Role of MmTRA1b/phospholipid scramblase1 gene expression in the induction of differentiation of human myeloid leukemia cells into granulocytes

Phospholipid scramblase 1: a protein with multiple functions via multiple molecular interactors

Phospholipid scramblase 1 (PLSCR1) is the most studied protein of the scramblase family. Originally, it was identified as a membrane protein involved in maintaining plasma membrane asymmetry. However, studies conducted over the past few years have shown the involvement of PLSCR1 in several other cellular pathways. Indeed, PLSCR1 is not only embedded in the plasma membrane but is also expressed in several intracellular compartments where it interacts with a diverse repertoire of effectors, mediators, and regulators contributing to distinct cellular processes. Although most PLSCR1 interactors are thought to be cell-type specific, PLSCR1 often exerts its regulatory functions through shared mechanisms, including the trafficking of different molecules within intracellular vesicles such as endosomes, liposomes, and phagosomes. Intriguingly, besides endogenous proteins, PLSCR1 was also reported to interact with exogenous viral proteins, thereby regulating viral uptake and spread. This review aims to summarize the current knowledge about the multiple roles of PLSCR1 in distinct cellular pathways.

Phospholipid Scramblases: Role in Cancer Progression and Anticancer Therapeutics

Phospholipid scramblases (PLSCRs) that catalyze rapid mixing of plasma membrane lipids result in surface exposure of phosphatidyl serine (PS), a lipid normally residing to the inner plasma membrane leaflet. PS exposure provides a chemotactic eat-me signal for phagocytes resulting in non-inflammatory clearance of apoptotic cells by efferocytosis. However, metastatic tumor cells escape efferocytosis through alteration of tumor microenvironment and apoptotic signaling. Tumor cells exhibit altered membrane features, high constitutive PS exposure, low drug permeability and increased multidrug resistance through clonal evolution. PLSCRs are transcriptionally up-regulated in tumor cells leading to plasma membrane remodeling and aberrant PS exposure on cell surface. In addition, PLSCRs interact with multiple cellular components to modulate cancer progression and survival. While PLSCRs and PS exposed on tumor cells are novel drug targets, many exogenous molecules that catalyze lipid scrambling on tumor plasma membrane are potent anticancer therapeutic molecules. In this review, we provide a comprehensive analysis of scramblase mediated signaling events, membrane alteration specific to tumor development and possible therapeutic implications of scramblases and PS exposure.

RNA interference-mediated downregulation of phospholipid scramblase 1 expression in primary liver cancer in vitro

Phospholipid scramblase 1 (PLSCR1) serves a function in the pathogenesis and progression of various types of cancer. However, the role of PLSCR1 in human primary liver cancer remains unknown. The aim of the present study was to evaluate the expression of PLSCR1 in primary liver cancer and analyse the clinical significance. In addition, the present study detected and compared the biological behaviours of HepG2 cells with different levels of activated PLSCR1 or silenced PLSCR1. PLSCR1 expression in primary liver cancer tissue samples was examined using immunohistochemistry. Cultured HepG2 cells were infected with lentiviruses to suppress or activate PLSCR1 expression. Reverse transcription-quantitative PCR and western blotting were performed to analyse the effects of silencing or activating PLSCR1 in cell lines at the mRNA and protein levels, respectively. The effects of PLSCR1 expression on cell proliferation, adhesion, migration and invasion were subsequently determined using Cell Counting Kit 8, adhesion, and Transwell migration and invasion assays. PLSCR1 expression in primary liver cancer tissue samples was higher compared with that in adjacent non-cancerous liver tissue samples and normal tissue samples, and positively correlated with the clinical stage. PLSCR1 was effectively downregulated or overexpressed in HepG2 cells using small interfering RNA and lentivirus techniques, respectively. PLSCR1 upregulation promoted cell proliferation, invasion and migration, while PLSCR1 downregulation inhibited these effects. PLSCR1 is highly expressed in primary liver cancer and associated with the clinical stage. Downregulating the expression of PLSCR1 significantly inhibited the proliferation, adhesion, migration and invasion of cancer cells, suggesting that PLSCR1 may be a potential therapeutic target for preventing the progression of primary liver cancer.

Phospholipid scramblase 1 interacts with influenza A virus NP, impairing its nuclear import and thereby suppressing virus replication

Transcription and replication of the influenza A virus (IAV) genome occur in the nucleus of infected cells and are carried out by the viral ribonucleoprotein complex (vRNP). As a major component of the vRNP complex, the viral nucleoprotein (NP) mediates the nuclear import of the vRNP complex via its nuclear localization signals (NLSs). Clearly, an effective way for the host to antagonize IAV infection would be by targeting vRNP nuclear import. Here, we identified phospholipid scramblase 1 (PLSCR1) as a binding partner of NP by using a yeast two-hybrid (Y2H) screen. The interaction between NP and PLSCR1 in mammalian cells was demonstrated by using co-immunoprecipitation and pull-down assays. We found that the stable overexpression of PLSCR1 suppressed the nuclear import of NP, hindered the virus life cycle, and significantly inhibited the replication of various influenza subtypes. In contrast, siRNA knockdown or CRISPR/Cas9 knockout of PLSCR1 increased virus propagation. Further analysis indicated that the inhibitory effect of PLSCR1 on the nuclear import of NP was not caused by affecting the phosphorylation status of NP or by stimulating the interferon (IFN) pathways. Instead, PLSCR1 was found to form a trimeric complex with NP and members of the importin α family, which inhibited the incorporation of importin β, a key mediator of the classical nuclear import pathway, into the complex, thus impairing the nuclear import of NP and suppressing virus replication. Our results demonstrate that PLSCR1 negatively regulates virus replication by interacting with NP in the cytoplasm and preventing its nuclear import.

Phospholipid scramblase 1 amplifies anaphylactic reactions in vivo

Mast cells are critical actors of hypersensitivity type I (allergic) reactions by the release of vasoactive and proinflammatory mediators following their activation by aggregation of the high-affinity receptor for immunoglobulin E (FcεRI). We have previously identified Phospholipid Scramblase 1 (PLSCR1) as a new molecular intermediate of FcεRI signaling that amplifies degranulation of the rat mast cell line RBL-2H3. Here we characterized primary mast cells from Plscr1-/- mice. The absence of PLSCR1 expression did not impact mast cell differentiation as evidenced by unaltered FcεRI expression, general morphology, amount of histamine stored and expression of FcεRI signal effector molecules. No detectable mast cell deficiency was observed in Plscr1-/- adult mice. In dose-response and time-course experiments, primary cultures of mast cells (bone marrow-derived mast cells and peritoneal cell-derived mast cells) generated from Plscr1-/- mice exhibited a reduced release of β-hexosaminidase upon FcεRI engagement as compared to their wild-type counterparts. In vivo, Plscr1-/- mice were protected in a model of passive systemic anaphylaxis when compared to wild-type mice, which was consistent with an observed decrease in the amounts of histamine released in the serum of Plscr1-/- mice during the reaction. Therefore, PLSCR1 aggravates anaphylactic reactions by increasing FcεRI-dependent mast cell degranulation. PLSCR1 could be a new therapeutic target in allergy.

Natural compounds and pharmaceuticals reprogram leukemia cell differentiation pathways

In addition to apoptosis resistance and cell proliferation capacities, the undifferentiated state also characterizes most cancer cells, especially leukemia cells. Cell differentiation is a multifaceted process that depends on complex regulatory networks that involve transcriptional, post-transcriptional and epigenetic regulation of gene expression. The time- and spatially-dependent expression of lineage-specific genes and genes that control cell growth and cell death is implicated in the process of maturation. The induction of cancer cell differentiation is considered an alternative approach to elicit cell death and proliferation arrest. Differentiation therapy has mainly been developed to treat acute myeloid leukemia, notably with all-trans retinoic acid (ATRA). Numerous molecules from diverse natural or synthetic origins are effective alone or in association with ATRA in both in vitro and in vivo experiments. During the last two decades, pharmaceuticals and natural compounds with various chemical structures, including alkaloids, flavonoids and polyphenols, were identified as potential differentiating agents of hematopoietic pathways and osteogenesis.

Roles and regulation of phospholipid scramblases

Phospholipid scramblase activity is involved in the collapse of phospholipid (PL) asymmetry at the plasma membrane leading to externalization of phosphatidylserine. This activity is crucial for initiation of the blood coagulation cascade and for recognition/elimination of apoptotic cells by macrophages. Efforts to identify gene products associated with this activity led to the characterization of PL scramblase (PLSCR) and XKR family members which contribute to phosphatidylserine exposure in response to apoptotic stimuli. Meanwhile, TMEM16 family members were identified to externalize phosphatidylserine in response to elevated calcium in Scott syndrome platelets, which is critical for activation of the coagulation cascade. Herein, we report their mechanisms of gene regulation, molecular functions independent of their scrambling activity, and their potential roles in pathogenic conditions.

Cotylenin A and arsenic trioxide cooperatively suppress cell proliferation and cell invasion activity in human breast cancer cells

Arsenic trioxide (ATO) is an approved treatment for acute promyelocytic leukemia (APL). It has also shown potential for treatment of multiple myeloma and various solid tumors including breast cancer. The requirement of high, toxic concentrations for the induction of apoptosis in non-APL and solid tumor cells is a major limitation for its use in other hematological malignancies and solid tumors. We have examined whether inducers of differentiation of leukemia cells can control the growth of solid tumor cells. In the present study, we found that cotylenin A, a plant growth regulator and a potent inducer of differentiation in myeloid leukemia cells, significantly potentiated both ATO-induced inhibition of cell growth in a liquid culture, and ATO-induced inhibition of anchorage-independent growth in a semi-solid culture in human breast cancer MCF-7 and MDA-MB-231 cells. ISIR-005 (a synthetic cotylenin A-derivative) was also able to enhance ATO-induced growth inhibition. The combined treatment with cotylenin A and ATO induced cleaved caspase-7 in MCF-7 cells at the concentrations which ATO alone scarcely induced and cotylenin A alone only weakly induced. Expression of survivin in MCF-7 cells was markedly decreased with the presence of both cotylenin A and ATO, although the expression of survivin was only slightly decreased by cotylenin A or ATO alone. The pretreatment with N-acetylcysteine significantly reduced the combination treatment-induced cell growth inhibition. These data suggest that induction of cleaved caspase-7, inhibition of survivin and oxidative responses are important events in the corporative inhibition in the growth of MCF-7 cells induced by both cotylenin A and ATO. Furthermore, we found that the combined treatment with cotylenin A and ATO also could be effective in suppressing the invasive capacity of MDA-MB-231 cells determined with the impedance-based xCELLigence Real-Time Cell Analysis technology. These results suggest that cotylenin A is an attractive enhancer for the ATO-induced anticancer activities in human breast cancer.

Wogonoside induces cell cycle arrest and differentiation by affecting expression and subcellular localization of PLSCR1 in AML cells

Wogonoside induces cell cycle arrest and differentiation. Wogonoside acts by changing PLSCR1 expression and subcellular localization in the nucleus and by PLSCR1-related molecular events.

Blockade of phospholipid scramblase 1 with its N-terminal domain antibody reduces tumorigenesis of colorectal carcinomas in vitro and in vivo

Background

Membrane-bound phospholipid scramblase 1 (PLSCR1) is involved in both lipid trafficking and cell signaling. Previously, we showed that PLSCR1 is overexpressed in many colorectal carcinomas (CRCs). In the present study, we investigated the tumorigenic role of PLSCR1 in CRC and suggest that it is a potential therapeutic target.

Methods

To identify PLSCR1 as a therapeutic target, we studied the tumorigenic properties of CRC cell lines treated with a monoclonal antibody (NP1) against the N-terminus of PLSCR1 in vitro and in vivo. We also investigated cell cycle status and epidermal growth factor receptor–related pathways and downstream effectors of PLSCR1 after blocking its function with NP1.

Results

Treating CRC cells with NP1 in vitro and in vivo decreased cell proliferation, anchorage-independent growth, migration, and invasion. Adding NP1 to the CRC cell line HT29 caused arrest at G1/S. Treating HT29 cells with NP1 significantly decreased the expression of cyclin D1 and phosphorylation levels of Src, the adaptor protein Shc, and Erks. The reduced level of cyclin D1 led to an increase in the activated form of the tumor suppressor retinoblastoma protein via dephosphorylation. These actions led to attenuation of tumorigenesis.

Conclusions

Therefore, PLSCR1 may serve as a potential therapeutic target for CRC.

Silencing phospholipid scramblase 1 expression by RNA interference in colorectal cancer and metastatic liver cancer

BACKGROUND

Phospholipid scramblase 1 (PLSCR1) not only participates in the transbilayer movement of phospholipids, but also plays a role in the pathogenesis and progression of cancers. The present study aimed to evaluate the effect of silencing PLSCR1 expression by RNA interference in colorectal cancer (CRC) and metastatic liver cancer.

METHODS

The expression of PLSCR1 in CRC and metastatic liver cancer samples was assessed by immunohistochemistry. The cultured cells with the highest expression were selected for subsequent experiments. We designed three siRNA oligonucleotide segments targeted at PLSCR1. Successful transfection was confirmed. The biological behavior of the cells in proliferation, adhesion, migration and invasion was determined.

RESULTS

PLSCR1 protein expression increased significantly in the majority of CRC and metastatic liver cancer samples compared with normal samples. Lovo cells had the highest expression of PLSCR1. The siRNA-390 oligonucleotide segment had the best silencing effect. After transfection, Lovo cell proliferation was significantly inhibited compared with the controls in the MTT assay. Laminin and fibronectin adhesion assays showed Lovo cell adhesion was also significantly inhibited. In the migration assay, the number of migrating cells in the PLSCR1 siRNA-390 group was 50+/-12, significantly lower than the number in the siRNA-N group (115+/-28) and in the control group (118+/-31). In an invasion test, the number of invading cells in the PLSCR1 siRNA-390 group was 60+/-18, significantly lower than that in the siRNA-N group (97+/-26) and the control group (103+/-24).

CONCLUSIONS

PLSCR1 is overexpressed in CRC and metastatic liver cancer. Silencing of PLSCR1 by siRNA inhibits the proliferation, adhesion, migration and invasion of Lovo cells, which suggests that PLSCR1 contributes to the tumorigenesis and tumor progression of CRC. PLSCR1 may be a potential gene therapy target for CRC and associated metastatic liver cancer.

Identification of phospholipid scramblase 1 as a biomarker and determination of its prognostic value for colorectal cancer

The purpose of this study was to examine the expression of phospholipid scramblase 1 (PLSCR1) in tumor tissues and plasma specimens of patients with colorectal cancer (CRC), as well as analyze its association with clinical parameters. The expression levels of PLSCR1 protein in 104 matched CRC and adjacent normal tissue sections and 50 pairs of CRC tissue blocks were determined by use of immunohistochemical and Western blot analyses, respectively. To evaluate the diagnostic potential of PLSCR1, the plasma levels of PLSCR1 were investigated in 111 additional subjects (59 CRC patients and 52 healthy controls) by Western blot. PLSCR1 was overexpressed in malignant adenocarcinoma tissues compared with normal colorectal mucosa (P < 0.001). In addition, the plasma level of PLSCR1 was not only significantly elevated in CRC patients compared with healthy individuals (P < 0.001), but it was also substantially increased in early stage CRC (P < 0.001). Importantly, the overall sensitivity and specificity of PLSCR1 for CRC detection were 80% and 59.6%, respectively. The area under the ROC curve of PLSCR1 for CRC diagnosis is 0.75, which increases to 0.8 if combined with the measurement of carcinoembryonic antigen. Univariate analysis with the Cox regression model revealed that elevated PLSCR1 expression indicated a poor prognosis for CRC. This study showed that PLSCR1 protein levels were significantly elevated in both the cancer tissue and plasma of CRC patients. Moreover, the plasma levels of PLSCR1 were significantly elevated in patients with early stage CRC compared with healthy individuals, suggesting that PLSCR1 might be used as a noninvasive serological diagnostic and prognostic biomarker for CRC.

Phospholipid scramblase 1 is secreted by a lipid raft-dependent pathway and interacts with the extracellular matrix protein 1 in the dermal epidermal junction zone of human skin

We examined the interaction of ECM1 (extracellular matrix protein 1) using yeast two-hybrid screening and identified the type II transmembrane protein, PLSCR1 (phospholipid scramblase 1), as a binding partner. This interaction was then confirmed by in vitro and in vivo co-immunoprecipitation experiments, and additional pull-down experiments with GST-tagged ECM1a fragments localized this interaction to occur within the tandem repeat region of ECM1a. Furthermore, immunohistochemical staining revealed a partial overlap of ECM1 and PLSCR1 in human skin at the basal epidermal cell layer. Moreover, in human skin equivalents, both proteins are expressed at the basal membrane in a dermal fibroblast-dependent manner. Next, immunogold electron microscopy of ultrathin human skin sections showed that ECM1 and PLSCR1 co-localize in the extracellular matrix, and using antibodies against ECM1 or PLSCR1 cross-linked to magnetic immunobeads, we were able to demonstrate PLSCR1-ECM1 interaction in human skin extracts. Furthermore, whereas ECM1 is secreted by the endoplasmic/Golgi-dependent pathway, PLSCR1 release from HaCaT keratinocytes occurs via a lipid raft-dependent mechanism, and is deposited in the extracellular matrix. In summary, we here demonstrate that PLSCR1 interacts with the tandem repeat region of ECM1a in the dermal epidermal junction zone of human skin and provide for the first time experimental evidence that PLSCR1 is secreted by an unconventional secretion pathway. These data suggest that PLSCR1 is a multifunctional protein that can function both inside and outside of the cell and together with ECM1 may play a regulatory role in human skin.

Calcium binding studies of peptides of human phospholipid scramblases 1 to 4 suggest that scramblases are new class of calcium binding proteins in the cell

BACKGROUND

Phospholipid scramblases are a group of four homologous proteins conserved from C. elegans to human. In human, two members of the scramblase family, hPLSCR1 and hPLSCR3 are known to bring about Ca2+ dependent translocation of phosphatidylserine and cardiolipin respectively during apoptotic processes. However, affinities of Ca2+/Mg2+ binding to human scramblases and conformational changes taking place in them remains unknown.

METHODS

In the present study, we analyzed the Ca2+ and Mg2+ binding to the calcium binding motifs of hPLSCR1-4 and hPLSCR1 by spectroscopic methods and isothermal titration calorimetry.

RESULTS

The results in this study show that (i) affinities of the peptides are in the order hPLSCR1>hPLSCR3>hPLSCR2>hPLSCR4 for Ca2+ and in the order hPLSCR1>hPLSCR2>hPLSCR3>hPLSCR4 for Mg2+, (ii) binding of ions brings about conformational change in the secondary structure of the peptides. The affinity of Ca2+ and Mg2+ binding to protein hPLSCR1 was similar to that of the peptide I. A sequence comparison shows the existence of scramblase-like motifs among other protein families.

CONCLUSIONS

Based on the above results, we hypothesize that the Ca2+ binding motif of hPLSCR1 is a novel type of Ca2+ binding motif.

GENERAL SIGNIFICANCE

Our findings will be relevant in understanding the calcium dependent scrambling activity of hPLSCRs and their biological function.

Wogonin induces the granulocytic differentiation of human NB4 promyelocytic leukemia cells and up-regulates phospholipid scramblase 1 gene expression

Previous studies have firmly demonstrated that wogonin, a naturally occurring monoflavonoid extracted from the root of the Chinese herb medicine Scutellaria baicalensis, could effectively inhibit the proliferation of several cancer cell lines. However, little is known about the effect of wogonin on differentiation induction of leukemic cells. Here we investigate the potential role of wogonin in the proliferation and differentiation of NB4, a human promyelocytic leukemia cell line derived from a patient with acute promyelocytic leukemia. Our results indicated that wogonin significantly suppressed the proliferation and efficiently induced the differentiation of NB4 cells. NB4 cell growth was inhibited by 55-60% after treatment with 50 microM wogonin for a period of 5 days. The results of the nitroblue tetrazolium (NBT) reduction test (with 67.13% positive cells by 50 microM wogonin for 5 days), Giemsa staining (with 67.24% positive cells by 50 microM wogonin for 5 days), and the expression of mature-related cell-surface differentiation antigens CD11b and CD14 (with 70.94% CD11b(+) and 5.82% CD14(+) cells by 50 microM wogonin for 5 days) demonstrated an increase in the differentiation-inducing action of wogonin on the NB4 cells, which was accompanied by an increase in mRNA and protein expression of phospholipids scramblase 1 (PLSCR1). Meanwhile, the level of phosphorylated PKC delta (Ser643) was dramatically increased in wogonin treated NB4 cells. Interestingly, wogonin treatment displayed little effect on the apoptosis of NB4 cells. Taken together, the results reported here demonstrated that wogonin could promote the granulocytic differentiation of NB4 cells by up-regulating the expression of PLSCR1 gene.

Phospholipid scramblases: An overview

Phospholipid scramblases are a group of homologous proteins that are conserved in all eukaryotic organisms. They are believed to be involved in destroying plasma membrane phospholipid asymmetry at critical cellular events like cell activation, injury and apoptosis. However, a detailed mechanism of phospholipid scrambling still awaits a proper understanding. The most studied member of this family, phospholipid scramblase 1 (PLSCR1) (a 37kDa protein), is involved in rapid Ca2+ dependent transbilayer redistribution of plasma membrane phospholipids. Recently the function of PLSCR1 as a phospholipids translocator has been challenged and evidences suggest that PLSCR1 acts as signaling molecule. It has been shown to be involved in protein phosphorylation and as a potential activator of genes in response to interferon and other cytokines. Interferon induced rapid biosynthesis of PLSCR1 targets some of the protein into the nucleus, where it binds to the promoter region of inositol 1,4,5-triphosphate (IP3) receptor type 1 (IP3R1) gene and induces its expression. Palmitoylation of PLSCR1 acts as a switch, controlling its localization either to the PM or inside the nucleus. In the present review, we discuss the current understanding of PLSCR1 in relation to its trafficking, localization and signaling functions.

Antileukemic roles of human phospholipid scramblase 1 gene, evidence from inducible PLSCR1-expressing leukemic cells

Phospholipid scramblase 1 (PLSCR1) is a multiply palmitoylated protein which is localized in either the cell membrane or nucleus depending on its palmitoylated state. The increasing evidence showed the biological roles of PLSCR1 in cell signaling, maturation and apoptosis. To investigate the functions of PLSCR1 in leukemic cells, we generated an inducible PLSCR1-expressing cell line using myeloid leukemic U937 cells. In this cell line, PLSCR1 was tightly regulated and induced upon tetracycline withdrawal. Our results showed that inducible PLSCR1 expression arrested the proliferation of U937 cells at G1 phase. Meanwhile, PLSCR1-overexpressing U937 cells also underwent granulocyte-like differentiation with increased sensitivity to etoposide-induced apoptosis. Furthermore, we also found that PLSCR1 induction increased cyclin-dependent kinase inhibitors p27(Kip1) and p21(Cip1) proteins, together with downregulation of S phase kinase-associated protein 2 (SKP2), an F-box subunit of the ubiquitin-ligase complex that targets proteins for degradation. Additionally, PLSCR1 induction significantly decreased c-Myc protein and antiapoptotic Bcl-2 protein. Although the exact mechanism by which PLSCR1 regulates these cellular events and gene expression remains unresolved, our results suggest that PLSCR1 plays the antagonistic role regarding leukemia development. These data will shed new insights into understanding the biochemical and biological functions of PLSCR1 protein.

Interferon-α-induced Expression of Phospholipid Scramblase 1 through STAT1 Requires the Sequential Activation of Protein Kinase Cδ and JNK

Phospholipid scramblase 1 (PLSCR1), a calcium-binding protein that either inserts into the plasma membrane or binds to genomic DNA in the nucleus, has been shown to contribute to the cell proliferation, differentiation, and apoptosis as well as antiviral activity of interferon (IFN). The expression of PLSCR1 protein is also known to be markedly increased in response to IFN and to some differentiation inducing agents such as all-trans retinoic acid, but the precise mechanisms of this response remain to be investigated. In this study, we show that the protein kinase Cdelta (PKCdelta)-specific inhibitor rottlerin and the dominant negative mutant of PKCdelta significantly antagonized IFN-induced PLSCR1 expression. The influence of PKCdelta on IFN-mediated induction of PLSCR1 was dependent upon the phosphorylation of STAT1 at Ser-727. Furthermore, PKCdelta-mediated activation of STAT1 required the activation of JNK, as the inhibition of JNK activity by its specific inhibitor or transfection of its dominant negative mutant suppressed both serine phosphorylation of STAT1 and PLSCR1 expression but not the activation of PKCdelta. In conclusion, our results suggest that the induction of PLSCR1 transcription through STAT1 depends upon sequential activation of PKCdelta and JNK.

Effects of combined treatment with rapamycin and cotylenin A, a novel differentiation-inducing agent, on human breast carcinoma MCF-7 cells and xenografts

INTRODUCTION

Rapamycin, an inhibitor of the serine/threonine kinase target of rapamycin, induces G1 arrest and/or apoptosis. Although rapamycin and its analogues are attractive candidates for cancer therapy, their sensitivities with respect to growth inhibition differ markedly among various cancer cells. Using human breast carcinoma cell line MCF-7 as an experimental model system, we examined the growth-inhibitory effects of combinations of various agents and rapamycin to find the agent that most potently enhances the growth-inhibitory effect of rapamycin.

METHOD

We evaluated the growth-inhibitory effect of rapamycin plus various agents, including cotylenin A (a novel inducer of differentiation of myeloid leukaemia cells) to MCF-7 cells, using either MTT assay or trypan blue dye exclusion test. The cell cycle was analyzed using propidium iodide-stained nuclei. Expressions of several genes in MCF-7 cells with rapamycin plus cotylenin A were studied using cDNA microarray analysis and RT-PCR. The in vitro results of MCF-7 cells treated with rapamycin plus cotylenin A were further confirmed in vivo in a mouse xenograft model.

RESULTS

We found that the sensitivity of rapamycin to MCF-7 cells was markedly affected by cotylenin A. This treatment induced growth arrest of the cells at the G1 phase, rather than apoptosis, and induced senescence-associated beta-galactosidase activity. We examined the gene expression profiles associated with exposure to rapamycin and cotylenin A using cDNA microarrays. We found that expressions of cyclin G2, transforming growth factor-beta-induced 68 kDa protein, BCL2-interacting killer, and growth factor receptor-bound 7 were markedly induced in MCF-7 cells treated with rapamycin plus cotylenin A. Furthermore, combined treatment with rapamycin and cotylenin A significantly inhibited the growth of MCF-7 cells as xenografts, without apparent adverse effects.

CONCLUSION

Rapamycin and cotylenin A cooperatively induced growth arrest in breast carcinoma MCF-7 cells in vitro, and treatment with rapamycin and cotylenin A combined more strongly inhibited the growth of MCF-7 cells as xenografts in vivo than treatment with rapamycin or cotylenin A alone, suggesting that this combination may have therapeutic value in treating breast cancer. We also identified several genes that were markedly modulated in MCF-7 cells treated with rapamycin plus cotylenin A.

Phospholipid Scramblase 1 Binds to the Promoter Region of the Inositol 1,4,5-Triphosphate Receptor Type 1 Gene to Enhance Its Expression

Phospholipid scramblase 1 (PLSCR1) is a multiply palmitoylated, endofacial membrane protein originally identified based on its capacity to promote accelerated transbilayer phospholipid movement in response to Ca(2+). Recent evidence suggests that this protein also participates in cell response to various growth factors and cytokines, influencing myeloid differentiation, tumor growth, and the antiviral activity of interferon. Whereas plasma membrane PLSCR1 was shown to be required for normal recruitment and activation of Src kinase by stimulated cell surface growth factor receptors, PLSCR1 was also found to traffic into the nucleus and to tightly bind to genomic DNA, suggesting a possible additional nuclear function. We now report evidence that PLSCR1 directly binds to the 5'-promoter region of the inositol 1,4,5-triphosphate receptor type 1 gene (IP3R1) to enhance expression of the receptor. Probing a CpG island genomic library with PLSCR1 as bait identified four clones with avidity for PLSCR1, including a 191-bp fragment of the IP3R1 promoter. Using electrophoretic mobility shift and transcription reporter assays, the PLSCR1-binding site in IP3R1 was mapped to residues (-101)GTAACCATGTGGA(-89), and the segment spanning Met(86)-Glu(118) in PLSCR1 was identified to mediate its transcriptional activity. The significance of this interaction between PLSCR1 and IP3R1 in situ was confirmed by comparing levels of IP3R1 mRNA and protein in matched cells that either expressed or were deficient in PLSCR1. These data suggest that in addition to its role at the plasma membrane, effects of PLSCR1 on cell proliferative and maturational responses may also relate to alterations in expression of cellular IP3 receptors.

Adiposity, dyslipidemia, and insulin resistance in mice with targeted deletion of phospholipid scramblase 3 (PLSCR3)

The phospholipid scramblases (PLSCR1 to PLSCR4) are a structurally and functionally unique class of proteins, which are products of a tetrad of genes conserved from Caenorhabditis elegans to humans. The best characterized member of this family, PLSCR1, is implicated in the remodeling of the transbilayer distribution of plasma membrane phospholipids but is also required for normal signaling through select growth factor receptors. Mice with targeted deletion of PLSCR1 display perinatal granulocytopenia due to defective response of hematopoietic precursors to granulocyte colony-stimulating factor and stem cell factor. To gain insight into the biologic function of another member of the PLSCR family, we investigated mice with targeted deletion of PLSCR3, a protein that like PLSCR1 is expressed in many blood cells but which, by contrast to PLSCR1, is also highly expressed in fat and muscle. PLSCR3(-/-) mice at 2 months of age displayed aberrant accumulation of abdominal fat when maintained on standard rodent chow, which was accompanied by insulin resistance, glucose intolerance, and dyslipidemia. Primary adipocytes and cultured bone-marrow-derived macrophages from PLSCR3(-/-) mice were engorged with neutral lipid, and adipocytes displayed defective responses to exogenous insulin. Plasma of PLSCR3(-/-) mice was elevated in non-high-density lipoproteins, cholesterol, triglycerides, nonesterified fatty acids, and leptin, whereas adiponectin was low. These data suggest that the expression of PLSCR3 may be required for normal adipocyte and/or macrophage maturation or function and raise the possibility that deletions or mutations affecting the PLSCR3(-/-) gene locus may contribute to the risk for lipid-related disorders in humans.

Protein kinase Cdelta mediates retinoic acid and phorbol myristate acetate-induced phospholipid scramblase 1 gene expression: its role in leukemic cell differentiation

Although phospholipid scramblase 1 (PLSCR1) was originally identified based on its capacity to promote transbilayer movement of membrane phospholipids, subsequent studies also provided evidence for its role in cell proliferation, maturation, and apoptosis. In this report, we investigate the potential role of PLSCR1 in leukemic cell differentiation. We show that all-trans retinoic acid (ATRA), an effective differentiation-inducing agent of acute promyelocytic leukemic (APL) cells, can elevate PLSCR1 expression in ATRA-sensitive APL cells NB4 and HL60, but not in maturation-resistant NB4-LR1 cells. ATRA- and phorbol 12-myristate 13-acetate (PMA)-induced monocytic differentiation is accompanied by increased PLSCR1 expression, whereas only a slight or no elevation of PLSCR1 expression is observed in U937 cells differentiated with dimethyl sulfoxide (DMSO), sodium butyrate, or vitamin D3. Cell differentiation with ATRA and PMA, but not with vitamin D3 or DMSO, results in phosphorylation of protein kinase Cdelta (PKCdelta), and the PKCdelta-specific inhibitor rottlerin nearly eliminates the ATRA- and PMA-induced expression of PLSCR1, while ectopic expression of a constitutively active form of PKCdelta directly increases PLSCR1 expression. Finally, decreasing PLSCR1 expression with small interfering RNA inhibits ATRA/PMA-induced differentiation. Taken together, these results suggest that as a protein induced upon PKCdelta activation, PLSCR1 is required for ATRA- and PMA-triggered leukemic cell differentiation.

MmTRA1b/phospholipid scramblase 1 gene expression is a new prognostic factor for acute myelogenous leukemia

We previously found that expression of the Mm-1 cell-derived transplantability-associated gene 1b (MmTRA1b)/phospholipid scramblase 1 gene was markedly induced during the granulocytic differentiation of human myeloid leukemia cells. To evaluate the role of MmTRA1b expression in human myeloid leukemia, we investigated the relative levels of MmTRA1b transcripts in 81 patients with acute myelogenous leukemia (AML) by the reverse transcriptase polymerase chain reaction. The expression of MmTRA1b in AML-M1, -M5a and -M5b was significantly lower than that in normal bone marrow cells. The levels of MmTRA1b expression in AML-M2 and -M4 varied among patients. Higher MmTRA1b mRNA levels were associated with significantly longer overall survival in AML, especially in AML-M4 patients, independent of chromosomal aberrations such as t(8;21) and inv(16). The present results suggest that the MmTRA1b mRNA level is a new prognostic factor for AML, especially the AML-M4 subtype.