AKAP350 Is involved in the development of apical "canalicular" structures in hepatic cells HepG2

Metformin and glucose starvation decrease the migratory ability of hepatocellular carcinoma cells: targeting AMPK activation to control migration

Hepatocellular carcinoma (HCC) is a highly metastatic cancer with very poor prognosis. AMP activated kinase (AMPK) constitutes a candidate to inhibit HCC progression. First, AMPK is downregulated in HCC. Second, glucose starvation induces apoptosis in HCC cells via AMPK. Correspondingly, metformin activates AMPK and inhibits HCC cell proliferation. Nevertheless, the effect of AMPK activation on HCC cell invasiveness remains elusive. Here, migration/invasion was studied in HCC cells exposed to metformin and glucose starvation. Cell viability, proliferation and differentiation, as well as AMPK and PKA activation were analyzed. In addition, invasiveness in mutants of the AMPKα activation loop was assessed. Metformin decreased cell migration, invasion and epithelial-mesenchymal transition, and interference with AMPKα expression avoided metformin actions. Those antitumor effects were potentiated by glucose deprivation. Metformin activated AMPK at the same time that inhibited PKA, and both effects were enhanced by glucose starvation. Given that AMPKα(S173) phosphorylation by PKA decreases AMPK activation, we hypothesized that the reduction of PKA inhibitory effect by metformin could explain the increased antitumor effects observed. Supporting this, in AMPK activating conditions, cell migration/invasion was further impaired in AMPKα(S173C) mutant cells. Metformin emerges as a strong inhibitor of migration/invasion in HCC cells, and glucose restriction potentiates this effect.

Microtubules regulate brush border formation

Most epithelial cells contain apical membrane structures associated to bundles of actin filaments, which constitute the brush border. Whereas microtubule participation in the maintenance of the brush border identity has been characterized, their contribution to de novo microvilli organization remained elusive. Hereby, using a cell model of individual enterocyte polarization, we found that nocodazole induced microtubule depolymerization prevented the de novo brush border formation. Microtubule participation in brush border actin organization was confirmed in polarized kidney tubule MDCK cells. We also found that centrosome, but not Golgi derived microtubules, were essential for the initial stages of brush border development. During this process, microtubule plus ends acquired an early asymmetric orientation toward the apical membrane, which clearly differs from their predominant basal orientation in mature epithelia. In addition, overexpression of the microtubule plus ends associated protein CLIP170, which regulate actin nucleation in different cell contexts, facilitated brush border formation. In combination, the present results support the participation of centrosomal microtubule plus ends in the activation of the polarized actin organization associated to brush border formation, unveiling a novel mechanism of microtubule regulation of epithelial polarity.

Akap350 Recruits Eb1 to The Spindle Poles, Ensuring Proper Spindle Orientation and Lumen Formation in 3d Epithelial Cell Cultures

The organization of epithelial cells to form hollow organs with a single lumen requires the accurate three-dimensional arrangement of cell divisions. Mitotic spindle orientation is defined by signaling pathways that provide molecular links between specific spots at the cell cortex and astral microtubules, which have not been fully elucidated. AKAP350 is a centrosomal/Golgi scaffold protein, implicated in the regulation of microtubule dynamics. Using 3D epithelial cell cultures, we found that cells with decreased AKAP350 expression (AKAP350KD) formed polarized cysts with abnormal lumen morphology. Analysis of mitotic cells in AKAP350KD cysts indicated defective spindle alignment. We established that AKAP350 interacts with EB1, a microtubule associated protein that regulates spindle orientation, at the spindle poles. Decrease of AKAP350 expression lead to a significant reduction of EB1 levels at spindle poles and astral microtubules. Conversely, overexpression of EB1 rescued the defective spindle orientation induced by deficient AKAP350 expression. The specific delocalization of the AKAP350/EB1complex from the centrosome decreased EB1 levels at astral microtubules and lead to the formation of 3D-organotypic structures which resembled AKAP350KD cysts. We conclude that AKAP350 recruits EB1 to the spindle poles, ensuring EB1 presence at astral microtubules and proper spindle orientation during epithelial morphogenesis.

AMPK and PKA interaction in the regulation of survival of liver cancer cells subjected to glucose starvation

The signaling pathways that govern survival response in hepatic cancer cells subjected to nutritional restriction have not been clarified yet. In this study we showed that liver cancer cells undergoing glucose deprivation both arrested in G0/G1 and died mainly by apoptosis. Treatment with the AMPK activator AICAR phenocopied the effect of glucose deprivation on cell survival, whereas AMPK silencing in HepG2/C3A, HuH-7 or SK-Hep-1 cells blocked the cell cycle arrest and the increase in apoptotic death induced by glucose starvation. Both AMPK and PKA were promptly activated after glucose withdrawal. PKA signaling had a dual role during glucose starvation: whereas it elicited an early decreased in cell viability, it later improved this parameter. We detected AMPK phosphorylation (AMPKα(Ser173)) by PKA, which was increased in glucose starved cells and was associated with diminution of AMPK activation. To better explore this inhibitory effect, we constructed a hepatocarcinoma derived cell line which stably expressed an AMPK mutant lacking that PKA phosphorylation site: AMPKα1(S173C). Expression of this mutant significantly decreased viability in cells undergoing glucose starvation. Furthermore, after 36 h of glucose deprivation, the index of AMPKα1(S173C) apoptotic cells doubled the apoptotic index observed in control cells. Two main remarks arise: 1. AMPK is the central signaling kinase in the scenario of cell cycle arrest and death induced by glucose starvation in hepatic cancer cells; 2. PKA phosphorylation of Ser173 comes out as a strong control point that limits the antitumor effects of AMPK in this situation.

RSK1 protects P-glycoprotein/ABCB1 against ubiquitin-proteasomal degradation by downregulating the ubiquitin-conjugating enzyme E2 R1

P-glycoprotein (P-gp) is a critical determinant of multidrug resistance in cancer. We previously reported that MAPK inhibition downregulates P-gp expression and that P-gp undergoes ubiquitin-proteasomal degradation regulated by UBE2R1 and SCF^Fbx15. Here, we investigated the crosstalk between MAPK inhibition and the ubiquitin-proteasomal degradation of P-gp. Proteasome inhibitors or knockdown of FBXO15 and/or UBE2R1 cancelled MEK inhibitor-induced P-gp downregulation. RSK1 phosphorylated Thr162 on UBE2R1 but did not phosphorylate FBXO15. MEK and RSK inhibitors increased UBE2R1-WT but not UBE2R1-T162D and -T162A expression. UBE2R1-T162D showed higher self-ubiquitination and destabilisation than UBE2R1-WT and -T162A. Unlike UBE2R1-WT and -T162A, UBE2R1-T162D did not induce P-gp ubiquitination. UBE2R1-WT or -T162A downregulated P-gp expression and upregulated rhodamine 123 level and sensitivity to vincristine and doxorubicin. However, UBE2R1-T162D did not confer any change in P-gp expression, rhodamine 123 accumulation and sensitivity to the drugs. These results suggest that RSK1 protects P-gp against ubiquitination by reducing UBE2R1 stability.

Centrosomal AKAP350 and CIP4 act in concert to define the polarized localization of the centrosome and Golgi in migratory cells

The acquisition of a migratory phenotype is central in processes as diverse as embryo differentiation and tumor metastasis. An early event in this phenomenon is the generation of a nucleus-centrosome-Golgi back-to-front axis. AKAP350 (also known as AKAP9) is a Golgi and centrosome scaffold protein that is involved in microtubule nucleation. AKAP350 interacts with CIP4 (also known as TRIP10), a cdc42 effector that regulates actin dynamics. The present study aimed to characterize the participation of centrosomal AKAP350 in the acquisition of migratory polarity, and the involvement of CIP4 in the pathway. The decrease in total or in centrosomal AKAP350 led to decreased formation of the nucleus-centrosome-Golgi axis and defective cell migration. CIP4 localized at the centrosome, which was enhanced in migratory cells, but inhibited in cells with decreased centrosomal AKAP350. A decrease in the CIP4 expression or inhibition of the CIP4-AKAP350 interaction also led to defective cell polarization. Centrosome positioning, but not nuclear movement, was affected by loss of CIP4 or AKAP350 function. Our results support a model in which AKAP350 recruits CIP4 to the centrosome, providing a centrosomal scaffold to integrate microtubule and actin dynamics, thus enabling centrosome polarization and ensuring cell migration directionality.

The centrosome: a prospective entrant in cancer therapy

INTRODUCTION

The centrosome plays an essential role in the cell cycle. The centrosome and its associated proteins assist in nucleating and organizing microtubules. A structural or a functional aberration in the centrosome is known to cause abnormal cell proliferation leading to tumors. Therefore, the centrosome is considered as a promising anti-cancer target.

AREAS COVERED

This review begins with a brief introduction to the centrosome and its role in the cell cycle. We elaborate on the centrosome-associated proteins that regulate microtubule dynamics. In addition, we discuss the centrosomal protein kinase targets such as cyclin-dependent, polo-like and aurora kinases. Inhibitors targeting these kinases are undergoing clinical trials for cancer chemotherapy. Further, we shed light on new approaches to target the centrosomal proteins for cancer therapy.

EXPERT OPINION

Insights into the functioning of the centrosomal proteins will be extremely beneficial in validating the centrosome as a target in cancer therapy. New strategies either as a single entity or in combination with current chemotherapeutic agents should be researched or exploited to reveal the promises that the centrosome holds for future cancer therapy.

Centrosomal AKAP350 modulates the G1/S transition

AKAP350 (AKAP450/AKAP9/CG-NAP) is an A-kinase anchoring protein, which recruits multiple signaling proteins to the Golgi apparatus and the centrosomes. Several proteins recruited to the centrosomes by this scaffold participate in the regulation of the cell cycle. Previous studies indicated that AKAP350 participates in centrosome duplication. In the present study we specifically assessed the role of AKAP350 in the progression of the cell cycle. Our results showed that interference with AKAP350 expression inhibits G₁/S transition, decreasing the initiation of both DNA synthesis and centrosome duplication. We identified an AKAP350 carboxyl-terminal domain (AKAP350CTD), which contained the centrosomal targeting domain of AKAP350 and induced the initiation of DNA synthesis. Nevertheless, AKAP350CTD expression did not induce centrosomal duplication. AKAP350CTD partially delocalized endogenous AKAP350 from the centrosomes, but increased the centrosomal levels of the cyclin-dependent kinase 2 (Cdk2). Accordingly, the expression of this AKAP350 domain increased the endogenous phosphorylation of nucleophosmin by Cdk2, which occurs at the G₁/S transition and is a marker of the centrosomal activity of the cyclin E-Cdk2 complex. Cdk2 recruitment to the centrosomes is a necessary event for the development of the G₁/S transition. Altogether, our results indicate that AKAP350 facilitates the initiation of DNA synthesis by scaffolding Cdk2 to the centrosomes, and enabling its specific activity at this organelle. Although this mechanism could also be involved in AKAP350-dependent modulation of centrosomal duplication, it is not sufficient to account for this process.

Altered lysophosphatidic acid (LPA) receptor expression during hepatic regeneration in a mouse model of partial hepatectomy

BACKGROUND

Hepatic regeneration requires coordinated signal transduction for efficient restoration of functional liver mass. This study sought to determine changes in lysophosphatidic acid (LPA) and LPA receptor (LPAR) 1-6 expression in regenerating liver following two-thirds partial hepatectomy (PHx).

METHODS

Liver tissue and blood were collected from male C57BL/6 mice following PHx. Circulating LPA was measured by enzyme-linked immunosorbent assay (ELISA) and hepatic LPAR mRNA and protein expression were determined.

RESULTS

Circulating LPA increased 72 h after PHx and remained significantly elevated for up to 7 days post-PHx. Analysis of LPAR expression after PHx demonstrated significant increases in LPAR1, LPAR3 and LPAR6 mRNA and protein in a time-dependent manner for up to 7 days post-PHx. Conversely, LPAR2, LPAR4 and LPAR5 mRNA were barely detected in normal liver and did not significantly change after PHx. Changes in LPAR1 expression were confined to non-parenchymal cells following PHx.

CONCLUSIONS

Liver regeneration following PHx is associated with significant changes in circulating LPA and hepatic LPAR1, LPAR3 and LPAR6 expression in a time- and cell-dependent manner. Furthermore, changes in LPA-LPAR post-PHx occur after the first round of hepatocyte division is complete.

Regulations of P-Glycoprotein/ABCB1/MDR1 in Human Cancer Cells

Multidrug resistance (MDR) in cancer cells is a phenotype whereby cells display reduced sensitivity to anticancer drugs, based on a variety of mechanisms, including an increase in drug efflux, the reduction of drug uptake, the activation of cell growth and survival signaling, the promotion of DNA repair, and the inhibition of apoptosis signaling. Increased expression of the plasma membrane drug efflux pumps, the ATP-binding cassette (ABC) transporters, is involved in MDR. P-Glycoprotein/ABCB1 is a member of the ABC transporter family, and facilitates the efflux of various anticancer drugs, including anthracyclines, vinca alkaloids, epipodophyllotoxins, taxanes, and kinase inhibitors, from cells. P-Glycoprotein is also expressed in normal tissues and cells, including the kidney, liver, colon, and adrenal gland, to transport and/or secrete substrates and at the blood-brain, blood-placenta, and blood-testis barriers to protect these tissues from toxic substances. To understand the mechanistic functions of P-glycoprotein and to overcome MDR, investigators have identified the substrates and competitive inhibitors of P-glycoprotein. Recently, we and other groups reported associations between cellular signaling pathways and the expression, stability, degradation, localization, and activity of P-glycoprotein. The present review summarizes the currently available information about the transcriptional and posttranslational regulation of P-glycoprotein expression and function.

Benznidazole modulates cell proliferation in acute leukemia cells

CONTEXT

We have previously reported that benznidazole (BZL), known for its trypanocidal action, has anti-proliferative activity against different cell lines like HeLa and Raw 264.7 among others. At the moment, it has not been reported if the anti-proliferative effect of BZL is similar for non-adherent hematopoietic cells like was reported for adherent cancer cell lines.

OBJECTIVE

We aimed to investigate the efficacy of BZL on the growth of the leukemic cell lines THP-1 and OCI/AML3.

MATERIALS AND METHODS

We evaluated cell proliferation by [³H]-thymidine incorporation and MTT reduction as well as cell death by lactate dehydrogenase (LDH) activity. We assessed apoptosis by flow cytometry for detection of annexin V-positive and propidium iodide-negative cells, along with nuclear morphology by diamidino-2-phenolindole (DAPI) staining. Western blot studies were performed to evaluate changes in cell cycle proteins in BZL-treated cells.

RESULTS

BZL significantly reduced proliferation of both cell lines without inducing cell death. Likewise it produced no significant differences in apoptosis between treated cells and controls. In addition, flow cytometry analysis indicated that BZL caused a larger number of THP-1 cells in G0/G1 phase and a smaller number of cells in S phase than controls. This was accompanied with an increase in the expression of the CDK inhibitor p27 and of cyclin D1, with no significant differences in the protein levels of CDK1, CDK2, CDK4, cyclins E, A and B as compared to controls.

CONCLUSION

BZL inhibits the proliferation of leukemic non-adherent cells by controlling cell cycle at G0/G1 cell phase through up-regulation of p27.

Golgi as an MTOC: making microtubules for its own good

In cells, microtubules (MTs) are nucleated at MT-organizing centers (MTOCs). The centrosome-based MTOCs organize radial MT arrays, which are often not optimal for polarized trafficking. A recently discovered subset of non-centrosomal MTs nucleated at the Golgi has proven to be indispensable for the Golgi organization, post-Golgi trafficking and cell polarity. Here, we summarize the history of this discovery, known molecular prerequisites of MT nucleation at the Golgi and unique functions of Golgi-derived MTs.

Cytoplasmic domains of the transporter associated with antigen processing and P-glycoprotein interact with subunits of the proteasome

The proteasome is a multi-protein complex that degrades cellular proteins as well as foreign proteins destined for antigen presentation. The latter function involves the immunoproteasome, in which several proteasome subunits are exchanged for gamma-interferon-induced subunits. The transporter associated with antigen processing (TAP) transports proteasome-generated peptides across the membrane of the endoplasmic reticulum (ER) prior to presentation on the plasma membrane. We demonstrate interactions between the cytoplasmic domains of TAP subunits and subunits of both the proteasome and the immunoproteasome, suggesting direct targeting of antigenic peptides to the ER via a TAP-proteasome association. We also show interaction between one of the cytoplasmic domains of P-glycoprotein and a proteasome subunit, but not the corresponding immunoproteasome subunit, suggesting a possible role for P-glycoprotein in the transport of proteasome-derived peptides.