Proteolytic receptor cleavage in the pathogenesis of blood rheology and co-morbidities in metabolic syndrome. Early forms of autodigestion

Abstract

Abnormal blood rheological properties seldom occur in isolation and instead are accompanied by other complications, often designated as co-morbidities. In the metabolic syndrome with complications like hypertension, diabetes and lack of normal microvascular blood flow, the underlying molecular mechanisms that simultaneously lead to elevated blood pressure and diabetes as well as abnormal microvascular rheology and other cell dysfunctions have remained largely unknown. In this review, we propose a new hypothesis for the origin of abnormal cell functions as well as multiple co-morbidities. Utilizing experimental models for the metabolic disease with diverse co-morbidities we summarize evidence for the presence of an uncontrolled extracellular proteolytic activity that causes ectodomain receptor cleavage and loss of their associated cell function. We summarize evidence for unchecked degrading proteinase activity, e.g. due to matrix metalloproteases, in patients with hypertension, Type II diabetes and obesity, in addition to evidence for receptor cleavage in the form of receptor fragments and decreased extracellular membrane expression levels. The evidence suggest that a shift in blood rheological properties and other co-morbidities may in fact be derived from a common mechanism that is due to uncontrolled proteolytic activity, i.e. an early form of autodigestion. Identification of the particular proteases involved and the mechanisms of their activation may open the door to treatment that simultaneously targets multiple co-morbidities in the metabolic syndrome.

Keywords: Matrix metalloproteinase; autodigestion; cell rheology; hypertension; insulin resistance; mechanotransduction; pancreatic digestive enzyme; rarefaction; serine protease; spontaneously hypertensive rat.

Fig. 1

(A) In situ zymography (with fluorescently quenched casein and gelatin in red and green fluorescence, respectively) of the small intestine of rats fed high fat diet (HFD, with 60% calorie intake from fat; Harlan, USA) and a normal control diet (CD) for 12 weeks. Sections are oriented with the muscularis at the bottom of each panel and villi pointing upwards. Casein degrading enzymes activity (left) was low in villi of animals on CD and is enhanced after HFD with penetration to the muscular layer (arrows). Gelatinases activity (right) was present along the villi in CD, but in HFD, elevated activity is recorded at the base of the villi, next to the muscular layer. (B) In-situ zymography on skeletal muscle (gastrocnemius) of rats on CD and HFD for 12 weeks. Conditions are the same as in Fig. 1(A).

Fig. 2

Immunohistochemical sections with primary antibody against the extracellular domain of the insulin receptor (InRα, N-20, sc-710 polyclonal antibody mapping to the N terminus, Santa Cruz Biotechnology) followed by biotin-avidin label [35] showing the expression density of the insulin receptor ectodomain in aortic rings from rats fed HFD (left) or CD (right) for 12 weeks.

Fig. 3

(A) Extracellular domain immunolabel density of the insulin receptor (same techniques as in Fig. 2). (B) Glucose uptake (non-hydrolyzable glucose analog 6-NBD-deoxyglucose; according to Methods described in [35]) in fresh isolated leukocytes after lysis of red blood cells from rats fed HFD and CD for 12 weeks with (C) digital fluorescent intensity measurements. The leukocytes were exposed to the fluorescent glucose analog for a period of 30 min. Note the reduced insulin receptor density and attenuated transmembrane glucose transport into the cells after exposure to plasma of rats on HFD compared to CD. *p < 0.05 HFD vs. CD by student t test. N = 4 rats/group.

Fig. 4

Western blot showing insulin receptor alpha in plasma of rats fed HFD and CD for 12 weeks. In addition to the expected upper band (thick arrow, 130 KDa), three additional bands (small arrows) are present in plasma of HFD rats and low or absent in the plasma of rats on CD. The bands indicate additional cleavage products of the insulin receptor alpha and require further mass-spec analysis. (Colors are visible in the online version of the article; http://dx.doi.org/10.3233/BIR-15045.)