**5. C-terminally truncated form of RAGE (soluble RAGE, sRAGE) as potential biomarkers for cardiovascular diseases, metabolic syndrome and insulin resistance**

## **5.1 Truncated form of RAGE**

Numerous truncated forms of RAGE have recently been described 12, 95-98 (Figure 1). Two major spliced variants of RAGE mRNA, N-terminal and C-terminal truncated forms, have been most extensively characterized 12. The N-truncated isoform of RAGE mRNA codes for a 303-amino-acid protein lacking the N-terminal signal sequence and the first V-like extracellular domain. The N-truncated form is incapable of binding with AGEs, since the V-domain is critical for binding of the ligand 71. The N-truncated form of RAGE appears to be expressed on the cell surface similar to the full-length RAGE, although its biological roles remain to be elucidated 99. It has been suggested that this form of RAGE could be involved in angiogenic regulation in a fashion independent of the classical RAGE signaling pathway 99.

### **5.2 Endogenous secretory RAGE (esRAGE)**

The C-terminal truncated form of RAGE lacks the exon 10 sequences encoding the transmembrane and intracytoplasmic domains 12. This spliced variant mRNA of RAGE

transporter member 4 (Glut 4) were identified to be downregulated in RAGE-/- adipose tissue. Figure 5A shows genes specifically suppressed in RAGE-/- adipose tissue (closed circles) in insulin signaling and adipocyte differentiation pathways. Real-time quantitative RT-PCR analyses confirmed the genes in the pathways were indeed down-regulated in RAGE-/- adipose tissue (Figure 5B). These results altogether suggest direct role of RAGE in adiposity. Although in which cell types RAGE is principally working, insulin signaling and adipocyte signaling pathway in adipose tissue appear to play important part in RAGE

Impaired insulin action, when assessed by fasting serum insulin levels or the homeostasis model assessment of insulin resistance (HOMA-IR) 92, is associated with atherosclerosis and an increased risk of myocardial infarction. Insulin resistance is associated with endothelial dysfunction 93 and may serve as a link between insulin resistance and atherosclerosis. Recent findings by Harja et al highlighted the involvement of RAGE in endothelial dysfunction 81. Endothelium-dependent vasorelaxation was tested in isolated mouse aortic rings from *apoE– /–* and *apoE–/–RAGE–/–* mice, and relaxation response to acetylcholine was significantly improved in the RAGE deficient mouse. Similarly, impaired endothelial function in diabetic obese mice was also shown to be mediated by AGEs/RAGE system, since blockade of AGE-RAGE interaction by soluble RAGE significantly improved endothelial function 94. Recent clinical observations by Linden et al 44 also implies AGEs/RAGE system is involved in impaired endothelial function in patients with chronic kidney diseases. Thus, not only by the interaction at the cellular signaling level, but RAGE appears to impair endothelial function and potentially blood flow in insulin target tissues, leading to insulin resistance in

**5. C-terminally truncated form of RAGE (soluble RAGE, sRAGE) as potential biomarkers for cardiovascular diseases, metabolic syndrome and insulin** 

Numerous truncated forms of RAGE have recently been described 12, 95-98 (Figure 1). Two major spliced variants of RAGE mRNA, N-terminal and C-terminal truncated forms, have been most extensively characterized 12. The N-truncated isoform of RAGE mRNA codes for a 303-amino-acid protein lacking the N-terminal signal sequence and the first V-like extracellular domain. The N-truncated form is incapable of binding with AGEs, since the V-domain is critical for binding of the ligand 71. The N-truncated form of RAGE appears to be expressed on the cell surface similar to the full-length RAGE, although its biological roles remain to be elucidated 99. It has been suggested that this form of RAGE could be involved in angiogenic regulation in a fashion independent of the classical RAGE

The C-terminal truncated form of RAGE lacks the exon 10 sequences encoding the transmembrane and intracytoplasmic domains 12. This spliced variant mRNA of RAGE

regulation of adiposity.

vivo.

**resistance** 

**5.1 Truncated form of RAGE** 

signaling pathway 99.

**5.2 Endogenous secretory RAGE (esRAGE)** 

**4.4 RAGE, endothelial dysfunction and insulin resistance** 

encodes a protein consisting of 347 amino acids with a 22-amino-acid signal sequence, and is released from cells. This C-truncated form is now known to be present in human circulation and is named endogenous secretory RAGE (esRAGE) 12. Regulation of alternative splicing of the RAGE is recently shown to be regulated through G-rich cis-elements and heterogenous nuclear ribonucleoprotein H 100. esRAGE was found to be capable of neutralizing the effects of AGEs on endothelial cells in culture 12. Adenoviral overexpression of esRAGE in vivo in mice reverses diabetic impairment of vascular dysfunction 101. Thus, the decoy function of esRAGE may exhibit a feedback mechanism by which esRAGE prevents the activation of RAGE signaling.
