NGAL – Neutrophil gelatinase-associated lipocalin

NGAL – Description, Expression and first Functional Analysis

According to the initial description of the molecule NGAL (neutrophil gelatinase-associated lipocalin) in neutrophils (25 kDa monomer, 46 kDa homodimer linked to MMP-9) by Triebel et al. (1992) and Kjeldsen et al. (1993), tissue expression analysis of Cowland et al. (1997) and Friedel et al. (1999) showed, that this neutrophil protein is also expressed in several other tissues, especially epithelial cells, and especially in organs, which are continuously exposed to microorganisms.

This is likely due to the bacteriostatic effect of NGAL. In a complex with a bacterial siderophore and possibly in complex with an expected but not yet discovered eukaryotic siderophore-like protein NGAL is able to bind iron whereby microorganisms are deprived from the survival factor Fe3+ (Goetz et al., 2002).
Similarly NGAL probably acts as an iron shuttle in extracellular spaces and also affects the intracellular iron concentration, which in turn influences the expression of many iron-dependent genes, including those which are not directly involved in iron metabolism. The effects on the cell may vary according to the iron levels and depending on the degree of differentiation, stage or other conditions.

Although the functions of NGAL are not fully understood, it seems that it comes to increased NGAL production in cells under “stress”, especially in areas with fast epithelial turnover, such as malignant or inflamed epithelia (Friedl et al., 1999; Stoesz et al., 1995; Nielsen et al., 1996), but also in atherosclerotic lesions and after ischemic injury (Hemdahl et al., 2006), or in back forming tissues (e.g. mouse uterus post-partum, mammary glands after weaning). Contrary to the assumption that NGAL is involved in apoptosis, it seems likely that the expression of NGAL is associated with a maintenance response. The increase of NGAL in the kidney (see below) could be due to an ongoing repair mechanism of the tubular epithelium, in which NGAL might serve as a differentiation factor for the conversion of metanephric mesenchymal cells into tubular epithelial cells (Yang et al., 2002).

The NGAL expression may be increased in various cancers (often together with MMP-9). This has been shown in several publications by detection of the expression of NGAL in tumor cells and on the basis of high urinary NGAL levels both, in free form and in a complex with MMP-9. It has been suggested to use urinary NGAL-MMP-9 as a marker of disease status in breast cancer patients (Fernández et al., 2005) and as indication for the presence of brain tumor (Smith et al., 2008).

At inflammation and infection NGAL is released from the secondary granules of activated neutrophil granulocytes (Kjeldsen et al., 1993) and shows in infectious diseases, especially bacterial infections, an increase in plasma levels (Xu et al., 1995).

However, the most interesting discovery was an early and extreme upregulation of NGAL in kidney cells, which was first dicovered in the homologous mouse protein 24p3 after SV40 infection (Hraba-Renevey et al., 1989) and later observed after ischemia-reperfusion injury in proximal tubule cells of rats (Matthew et al., 2001).
In humans increased NGAL plasma levels in patients with kidney damage due to a systemic vasculitis showed a strong correlation with decreased renal function (Ohlsson et al., 2003), and an NGAL increase could be confirmed in ischemia-reperfusion injury as well as extended to nephrotoxic substances (Mishra et al., 2003; Armin et al., 2004; Mishra et al., 2004).

Mishra et al. showed in 2005, that NGAL levels in urine in children could serve as an early marker for ischemic renal injury after cardiopulmonary bypass. An increase of NGAL levels in human urine or plasma can already be detected 2 hours after the onset of kidney injury (Mishra et al., 2005), which represents a significant time saving for the detection of acute renal failure compared to conventional markers such as Creatinine or Cystatin C.

These publications build the starting point for global studies on the use of NGAL as renal biomarker in different clinical questions.

NGAL article on CLI-Online