The MCP-1/CCR2 System

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MCP-1 (also known as CCL2), the most thoroughly characterized CC chemokine, is secreted by mononuclear cells and by a variety of mesenchymal cells, including renal resident cells, and regulates the recruitment and activation of monocytes by binding to CC-chemokine receptor-2 (CCR2). In experimental settings, enhanced expression of MCP-1 has been demonstrated within the diabetic glomeruli. This occurs in an early phase of the disease, but persists during disease progression and correlates with macrophage infiltration [14, 18, 23, 74]. Upregulation of MCP-1 was also observed in tubular cells in both experimental diabetes and human DN [18, 75]. Furthermore, glomerular expression of CCR2 is strongly upregulated in renal biopsies from type 2 diabetic patients with overt nephropathy and closely correlates with the extent of proteinuria [76], indicating that in DN there is an increase in both expression of MCP-1 and responsiveness to MCP-1.

In vitro studies have clarified that diabetes-related insults, such as high glucose [77-82], advanced glycation end products (AGEs) [83-85], angiotensin II [86], mechanical stretch [86], which mimics glomerular capillary hypertension, and TGF-b1 [87, 88] are potent MCP-1 inducers in glomerular cells, providing a cellular mechanism for MCP-1 upregulation. Inflammatory cytokines also contribute as both IL-1 and TNF-a can enhance MCP-1 expression [44, 89-90]. Furthermore, in established DN both excess mesangial matrix deposition and plasma protein leaking from injured glomeruli may further increase MCP-1 expression [91] as mesangial cell adhesion to extracellular matrix components promotes MCP-1 expression and protein overload up-regulates MCP-1 expression in tubular cells [92, 93]. The transcription factor NF-kB is a convergence point for insults inducing MCP-1 overexpression. Consistently, we have shown that a ligand of Peroxisome Proliferator-Activated receptor-g (PPAR-g) prevents MCP-1 secretion in response to both stretch and high glucose by inhibiting NF-kB activity [86].

Local recruitment/activation of monocytes/macrophages is considered the predominant way by which MCP-1 contributes to the pathogenesis and the progression of DN through the mechanisms described above. However, we have demonstrated that direct effects of MCP-1 on glomerular cells also play an important role [94]. The MCP-1 receptor CCR2 is exposed by resident glomerular cells both in vitro [76, 95, 96] and in vivo [76, 96, 97]. In mesangial cells MCP-1 binding to CCR2 induces ICAM-1 upregulation and has thus a direct pro-inflammatory activity [95]. Furthermore, it enhances fibronectin production through a NF-kB-TGF-b1-dependent mechanism, resulting in prosclerotic effects [98]. Finally, MCP-1 mediates at least in part high-glucose-induced TGF-b1, fibronectin, and collagen type IV production [99]. There is also evidence that MCP-1 has direct deleterious effects in podocytes as activation of the CCR2 receptor by MCP-1 down-regulates nephrin expression via a CCR2-Rho-kinase-dependent mechanism [76], enhances apoptosis via TGF-b1 [100], and is the mediator of high glucose-induced podocyte apoptosis [100]. MCP-1 also promotes podocyte migration [96] that is of relevance in the setting of diabetes as podocyte foot process effacement is considered a migratory event. Consistently, TGF-β has been shown to induce expression of MCP-1 in culture podocytes and MCP-1, in turn, causes rearrangement of the actin cytoskeleton, cellular motility, and increased podocyte permeability to albumin [101]. Taken together these data indicate that in mesangial cells and podocytes MCP-1 can directly induce pleiotropic effects on resident glomerular cells that may contribute to the development of the phenotypic abnormalities characteristic of DN.

Studies in experimental diabetes have convincingly demonstrated the causative role of MCP-1 in the pathogenesis of DN. We and others have shown that deletion of the MCP-1 gene prevents the development of albuminuria and the rise in serum creatinine in STZ-induced diabetic mice [18, 76] and significantly reduces albumin leakage in obese ob/ob diabetic animals [24]. Furthermore, we found that nephrin downregulation was completely prevented [76] and overexpression of both fibronectin and collagen type IV significantly reduced in STZ-induced diabetic mice lacking MCP-1 [98], providing a mechanism for the anti-proteinuric and renoprotective effect of MCP-1 deprivation. Consistently, gene transfer of the 7ND gene, a N-terminal deletion mutant of human MCP-1, ameliorates glomerulosclerosis in iNOS transgenic diabetic mice [102] and attenuates diabetes-induced glomerular hypertrophy and glomerulosclerosis in STZ-induced diabetic rats [103]. An alternative approach used to lower MCP-1 signalling is to block the MCP-1 receptor CCR2. In keeping with studies directly targeting MCP-1, diabetic CCR2 knockout mice show less albuminuria and reduced expression of both fibronectin and inflammatory cytokines [104]. More recently, oral CCR2 antagonists, such as RS504393, RS102895, RO5234444, TLK-19705, have been shown to ameliorate DN functional/structural alterations in both Ins2Akita mice and db/db mice by reducing macrophage infiltration, inflammation, oxidative stress, and fibrosis [105-107]. Finally, treatment with CCX140­B that at variance with other CCR2 antagonists does not rise MCP-1 circulating levels has been shown to ameliorate glomerular hypertrophy, podocyte loss, and renal function in experimental DN [108]. Blood glucose control is, however, improved by CCR2 antagonists and likely contributes to the clinical benefit [109]. Collectively these preclinical data strongly support the hypothesis of a key role of the MCP-1/CCR2 in the pathogenesis of DN and have open the way to studies testing potential clinical applications in humans. However, agents modulating the MCP-1/CCR2 may behave differently in two species because murine MCP-1 is similar to human MCP-1, but not functionally analogous [110].