Low-level C-reactive protein levels exert cytoprotective actions on human podocytes

Albuminuria and elevated C-reactive protein (CRP) levels are common manifestations of many inflammatory diseases. Cardiovascular-based drugs, with secondary anti-inflammatory actions, such as angiotensin-converting enzyme-inhibitors are able to reduce both proteinuria and CRP levels, raising the question of whether CRP directly influences the processes that result in proteinuria. As proteinuria is thought to be induced as a result of podocyte dysfunction, we investigated whether there is a pathomechanistic link with CRP.

Podocytes were analysed for evidence of endogenous CRP production in response to inflammatory agents. In addition, they were incubated in the presence of various concentrations of exogenous CRP and analysed for evidence of a response to treatment.

Our results demonstrated that inflammatory agents such as macrophage-conditioned medium and interleukin-1β induced the expression of CRP messenger RNA in podocytes. However, they were unable to induce CRP protein. Stimulation of podocytes with exogenous CRP demonstrated that 10 μg/mL CRP induced a low but significant level of interleukin-6 secretion. Tumour necrosis factor α, however, was not detected. CRP did up-regulate the expression of the slit diaphragm proteins nephrin and CD2AP, as well as the structural proteins ezrin and podocalyxin-like protein-1, proteins known to be involved in signalling via the phosphotidylinositol-3 (PI-3) kinase pathway. CRP exposure reduced caspase-3 enzyme activity and up-regulated the expression of the anti-apoptotic protein Bcl-2. In the presence of the PI-3 kinase inhibitor LY294002, the ability of CRP to suppress caspase-3 activity was significantly reduced.

Taken together, these data suggest that rather than inducing podocyte damage, CRP may be a survival factor for podocytes by maintaining their structural integrity and initiating a survival cascade, which may facilitate podocyte recovery from injury.