Arterial haptoglobin : expression, regulation and function

Abstract

Several studies have demonstrated that collagen turnover is important during arterial restructuring. However, the precise mediators involved in collagen turnover are still not known. In order to develop specific inhibitors to prevent arterial restructuring, it is essential to get a better understanding of the molecular pathways involved in collagen turnover. In this thesis, we used a subtraction PCR to identify new candidate proteins that are involved in arterial restructuring and to get a better insight in these pathways. One of the identified proteins using subtraction PCR was the acute phase glycoprotein haptoglobin. Haptoglobin was found to be expressed by adventitial fibroblasts, early after flow changes. In vitro assays demonstrated that haptoglobin is a gelatinase inhibitor and important for cell migration. Furthermore, haptoglobin knockout mice demonstrated delayed arterial restructuring after flow changes. As NO plays an important role in flow-induced arterial restructuring, we investigated whether arterial haptoglobin expression was regulated via NO. Inhibition of NO synthesis by a aspecific NOS-inhibitor resulted in decreased arterial haptoglobin expression. This coincided with decreased arterial IL-6 expression, which is a well described regulator of haptoglobin expression in the liver. IL-6 knockout mice were used to confirm a regulatory role of IL-6 in arterial haptoglobin expression. However, haptoglobin expression was normal in IL-6 knockout mice, pointing to back-up mechanisms for the regulation of haptoglobin expression. Previously, sero-epidemiological studies have demonstrated that arthritis and cancer are characterized by increased serum haptoglobin levels. In this thesis, we demonstrated that local expression of haptoglobin is increased in these two pathological tissues that are characterized by enhanced cell migration and matrix turnover. As haptoglobin is present at high concentrations in the serum, the function of local haptoglobin expression is not completely understood. We therefore investigated whether alterations in haptoglobin glycosylation could explain the necessity of local haptoglobin expression. Haptoglobin expression and glycosylation was studied in balloon dilated rabbit arteries. Balloon dilation increased arterial haptoglobin expression within the first two weeks after injury whereas liver haptoglobin expression remained constant. There were no differences in arterial haptoglobin glycosylation patterns after balloon dilation. However, arterial haptoglobin was differentially glycosylated compared to liver haptoglobin and these arterial haptoglobin glycoforms could be detected in the serum. Previously, alterations in serum haptoglobin glycosylation have been associated with the progression and outcome of different diseases. As arterial haptoglobin has a characteristic glycosylation pattern, we investigated whether serum haptoglobin glycosylation patterns could serve as serum marker for atherosclerotic disease. There were, however, no differences were found in total serum haptoglobin levels or serum haptoglobin glycosylation patterns between patients with atherosclerosis and patients with only risk factors. In addition, no relation was found between total serum haptoglobin levels and glycosylation patterns with intima-media thickness in the carotid artery, which is a surrogate measure for the extent of atherosclerosis. These results indicate that serum haptoglobin glycosylation patterns are not suitable to use as serum marker for atherosclerotic disease