SUMMARY The mechanistic underpinnings of metastatic dormancy and reactivation are poorly understood. A gain-of-function cDNA screen reveals that Coco, a secreted antagonist of TGF-β ligands, induces dormant breast cancer cells to undergo reactivation in the lung. Mechanistic studies indicate that Coco exerts this effect by blocking lung-derived BMP ligands. Whereas Coco enhances the manifestation of traits associated with cancer stem cells, BMP signaling suppresses it. Coco induces a discrete gene expression signature, which is strongly associated with metastatic relapse to the lung but not to the bone or brain in patients. Experiments in mouse models suggest that these latter organs contain niches devoid of bioactive BMP. These findings reveal that metastasis-initiating cells need to overcome organ-specific anti-metastatic signals in order to undergo reactivation.
We propose a limiting component hypothesis, in which the volume of the cell sets centrosome size by limiting the total amount of centrosome components. This idea could be a general mechanism for setting the size of intracellular organelles during development.
Centrosomes are highly dynamic, spherical organelles without a membrane. Their physical nature and their assembly are not understood. Using the concept of phase separation, we propose a theoretical description of centrosomes as liquid droplets. In our model, centrosome material occurs in a form soluble in the cytosol and a form that tends to undergo phase separation from the cytosol. We show that an autocatalytic chemical transition between these forms accounts for the temporal evolution observed in experiments. Interestingly, the nucleation of centrosomes can be controlled by an enzymatic activity of the centrioles, which are present at the core of all centrosomes. This nonequilibrium feature also allows for multiple stable centrosomes, a situation that is unstable in equilibrium phase separation. Our theory explains the growth dynamics of centrosomes for all cell sizes down to the eight-cell stage of the Caenorhabditis elegans embryo, and it also accounts for data acquired in experiments with aberrant numbers of centrosomes and altered cell volumes. Furthermore, the model can describe unequal centrosome sizes observed in cells with perturbed centrioles. We also propose an interpretation of the molecular details of the involved proteins in the case of C. elegans. Our example suggests a general picture of the organization of membraneless organelles.
Caveolin-1 (CAV1), a highly conserved membrane-associated protein, is a putative regulator of cellular transformation. CAV1 is localized in the plasmalemma, secretory vesicles, Golgi, mitochondria, and endoplasmic reticulum membrane and associates with the microtubule cytoskeleton. Taxanes such as paclitaxel (Taxol) are potent anti-tumor agents that repress the dynamic instability of microtubules and arrest cells in the G 2 /M phase. Src phosphorylation of Tyr-14 on CAV1 regulates its cellular localization and function. We report that phosphorylation of CAV1 on Tyr-14 regulates paclitaxel-mediated apoptosis in MCF-7 breast cancer cells. Befitting its role as a multitasking molecule, we show that CAV1 sensitizes cells to apoptosis by regulating cell cycle progression and activation of the apoptotic signaling molecules BCL2, p53, and p21. We demonstrate that phosphorylated CAV1 triggers apoptosis by inactivating BCL2 and increasing mitochondrial permeability more efficiently than non-phosphorylated CAV1. Furthermore, expression of p21, which correlates with taxane sensitivity, is regulated by CAV1 phosphorylation in a p53-dependent manner. Collectively, our findings underscore the importance of CAV1 phosphorylation in apoptosis and suggest that events that negate CAV1 tyrosine phosphorylation may contribute to anti-microtubule drug resistance. Caveolin-1 (CAV1)2 is a 21-24-kDa protein and the prototype of a family of integral membrane proteins that associate with specific cholesterol-and sphingolipid-rich domains to form the structural foundation of membrane invaginations called caveolae. Caveolae act as sites of signal transduction in various cell types (1). CAV1 is thought to regulate the activity of proteins such as Src kinases, epidermal growth factor tyrosine kinase, Her2/neu (ErbB2) kinase, ERK (extracellular signal-regulated kinase), H-Ras, endothelial nitric-oxide synthase, and G proteins (1, 2) involved in survival pathways. In human breast tumors, CAV1 levels inversely correlate with tumor size (3), and CAV1 expression reduces the growth of mouse mammary tumors and their spontaneous metastasis to lung and bone (4). However, in breast cancer cell culture models, CAV1 is downregulated in non-invasive human breast cancer cells but upregulated in cells with an invasive phenotype (5-7).Taxanes are potent anti-tumor agents that function by binding to the  subunits of tubulin and repressing the dynamic instability of spindles (8, 9), activities that lead to cell cycle arrest in the G 2 /M phase (10). Taxanes such as paclitaxel (Taxol) or docetaxel (Taxotere) are routinely used in the firstline treatment of metastatic breast, lung, ovarian, and digestive cancers (11). In primary breast cancer, inclusion of taxane in adjuvant chemotherapy reduces the relative risk of recurrence and improves overall survival (12). Acquired resistance through cellular adaptations or mutations in neoplastic cells remains a major problem in chemotherapy. Although taxanes are substrates for ABC transporters, other resistance mechanis...
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