Abstract A08: PI3Kbeta regulates beta-1 integrin signaling in invadopodia through formation of PI(3,4)P2

The PI3Kbeta isoform of PI 3-kinase, which is activated downstream from both RTKs and GPCRs, is an important regulator of invadopodia formation. We have previously reported that in MDA-MB-231 breast cancer cells, replacement of endogenous PI3Kbeta with kinase dead (KD) or GPCR-uncoupled (KK-DD) mutants leads to defects in invadopodia-mediated gelatin degradation. We have now studied the mechanism by which loss of PI3Kbeta signaling affects invadopodia. While the number of invadopodia precursors is unaffected by mutation of PI3Kbeta, formation of mature invadopodia is reduced in cells expressing KK-DD and KD PI3Kbeta mutants. Previous studies have demonstrated a role for beta-1 integrins in invadopodia maturation, and PI3Kbeta has been implicated in integrin signaling in platelets. We therefore tested whether mutation of PI3Kbeta would affect integrin signaling in breast cancer cells. In both haptotaxis and cell spreading assays on collagen I, cells expressing KD or KK-DD PI3Kbeta showed a significant impairment. Haptotaxis and cell spreading was also inhibited by TGX221 (PI3Kbeta inhibitor) and pertussis toxin (PTX), but not by specific inhibitors of other Class I PI3Ks. To distinguish whether PI3Kbeta was acting on integrins by inside-out versus outside-in mechanisms, we treated cells with the activating beta-1 integrin antibody TS2/16, which bypasses inside-out regulation of integrin binding. TS2/16 increased cell spreading on gelatin, but this was blocked by TGX221, suggesting that PI3Kbeta acts downstream of beta-1 integrins. Importantly, haptotaxis and cell spreading were not affected by an Akt inhibitor, but were blocked by an inhibitor of SHIP2, which hydrolyzes PIP3 to produce PI[3,4]P2. These data suggest that integrin signaling requires the coupling of Gbeta/gamma-mediated activation of PI3Kbeta to the production of PI[3,4]P2. Finally, to determine whether PI3Kbeta was involved in beta-1 integrin signaling in invadopodia, we plated starved cells on high-density fibrillar collagen (HDFC), which drives invadopodia formation through beta-1 integrin activation. Collagen degradation on HDFC was blocked by inhibition of PI3Kbeta or SHIP2, but not PI3Kalpha. Moreover, recruitment of the PI(3,4)P2 binding protein lamellipodin to invadopodia was reduced by inhibition of PI3Kbeta. In summary, beta-1 integrin signaling in the context of invadopodia maturation requires signaling through PI3Kbeta. We propose that heterotrimeric G-protein activation of PI3Kbeta is necessary for formation of PI(3,4)P2 in invadopodia, which is required for invadopodial maturation.

Citation Format: Zahra Erami, Anne R. Bresnick, Jonathan M. Backer. PI3Kbeta regulates beta-1 integrin signaling in invadopodia through formation of PI(3,4)P2 [abstract]. In: Proceedings of the AACR Special Conference on Targeting PI3K/mTOR Signaling; 2018 Nov 30-Dec 8; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(10_Suppl):Abstract nr A08.

PI3Kβ is selectively required for growth factor-stimulated macropinocytosis

Macropinocytosis is an actin-dependent but clathrin-independent endocytic process by which cells nonselectively take up large aliquots of extracellular material. Macropinocytosis is used for immune surveillance by dendritic cells, as a route of infection by viruses and protozoa, and as a nutrient uptake pathway in tumor cells. In this study, we explore the role of class I phosphoinositide 3-kinases (PI3Ks) during ligand-stimulated macropinocytosis. We find that macropinocytosis in response to receptor tyrosine kinase activation is strikingly dependent on a single class I PI3K isoform, namely PI3Kβ (containing the p110β catalytic subunit encoded by PIK3CB). Loss of PI3Kβ expression or activity blocks macropinocytosis at early steps, before the formation of circular dorsal ruffles, but also plays a role in later steps, downstream from Rac1 activation. PI3Kβ is also required for the elevated levels of constitutive macropinocytosis found in tumor cells that are defective for the PTEN tumor suppressor. Our data shed new light on PI3K signaling during macropinocytosis, and suggest new therapeutic uses for pharmacological inhibitors of PI3Kβ.

PI3Kβ links integrin activation and PI(3,4)P2 production during invadopodial maturation

The invasion of tumor cells from the primary tumor is mediated by invadopodia, actin-rich protrusive organelles that secrete matrix metalloproteases and degrade the extracellular matrix. This coupling between protrusive activity and matrix degradation facilitates tumor invasion. We previously reported that the PI3Kβ isoform of PI 3-kinase, which is regulated by both receptor tyrosine kinases and G protein-coupled receptors, is required for invasion and gelatin degradation in breast cancer cells. We have now defined the mechanism by which PI3Kβ regulates invadopodia. We find that PI3Kβ is specifically activated downstream from integrins, and is required for integrin-stimulated spreading and haptotaxis as well as integrin-stimulated invadopodia formation. Surprisingly, these integrin-stimulated and PI3Kβ-dependent responses require the production of PI(3,4)P2 by the phosphoinositide 5'-phosphatase SHIP2. Thus, integrin activation of PI3Kβ is coupled to the SHIP2-dependent production of PI(3,4)P2, which regulates the recruitment of PH domain-containing scaffolds such as lamellipodin to invadopodia. These findings provide novel mechanistic insight into the role of PI3Kβ in the regulation of invadopodia in breast cancer cells.

Abstract 3165: PI 3-kinase-beta regulates invadopodia maturation and beta-1 integrin signaling

The invasion of tumors cells during metastasis is mediated by invadopodia, actin rich protrusive organelles that secrete matrix metalloproteases and mediate matrix degradation. The PI3K-beta isoform of PI 3-kinase, which is regulated by both receptor tyrosine kinases and GPCRs, is an important regulator of invadopodia formation. We have previously reported that in MDA-MB-231 breast cancer cells, replacement of endogenous PI3K-beta with kinase dead (p110-betaK799R; KD) or GPCR-uncoupled (p110-beta526KK-DD; KK-DD) mutants leads to defects in gelatin degradation. We have now studied the mechanism by which loss of PI3K-beta signaling affects invadopodia formation and matrix degradation. While the number of invadopodia precursors (cortactin-Tks5 positive, degradation negative) is unaffected by mutation of PI3K-beta, formation of mature invadopodia (cortactin-Tks5 positive, degradation positive) is reduced in cells expressing KK-DD and KD PI3K-beta. Previous studies have demonstrated a role for beta-1 integrins in invadopodia maturation, and PI3K-beta has been implicated in integrin signaling in platelets. We therefore tested whether mutation of PI3K-beta would affect integrin signaling in breast cancer cells. In a haptotaxis assay, MDA-MB-231 cells expressing KD or KK-DD PI3K-beta showed a significant decrease in migration. Consistent with these data, cell spreading on collagen I was significantly impaired in cells expressing KD or KK-DD PI3K-beta, and was inhibited by TGX221 (PI3K-beta inhibitor) and LY294002 (pan-PI3K inhibitor), but not by specific inhibitors of other Class I PI3Ks (alpha, delta and gamma). Spreading was also impaired in cells treated with pertussis toxin, which blocks GPCR activation of PI3K-beta. To distinguish whether PI3K-beta was acting on integrins by inside-out versus outside-in mechanisms, we treated cells with the activating beta-1 integrin antibody TS2/16, which bypasses inside-out regulation of integrin binding. TS2/16 increased cell spreading on collagen I, but TS2/16-stimulated spreading was blocked by treatment of cells with TGX221, suggesting that PI3K-beta acts downstream of beta-1 integrins. Finally, to determine whether PI3K-beta was involved in beta-1 integrin signaling in invadopodia, we plated cells on high density fibrillar collagen (HDFC), which enhances invadopodia formation through beta-1 integrin activation. Invadopodia formation on HDFC was blocked by treatment of cells with TGX221. In summary, beta-1 integrin signaling in the context of invadopodia requires GPCR signaling to PI3K-beta. We propose that GPCR activation of PI3K-beta is a critical downstream component of beta-1 integrin signaling in invadopodia.

Citation Format: Zahra Erami, Anne R. Bresnick, Jonathan M. Backer. PI 3-kinase-beta regulates invadopodia maturation and beta-1 integrin signaling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3165.

In-Depth Proteomics Identifies a Role for Autophagy in Controlling Reactive Oxygen Species Mediated Endothelial Permeability

Endothelial cells (ECs) form the inner layer of blood vessels and physically separate the blood from the surrounding tissue. To support tissues with nutrients and oxygen, the endothelial monolayer is semipermeable. When EC permeability is altered, blood vessels are not functional, and this is associated with disease. A comprehensive knowledge of the mechanisms regulating EC permeability is key in developing strategies to target this mechanism in pathologies. Here we have used an in vitro model of human umbilical vein endothelial cells mimicking the formation of a physiologically permeable vessel and performed time-resolved in-depth molecular profiling using stable isotope labeling by amino acids in cell culture mass spectrometry (MS)-proteomics. Autophagy is induced when ECs are assembled into a physiologically permeable monolayer. By using siRNA and drug treatment to block autophagy in combination with functional assays and MS proteomics, we show that ECs require autophagy flux to maintain intracellular reactive oxygen species levels, and this is required to maintain the physiological permeability of the cells.

Intravital FRAP Imaging using an E-cadherin-GFP Mouse Reveals Disease- and Drug-Dependent Dynamic Regulation of Cell-Cell Junctions in Live Tissue

E-cadherin-mediated cell-cell junctions play a prominent role in maintaining the epithelial architecture. The disruption or deregulation of these adhesions in cancer can lead to the collapse of tumor epithelia that precedes invasion and subsequent metastasis. Here we generated an E-cadherin-GFP mouse that enables intravital photobleaching and quantification of E-cadherin mobility in live tissue without affecting normal biology. We demonstrate the broad applications of this mouse by examining E-cadherin regulation in multiple tissues, including mammary, brain, liver, and kidney tissue, while specifically monitoring E-cadherin mobility during disease progression in the pancreas. We assess E-cadherin stability in native pancreatic tissue upon genetic manipulation involving Kras and p53 or in response to anti-invasive drug treatment and gain insights into the dynamic remodeling of E-cadherin during in situ cancer progression. FRAP in the E-cadherin-GFP mouse, therefore, promises to be a valuable tool to fundamentally expand our understanding of E-cadherin-mediated events in native microenvironments.

Abstract B17: In-depth proteomics unveils fatty acid oxidation role in controlling vascular permeability

Endothelial cells (ECs) play a key role in maintaining vascular functionality. Alterations in vessel stability and permeability are hallmarks of tumor angiogenesis and may affect the efficacy of anti-cancer therapy. Modulating EC metabolism has emerged as a promising strategy to target angiogenesis in pathological conditions, but so far, little is known about the role of EC metabolism in the regulation of vessel stability and permeability.

In this work we took advantage of a simplified in-vitro model of angiogenesis, where HUVECs (human umbilical vein endothelial cells) form a vascular-like network on a tridimensional matrix, to elucidate the role of EC metabolism during this process.

By integrating high resolution proteomic data with a human genome scale metabolic model we built up the first predictive model of metabolic fluxes occurring in ECs forming a complex network and identified increased fluxes for reactions involving fatty acid oxidation (FAO) enzymes when ECs are assembled into a fully formed network. Upon inhibition of CPT1, the FAO rate-limiting enzyme, we disrupted the network and reduced cellular ATP levels and oxygen consumption, which were restored by replenishing the tricarboxylic acid cycle (TCAc). Remarkably, phosphoproteomic changes measured upon CPT1A inhibition evoked those triggered by thrombin, a potent inducer of EC permeability through calcium signaling. Indeed, acute CPT1A inhibition increased EC permeability in-vitro and leakage of fully formed blood vessel in-vivo, which were restored by replenishing the TCAc or inhibiting calcium influx.

FAO emerges as central regulator of blood vessel stability, revealing the possibility of targeting FAO to interfere with tumor vessel permeability.

Citation Format: Francesca Patella, Zachary T. Schug, Erez Persi, Lisa J. Neilson, Zahra Erami, Daniele Avanzato, Federica Maione, Juan R. Hernandez-Fernaud, Gillian Mackay, Liang Zheng, Steven Reid, Christian Frezza, Enrico Giraudo, Alessandra Fiorio Pla, Kurt Anderson, Eytan Ruppin, Eyal Gottlieb, Sara Zanivan. In-depth proteomics unveils fatty acid oxidation role in controlling vascular permeability. [abstract]. In: Proceedings of the AACR Special Conference: Tumor Angiogenesis and Vascular Normalization: Bench to Bedside to Biomarkers; Mar 5-8, 2015; Orlando, FL. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl):Abstract nr B17.

There are four dynamically and functionally distinct populations of E-cadherin in cell junctions

E-cadherin is a trans-membrane tumor suppressor responsible for epithelial cell adhesion. E-cadherin forms adhesive clusters through combined extra-cellular cis- and trans-interactions and intracellular interaction with the actin cytoskeleton. Here we identify four populations of E-cadherin within cell junctions based on the molecular interactions which determine their mobility and adhesive properties. Adhesive and non-adhesive populations of E-cadherin each consist of mobile and immobile fractions. Up to half of the E-cadherin immobilized in cell junctions is non-adhesive. Incorporation of E-cadherin into functional adhesions require all three adhesive interactions, with deletion of any one resulting in loss of effective cell-cell adhesion. Interestingly, the only interaction which could independently slow the diffusion of E-cadherin was the tail-mediated intra-cellular interaction. The adhesive and non-adhesive mobile fractions of E-cadherin can be distinguished by their sensitivity to chemical cross-linking with adhesive clusters. Our data define the size, mobility, and adhesive properties of four distinct populations of E-cadherin within cell junctions, and support association with the actin cytoskeleton as the first step in adhesion formation.

Proteomics-based metabolic modeling reveals that fatty acid oxidation (FAO) controls endothelial cell (EC) permeability

Endothelial cells (ECs) play a key role to maintain the functionality of blood vessels. Altered EC permeability causes severe impairment in vessel stability and is a hallmark of pathologies such as cancer and thrombosis. Integrating label-free quantitative proteomics data into genome-wide metabolic modeling, we built up a model that predicts the metabolic fluxes in ECs when cultured on a tridimensional matrix and organize into a vascular-like network. We discovered how fatty acid oxidation increases when ECs are assembled into a fully formed network that can be disrupted by inhibiting CPT1A, the fatty acid oxidation rate-limiting enzyme. Acute CPT1A inhibition reduces cellular ATP levels and oxygen consumption, which are restored by replenishing the tricarboxylic acid cycle. Remarkably, global phosphoproteomic changes measured upon acute CPT1A inhibition pinpointed altered calcium signaling. Indeed, CPT1A inhibition increases intracellular calcium oscillations. Finally, inhibiting CPT1A induces hyperpermeability in vitro and leakage of blood vessel in vivo, which were restored blocking calcium influx or replenishing the tricarboxylic acid cycle. Fatty acid oxidation emerges as central regulator of endothelial functions and blood vessel stability and druggable pathway to control pathological vascular permeability.

Organotypic collagen I assay: a malleable platform to assess cell behaviour in a 3-dimensional context

Cell migration is fundamental to many aspects of biology, including development, wound healing, the cellular responses of the immune system, and metastasis of tumor cells. Migration has been studied on glass coverslips in order to make cellular dynamics amenable to investigation by light microscopy. However, it has become clear that many aspects of cell migration depend on features of the local environment including its elasticity, protein composition, and pore size, which are not faithfully represented by rigid two dimensional substrates such as glass and plastic ¹. Furthermore, interaction with other cell types, including stromal fibroblasts ² and immune cells ³, has been shown to play a critical role in promoting the invasion of cancer cells. Investigation at the molecular level has increasingly shown that molecular dynamics, including response to drug treatment, of identical cells are significantly different when compared in vitro and in vivo⁴.

Ideally, it would be best to study cell migration in its naturally occurring context in living organisms, however this is not always possible. Intermediate tissue culture systems, such as cell derived matrix, matrigel, organotypic culture (described here) tissue explants, organoids, and xenografts, are therefore important experimental intermediates. These systems approximate certain aspects of an in vivo environment but are more amenable to experimental manipulation such as use of stably transfected cell lines, drug treatment regimes, long term and high-resolution imaging. Such intermediate systems are especially useful as proving grounds to validate probes and establish parameters required to image the dynamic response of cells and fluorescent reporters prior to undertaking imaging in vivo ⁵. As such, they can serve an important role in reducing the need for experiments on living animals.