A new aspect of cell membrane structure is presented, based on the dynamic clustering of sphingolipids and cholesterol to form rafts that move within the fluid bilayer. It is proposed that these rafts function as platforms for the attachment of proteins when membranes are moved around inside the cell and during signal transduction.

Functional rafts in cell membranes

Recent discoveries of endogenous negative regulators of angiogenesis, thrombospondin, angiostatin and glioma-derived angiogenesis inhibitory factor, all associated with neovascularized tumours, suggest a new paradigm of tumorigenesis. It is now helpful to think of the switch to the angiogenic phenotype as a net balance of positive and negative regulators of blood vessel growth. The extent to which the negative regulators are decreased during this switch may dictate whether a primary tumour grows rapidly or slowly and whether metastases grow at all.

Angiogenesis in cancer, vascular, rheumatoid and other disease

A fluid mosaic model is presented for the gross organization and structure of the proteins and lipids of biological membranes. The model is consistent with the restrictions imposed by thermodynamics. In this model, the proteins that are integral to the membrane are a heterogeneous set of globular molecules, each arranged in an amphipathic structure, that is, with the ionic and highly polar groups protruding from the membrane into the aqueous phase, and the nonpolar groups largely buried in the hydrophobic interior of the membrane. These globular molecules are partially embedded in a matrix of phospholipid. The bulk of the phospholipid is organized as a discontinuous, fluid bilayer, although a small fraction of the lipid may interact specifically with the membrane proteins. The fluid mosaic structure is therefore formally analogous to a two-dimensional oriented solution of integral proteins (or lipoproteins) in the viscous phospholipid bilayer solvent. Recent experiments with a wide variety of techniqes and several different membrane systems are described, all of which abet consistent with, and add much detail to, the fluid mosaic model. It therefore seems appropriate to suggest possible mechanisms for various membrane functions and membrane-mediated phenomena in the light of the model. As examples, experimentally testable mechanisms are suggested for cell surface changes in malignant transformation, and for cooperative effects exhibited in the interactions of membranes with some specific ligands. Note added in proof: Since this article was written, we have obtained electron microscopic evidence (69) that the concanavalin A binding sites on the membranes of SV40 virus-transformed mouse fibroblasts (3T3 cells) are more clustered than the sites on the membranes of normal cells, as predicted by the hypothesis represented in Fig. 7B. T-here has also appeared a study by Taylor et al. (70) showing the remarkable effects produced on lymphocytes by the addition of antibodies directed to their surface immunoglobulin molecules. The antibodies induce a redistribution and pinocytosis of these surface immunoglobulins, so that within about 30 minutes at 37 degrees C the surface immunoglobulins are completely swept out of the membrane. These effects do not occur, however, if the bivalent antibodies are replaced by their univalent Fab fragments or if the antibody experiments are carried out at 0 degrees C instead of 37 degrees C. These and related results strongly indicate that the bivalent antibodies produce an aggregation of the surface immunoglobulin molecules in the plane of the membrane, which can occur only if the immunoglobulin molecules are free to diffuse in the membrane. This aggregation then appears to trigger off the pinocytosis of the membrane components by some unknown mechanism. Such membrane transformations may be of crucial importance in the induction of an antibody response to an antigen, as well as iv other processes of cell differentiation.

The fluid mosaic model of the structure of cell membranes.

A simple, rapid, and sensitive DNA assay procedure.

Endostatin: an endogenous inhibitor of angiogenesis and tumor growth.

Chronic fatigue illness and Operation Desert Storm.

The most common complaints of cancer patients undergoing chemo- or radiotherapy are fatigue, nausea, vomiting, malaise, diarrhea and headaches. These adverse effects are thought to be due to damage of normal tissues during the course of therapy. In addition, recent evidence indicates that fatigue is related to reduced mitochondrial function through loss of efficiency in the electron transport chain caused by membrane oxidation, and this occurs during aging, in fatiguing illnesses and in cancer patients during cytotoxic therapy. Lipid Replacement Therapy administered as a nutritional supplement with antioxidants can prevent oxidative membrane damage to normal tissues, restore mitochondrial and other cellular membrane functions and reduce the adverse effects of cancer therapy. Recent clinical trials using patients with chronic fatigue have shown the benefit of Lipid Replacement Therapy plus antioxidants in restoring mitochondrial electron transport function and reducing moderate to severe chronic fatigue by protecting mitochondrial and other cellular membranes from oxidative and other damage. In cancer patients a placebo-controlled, cross-over clinical trial using Lipid Replacement Therapy plus antioxidants demonstrated that the adverse effects of chemotherapy can be reduced in 57-70% of patients. Dietary use of unoxidized membrane lipids plus antioxidants is recommended for patients undergoing cancer therapy to improve quality of life but should not be taken at the same time of day as the therapy.

http://www.immed.org/publications/PORnicolson_copy.pdf

Lipid replacement/antioxidant therapy as an adjunct supplement to reduce the adverse effects of cancer therapy and restore mitochondrial function.

The morbidity in most cancer patients is not due to their primary cancers; it is due to metastatic disease. Thus understanding the progression of tumors to the metastatic state and the changes that take place in highly malignant cells are important in the development of new therapeutic approaches to diagnose, prognostically assess and treat highly progressive malignancies. Changes in the expression of certain genes or alterations of gene structures and encoded products can result in benign tumor cells progressing to the invasive and metastatic states. This has been shown in the laboratory by transferring dominantly acting oncogenes into susceptible cells and then testing the malignant properties of these cells in vivo. Usually such rapid qualitative changes in malignant state occur only rarely; the natural progression of tumor cells to the invasive or metastatic state occurs through a slow stepwise process of change. Tumor progression, in some instances, can be reversible, involving changes in dominantly acting oncogenes or tumor suppressor genes. The natural progression of tumors to highly malignant states also involves their ability to circumvent host microenvironmental controls that regulate cellular growth and diversity. Quantitative changes in gene expression rather than qualitative changes in gene structure are important in microenvironmental effects on progression. One of the important mechanisms of cellular regulation in epithelial tissues, such as breast epithelium, appears to be mediated by intercellular junctional communication. Changes in gene expression can result in loss of junctional communication, followed by cellular diversification and progression. Highly malignant tumor cells that have slowly evolved in vivo with only a few qualitative changes in gene structure have probably undergone extensive cycles of diversification and have multiple quantitative differences in gene expression. Some of these genes are related to metastasis. For example, we have identified a novel gene called mta1 (rat) or MTA1 (human) that appears to be involved in mammary cell motility and growth regulation. This may be an example of a gene that regulates highly malignant cellular phenotypes. When coupled with other cellular changes, such as loss of intercellular communication, specific changes in gene expression may result in cellular diversification and tumor progression.

Metastasis-Associated genes and metastatic tumor progression.

Adhesion and stabilization of circulating tumor cells to endothelial cells in target blood vessels play an important role in the complex process of metastasis. We examined the cell surface receptors involved in the liver-metastatic adhesive interactions of murine RAW117 large-cell lymphoma cells to unstimulated hepatic sinusoidal endothelial cells (HSE) under physiological flow conditions. Flow cytometric analysis indicated that VCAM-1, ICAM-1 and PECAM-1 are constitutively expressed on the surfaces of both HSE and RAW117 cells. However, monoclonal antibody (mAb) blockade studies showed that ICAM-1 and PECAM-1 affected neither the attachment nor the stabilization step of the adhesion of RAW117 cells to HSE cell monolayers under flow. In contrast, RAW117 cells required a significantly lower shear stress to establish adhesion to HSE cells when VCAM-1 receptors on HSE cells were blocked with mAb. Furthermore, the presence of the anti-VCAM-1 mAb significantly decreased the extent of adhesion compared to that of the control, without affecting adherent cell stabilization times. Blocking the α4integrin subunits present mainly on RAW117 cells produced similar results to those previously observed with anti-VCAM-1 mAb. Although constitutively present mainly on the surfaces of RAW117 cells, MAdCAM-1 and β7 integrin subunit do not appear to play a role in either the arrest or stabilization of RAW117 cells on HSE cell monolayers. However, blocking the β1integrin subunit on the RAW117-H10 cells reduced adhesion to the same extent as anti-α4 and anti-VCAM-1 treatments. These observations suggest that an interaction of integrin α4β1on RAW117 cells with liver endothelial VCAM-1 occurs during the early stages of the adhesion process and may be important in liver metastasis.

Integrin α4β1/VCAM-1 pathway mediates primary adhesion of RAW117 lymphoma cells to hepatic sinusoidal endothelial cells under flow

The Fluid-Mosaic Membrane Model of cell membrane structure was based on thermodynamic principals and the available data on component lateral mobility within the membrane plane [Singer SJ, Nicolson GL. The Fluid Mosaic Model of the structure of cell membranes. Science 1972; 175: 720-731]. After more than forty years the model remains relevant for describing the basic nano-scale structures of a variety of biological membranes. More recent information, however, has shown the importance of specialized membrane domains, such as lipid rafts and protein complexes, in describing the macrostructure and dynamics of biological membranes. In addition, membrane-associated cytoskeletal structures and extracellular matrix also play roles in limiting the mobility and range of motion of membrane components and add new layers of complexity and hierarchy to the original model. An updated Fluid-Mosaic Membrane Model is described, where more emphasis has been placed on the mosaic nature of cellular membranes where protein and lipid components are more crowded and limited in their movements in the membrane plane by lipid-lipid, protein-protein and lipid-protein interactions as well as cell-matrix, cellcell and cytoskeletal interactions. These interactions are important in restraining membrane components and maintaining the unique mosaic organization of cell membranes into functional, dynamic domains.

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Garth L. Nicolson

Papers

Chronic fatigue illness and Operation Desert Storm.

Lipid replacement/antioxidant therapy as an adjunct supplement to reduce the adverse effects of cancer therapy and restore mitochondrial function.

Metastasis-Associated genes and metastatic tumor progression.

Integrin α4β1/VCAM-1 pathway mediates primary adhesion of RAW117 lymphoma cells to hepatic sinusoidal endothelial cells under flow

Update of the 1972 Singer-Nicolson Fluid-Mosaic Model of Membrane Structure.