RESUMEN

Cancer cells hijack autophagy pathway to evade anti-cancer therapeutics. Many molecular signaling pathways associated with drug-resistance converge on autophagy induction. Honokiol (HNK), a natural phenolic compound purified from Magnolia grandiflora, has recently been shown to impede breast tumorigenesis and, in the present study, we investigated whether breast cancer cells evoke autophagy to modulate therapeutic efficacy and functional networks of HNK. Indeed, breast cancer cells exhibit increased autophagosomes-accumulation, MAP1LC3B-II/LC3B-II-conversion, expression of ATG proteins as well as elevated fusion of autophagosomes and lysosomes upon HNK treatment. Breast cancer cells treated with HNK demonstrate significant growth inhibition and apoptotic induction, and these biological processes are blunted by macroautophagy/autophagy. Consequently, inhibiting autophagosome formation, abrogating autophagosome-lysosome fusion or genetic-knockout of BECN1 and ATG7 effectively increase HNK-mediated apoptotic induction and growth inhibition. Next, we explored the functional impact of tumor suppressor STK11 in autophagy induction in HNK-treated cells. STK11-silencing abrogates LC3B-II-conversion, and blocks autophagosome/lysosome fusion and lysosomal activity as illustrated by LC3B-Rab7 co-staining and DQ-BSA assay. Our results exemplify the cytoprotective nature of autophagy invoked in HNK-treated breast cancer cells and put forth the notion that a combined strategy of autophagy inhibition with HNK would be more effective. Indeed, HNK and chloroquine (CQ) show synergistic inhibition of breast cancer cells and HNK-CQ combination treatment effectively inhibits breast tumorigenesis and metastatic progression. Tumor-dissociated cells from HNK-CQ treated tumors exhibit abrogated invasion and migration potential. Together, these results implicate that breast cancer cells undergo cytoprotective autophagy to circumvent HNK and a combined treatment with HNK and CQ can be a promising therapeutic strategy for breast cancer.

RESUMEN

[This corrects the article DOI: 10.1371/journal.pone.0088162.].

RESUMEN

Obscurins are a family of RhoGEF-containing proteins with tumor and metastasis suppressing roles in breast epithelium. Downregulation of giant obscurins in normal breast epithelial cells leads to reduced levels of active RhoA and of its downstream effectors. Herein, we elucidate how depletion of giant obscurins affects the response of breast epithelial cells to changes in the mechanical properties of the microenvironment. We find that knockdown of obscurins increases cell morphodynamics, migration speed, and diffusivity on polyacrylamide gels of ≥ 1 kPa, presumably by decreasing focal adhesion area and density as well as cell traction forces. Depletion of obscurins also increases cell mechanosensitivity on soft (0.4-4 kPa) surfaces. Similar to downregulation of obscurins, pharmacological inhibition of Rho kinase in breast epithelial cells increases migration and morphodynamics, suggesting that suppression of Rho kinase activity following obscurin knockdown can account for alterations in morphodynamics and migration. In contrast, inhibition of myosin light chain kinase reduces morphodynamics and migration, suggesting that temporal changes in cell shape are required for efficient migration. Collectively, downregulation of giant obscurins facilitates cell migration through heterogeneous microenvironments of varying stiffness by altering cell mechanobiology.

RESUMEN

Obscurins are a family of giant cytoskeletal proteins, originally identified in striated muscles where they have structural and regulatory roles. We recently showed that obscurins are abundantly expressed in normal breast epithelial cells where they play tumor and metastasis suppressing roles, but are nearly lost from advanced stage breast cancer biopsies. Consistent with this, loss of giant obscurins from breast epithelial cells results in enhanced survival and growth, epithelial to mesenchymal transition (EMT), and increased cell migration and invasion in vitro and in vivo. In the current study, we demonstrate that loss of giant obscurins from breast epithelial cells is associated with significantly increased phosphorylation and subsequent activation of the PI3K signaling cascade, including activation of AKT, a key regulator of tumorigenesis and metastasis. Pharmacological and molecular inhibition of the PI3K pathway in obscurin-depleted breast epithelial cells results in reversal of EMT, (re)formation of cell-cell junctions, diminished mammosphere formation, and decreased cell migration and invasion. Co-immunoprecipitation, pull-down, and surface plasmon resonance assays revealed that obscurins are in a complex with the PI3K/p85 regulatory subunit, and that their association is direct and mediated by the obscurin-PH domain and the PI3K/p85-SH3 domain with a KD of ~50 nM. We therefore postulate that giant obscurins act upstream of the PI3K cascade in normal breast epithelial cells, regulating its activation through binding to the PI3K/p85 regulatory subunit.

Asunto(s)

Neoplasias de la Mama/patología , Fosfatidilinositol 3-Quinasa Clase Ia/metabolismo , Transición Epitelial-Mesenquimal/fisiología , Factores de Intercambio de Guanina Nucleótido Rho/metabolismo , Neoplasias de la Mama/metabolismo , Línea Celular Tumoral , Células Epiteliales/metabolismo , Células Epiteliales/patología , Femenino , Humanos , Fosfatidilinositol 3-Quinasas/metabolismo , Proteínas Serina-Treonina Quinasas

RESUMEN

Obscurins comprise a family of proteins originally identified in striated muscles, where they play essential roles in myofibrillogenesis, cytoskeletal organization, and Ca(2+) homeostasis. They are encoded by the single OBSCN gene, and are composed of tandem adhesion domains and signaling motifs. To date, two giant obscurin isoforms have been described in detail that differ only at the extreme COOH-terminus; while obscurin-A (∼720 kDa) contains a non-modular COOH-terminus that harbors binding sites for the adaptor proteins ankyrins, obscurin-B (∼870 kDa) contains two COOH-terminal serine-threonine kinase domains preceded by adhesion motifs. Besides the two known giant obscurins, a thorough search of transcript databases suggests that complex alternative splicing of the obscurin transcript results in the generation of additional giant as well as small isoforms with molecular masses ranging between ∼50-970 kDa. These novel isoforms share common domains with the characterized isoforms, but also contain unique regions. Using a panel of highly specific antibodies directed against epitopes spanning the entire length of giant obscurins, we employed western blotting and immunohistochemistry to perform a systematic and comprehensive characterization of the expression profile of obscurins in muscle and non-muscle tissues. Our studies demonstrate for the first time that obscurins are not restricted to striated muscles, but are abundantly expressed in several tissues and organs including brain, skin, kidney, liver, spleen, and lung. While some obscurin isoforms are ubiquitously expressed, others are preferentially present in specific tissues and organs. Moreover, obscurins are present in select structures and cell types where they assume nuclear, cytosolic, and membrane distributions. Given the ubiquitous expression of some obscurins, along with the preferential expression of others, it becomes apparent that obscurins may play common and unique roles, respectively, in the regulation and maintenance of cell homeostasis in various tissues and organs throughout the body.

Asunto(s)

Factores de Intercambio de Guanina Nucleótido/metabolismo , Proteínas Musculares/metabolismo , Músculos/metabolismo , Animales , Epítopos/inmunología , Femenino , Perfilación de la Expresión Génica , Humanos , Ratones Endogámicos C57BL , Músculo Estriado/metabolismo , Especificidad de Órganos/genética , Isoformas de Proteínas/metabolismo , Proteínas Serina-Treonina Quinasas , Transporte de Proteínas , Ratas Sprague-Dawley , Factores de Intercambio de Guanina Nucleótido Rho , Fracciones Subcelulares/metabolismo

RESUMEN

Discovered about a decade ago, obscurin (~720 kDa) is a member of a family of giant proteins expressed in striated muscle that are essential for normal muscle function. Much of what we understand about obscurin stems from its functions in cardiac and skeletal muscle. However, recent evidence has indicated that variants of obscurin ("obscurins") are expressed in diverse cell types, where they contribute to distinct cellular processes. Dysfunction or abrogation of obscurins has also been implicated in the development of several pathological conditions, including cardiac hypertrophy and cancer. Herein, we present an overview of obscurins with an emphasis on novel findings that demonstrate their heretofore-unsuspected importance in cell signaling and disease progression.

Asunto(s)

Factores de Intercambio de Guanina Nucleótido Rho/fisiología , Animales , Cardiomiopatía Hipertrófica Familiar/genética , Humanos , Desarrollo de Músculos , Músculo Esquelético/metabolismo , Mutación Missense , Neoplasias/genética , Especificidad de Órganos , Mapas de Interacción de Proteínas , Proteínas Serina-Treonina Quinasas

RESUMEN

Obscurins (∼70 - 870 kDa), encoded by the single OBSCN gene, are cytoskeletal proteins originally identified in striated muscles with structural and regulatory roles. Recently, analysis of 13,023 genes in breast and colorectal cancers identified OBSCN as one of the most frequently mutated genes, implicating it in cancer formation. Herein we studied the expression profile of obscurins in breast, colon, and skin cancer cell lines and their involvement in cell survival. Immunoblot analysis demonstrated significant reduction of obscurin proteins [corrected] in cancer cells, resulting from decreased mRNA levels and/or the presence of mutant transcripts. In normal epithelium, obscurins localize in cytoplasmic puncta, the cell membrane, and the nucleus. Accordingly, subcellular fractionation demonstrated the presence of 2 novel nuclear isoforms of ∼110 and ∼120 kDa. Nontumorigenic MCF10A breast epithelial cells stably transduced with shRNAs targeting giant obscurins exhibited increased viability (∼30%) and reduced apoptosis (∼20%) following exposure to the DNA-damaging agent etoposide. Quantitative RT-PCR further indicated that the antiapoptotic genes BAG4 and HAX1 were up-regulated (1.5- and 1.4-fold, respectively), whereas initiator caspase-9 and death caspase-3 transcripts were down-regulated (0.8- and 0.6-fold, respectively). Our findings are the first to pinpoint critical roles for obscurins in cancer development by contributing to the regulation of cell survival.

Asunto(s)

Mama/citología , Mama/metabolismo , Factores de Intercambio de Guanina Nucleótido/genética , Factores de Intercambio de Guanina Nucleótido/fisiología , Proteínas Musculares/genética , Proteínas Musculares/fisiología , Apoptosis/genética , Apoptosis/fisiología , Secuencia de Bases , Neoplasias de la Mama/genética , Neoplasias de la Mama/metabolismo , Línea Celular , Línea Celular Tumoral , Núcleo Celular/metabolismo , Supervivencia Celular/genética , Supervivencia Celular/fisiología , Transformación Celular Neoplásica/genética , Neoplasias del Colon/genética , Neoplasias del Colon/metabolismo , Daño del ADN , Células Epiteliales/citología , Células Epiteliales/metabolismo , Etopósido/toxicidad , Femenino , Expresión Génica , Factores de Intercambio de Guanina Nucleótido/antagonistas & inhibidores , Factores de Intercambio de Guanina Nucleótido/deficiencia , Humanos , Proteínas Musculares/antagonistas & inhibidores , Proteínas Musculares/deficiencia , Isoformas de Proteínas/genética , Isoformas de Proteínas/metabolismo , Proteínas Serina-Treonina Quinasas , ARN Mensajero/genética , ARN Mensajero/metabolismo , ARN Neoplásico/genética , ARN Neoplásico/metabolismo , ARN Interferente Pequeño/genética , Factores de Intercambio de Guanina Nucleótido Rho , Neoplasias Cutáneas/genética , Neoplasias Cutáneas/metabolismo

RESUMEN

The 5-year survival rate is very low when breast cancer becomes metastatic. The metastatic process is governed by a network of molecules of which SLUG is known to play a major role as a regulator of epithelial-to-mesenchymal transition (EMT). Prostate-derived ETS factor (PDEF) has been proposed as a tumor suppressor, possibly through inhibition of invasion and metastasis; therefore, understanding the mechanism of PDEF regulation may help to better understand its role in breast cancer progression. This study shows for the first time that the transcription factor SLUG is a direct target of PDEF in breast cancer. We show that the expression of PDEF is able to suppress/dampen EMT through the negative regulation of SLUG. In addition, we show that PDEF is also able to regulate downstream targets of SLUG, namely E-cadherin, in both SLUG-dependent and -independent manners, suggesting a critical role for PDEF in regulating EMT.