Apolipoprotein E suppresses glial cell secretion of TNFα

Abstract

Apolipoprotein E (apoE) is a 299 amino acid protein with multiple biological functions. Initially described in the context of cholesterol metabolism, apoE also has immunomodulatory properties and recent evidence has implicated a role for apoE in neurological disease. One possibility is that apoE, which is the predominant apolipoprotein produced intra-axially, may modify the CNS response to acute and chronic injury. We prepared mixed neuronal-glial cultures from apoE deficient mouse pups and measured secretion of TNFα after stimulation with lipopolysaccharide (LPS) in the presence and absence of human recombinant apoE3 and E4. We demonstrate that preincubation with apoE blocks glial secretion of TNFα in a dose-dependent manner. This effect is independent of any direct effect of apoE on cell viability and is greatest when apoE is preincubated with the cell culture for 24 h.

Introduction

Apolipoprotein E (apoE) is a 299 amino acid lipid-carrying protein that was originally described in the context of cholesterol metabolism. ApoE also has immunomodulatory properties and is locally secreted by macrophage after peripheral nerve injury and by astrocytes and oligodendrocytes after CNS injury, suggesting a role in response to neural injury or repair (Stoll et al., 1989; Stoll and Mueller, 1986; Skene and Shooter, 1983). The human apoE protein has three major isoforms, E2, E3 and E4, which differ by a single amino acid substitution at positions 112 and 158. ApoE3, the most common isoform, differs from E4 by a cysteine–arginine interchange at position 112, whereas E2, the least prevalent isoform, contains a cysteine at position 158. The apoE4 allele is a major determinant of risk for sporadic and late onset familial Alzheimer's disease (Strittmatter et al., 1993). In addition to this well established association, apoE genotype may effect the outcome in a variety of acute neurological conditions, including cerebral hemorrhage, closed head injury, stroke and cognitive deterioration after cardiopulmonary bypass (Alberts et al., 1996; Connolly et al., 1996; Sorbi et al., 1996; Newman et al., 1995).

The brain has a limited repertoire of responses to diverse forms of injury and microglial activation is the cornerstone of the CNS inflammatory response (Streit et al., 1988). Activated microglia may contribute directly to neurotoxicity and tissue destruction in a variety of ways, including generation of reactive oxygen species and release of glutamate. Activated microglia also contribute secondarily to inflammation-mediated tissue destruction via the release of cytokines such as IL-1β, IL-6 and TNFα. In particular, TNFα has been implicated in gliosis, demyelination, cerebral edema and exacerbation of amyloid neurotoxicity (Arvin et al., 1995; Meda et al., 1995; Selmaj and Raine, 1988; Shohami et al., 1993). Paradoxically, TNFα may also play a beneficial role after CNS injury by causing enhanced secretion of neurotrophic factors, increased neuronal adhesion and enhanced neurite outgrowth (Hattori et al., 1993; Lotan et al., 1994; Munoz-Fernandez et al., 1991; Schwartz et al., 1991).

In addition to its potential role in CNS injury, apoE is known to have immunomodulatory functions. For example, apoE decreases lymphocyte proliferation after mitogenic challenge and reduces pokeweed-mitogen stimulated immunoglobulin synthesis of B-lymphocytes (Pepe and Curtiss, 1986; Curtiss and Edgington, 1979). The in vivo functional significance of apoE in downregulating the immune response remains to be elucidated, but in theory, apoE might play a unique role in the CNS, where it is the primary apolipoprotein produced. One possibility consistent with apoE's immunomodulatory effects and its putative role in neurological disease is that apoE modulates the CNS response to acute and chronic injury. We now demonstrate that apoE suppresses secretion of TNFα in glial cultures stimulated with lipopolysaccharide (LPS).