Inflammatory mediators and their relevance to allergic rhinitis 1 Kinins

Historically, the first reports relating to the kinin system refer to a substance in rabbit and human urine that caused hypotension in anaesthetised dogs (Abelous & Hardier, 1909). As this substance was believed to be derived from the pancreas, it was named kallikrein from the Greek word for pancreas, kallikreas. Werle and colleagues (1937) incubated kallikrein with plasma and found a substance capable of contracting the guinea-pig ileum and causing hypotension: it was identified as the peptide kallidin. Snake venom incubated with plasma leads to the formation of a substance causing a slow, delayed contraction of the guinea-pig isolated ileum: this substance was named bradykinin from the Greek for movement, kinin and slow, brady (Rocha E Silva et a i, 1949).

The kinins, bradykinin and the closely related peptide kallidin are, respectively, nine and ten amino acid peptides formed by the action of enzymes called kallikreins on precursors called kininogens.

Lys-Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg Kallidin Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg Bradykinin

There are two types of kallikrein enzymes: plasma kallikrein which is present in the circulation and cleaves bradykinin from high-molecular weight kininogen and a kallikrein present in various tissues which cleaves lys-bradykinin (kallidin) from high- and low- molecular weight kininogens. Once formed, the kinins mediate most of their actions via

activation of two membrane receptor types termed Bi and B2. The actions of bradykinin

and kallidin include vasodilatation, increased vascular permeability, stimulation of nerves, stimulation of epithelial cell ion transport and contraction of intestinal and uterine smooth muscle. The kinins are broken down by two families of enzymes, kininase I and n. Each factor involved in the generation of the kinins will now be reviewed in more detail.

1.6.1.1 The generation and breakdown o f bradykinin

1.6.1.1.1 Kininogens

Three types of kininogen have been identified in mammals: high-molecular weight (HMW) kininogen which is present in the blood, low-molecular weight (LMW) kininogen in the blood and tissues and T-kininogen which is found exclusively in the rat (Habermann, 1963; Suzuki et al., 1967; Jacobsen, 1966; Okamoto & Greenbaum, 1983). Both HMW-ki ni nogen and LMW-kininogen are the product of a single gene in humans, their differing primary structures being produced by alternate splicing of the gene transcript (Kitamura et al., 1985). The human kininogen gene contains 11 exons, nine of which, at the 5'-terminal, code for the mRNA producing the heavy chain common to both HMW and LMW kininogen (Kitamura et at., 1985). Exon 10 contains the sequence producing the kinin-containing region of the kininogens together with a sequence which produces the HMW-kininogen light-chain. Exon 11 codes for the LMW kininogen light chain. The heavy chain, which has the same basic structure in both HMW and LMW kininogen, has the ability to inhibit cysteine proteases (Sueyoshi et al., 1985). The light chain of HMW kininogen contains a domain rich in histidine, proline and lysine which binds to damaged endothelial surfaces, crystals and degraded cartilage products. Once plasma kallikrein releases bradykinin from the region between the light and heavy chains, the HMW kininogen light chain enhances clotting by activation of clotting factors (Kaplan, 1978). The light chain of HMW kininogen also contains the domain which binds plasma kallikrein or clotting factor XI (Sugo et al., 1980). Both the heavy and light chains participate in the binding of HMW kininogen to endothelial cells (Jiang et al., 1992;

Chapter 1________________________________________________________________________________ Introduction

Asakura et a l, 1992), platelets (Meloni et a l, 1992) and neutrophils (Wachtfogel et al., 1994). The function of the light chain of LMW kininogen remains unclear.

1.6.1.1.2 Plasma kallikrein

Plasma kallikrein is the product of a single gene (Seidah et al., 1989) and its mRNA is expressed exclusively in the liver. The inactive form, plasma prekallikrein, is secreted by hepatocytes and circulates in the plasma bound to high-molecular weight kininogen (Mandle et a l, 1976). Human plasma prekallikrein is a single-chain polypeptide of 619 amino acids that is converted to the active, plasma kallikrein by cleavage of an internal Arg-He bond (Chung et a l, 1986). Plasma kallikrein is composed of a heavy chain (371 amino acids) and a light chain (248 amino acids) held together by a disulphide bond. It is the light chain of plasma kallikrein which contains its catalytic activity: being homologous to trypsin proteases (Chung et a l, 1986). Plasma kallikrein hydrolyses Lys- Arg and Arg-Ser bonds in high-molecular weight kininogen to release bradykinin. Low molecular weight kininogen is a poor substrate for plasma kallikrein, bradykinin can be generated from this kininogen in the presence of neutrophil elastase (Sato & Nagasawa,

1988).

1.6.1.1.3 Tissue kallikrein

Three genes clustered on chromosome 19 have been identified as belonging to the human kallikrein gene family (Baker & Shine, 1985; Fukushima et a l, 1985; Schedlich et a l,

1987; Evans et a l, 1988), but more may exist as 19 genomic clones have been identified which hybridise with monkey kallikrein cDNA probes (Murray et a l, 1990). Tissue kallikrein is present in many human tissues, particularly the pancreas, kidney, salivary glands and the mucosa of the airways and gastrointestinal tract. Human urinary tissue kallikrein has been widely studied and appears to be synthesised bound to 17 amino acids which are cleaved off to form an inactive precursor. The inactive form of human tissue kallikrein is activated by cleaving seven amino acids from the NH2-terminal (Takahashi

et a l, 1986). Active tissue kallikrein consists of 238 amino acids with multiple glycosylation sites (Takahashi et a l, 1988; Lu et a l, 1989; Kellermann et a l, 1988), and this variable glycosylation may explain the heterogeneity in the observed molecular weights of tissue kallikrein (Fiedler & Hirschauer, 1981). Tissue kallikrein releases kallidin by hydrolysis of Met-Lys and Arg-Ser bonds in the high and low molecular weight kininogen molecules. The failure of tissue kallikrein to generate bradykinin results from the inability to accommodate the Lys-Arg-Pro sequence present in the kininogen molecule. Tissue kallikrein is, therefore, unable to cleave the Lys-Arg bond and produce bradykinin (Bhoola gf a/., 1992).

1.6.1.1.4 Mechanisms o f kinin formation

The formation of bradykinin in the plasma is dependent upon coagulation factor XII (Hageman factor). Hageman factor, in its unactivated state, is a single-chain globulin with a molecular weight of 80,000 (Cochrane & Wuepper, 1971). Upon contact with certain negatively charged surfaces, for example glass, silicates or heparin (Proud & Kaplan, 1988), Hageman factor becomes activated and converts plasma prekallikrein to kallikrein; and bradykinin is cleaved from HMW-kininogen. Activated Hageman factor also activates coagulation factor XI, which circulates bound to HMW-kininogen. Activated plasma kallikrein has a powerful positive feedback effect in that it further activates Hageman factor. Tissue kallikrein can be activated by a variety of proteolytic enzymes, for example trypsin, plasmin or plasma kallikrein (Proud & Kaplan, 1988). There have been reports that a number of other proteases generated in the inflammatory response may be able to liberate kinins from kininogen: these include mast cell tryptase (Proud et a l,

1987b), calpains (Higahiyama et a l, 1986), eosinophil cationic protein (Venge et a l, 1979) and proteases derived from the house-dust mite (Takahashi et a l, 1990). The mechanisms of bradykinin formation by plasma and tissue pathways is shown in figure

Chapter 1 Introduction