This study was designed to investigate the effect of CLD and CKD on RBP4 isoforms and to identify factors influencing and/or generating RBP4 isoforms. We were able to show that the relative amount of RBP4 isoforms (apo-RBP4, RBP4-L, RBP4-LL) was increased in CKD patients, but not in CLD patients, in comparison to controls. Our results also show that RBP4 levels were significantly elevated in serum of CKD patients compared to both, CLD patients and controls. In contrast, RBP4, TTR and ROH levels were significantly decreased in CLD patients, as compared to CKD patients and controls.
Jaconi et al.  investigated RBP4-L and RBP4-LL in the serum of hemo-dialysis patients and regarded the occurrence of RBP4 isoforms to be specific for CKD [11, 17]. To date, RBP4 isoforms have been investigated exclusively in a small number of patients ( and , respectively) suffering from CKD [11, 17] and not in CLD patients. Our data show that RBP4-L and RBP4-LL, which are truncated at the C-terminal end of the molecule, were increased in CKD (Figure 1). In contrast to that, in CLD patients – irrespective of the kind of liver disease – there were no increased amounts of RBP4-L and RBP4-LL, thus supporting the relation between RBP4 isoforms and kidney function. The increased survival and retention of RBP4 in the circulation during CKD may contribute to the increased truncation of RBP4. Although there is evidence that a specific carboxypeptidase is responsible for the truncation [17, 20], the physiological impact of RBP4-L and RBP4-LL is not known. However, RBP4-L and RBP4-LL isolated from CKD serum, inhibit chemotaxis and oxidative metabolism of polymorphonuclear leucocytes. These alterations in leucocytes activity may disturb immune defense in these patients . In addition, the C-terminal end of RBP4 is involved in ROH binding, and therefore, RBP4 modifications may also influence the interaction with TTR [7, 25].
Additionally, we confirmed that RBP4, TTR and ROH levels in various liver diseases were markedly depressed, particularly in patients with c2-cirrhosis or hepato-cellular carcinoma, which is in accordance with results of previously published studies [16, 26–28]. This decrease is due to a loss of functional hepatic tissue resulting in decreased synthesis of RBP4 and TTR and decreased release of the ROH-transport complex into the circulation [23, 27].
In patients with CKD, the levels of RBP4 were markedly elevated and therefore the molar ratio of RBP4 to TTR was increased. In healthy states, TTR is present in a 3–5 fold molar excess in plasma and the serum RBP4/TTR ratio is approximately 0.4 whereas in CKD patients an increase in the RBP4/TTR molar ratio up to 1.06 has been reported [16, 18, 29, 30]. This is consistent with the 3-fold elevated RBP4/TTR ratio from 0.36 in controls to 0.96 in CKD in our study. Due to the increase of RBP4 and the simultaneous fall in TTR levels in CKD, almost one molecule of TTR and one molecule of RBP4 are present in the circulation [16, 18, 31]. The decrease in TTR levels in CKD may be due to malnutrition and/or infectious disease [16, 29].
The kidneys play an important role in the recycling of RBP4 since RBP4 catabolism is disturbed in CKD patients [16, 31]. According to previous studies elevated serum creatinine levels, a marker for kidney dysfunction, are associated with high serum concentrations of RBP4 [16, 32]. This is due to the loss of functional tissue and/or the entire nephron in kidney failure, which leads to decreased filtration of creatinine and abnormal survival of small serum proteins resulting in an increase of their serum levels [10, 33]. This might explain the increased RBP4 levels in CKD (Table 2). Under physiological conditions 98% of RBP4 is bound to ROH (holo-RBP4) and 2% circulate ROH free as apo-RBP4 [18, 34]. In this study we show that the percentage of plasma apo-RBP4 is highly increased in CKD patients compared to controls and CLD patients, thus supporting early findings [20, 35]. Nearly all of the apo-RBP4 is normally glomerularly filtered and reabsorbed by the kidney proximal convoluted tubules. The increase in the molar ratio of RBP4 to ROH in our CKD patients indicates an excess of RBP4 over ROH, leading to an increase in RBP4 unbound to ROH, which is consistent with the increase in apo-RBP4. The altered holo- to apo-RBP4-ratio in CKD complies also with previous results indicating that impaired kidney function compromises sufficient metabolisation of apo-RBP4 from serum [14, 20, 31, 36]. This finding is confirmed by the correlation of apo-RBP4 and serum creatinine in our study.
The alterations in RBP4 metabolism during CKD are of interest in relation to T2DM since T2DM patients are exposed to increased oxidative stress which has been reported to be linked to endothelial dysfunction . It is known that T2DM patients often suffer from kidney dysfunction  and therefore the RBP4-L and RBP4-LL may further enhance oxidative stress through their action on polymorphonuclear leucocytes .