Investigation of the transforming growth factor-beta 1 signalling pathway as a possible link between hyperphosphataemia and renal fibrosis in feline chronic kidney disease.

Chronic kidney disease (CKD) is common in geriatric cats, and is characterised in the majority of cases by tubulointerstitial inflammation and fibrosis. Hyperphosphataemia is a frequent complication of CKD and is independently associated with severity of renal fibrosis and disease progression. Transforming growth factor-beta 1 (TGF-β1) signalling is thought to be a convergent pathway which mediates the progression of renal fibrosis in CKD. The aims of this study were to explore the interaction between increased extracellular phosphate and the TGF-β1 signalling pathway by investigating: (a) the effect of a commercially available, phosphate-restricted, diet on urinary TGF-β1 excretion in cats with CKD; and (b) the role of increased extracellular phosphate in regulating proliferation, apoptosis, and expression of genes related to TGF-β1 signalling and extracellular matrix (ECM) production in feline proximal tubular epithelial cells (FPTEC) and cortical fibroblasts from cats with azotaemic CKD (CKD-FCF). The dietary intervention study revealed no effect of dietary phosphate restriction on urinary active TGF-β1 excretion after 4-8 weeks (P=0.98), despite significantly decreasing serum phosphate (P<0.001). There was no effect of increased growth media phosphate concentration (from 0.95mM to 2mM and 3.5mM) on proliferation (P=0.99) and apoptotic activity in FPTEC (P=0.22), or expression of genes related to ECM production and the TGF-β1 signalling pathway in FPTEC and CKD-FCF (P>0.05). These findings suggest the beneficial effects of dietary phosphate restriction on progression of feline CKD may not occur through modulation of renal TGF-β1 production, and do not support a direct pro-fibrotic effect of increased extracellular phosphate on feline renal cells.