Howard R. Petty

Integrins as promiscuous signal transduction devices

Recent studies suggest physical and functional interactions of glycosylphosphatidylinositol (GPI)-linked proteins (CD14, CD16b and CD87) with leukocyte beta 2 integrins. As discussed in this article, it now appears that beta 2 integrins relay proinflammatory information from GPI-anchored membrane receptors to the cytoplasm via exodomain interactions.

Function of the urokinase receptor (CD87) in neutrophil chemotaxis.

During recruitment, leukocytes respond to chemotaxins and traverse matrix barriers. Urokinase‐type plasminogen activator (uPA), bound to its receptor (uPAR; CD87) facilitates plasmin formation, which promotes matrix proteolysis. Polymorphonuclear leukocytes (PMNs) are critical to the inflammatory response and express both uPA and CD87. To determine whether uPA and CD87 are required for PMN Chemotaxis, PMNs were pretreated with an anti‐CD87 monoclonal antibody (mAb), a neutralizing anti‐uPA mAb, or uPA. PMN Chemotaxis was profoundly suppressed by the anti‐CD87 mAb but was unaffected by anti‐uPA mAb or uPA. The role CD87 plays in Chemotaxis may be related to its ability to associate with CR3. CD87/CR3 coupling can be disrupted by specific saccharides. The same saccharides that disrupt CD87/CR3 coupling (NADC, D‐mannose, and mannoside) inhibit PMN Chemotaxis. We conclude that CD87 plays a crucial role in PMN Chemotaxis in vitro that is independent of uPA enzyme activity but may be related to the ability of CD87 to interact with CR3.

Molecular Proximity of Kv1.3 Voltage-gated Potassium Channels and β1-Integrins on the Plasma Membrane of Melanoma Cells: Effects of Cell Adherence and Channel Blockers

Tumor cell membranes have multiple components that participate in the process of metastasis. The present study investigates the physical association of β1-integrins and Kv1.3 voltage-gated potassium channels in melanoma cell membranes using resonance energy transfer (RET) techniques. RET between donor-labeled anti–β1-integrin and acceptor-labeled anti-Kv1.3 channels was detected on LOX cells adherent to glass and fibronectin-coated coverslips. However, RET was not observed on LOX cells in suspension, indicating that molecular proximity of these membrane molecules is adherence-related. Several K+ channel blockers, including tetraethylammonium, 4-aminopyridine, and verapamil, inhibited RET between β1-integrins and Kv1.3 channels. However, the irrelevant K+ channel blocker apamin had no effect on RET between β1-integrins and Kv1.3 channels. Based on these findings, we speculate that the lateral association of Kv1.3 channels with β1-integrins contributes to the regulation of integrin function and that channel blockers might affect tumor cell behavior by influencing the assembly of supramolecular structures containing integrins.

Toll-like receptor 4 (TLR4) of retinal pigment epithelial cells participates in transmembrane signaling in response to photoreceptor outer segments.

Retinal pigment epithelial (RPE) cells mediate the recognition and clearance of effete photoreceptor outer segments (POS), a process central to the maintenance of normal vision. Given the emerging importance of Toll-like receptors (TLRs) in transmembrane signaling in response to invading pathogens as well as endogenous substances, we hypothesized that TLRs are associated with RPE cell management of POS. TLR4 clusters on human RPE cells in response to human, but not bovine, POS. However, TLR4 clustering could be inhibited by saturating concentrations of an inhibitory anti-TLR4 mAb. Furthermore, human POS binding to human RPE cells elicited transmembrane metabolic and calcium signals within RPE cells, which could be blocked by saturating doses of an inhibitory anti-TLR4 mAb. However, the heterologous combination of bovine POS and human RPE did not trigger these signals. The pattern recognition receptor CD36 collected at the POS–RPE cell interface for both homologous and heterologous samples, but human TLR4 only collected at the human POS–human RPE cell interface. Kinetic experiments of human POS binding to human RPE cells revealed that CD36 arrives at the POS–RPE interface followed by TLR4 accumulation within 2 min. Metabolic and calcium signals immediately follow. Similarly, the production of reactive oxygen metabolites (ROMs) was observed for the homologous human system, but not the heterologous bovine POS–human RPE cell system. As (a) the bovine POS/human RPE combination did not elicit TLR4 accumulation, RPE signaling, or ROM release, (b) TLR4 arrives at the POS–RPE cell interface just before signaling, (c) TLR4 blockade with an inhibitory anti-TLR4 mAb inhibited TLR4 clustering, signaling, and ROM release in the human POS–human RPE system, and (d) TLR4 demonstrates similar clustering and signaling responses to POS in confluent RPE monolayers, we suggest that TLR4 of RPE cells participates in transmembrane signaling events that contribute to the management of human POS.

A Model of the Oscillatory Metabolism of Activated Neutrophils

We present a two-compartment model to explain the oscillatory behavior observed experimentally in activated neutrophils. Our model is based mainly on the peroxidase-oxidase reaction catalyzed by myeloperoxidase with melatonin as a cofactor and NADPH oxidase, a major protein in the phagosome membrane of the leukocyte. The model predicts that after activation of a neutrophil, an increase in the activity of the hexose monophosphate shunt and the delivery of myeloperoxidase into the phagosome results in oscillations in oxygen and NAD(P)H concentration. The period of oscillation changes from >200 s to 10-30 s. The model is consistent with previously reported oscillations in cell metabolism and oxidant production. Key features and predictions of the model were confirmed experimentally. The requirement of the hexose monophosphate pathway for 10 s oscillations was verified using 6-aminonicotinamide and dexamethasone, which are inhibitors of glucose-6-phosphate dehydrogenase. The role of the NADPH oxidase in promoting oscillations was confirmed by dose-response studies of the effect of diphenylene iodonium, an inhibitor of the NADPH oxidase. Moreover, the model predicted an increase in the amplitude of NADPH oscillations in the presence of melatonin, which was confirmed experimentally. Successful computer modeling of complex chemical dynamics within cells and their chemical perturbation will enhance our ability to identify new antiinflammatory compounds.

Function of the lectin domain of mac-1/complement receptor type 3 (CD11b/CD18) in regulating neutrophil adhesion

A lectin function within CD11b mediates both cytotoxic priming of Mac-1/complement receptor type 3 (CR3) by β-glucan and the formation of transmembrane signaling complexes with GPI-anchored glycoproteins such as CD16b (FcγRIIIb). A requirement for GPI-anchored urokinase plasminogen activator receptor (uPAR; CD87) in neutrophil adhesion and diapedesis has been demonstrated with uPAR-knockout mice. In this study, neutrophil activation conditions generating high-affinity (H-AFN) or low-affinity (L-AFN) β2 integrin adhesion were explored. A role for the Mac-1/CR3 lectin domain and uPAR in mediating H-AFN or L-AFN adhesion was suggested by the inhibition of Mac-1/CR3-dependent adhesion to ICAM-1 or fibrinogen by β-glucan or anti-uPAR. The formation of uPAR complexes with Mac-1/CR3 activated for L-AFN adhesion was demonstrated by fluorescence resonance energy transfer. Conversely, Jurkat cell LFA-1 H-AFN-adhesion to ICAM-1 was not associated with uPAR/LFA-1 complexes, any requirement for GPI-anchored glycoproteins, or inhibition by β-glucan. A single CD11b lectin site for β-glucan and uPAR was suggested because the binding of either β-glucan or uPAR to Mac-1/CR3 selectively masked two CD11b epitopes adjacent to the transmembrane domain. Moreover, treatment with phosphatidylinositol-specific phospholipase C that removed GPI-anchored proteins increased CD11b-specific binding of 125I-labeled β-glucan by 3-fold and this was reversed with soluble recombinant uPAR. Conversely, neutrophil activation for generation of Mac-1/CR3/uPAR complexes inhibited CD11b-dependent binding of 125I-labeled β-glucan by 75%. These data indicate that the same lectin domain within CD11b regulates both the cytotoxic and adhesion functions of Mac-1/CR3.

Identification of Channels Promoting Calcium Spikes and Waves in HT1080 Tumor Cells Their Apparent Roles in Cell Motility and Invasion

Intracellular Ca2+ signals have been associated with cell polarization and locomotion. As cell motility underlies metastasis, we have sought to better characterize the Ca2+ signaling events in HT1080 fibrosarcoma cells. We have tested the hypothesis that low voltage-activated (LVA) and nonvoltage-gated (NVG) channels of HT1080 cells participate in dynamic Ca2+-signaling events leading to cell migration and invasion. Immunofluorescence microscopy has shown that HT1080 cells express LVA T-type Ca2+ channels uniformly about the cell periphery, whereas the transient receptor potential-1 (a NVG cation channel) protein appears as punctate spots about a cell’s periphery. HT1080 cells exhibit periodic intracellular Ca2+ spikes. High-speed imaging revealed that the Ca2+ spikes were composed of a single Ca2+ wave traveling unidirectionally about the periphery of the cytoplasm in a clockwise fashion (as viewed from basal to apical surfaces). The T-type Ca2+ channel blocker mibefradil inhibited Ca2+ spikes and waves on cells and, in parallel, inhibited cell motility and invasion in a dose-dependent manner. Similar changes were noted with the NVG cation channel blockers Gd3+ and carboxyamido-triazole. The combination of LVA and NVG blockers further reduced Matrigel invasiveness. However, the Ca2+ channel blockers nicardipine, SKF96365, diltiazem, and verapamil had no effect at appropriate doses. These results indicate that certain LVA and NVG channels regulate HT1080 cell motility. In addition to providing novel information regarding cancer cell motility, we suggest that it may be possible to design drugs that inhibit a key Ca2+ wave, thereby enhancing the efficacy of emerging therapeutic protocols.

Interactions of integrins with their partner proteins in leukocyte membranes

Integrins participate in many aspects of immunologic and inflammatory responses, especially those involving cell migration, adherence, and activation. Although leukocyte integrins such as complement receptor type 3 (CR3) are known to carry out certain functions without the intervention of other plasma membrane receptors, many plasma membrane proteins are now known to physically interact and functionally cooperate with integrins. Several of these interactions are highly dynamic within cell membranes; thus integrin-partner protein interactions change during certain physiological processes. This allows an extraordinary adaptability of the system to prime and promote proinflammatory signaling. Since our discovery of the CR 3-FcyRIIIB interaction, the plasma membrane protein repertoire of β1, β2, and β3 integrins has grown to include: FcγRIIA (CD32), uPAR (urokinase-type plasminogen activator receptor; CD87), CD14, voltage-gated K+ channels (Kvl.3), integrin-associated protein (IAP), CD98, tetraspans (TM4SF), insulin receptors, and PDGFβ receptors. In this article we will highlight certain features of this growing field of research, especially with regard to their relevance in immunology and inflammation.

Receptor-receptor interactions of complement receptor type 3 in neutrophil membranes.

The leukocyte integrin CR3 (GDllb/CD18) is known to participate in a variety of cell functions. Recent studies have indicated that CR3 may communicate with other plasma membrane receptors to carry out several cell functions. In this review we discuss these potential receptor‐receptor interactions of CR3 and present a unifying model of GR3s diverse functions.

Apparent role of traveling metabolic waves in oxidant release by living neutrophils

Cell metabolism self-organizes into two types of dissipative structures: chemical oscillations and traveling metabolic waves. In the present study we test the hypothesis that traveling NAD(P)H waves within neutrophils are associated spatially and temporally with the release of reactive oxygen metabolites (ROMs). Using high-speed optical microscopy and taking advantage of the autofluorescence of NAD(P)H, we have observed the propagation of NAD(P)H waves within cells. When NAD(P)H waves reach the lamellipodium of morphologically polarized neutrophils, a diffusing plume of superoxide is released as evidenced by the conversion of hydroethidine in the extracellular environment to ethidium bromide. Parallel results were obtained by using high-speed emission microspectrophotometry. These experiments indicate that the spatial and temporal properties of NAD(P)H waves are transformed into ROM pulses in the extracellular environment. Propagating NAD(P)H waves allow neutrophils to specifically deliver substrate to the lamellipodium at high concentrations, thus facilitating the local and periodic release of ROMs in the direction of cell movement and/or a target.