Endothelial cells of the rat brain vasculature express cyclooxygenase-2 mRNA in response to systemic interleukin-1β: a possible site of prostaglandin synthesis responsible for fever

Abstract

We previously showed that intraperitoneal injection of lipopolysaccharide induced cyclooxygenase-2 (COX-2) mRNA in as yet unidentified cells of blood vessels and leptomeninges in the rat brain and proposed a possible role of these cells as the source of prostaglandin E₂ in the genesis of fever (Cao et al., Brain Res., 697 (1995) 187–196). In the present study, to proceed further with this line of research, we addressed the following two questions: first, does a pyrogenic dose of interleukin-1β (IL-1β), an endogenous pyrogen, induce COX-2 mRNA in the brain blood vessels and leptomeninges? Secondly, if it does, what type of cells are positive for COX-2 mRNA? Intraperitoneal injection of recombinant human IL-1β (30 μg/kg) induced fever in rats and an in situ hybridization study revealed that faint but significant COX-2 mRNA signals appeared in the blood vessels and leptomeninges at 1.5 h after the injection (the early rising phase of fever). The mRNA signals increased in number and intensity at 4 h (early plateau phase), decreased at 6.5 h (early recovery phase), and completely disappeared by 10 h after the injection (late recovery phase). The COX-2 mRNA positive cells in the blood vessels were likely to be the endothelial cells since the corresponding cells in the adjacent mirror-imaged section also expressed mRNAs for intracellular adhesion molecule-1 and the type-I interleukin-1 receptor, although those in the leptomeninges still remained unidentified. These results imply that circulating IL-1β acts on its receptor on the endothelial cells of the brain vasculature to induce COX-2 mRNA, which is possibly responsible for the elevated level of PGE₂ seen during fever.

Introduction

Prostaglandin E₂ (PGE₂) has been considered to be an essential humoral factor that acts on the central thermoregulatory network to evoke fever [35]. Although it is widely accepted that the preoptic area or its vicinity is the site of PGE₂ action to evoke fever (review by Milton [27]), the exact site of PGE₂ production after pyrogen challenge remains unclear. One of the possible approaches to this question is to locate the rate-limiting enzyme(s) involved in prostaglandin biosynthesis. Especially, if the enzyme is induced by pyrogen challenge in a certain brain area, this area would be more likely involved in the patho-physiological responses to the pyrogen. As a candidate for the rate-limiting enzyme, we focused on cyclooxygenase-2 (COX-2).

COX-2 was first reported in 1991 by Kujubu et al. [22]as a phorbol ester tumor promoter-inducible enzyme possessing cyclooxygenase activity. COX-2 is also induced by inflammatory stimulants, such as lipopolysaccharide (LPS) or interleukin-1 (IL-1), and plays a major role in the rapid production of prostaglandins during inflammation (reviewed by Goppelt-Struebe [14]). In contrast, COX-1 is constitutively expressed in several organs and is relatively unaffected by inflammatory stimulation.

The brain, however, is an exceptional organ where COX-2 mRNA and its protein are constitutively expressed in the telencephalic neurons even under the unstimulated (normal) condition 4, 5, 44. After LPS administration, the COX-2 mRNA level in these telencephalic neurons became slightly elevated (maximally 1.8-fold), but that in the neurons or glial cells of other brain regions, including the preoptic area, the presumable site of febrile action of PGE₂, remained undetectable [5]. Therefore, it is unlikely that neurons or glial cells in the preoptic area are the major source of PGE₂ responsible for fever. In contrast, marked induction of COX-2 mRNA occurred in a certain type of cranial blood vessels and leptomeninges, where COX-2 mRNA was hardly detectable before LPS administration [5]. COX-2 induction in these non-neuronal and non-glial cells took place in the entire brain regions including the preoptic area. The time course of its induction was in accordance with that of fever. Based on this finding and available evidence reported previously, we emphasized a possible role of the blood vessels and leptomeninges in the production of PGE₂ responsible for fever [5].

To further assess this possibility, we conducted the present study using human IL-1β as a pyrogen. IL-1, one of the cytokines, has been considered to be an endogenous mediator of fever, and is released from macrophages in response to LPS [9]. The purpose of the present study was (1) to determine if a pyrogenic dose of IL-1β induces COX-2 mRNA in the blood vessels and leptomeninges and, if so, (2) to identify the type of cells that express COX-2 mRNA in response to IL-1β.