Background: Proximal tubule epithelial cells (PTEC) play a central role in the response of the kidney to insult by virtue of their production of chemokines and cytokines that signal an inflammatory response. Bone morphogenic protein-7 (BMP-7/OP-1), a member of the transforming growth factor-beta (TGF-beta) superfamily, has previously been demonstrated to reduce macrophage infiltration and tissue damage in animal models of acute and chronic renal failure. The present study was designed to define the molecular mechanism of BMP-7 action in human PTEC.
Methods: Expression of BMP-7 in the adult mouse kidney was determined indirectly through X-gal staining of heterozygous BMP-7/lacZ mice in combination with cell-type specific markers. Primary human PTEC were cultured in the presence of the pro-inflammatory cytokine, tumor necrosis factor-alpha (TNF-alpha), with and without BMP-7. RNA isolated from these two populations was then used to identify differentially regulated genes via gene-array analysis. Modulation of potential target genes was subsequently confirmed through ELISA and/or quantitative PCR.
Results: Expression from the BMP-7/lacZ transgene was detected in the collecting duct, thick ascending limb, distal convoluted tubule, and podocytes within glomeruli. No expression was detected within PTEC; however, these cells were found to express mRNA for BMP receptors including, ActR-I, BMPR-IA, ActR-II, ActR-IIB, and BMPR-II. BMP-7 significantly reduced TNF-alpha stimulated increases in mRNA for the pro-inflammatory genes, interleukin-6 (IL-6) and interleukin-1beta (IL-1beta), and the chemoattractants monocyte chemotactic protein-1 (MCP-1) and interleukin-8 (IL-8) in primary human PTEC. In addition, BMP-7 also reduced the expression of mRNA for endothelin-2 (ET-2), a vasoconstrictor, and increased the expression of mRNA for heme oxygenase-1 (HO-1), a vasodilator, although the latter was not statistically significant. In experiments designed to examine MCP-1 and IL-6 protein levels in response to additional TGF-beta superfamily members, TGF-beta1 was unable to mimic the effects of BMP-7 in reducing IL-6 production. However, the closely related BMP-6 exhibited similar properties to those of BMP-7. Each of the factors reduced MCP-1 expression.
Conclusions: BMP-7 represses the basal and TNF-alpha-stimulated expression of the pro-inflammatory cytokines IL-6 and IL-1beta, the chemokines MCP-1 and IL-8, and the vasoconstrictor ET-2 in PTEC. This data are consistent with the in vivo observations that BMP-7 administration in a model of chronic and acute renal failure results in a reduction in the infiltration of macrophages in the renal interstitium. Taken together, these observations suggest that BMP-7 may be a novel therapeutic agent for kidney disorders involving inflammation and ischemic damage of PTEC.