endogenous Bit1 in Hela and B16F1 cancer cell lines. The enhanced migratory and adhesive potential of Bit1 knockdown cells is in part dependent on their high level of Erk activation since down-regulating Erk in these cells attenuated their enhanced motility and adhesive properties. The Bit1 knockdown pools also showed a statistically highly significant increase in experimental lung metastasis, with no differences in tumor growth relative to control clones in vivo using a BALB/c nude mouse model system. Importantly, the pulmonary metastases of Bit1 knockdown cells exhibited increased phospho-Erk staining.

Conclusions/Significance

These findings indicate that downregulation of Bit1 conferred cancer cells with enhanced anoikis resistance, adhesive and migratory properties in vitro and specifically potentiated tumor metastasis in vivo. These results underscore the therapeutic importance of restoring Bit1 expression in cancer cells to circumvent metastasis at least in part through inhibition of the Erk pathway.

Apoptosis signaling kinases: from stress response to health outcomes

Apoptosis is a highly regulated process essential for the development and homeostasis of multicellular organisms. Whereas caspases, a large family of intracellular cysteine proteases, play central roles in the execution of apoptosis, other proapoptotic and antiapoptotic regulators such as the members of the Bcl-2 family are also critically involved in the regulation of apoptosis. A large body of evidence has revealed that a number of protein kinases are among such regulators and regulate cellular sensitivity to various proapoptotic signals at multiple steps in apoptosis. However, recent progress in the analysis of these apoptosis signaling kinases demonstrates that they generally act as crucial regulators of diverse cellular responses to a wide variety of stressors, beyond their roles in apoptosis regulation. In this review, we have cataloged apoptosis signaling kinases involved in cellular stress responses on the basis of their ability to induce apoptosis and discuss their roles in stress responses with particular emphasis on health outcomes upon their dysregulation.

Bit-1 mediates integrin-dependent cell survival through activation of the NFkappaB pathway

Loss of properly regulated cell death and cell survival pathways can contribute to the development of cancer and cancer metastasis. Cell survival signals are modulated by many different receptors, including integrins. Bit-1 is an effector of anoikis (cell death due to loss of attachment) in suspended cells. The anoikis function of Bit-1 can be counteracted by integrin-mediated cell attachment. Here, we explored integrin regulation of Bit-1 in adherent cells. We show that knockdown of endogenous Bit-1 in adherent cells decreased cell survival and re-expression of Bit-1 abrogated this effect. Furthermore, reduction of Bit-1 promoted both staurosporine and serum-deprivation induced apoptosis. Indeed knockdown of Bit-1 in these cells led to increased apoptosis as determined by caspase-3 activation and positive TUNEL staining. Bit-1 expression protected cells from apoptosis by increasing phospho-IκB levels and subsequently bcl-2 gene transcription. Protection from apoptosis under serum-free conditions correlated with bcl-2 transcription and Bcl-2 protein expression. Finally, Bit-1-mediated regulation of bcl-2 was dependent on focal adhesion kinase, PI3K, and AKT. Thus, we have elucidated an integrin-controlled pathway in which Bit-1 is, in part, responsible for the survival effects of cell-ECM interactions.

Protein kinase D as a potential new target for cancer therapy

Protein kinase D is a novel family of serine/threonine kinases and diacylglycerol receptors that belongs to the calcium/calmodulin-dependent kinase superfamily. Evidence has established that specific PKD isoforms are dysregulated in several cancer types, and PKD involvement has been documented in a variety of cellular processes important to cancer development, including cell growth, apoptosis, motility, and angiogenesis. In light of this, there has been a recent surge in the development of novel chemical inhibitors of PKD. This review focuses on the potential of PKD as a chemotherapeutic target in cancer treatment and highlights important recent advances in the development of PKD inhibitors.

MAPK scaffolding by BIT1 in the Golgi complex modulates stress resistance

The endoplasmic reticulum (ER) is an essential organelle whose major functions are to ensure proper secretory protein folding and trafficking. These mechanisms involve the activation of specific ER-resident molecular machines, which might be regulated by their membranous environments. Based on this observation, we aimed to characterize the proteome of ER-membrane microdomains to identify new components of the ER that have a role in secretory pathway-associated functions. Using this approach with dog pancreatic rough microsomes, we found that mitochondrial Bcl-2 inhibitor of transcription (BIT1) localized in the early secretory pathway and accumulated in the Golgi complex. Using both a chimeric protein of the luminal and transmembrane domains of ER-resident TRAPalpha and the cytosolic domain of BIT1, and silencing of BIT1 expression, we perturbed endogenous BIT1 oligomerization and localization to the Golgi. This led to enhanced ERK signaling from the Golgi complex, which resulted in improved stress resistance. This work provides the first evidence for the existence of ER microdomains that are involved in the regulation of BIT1 structure and trafficking, and identifies BIT1 as a negative regulator of the ERK-MAPK signaling pathway in the Golgi.

Monoclonal antibodies against human bit1, an apoptosis-associated mitochondrial protein

Bit1 (Bcl2-inhibitor of transcription 1) is a mitochondrial protein that was found eliciting caspase-independent apoptosis when released into the cytoplasm, where it forms a complex with AES (amino-terminal enhancer of split). In our research, cytosolic fraction of suspended cultured human ovarian cancer cells was taken as immunogen, and recombined protein Bit1-His was used to select hybridomas. Two hybridoma cell lines secreting monoclonal antibodies (MAbs) against Bit1 were obtained by routine murine hybridoma technique. The MAbs were characterized by indirect ELISA, Western blotting, and immunohistochemistry, and it was found that they recognized distinct epitopes. The sandwich method to detect sera Bit1 was established, and a distinct difference of sera Bit1 between the ovarian carcinoma patients and normal controls was found. To conclude, these MAbs against human Bit1 may be useful for exploring the tumor apoptosis mechanism and monitoring patients' clinical data.