A new effector of lipid metabolism: Complement factor properdin

Abstract

Background

The complement system is well known for its role in innate immunity via the classical, the alternative and the lectin pathways, although recent investigations suggest expanding roles in adipose tissue. Properdin stabilizes C3 convertase following alternative complement activation. Properdin is also present in adipose tissue, localized to adipocyte membranes.

Aim

We evaluated the potential role of properdin in energy metabolism using properdin deficient (PKO) mice and cell based assays.

Results

PKO mice have a diet-dependent increase in weight gain compared to wild-type (WT) littermates on a high fat diet (P < 0.05), directly related to 51% increase in relative fat mass (PKO: 35.8 ± 2.2% body fat vs. WT: 23.6 ± 2.2%, P < 0.01). PKO mice have decreased energy expenditure (P < 0.01), and altered postprandial lipid clearance (P < 0.01). However glucose metabolism was unchanged after a glucose tolerance test vs. WT mice. In murine 3T3-L1 adipocytes, addition of properdin had no effect on C3 or ASP production but almost completely inhibited the insulin-mediated stimulation of fatty acid uptake and incorporation into TG. Properdin had no effect on basal or insulin-stimulated glucose transport in either 3T3-L1 adipocytes or L6 rat skeletal muscle cells.

Conclusion

Thus properdin may be added to the growing list of complement proteins (C3, adipsin, factor B, ASP (C3adesArg), factor H, C1q and C3aR) which influence lipid metabolism, energy storage and insulin resistance, and further support the hypothesis of a dual role of complement in adipose tissue.

Introduction

The complement system is a complex enzymatic cascade consisting of more than 30 plasma proteins. Activation of complement can occur through three major pathways: classical, lectin and alternative pathways. Alternative pathway (AP) activation is antibody-independent and is activated through binding to various substances, including viruses, bacteria, fungi but also C-reactive protein, protein A, polysaccharides, tumor cells and damaged cells. Moreover, spontaneous hydrolysis of the complement C3 can also lead to this activation (Ehrnthaller et al., 2011). Recent investigations suggest that activation of the complement system can occur via additional pathways involving coagulation factors, direct cellular interaction (Huber-Lang et al., 2002) or on specific targets by properdin (Kemper et al., 2010, Kimura et al., 2008). Proximal activation leads to production of C3a and C3adesArg (Acylation Stimulating Protein) and subsequently C5a, while distal activation induces the assembly of the membrane attack complex which leads to formations of pores in target membranes resulting in the death of cells by lysis (Walport, 2001).

Physiological functions of complement include defense against infection, interfacing between innate and adaptive immune systems, and the disposal of cellular debris (Walport, 2001). Moreover, complement is also involved in reproductive success, embryonic implantation and tissue regeneration, and the presence of the strong estrogen-response elements in C3 may underlie functions that seem far removed from infectious or inflammatory responses (Pattrick et al., 2009).

Properdin is a protein of 53 kDa composed of identical subunits that associate head-to-tail to form dimers, trimers and tetramers (Smith et al., 1984). Each subunit is composed of six globular domains, which are homologous to thrombospondin-1 (TSP-1) (Nolan et al., 1991). Properdin binds to C3b and is involved in alternative complement activation. C3 undergoes spontaneous hydrolysis to form C3(H₂O) (functionally similar to C3b) which associates reversibly with factor B. This association allows factor D (adipsin or fD) to cleave factor B (fB), generating C3 convertase (C3bfBb). The active enzyme C3 convertase cleaves C3, generating C3a and C3b, enhancing the cycle. C3a is then converted to C3adesArg via carboxypeptidase. Properdin protects C3b from catalysis by complement regulator factors H and I and promotes stabilization of C3 convertase up to 10 fold (Kemper and Hourcade, 2008).

In addition to its role in innate immunity, a dual role for complement has been proposed in adipocyte biology (Pattrick et al., 2009, Schaffler and Scholmerich, 2010, van Greevenbroek, 2009). The connection between complement and adipose tissue (AdT) has a long history (Sissons et al., 1976), as reviewed recently (Pattrick et al., 2009). Early descriptions of lipodystrophy coinciding with hypocomplementemia occurred prior to the first description of familial C3 deficiency presenting with acquired partial lipodystrophy (McLean and Hoefnagel, 1980, Misra et al., 2004, Savage et al., 2009, Singer et al., 1994, Sissons et al., 1976). Moreover Choy et al. and Peake et al. showed independently that alternative complement proteins are produced by the murine 3T3-L1 and 3T3-F442A adipocyte (Choy et al., 1992, Peake et al., 1997), and many other complement proteins have now been identified (Pattrick et al., 2009). The complement product C3adesArg (aka adipokine acylation stimulating protein, ASP) has a profound effect on fat storage and energy metabolism (Cui et al., 2007, Paglialunga et al., 2008, Roy et al., 2008). Recently, the production of classical complement proteins by adipose cells has been demonstrated, with implications in obesity and insulin resistance (Zhang et al., 2007). Some components of the complement system and adipocytokines have intertwining actions on the immune system, energy metabolism and insulin resistance (Mamane et al., 2009, Peake and Shen, 2010, Peake et al., 2008). Finally, properdin mRNA is expressed in adipose tissue, and the protein itself localizes to the adipocyte plasma membrane (Pattrick et al., 2009).Given the role of properdin as regulator in the alternative complement pathway and its presence in adipocytes, properdin may also play a role in adipocyte biology. In the present study this was demonstrated using properdin deficient mice and in vitro studies.