Phil Oh

Subtractive proteomic mapping of the endothelial surface in lung and solid tumours for tissue-specific therapy.

The molecular complexity of tissues and the inaccessibility of most cells within a tissue limit the discovery of key targets for tissue-specific delivery of therapeutic and imaging agents in vivo. Here, we describe a hypothesis-driven, systems biology approach to identifying a small subset of proteins induced at the tissue–blood interface that are inherently accessible to antibodies injected intravenously. We use subcellular fractionation, subtractive proteomics and bioinformatics to identify endothelial cell surface proteins exhibiting restricted tissue distribution and apparent tissue modulation. Expression profiling and γ-scintigraphic imaging with antibodies establishes two of these proteins, aminopeptidase-P and annexin A1, as selective in vivo targets for antibodies in lungs and solid tumours, respectively. Radio-immunotherapy to annexin A1 destroys tumours and increases animal survival. This analytical strategy can map tissue- and disease-specific expression of endothelial cell surface proteins to uncover novel accessible targets useful for imaging and therapy.

Tumor cell growth inhibition by caveolin re-expression in human breast cancer cells.

Cancer development is a multistage process that results from the step-wise acquisition of somatic alterations in diverse genes. Recent studies indicate that caveolin-1 expression correlates with the level of oncogenic transformation in NIH3T3 cells, suggesting that caveolin in caveolae may regulate normal cell proliferation. In order to better understand potential functions of caveolin-1 in cancer development, we have studied expression levels of caveolin-1 in human breast cancer cells, and have found that caveolin expression is significantly reduced in human breast cancer cells compared with their normal mammary epithelial counterparts. When the caveolin cDNA linked to the CMV promoter is transfected into human mammary cancer cells having no detectable endogenous caveolin, overexpression of caveolin-1 resulted in substantial growth inhibition, as seen by the 50% decrease in growth rate and by ∼l5-fold reduction in colony formation in soft agar. In addition, characterization of caveolin-1 expression during cell cycle progression indicates that expression of α-caveolin-1 is regulated during cell cycle. Furthermore p53-deficient cells showed a loss in caveolin expression. In summary, the overall expression patterns, its ability to inhibit tumor growth in culture, its regulation during the cell cycle, and the loss of expression in p53-deficient cells all are consistent with an important growth regulating function for caveolin-1 in normal human mammary cells, that needs to be repressed in oncogenic transformation and tumor cell growth.

Direct proteomic mapping of the lung microvascular endothelial cell surface in vivo and in cell culture

Endothelial cells can function differently in vitro and in vivo; however, the degree of microenvironmental modulation in vivo remains unknown at the molecular level largely because of analytical limitations. We use multidimensional protein identification technology (MudPIT) to identify 450 proteins (with three or more spectra) in luminal endothelial cell plasma membranes isolated from rat lungs and from cultured rat lung microvascular endothelial cells. Forty-one percent of proteins expressed in vivo are not detected in vitro. Statistical analysis measuring reproducibility reveals that seven to ten MudPIT measurements are necessary to achieve ≥95% confidence of analytical completeness with current ion trap equipment. Large-scale mapping of the proteome of vascular endothelial cell surface in vivo, as demonstrated here, is advisable because distinct protein expression is apparently regulated by the tissue microenvironment that cannot yet be duplicated in standard cell culture.

Label-free, normalized quantification of complex mass spectrometry data for proteomic analysis

Replicate mass spectrometry (MS) measurements and the use of multiple analytical methods can greatly expand the comprehensiveness of shotgun proteomic profiling of biological samples. However, the inherent biases and variations in such data create computational and statistical challenges for quantitative comparative analysis. We developed and tested a normalized, label-free quantitative method termed the normalized spectral index (SIN), which combines three MS abundance features: peptide count, spectral count and fragment-ion (tandem MS or MS/MS) intensity. SIN largely eliminated variances between replicate MS measurements, permitting quantitative reproducibility and highly significant quantification of thousands of proteins detected in replicate MS measurements of the same and distinct samples. It accurately predicts protein abundance more often than the five other methods we tested. Comparative immunoblotting and densitometry further validate our method. Comparative quantification of complex data sets from multiple shotgun proteomics measurements is relevant for systems biology and biomarker discovery.

Targeting endothelium and its dynamic caveolae for tissue-specific transcytosis in vivo: A pathway to overcome cell barriers to drug and gene delivery

Site-directed pharmacodelivery is a desirable but elusive goal. Endothelium and epithelium create formidable barriers to endogenous molecules as well as targeted therapies in vivo. Caveolae provide a possible, yet unproven, transcellular pathway to overcome such barriers. By using an antibody- and subfractionation-based strategy, we generated a monoclonal antibody specific for lung caveolae (TX3.833) that targets rat lungs after i.v. injection (up to 89% of dose in 30 min). Unlike control antibodies (nonbinding or to lipid rafts), TX3.833 targets lung caveolae that bud to form free vesicles for selective and quantal transendothelial transport to underlying tissue cells in vivo. Rapid sequential transcytosis can occur to the alveolar air space via epithelial caveolae. Conjugation to TX3.833 increases drug delivery to the lung up to 172-fold and achieves rapid, localized bioefficacy. We conclude that: (i) molecular heterogeneity of the endothelium and its caveolae permits vascular targeting to achieve theoretical expectations of tissue-specific delivery and bioefficacy; (ii) caveolae can mediate selective transcytosis in vivo; and (iii) targeting caveolae may provide a tissue-specific pathway for overcoming key cell barriers to many drug and gene therapies in vivo.

Calcium-dependent membrane association sensitizes soluble guanylyl cyclase to nitric oxide

Nitric oxide (NO) is a ubiquitous, cell-permeable intercellular messenger. The current concept assumes that NO diffuses freely through the plasma membrane into the cytoplasm of a target cell, where it activates its cytosolic receptor enzyme, soluble guanylyl cyclase (sGC). Recent evidence, however, suggests that cellular membranes are not only the predominant site of calcium-dependent NO synthesis, but also the site of its distribution and binding. Here we extend this concept to NO signalling to show that active sGC is partially associated with the plasma membrane in a state of enhanced NO sensitivity. After cellular activation, sGC further translocates to the membrane fraction in human platelets and associates with the NO-synthase-containing caveolar fraction in rat lung endothelial cells, in a manner that is dependent on the concentration of intracellular calcium. Our data suggest that the entire NO signalling pathway is more spatially confined than previously assumed and that sGC dynamically translocates to the plasma membrane, where it is sensitized to NO.

Subcellular localization of intracellular protein tyrosine phosphatases in T cells.

A high protein tyrosine phosphatase (PTPase) activity is required to maintain circulating T lymphocytes in a resting phenotype, and to limit the initiation of T cell activation. We report that 15 of the currently known 24 intracellular PTPases are expressed in T cells, namely HePTP, TCPTP, SHP1, SHP2, PEP, PTP‐PEST, PTP‐MEG2, PTEN, PTPH1, PTP‐MEG1, PTP36, PTP‐BAS, LMPTP, PRL‐1 and OV‐1. Most were found in the cytosol and many were enriched at the plasma membrane. Only TCPTP and PTP‐MEG2 had subcellular localizations that essentially excludes them from a direct role in early T cell antigen receptor signaling events. Overexpression of 6 of the PTPases reduced IL‐2 gene activation, 3 of them thereby identified as novel candidates for negative regulators of TCR signaling. Our findings expand the repertoire of PTPases that should be considered for a regulatory role in T cell activation.

Transient Mechanoactivation of Neutral Sphingomyelinase in Caveolae to Generate Ceramide

The vascular endothelium acutely autoregulates blood flow in vivo in part through unknown mechanosensing mechanisms. Here, we report the discovery of a new acute mechanotransduction pathway. Hemodynamic stressors from increased vascular flow and pressure in situ rapidly and transiently induce the activity of neutral sphingomyelinase but not that acid sphingomyelinase in a time- and flow rate-dependent manner, followed by the generation of ceramides. This acute mechanoactivation occurs directly at the luminal endothelial cell surface primarily in caveolae enriched in sphingomyelin and neutral sphingomyelinase, but not acid sphingomyelinase. Scyphostatin, which specifically blocks neutral but not acid sphingomyelinase, inhibits mechano-induced neutral sphingomyelinase activity as well as downstream activation of extracellular signal-regulated kinase 1 and 2 (ERK1 and ERK2) by increased flow in situ. We postulate a novel physiological function for neutral sphingomyelinase as a new mechanosensor initiating the ERK cascade and possibly other mechanotransduction pathways.

Enhancing Identifications of Lipid-embedded Proteins by Mass Spectrometry for Improved Mapping of Endothelial Plasma Membranes in Vivo

Lipid membranes structurally define the outer surface and internal organelles of cells. The multitude of proteins embedded in lipid bilayers are clearly functionally important, yet they remain poorly defined. Even today, integral membrane proteins represent a special challenge for current large scale shotgun proteomics methods. Here we used endothelial cell plasma membranes isolated directly from lung tissue to test the effectiveness of four different mass spectrometry-based methods, each with multiple replicate measurements, to identify membrane proteins. In doing so, we substantially expanded this membranome to 1,833 proteins, including >500 lipid-embedded proteins. The best method combined SDS-PAGE prefractionation with trypsin digestion of gel slices to generate peptides for seamless and continuous two-dimensional LC/MS/MS analysis. This three-dimensional separation method outperformed current widely used two-dimensional methods by significantly enhancing protein identifications including single and multiple pass transmembrane proteins; >30% are lipid-embedded proteins. It also profoundly improved protein coverage, sensitivity, and dynamic range of detection and substantially reduced the amount of sample and the number of replicate mass spectrometry measurements required to achieve 95% analytical completeness. Such expansion in comprehensiveness requires a trade-off in heavy instrument time but bodes well for future advancements in truly defining the ever important membranome with its potential in network-based systems analysis and the discovery of disease biomarkers and therapeutic targets. This analytical strategy can be applied to other subcellular fractions and should extend the comprehensiveness of many future organellar proteomics pursuits.

In vivo proteomic imaging analysis of caveolae reveals pumping system to penetrate solid tumors

Technologies are needed to map and image biological barriers in vivo that limit solid tumor delivery and, ultimately, the effectiveness of imaging and therapeutic agents. Here we integrate proteomic and imaging analyses of caveolae at the blood-tumor interface to discover an active transendothelial portal to infiltrate tumors. A post-translationally modified form of annexin A1 (AnnA1) is selectively concentrated in human and rodent tumor caveolae. To follow trafficking, we generated a specific AnnA1 antibody that targets caveolae in the tumor endothelium. Intravital microscopy of caveolae-immunotargeted fluorophores even at low intravenous doses showed rapid and robust pumping across the endothelium to enter mammary, prostate and lung tumors. Within 1 h, the fluorescence signal concentrated throughout tumors to exceed the peak levels in blood. This transvascular pumping required the expression of caveolin 1 and annexin A1. Tumor uptake with other antibodies were >100-fold less. This proteomic imaging strategy reveals a unique target, antibody and caveolae pumping system for solid tumor penetration.