EL CURRICULO

There is strong evidence supporting the central role of IL-6 in the inflammatory response (Papanicolaou et al 1998; Woods et al 2000). Figure 1.9 demonstrate some of the pathways which links IL-6 in the inflammatory response that lead to the development of CHD Several studies have found a direct linked between IL-6 levels and cardiovascular disease (Biasucci et al 1996; Harris et al 1999; Ridker et al 2000b) (figure 1.10). IL-6 has been implicated in the pathogenesis of CHD in patients with MI and unstable angina (Berk et al 1990; Miyao et al 1993; Liuzzo et al 1994; 1998). Elevated IL-6 levels appears to be predictive of future heart disease (Harris et al 1999), and are common in patients with unstable angina compared with those with angina, particularly in smokers (Bataille and Klein 1992; de Maat et al 1996; Biasucci et al 1996). IL-6 levels also showed strong correlation with levels for fibrinogen and CRP (de Maat et al 1996; Bataille and Klein 1992). Experimental studies indicate that vascular endothelial and smooth muscles cells fi-om normal and aneurysms arteries produce IL-6 (Loppnow and Libby 1989a; 1989b; Szekanecz et al 1994), that IL-6 gene transcripts are expressed in human atherosclerotic lesions (Seino et al 1994; Rus et al 1996), and that IL-6 may have procoagulant effects (Mestries et al 1994; Van der Poil et al 1994; Stouthard et al 1996). Evidence from Ridker et al 2000b showed that median IL-6 concentrations were higher among men who subsequently had an MI compared to healthy men matched for age and smoking status. This study also demonstrates that the risk of future MI increased with increasing quartile of baseline IL-6 concentration such that men in the highest quartile had a relative risk 2.3 times higher than those in the lowest quartile with a 38% increased in risk for every quartile rise in IL-6 levels (table 1.7).

Figure 1.9 Central roles of IL-6 and its effect on the development of CHD (adapted from Wood et al 2000).

PSYCHOSOCIAL STRESS

MONOCYTES/MACROPHAGES

In inflammatory state (smokers lung, atheroma plaque)

ADIPOSE TISSUE

iL P L

INTERLEUKIN-6

LIVER

PLATELETS

ENDOTHELIUM

Acute phase response ^Fibrinogen

iH D L

fAggregability fAdhesion

molecules

Figure 1.10 Median IL-6 levels in healthy men, MI survivors and unstable angina patients (adapted from Ridker et al 2000b; Biasucci et al 1996). IL-6 levels are from different studies and are only used here as illustration to show differences in MI and UA patients compared to normal healthy individuals.

Normal MI

M I=M yocardial infarction, UA=Unstable angina

UA

Table 1.7 Adjusted relative risks of future myocardial infarction among apparently healthy men according to baseline level of IL-6 from the Physician Health Studies (Ridker et al 2000b).

Quartile of IL-6 (range, pg/ml I J<1.041 2 (1.04-1.46) 3 (1.47-2.28) 4 (>2.28) P for Trend Total cohort RR 1.0 1.9 3.5 2.3 0.01 95% Cl - 0.9-3.9 1.8-6.9 1.1-4.6 P - 0.08 0.001 0.03 Nonsmokers RR 1.0 2.1 3.1 2.7 0.009 95% Cl - 1.0-4.5 1.6-6.7 1.1-5.7 P - 0.06 0.002 0.02

RR indicates relative risk. P=p-values for RR compared to 1 quartile

Adjusted for total HDL cholesterol, BMI, diastolic blood pressure, diabetes, family history of premature coronary artery disease, alcohol use and exercise frequency

L 7,1 Impaired coagulation and fibrinolysis

Inflammation at the site of vessel wall injury, triggered by a combination of genetic and environmental risk factors can lead to low grade acute phase response and progressively shift the balance of haemostasis towards a prothrombotic state. This, coupled with plaque growth and fissuring, eventually triggers thrombosis and further amplifies the inflammatory response (Harrison et al 1997).

IL-6 has been well characterized as an inducer of acute phase proteins, synthesized from the liver, which are prothrombotic and promote increase blood viscosity (e.g. fibrinogen) (Lowe and Rumley 1999; Woodward et al 1999). Many studies have demonstrated a positive correlation between plasma fibrinogen levels and those of IL-6, thereby altering the haemostatic balance towards that of a prothrombotic state (Mendall et al 1997; Erren et al 1999; Koukkunen et al 2001). Elevated levels o f fibrinogen have been shown to substantially influence the risk of venous and arterial thrombosis as well as cardiovascular disease risk in the general population (Kannel et al 1987; 1997; Koenig and Ernest 1993; Meade et al 1986; Thompson et al 1995; Koster et al 1994; Heinrich et al 1994; Folsom et al 1997; Woodward et al 1998; Eriksson et al 1999). Raised fibrinogen levels may also contribute towards chronic atherosclerotic plaque growth and risk of thrombosis by increasing plasma viscosity, red blood cell and platelet aggregation. Fibrinogen may be directly incorporated into the injured endothelium where it localises with LDL and promotes regional fibrin deposition (Ernst et al 1988a; 1988b Koenig and Ernest 1993; Smith and Thompson 1994; Woodward et al 1999). Coupled with increases in the levels for tissue factor (TF), plasminogen activator inhibitor 1 (PAI-1), thrombin generation, and antithrombin productions via activated protein C (APC), fibrin clot formation is favoured and the risk of thrombosis and MI increased (Neumann et al 1997; Samad et al 1994; Levi et al 1998; Kowal et al 1998; Hooper et al 1998).

1,7.2 CRP

risk of ischaemic heart disease (IHD) and severity o f atherosclerosis (de Maat et 1996; Berk et al 1990; Woodhouse et al 1994; Biasucci et al 1996). Elevated CRP and IL-6 levels have been reported in patients with unstable angina (Biasucci et al 1996) and predict risks of future cardiovascular disease in men and women (Ridker et al 2000a; 2000b). It has also been shown that CRP levels are associated with CHD in apparently healthy subjects, both in a cross sectional study in general practice (Mendall et al 1996), and longitudinally in the US Physicians Health Study (Ridker et al 1997), the MONICA- Augsburg Cohort Study (Koenig et al 1997) and the MRFIT Study (Kuller et al 1996), where CRP levels predicted cardiovascular events or CHD mortality during a follow up of between 2 and 17 years. The precise role of CRP in cardiovascular disease is still debatable, but it is believed that CRP maybe more than just a marker of the acute phase response. Evidence suggests that CRP facilitates the uptake of lipids by macrophages accumulating in the atherosclerotic lesion (Hatanaka et al 1995).

i. 7,3 Connective tissue remodelling

Extracellular tissue remodelling is regulated by key matrix metalloproteinases (e.g. collagenases, elastases and stromelysin-1) and their inhibitors, ‘tissue inhibitor of metalloproteinases’ (e.g. TIMP-1) during tissue injury (Woessener 1991). The upregulation of TIMP-1 synthesis by IL-6 may shift the balance in favour of matrix proteins deposition leading to atherothrombosis and the progression of CHD (Kordula et al 1992; Roeb et al 1994; Ye et al 1995; Humphries et al 1998; de Maat et al 1999). Evidence from immunohistochemistry showed IL-6 mRNA expression was localised to the human arterial atherosclerotic wall as cellular and extracellular deposits in the connective tissue matrix, with the fibrous plaque having significant higher level o f IL-6 than the intima and media (Rus et al 1996).

Data from studies of apolipoprotein E (ApoE) knockout mice model, which develop atherosclerotic plaques in the aorta also showed elevated levels of IL-6 mRNA predominate mainly in the plaque area compared to normal mice. The treatment of these

ApoE knockout mice with estrogens (e.g. 17-P-estradiol) demonstrate an anti- atherosclerotic effect where regression of plaque sizes was seen (Sukovich et al 1998).

i. 7,4 Dyslipidaemia

Further evidence for the role of IL-6 in coronary heart disease came from studies showing secretion of IL-6 by subcutaneous adipose tissues (Mohamed-Ali et al 1997; Fried et al 1998) and IL-6 suppression of macrophage lipoprotein lipase secretion from the murine J774.2 cell line (Tenghu-Muhammad et al 1998). IL-6 has also been shown to reduce lipoprotein lipase (LPL) activity in adipose tissues, leading to reduced triglyceride uptake (Greenberg et al 1992). At present, little is known about the effects of IL-6 on adipose tissue. One possibility is that the downregulation of adipose tissue LPL results in increased hepatic triglyceride secretion (Nonogaki et al 1995) and may contribute to hypertriglyceridaemia. Evidence from Tanaka et al 1997 showed that C/EBPp (formerly known as NF-IL6) is required for adipocyte differentiation in mice. C/EBPp -/- mice did not accumulate lipid droplets. One can postulate that elevated levels of IL-6 facilitate the uptake of lipids by macrophages and IL-6 accumulation in the atherosclerotic lesions may be mediated through C/EBPp, resulting in atherogenesis and greater CHD risk. The regulation and mechanisms underlying this relationship are poorly understood. It is however still not known if elevated IL-6 levels are a response to disease and/or promote disease progression.