By David N. Leff
Autoimmunity is one thing, heart disease quite another. Or is it?
Cardiovascular attacks are the main cause of death in the industrialized world.
Autoimmune diseases, which misguidedly train their friendly fire on the body¿s own cells, tissues and organs, are among the most hotly researched targets in medical science, but remain black boxes. Their prime culprit is alleged to be the immune system¿s cytotoxic T lymphocytes. Three of the leading autoimmune maladies ¿ multiple sclerosis, rheumatoid arthritis and systemic lupus erythematosus ¿ number 3.33 million patients in the U.S. alone.
A ¿Perspective¿ editorial in the current issue of Science, dated July 27, 2001, is titled, ¿The push-me pull-you of T cell activation.¿ Its author is molecular biologist David Lo, who is Vice President, integrative biology at Digital Gene Technologies, Inc. in La Jolla, Calif. He advances two plausible theories as to autoimmunity¿s rogue strategy:
¿Immune cells activated in response to a pathogen protein could aberrantly attack a healthy tissue that expresses a structurally similar protein [molecular mimicry]. Alternatively, disregulation of immunoregulatory molecules could lead to recruitment and activation of self-reactive T cells within the healthy tissue.¿
Yet, Lo finds it ¿difficult to understand how either of these processes could explain chronic, intermittent multi-organ autoimmunity, typified by lupus.¿ He deems this puzzle ¿partly solved¿ by the article in the same issue of Science, on which his commentary focuses. It bears the title: ¿Lysophosphatidylcholine as a ligand for the immunoregulatory receptor G2A.¿ The paper¿s co-senior authors are immunologist Owen Witte, a Howard Hughes Medical Institute investigator at the University of California at Los Angeles, and biochemist/molecular biologist Yan Xu, a staff associate at the Cleveland Clinic.
¿Our main finding,¿ Xu told BioWorld Today, ¿is connecting two key molecules together, which had never been done before. They are LPC [lysophosphatidylcholine] and SPC [sphingosylphosphorylcholine]. Independent of their receptors, the two ligands are involved in inflammatory diseases, such as atherosclerosis, and immune system regulation.¿ Atherosclerosis is the cholesterol-driven coronary artery blockage that causes heart attacks.
Making Cells Do The Right Thing
¿We identified the ligands for their receptors,¿ Xu observed. ¿The receptor is sitting on the membrane of the T cells. They belong to a big family called the G protein-coupled receptors (GPC). It¿s a large family of genes, one of which is G2A. It encodes a specific receptor, which binds these two small lipids, LPC and SPC. Those receptors have very broad functions. They cover just about every process in the body.
¿But their function was not known previously,¿ Xu continued. ¿We did collaborative work with Owen Witte¿s group at UCLA. They cloned this G2A gene, and also made a mouse knockout, so they could delete it. They found that mice without this gene developed autoimmune diseases. The immune system reacts against its own proteins, its own antigens. By knocking out that gene in those mice, the animals developed autoimmune disease.¿ She made the point that, ¿At this early stage in research, linkage to human atherosclerosis and lupus is unproven speculation.
¿These receptors are expressed at high levels in the T cells, which are immune-defensive killer cells,¿ Xu said. ¿When you get infected by bacteria or some other bad-guy antigens, those T cells are activated and start to fight back. T cells have been shown to be involved in atherosclerosis, but their functions in autoimmune diseases are still very complex. T cells sometimes play positive roles, sometimes negative roles. Also, the LPC ligand is elevated in atherosclerosis. During disease development, LDLs ¿ low-density lipoproteins, the bad-guy cholesterol ¿ gets into the act, when LPC levels are increased.
¿The purpose of our paper was to establish the ligand-receptor relationship. So, we used classical pharmacological and biochemical in vitro methods,¿ Xu said. ¿The first assay was for calcium mobilization. Calcium is a very important signaling molecule inside the cells. The increase or decrease of calcium levels will regulate many cellular proteins ¿ cell proliferation, for example.
¿We found,¿ she continued, ¿that the cells do not express these receptors, and do not respond to lipids. So we artificially put those receptors on the cells and stimulated them till they started to respond to lipids ¿ to give those calcium signals. This is a strong first indication that those lipids are ligands for this receptor. Other molecules won¿t do it.¿
When Cells Get A Move On
¿Then, in a second experiment,¿ Xu recounted, ¿we did a study to see how these ligands bind to the receptor. Finally, we ran a cell migration test, using a leukemic T-cell line. So we put these receptors in those T lymphocytes, and found that those cells indeed responded to them. They moved, migrated, walked the lipids. That chemoattractant effect attracted cells to walk the gradient, the concentration where they had these ligands. And those cells overexpressed their receptor.
¿When signaled by intracellular messengers to start migrating,¿ she said, ¿their moving direction has to be by elongation. The cells¿ motion resembles that of a snail or an inchworm.¿ She cited two motivations for migration: ¿For lymphocytes ¿ T cells, B cells ¿ on the one hand you get invading bacteria. T cells move out to kill them by immune response. On the other hand, another motive is cancer metastasis.¿
Her laboratory is now shifting its focus to extend its receptor work to malignancy. ¿We don¿t really know,¿ she observed, ¿but we believe those receptors should be involved in cancer. Partly because this G2A is a cell-cycle regulator, which means that it actually blocks a cell from going through the cell cycle to proliferate. Proliferation is one of the most important features of tumor cells. They¿ve lost control of their proliferation ¿ just go crazy. And the genes seem to have some function as a brake at a certain point. It could be called a tumor suppressor. We¿ve started to accumulate some indications that it¿s probably involved in cancer development. So that¿s the direction we want to go.¿
¿The effects of the physiologically high concentrations of LPC in body fluids and serum,¿ her paper concluded, ¿as well as possible functional redundancy with G2A receptor analogs, may determine the suitability of these GPC receptors in the treatment of disease.¿