Mechanisms promoting escape from mitotic stress-induced tumor cell death

Non-small cell lung cancer (NSCLC) is notorious for its paltry responses to first-line therapeutic regimens. In contrast to acquired chemoresistance, little is known about the molecular underpinnings of the intrinsic resistance of chemo-naïve NSCLC. Here we report that intrinsic resistance to paclitaxel in NSCLC occurs at a cell-autonomous level because of the uncoupling of mitotic defects from apoptosis. To identify components that permit escape from mitotic stress-induced death, we used a genome-wide RNAi-based strategy, which combines a high-throughput toxicity screen with a live-cell imaging platform to measure mitotic fate. This strategy revealed that prolonging mitotic arrest with a small molecule inhibitor of the APC/cyclosome could sensitize otherwise paclitaxel-resistant NSCLC. We also defined novel roles for CASC1 and TRIM69 in supporting resistance to spindle poisons. CASC1, which is frequently co-amplified with KRAS in lung tumors, is essential for microtubule polymerization and satisfaction of the spindle assembly checkpoint. TRIM69, which associates with spindle poles and promotes centrosomal clustering, is essential for formation of a bipolar spindle. Notably, RNAi-mediated attenuation of CASC1 or TRIM69 was sufficient to inhibit tumor growth in vivo. On the basis of our results, we hypothesize that tumor evolution selects for a permissive mitotic checkpoint, which may promote survival despite chromosome segregation errors. Attacking this adaptation may restore the apoptotic consequences of mitotic damage to permit the therapeutic eradication of drug-resistant cancer cells.

SRC-family tyrosine kinases in oogenesis, oocyte maturation and fertilization: an evolutionary perspective

The oocyte is a highly specialized cell poised to respond to fertilization with a unique set of actions needed to recognize and incorporate a single sperm, complete meiosis, reprogram maternal and paternal genomes and assemble them into a unique zygotic genome, and finally initiate the mitotic cell cycle. Oocytes accomplish this diverse series of events through an array of signal transduction pathway components that include a characteristic collection of protein tyrosine kinases. The src-family protein kinases (SFKs) figure importantly in this signaling array and oocytes characteristically express certain SFKs at high levels to provide for the unique actions that the oocyte must perform. The SFKs typically exhibit a distinct pattern of subcellular localization in oocytes and perform critical functions in different subcellular compartments at different steps during oocyte maturation and fertilization. While many aspects of SFK signaling are conserved among oocytes from different species, significant differences exist in the extent to which src-family-mediated pathways are used by oocytes from species that fertilize externally vs those which are fertilized internally. The observation that several oocyte functions which require SFK signaling appear to represent common points of failure during assisted reproductive techniques in humans, highlights the importance of these signaling pathways for human reproductive health.

Molecular mechanisms of asymmetric division in oocytes

In contrast to symmetric division in mitosis, mammalian oocyte maturation is characterized by asymmetric cell division that produces a large egg and a small polar body. The asymmetry results from oocyte polarization, which includes spindle positioning, migration, and cortical reorganization, and this process is critical for fertilization and the retention of maternal components for early embryo development. Although actin dynamics are involved in this process, the molecular mechanism underlying this remained unclear until the use of confocal microscopy and live cell imaging became widespread in recent years. Information obtained through a PubMed database search of all articles published in English between 2000 and 2012 that included the phrases "oocyte, actin, spindle migration," "oocyte, actin, polar body," or "oocyte, actin, asymmetric division" was reviewed. The actin nucleation factor actin-related protein 2/3 complex and its nucleation-promoting factors, formins and Spire, and regulators such as small GTPases, partitioning-defective/protein kinase C, Fyn, microRNAs, cis-Golgi apparatus components, myosin/myosin light-chain kinase, spindle stability regulators, and spindle assembly checkpoint regulators, play critical roles in asymmetric cell division in oocytes. This review summarizes recent findings on these actin-related regulators in mammalian oocyte asymmetric division and outlines a complete signaling pathway.

Morphological and molecular markers are correlated with maturation-competence of human oocytes

STUDY QUESTION

Does the position of the germinal vesicle (GV) in human oocytes correlate with molecular and morphological parameters as well as with maturation-competence?

SUMMARY ANSWER

The position of GV in human oocytes correlates with density of microtubule (MT) filaments, concentration of Fyn, nucleolus localization and the ability of the oocytes to complete maturation following GV breakdown (GVBD).

WHAT IS KNOWN ALREADY

Our knowledge is confined to oocytes of young mice where maturation-competence is correlated with a central GV and regulated by MTs and the presence of a chromatin ring. Fyn kinase is localized at the spindle and cortex of mouse oocytes and plays a role in both maturation and MT stabilization.

STUDY DESIGN, SIZE, DURATION

Spatial localization of the GV and nucleolus (central or peripheral), the presence of a chromatin ring, the localization of Fyn, MT density and oocyte maturation were assessed in 153 human oocytes, 335 oocytes from young mice (2-month-old) and 146 oocytes from old mice (12-month-old).

PARTICIPANTS/MATERIALS, SETTING, METHODS

GV human oocytes were donated by consenting female patients (n = 57), 21-45-year-old undergoing IVF/ICSI. As a control, GV mouse oocytes were collected from female mice after injection of pregnant mares' serum gonadotrophin. Human and mouse GV oocytes allocated for immunocytochemistry were fixed on day of retrieval, stained with specific antibodies and imaged using a confocal laser-scanning microscope. Human and mouse oocytes allocated for maturation were incubated for 48 and 24 h, respectively. GVBD and extrusion of the first polar body (PBI) were assessed using differential interference contrast optics.

MAIN RESULTS AND THE ROLE OF CHANCE

GV location was peripheral and independent of age in 69.9% of the human oocytes, but GV location did vary with age in mice oocytes; it was central in 89.9% of the oocytes retrieved from young-mice and peripheral in 52.1% of the oocytes retrieved from old mice (P < 0.05). A central GV, whether in human or mouse oocytes, was highly correlated with a central nucleolus, absence of Fyn at the GV and a dense MT network (P < 0.05), whereas a peripheral GV correlated with peripheral nucleolus, presence of Fyn at the GV and a flimsy MT network. After 48 h in culture, no degeneration was observed in human central-GV oocytes, however, 12/95 (12.6%) of the peripheral-GV oocytes degenerated (P < 0.05). No correlation was observed between GV position and presence of a chromatin ring. The percentage of human oocytes that extruded the PBI after completing GVBD was significantly higher (73.7%) in central than in peripheral-GV oocytes (45.8%; P < 0.05). In mice oocytes, central location of the GV correlated with maturation competence in young (P < 0.05) but not old mice.

LIMITATIONS, REASONS FOR CAUTION

The fact that the human GV oocytes used in this study were exposed to gonadotrophic stimulation but failed to mature in vivo might be a sign of their low quality and this should be considered when drawing conclusions from the data. Furthermore, our observation that only peripheral-GV human oocytes were degraded may indicate that they are of a lower quality than central-GV human oocytes.

WIDER IMPLICATIONS OF THE FINDINGS

We suggest that the central location of GV within the oocytes, which is associated with an absence of Fyn at the GV and the presence of thick filamentous MTs in the ooplasm, may serve as a predictor of successful maturation and provide new insights for the use of IVM.

Cholesterol depletion disorganizes oocyte membrane rafts altering mouse fertilization

Drastic membrane reorganization occurs when mammalian sperm binds to and fuses with the oocyte membrane. Two oocyte protein families are essential for fertilization, tetraspanins and glycosylphosphatidylinositol-anchored proteins. The firsts are associated to tetraspanin-enriched microdomains and the seconds to lipid rafts. Here we report membrane raft involvement in mouse fertilization assessed by cholesterol modulation using methyl-β-cyclodextrin. Cholesterol removal induced: (1) a decrease of the fertilization rate and index; and (2) a delay in the extrusion of the second polar body. Cholesterol repletion recovered the fertilization ability of cholesterol-depleted oocytes, indicating reversibility of these effects. In vivo time-lapse analyses using fluorescent cholesterol permitted to identify the time-point at which the probe is mainly located at the plasma membrane enabling the estimation of the extent of the cholesterol depletion. We confirmed that the mouse oocyte is rich in rafts according to the presence of the raft marker lipid, ganglioside GM1 on the membrane of living oocytes and we identified the coexistence of two types of microdomains, planar rafts and caveolae-like structures, by terms of two differential rafts markers, flotillin-2 and caveolin-1, respectively. Moreover, this is the first report that shows characteristic caveolae-like invaginations in the mouse oocyte identified by electron microscopy. Raft disruption by cholesterol depletion disturbed the subcellular localization of the signal molecule c-Src and the inhibition of Src kinase proteins prevented second polar body extrusion, consistent with a role of Src-related kinases in fertilization via signaling complexes. Our data highlight the functional importance of intact membrane rafts for mouse fertilization and its dependence on cholesterol.

microRNA-125a-3p reduces cell proliferation and migration by targeting Fyn

Fyn, a member of the Src family kinases (SFKs), has a pivotal role in cell adhesion, proliferation, migration and survival, and its overexpression is associated with several types of cancer. MicroRNAs (miRNAs) play a major role in post-transcriptional repression of protein expression. In light of the significant functions of Fyn, together with studies demonstrating miR-125a as a tumor-suppressing miRNA that is downregulated in several cancer cell types and on our bioinformatics studies presented here, we chose to examine the post-transcription regulation of Fyn by miR-125a-3p in the HEK 293T cell line. We show that Fyn expression can be dramatically reduced by elevated levels of miR-125a-3p. Following this reduction, the activity of proteins downstream of Fyn, such as FAK, paxillin and Akt (proteins known to be overexpressed in various tumors), is also reduced. On a broader level, we show that miR-125a-3p causes an arrest of the cell cycle at the G2/M stage and decreases cell viability and migration, probably in a Fyn-directed manner. The results are reinforced by control experiments conducted using Fyn siRNA and anti-miR-125a-3p, as well as by the fact that numerous cancer cell lines show a significant downregulation of Fyn after mir-125a-3p overexpression. Collectively, we conclude that miR-125a-3p has an important role in the regulation of Fyn expression and of its signaling pathway, which implies that it has a therapeutic potential in overexpressed Fyn-related diseases.

Protein tyrosine kinase signaling in the mouse oocyte cortex during sperm-egg interactions and anaphase resumption

Fertilization triggers activation of a series of pre-programmed signal transduction pathways in the oocyte that establish a block to polyspermy, induce meiotic resumption, and initiate zygotic development. Fusion between sperm and oocyte results in rapid changes in oocyte intracellular free-calcium levels, which in turn activate multiple protein kinase cascades in the ooplasm. The present study examined the possibility that sperm-oocyte interaction involves localized activation of oocyte protein tyrosine kinases, which could provide an alternative signaling mechanism to that triggered by the fertilizing sperm. Confocal immunofluorescence analysis with antibodies to phosphotyrosine and phosphorylated protein tyrosine kinases allowed detection of minute signaling events localized to the site of sperm-oocyte interaction that were not amenable to biochemical analysis. The results provide evidence for localized accumulation of phosphotyrosine at the site of sperm contact, binding, or fusion, which suggests active protein tyrosine kinase signaling prior to and during sperm incorporation. The PYK2 kinase was found to be concentrated and activated at the site of sperm-oocyte interaction, and likely participates in this response. Widespread activation of PYK2 and FAK kinases was subsequently observed within the oocyte cortex, indicating that sperm incorporation is followed by more global signaling via these kinases during meiotic resumption. The results demonstrate an alternate signaling pathway triggered in mammalian oocytes by sperm contact, binding, or fusion with the oocyte.

Intersecting roles of protein tyrosine kinase and calcium signaling during fertilization

The oocyte is a highly specialized cell that must respond to fertilization with a preprogrammed series of signal transduction events that establish a block to polyspermy, trigger resumption of the cell cycle and execution of a developmental program. The fertilization-induced calcium transient is a key signal that initiates the process of oocyte activation and studies over the last several years have examined the signaling pathways that act upstream and downstream of this calcium transient. Protein tyrosine kinase signaling was found to be an important component of the upstream pathways that stimulated calcium release at fertilization in oocytes from animals that fertilize externally, but a similar pathway has not been found in mammals which fertilize internally. The following review will examine the diversity of signaling in oocytes from marine invertebrates, amphibians, fish and mammals in an attempt to understand the basis for the observed differences. In addition to the pathways upstream of the fertilization-induced calcium transient, recent studies are beginning to unravel the role of protein tyrosine kinase signaling downstream of the calcium transient. The PYK2 kinase was found to respond to fertilization in the zebrafish system and seems to represent a novel component of the response of the oocyte to fertilization. The potential impact of impaired PTK signaling in oocyte quality will also be discussed.

Src protein kinases in mouse and rat oocytes and embryos

Meiosis of the mammalian oocytes is a specialized cell division, initiated during the female's embryonic life. It arrests at the germinal vesicle (GV) stage and resumes with GV breakdown, followed by segregation of the chromosomes and extrusion of the first polar body in an asymmetric cell division that concludes the first meiotic division, before arresting at metaphase of the second meiotic division (MII). Once fertilized, the oocyte exits from MII, extrudes the second polar body, and the developing zygote will continue dividing to create a blastocyst. Although the two processes of meiosis and mitosis have different developmental functions, it is believed that they share similar mechanisms. Src family kinases (SFKs) are nine non-receptor protein tyrosine kinases that regulate many key cellular functions including meiotic and mitotic cell cycles. In this review we discuss the involvement of SFKs in meiotic and mitotic cell cycle key processes as nuclear envelope breakdown, spindle stabilization, karyokinetic exit from metaphase, regulation of cortical actin, and cytokinetic cleavage furrow ingression.