Transforming growth factor beta signal transduction in the kidney

Epithelial-mesenchymal transition in renal tubular cells in the pathogenesis of progressive tubulo-interstitial fibrosis

Epithelial-mesenchymal transition (EMT) plays an important role in embryogenesis and organ formation. Over the last 10-15 years it has been established that EMT is a significant mechanism of tumor progression and metastasis formation and also of progressive tissue fibrosis in the kidney, liver and lung. EMT seen in these diverse physiological and pathophysiological contexts shares a number of stages and modules, but also carries distinct, context specific characteristics. EMT in tissue fibrosis is a form of reverse embryogenesis, when highly specialized epithelial cells in the specific organs will respond to injury with loosing their epithelial characteristics and functions and regaining characteristics of the cells from which they originated. EMT in the context of tissue fibrosis can be induced by different forms of injury or a set of humoral factors. The process is regulated by a complex balance of humoral and microenvironmental stimuli, in which cell-cell contacts and interaction of the transitioning cell with the extracellular matrix components is very important. Intense research in this exciting field yielded good understanding of many of the details of this fascinating process, although numerous questions still await proper answers. There is indication that understanding of the molecular mechanisms underlying "fibrotic" EMT may lead to the design of specific and effective therapeutic measures for progressive tissue fibrosis.

Beta-hydroxybutyrate-induced growth inhibition and collagen production in HK-2 cells are dependent on TGF-beta and Smad3

BACKGROUND

Ketonuria is common in diabetes. The major form of ketone body is beta-hydroxybutyrate (beta-HB), which is metabolized by the proximal tubule. Transforming growth factor beta (TGF-beta) and tubulopathy are important in diabetic nephropathy. Thus, the role of TGF-beta and the downstream Smad3 in beta-HB-induced effects in the human proximal tubule (HK-2 cell) was studied.

METHODS

Effects of beta-HB (0.1 to 10 mmol/L) on HK-2 cells were determined for: proliferation, cell cycle distribution, collagen production, tubular transdifferentiation [expression of alpha-smooth muscle actin (alpha-SMA) protein], TGF-beta, Smad2/3, p21WAF1, and p27kip1.

RESULTS

Beta-HB (0.1 to 10 mmol/L) dose dependently decreased proliferation, arrested the cells in G0/G1 phase of the cell cycle, and increased p21WAF1/p27kip1 protein expression at 48 hours (without affecting p21WAF1/p27kip1 mRNA and transcription). beta-HB (1 mmol/L) increased p21WAF1/p27kip1 protein half-lives. Beta-HB (1 mmol/L) increased TGF-beta transcription at 24 hours and TGF-beta1 mRNA/bioactivity at 48 hours. Beta-HB (1 mmol/L) increased nuclear Smad2/3 protein expression and increased collagen production (without affecting tubular transdifferentiation), which were reversed by Smad7, dominant-negative Smad3, and N-acetylcysteine. Dominant-negative Smad3 reversed beta-HB-induced TGF-beta transcription at 24 hours, and reversed TGF-beta1 bioactivity at 48 hours. Dominant-negative Smad3 reversed beta-HB-induced p21WAF1/p27kip1 protein expression at 48 hours. Finally, N-acetylcysteine, TGF-beta antibody, Smad7, and dominant-negative Smad3 reversed beta-HB (1 mmol/L)-induced growth inhibition at 48 hours.

CONCLUSION

Beta-HB activated Smad 2/3 by oxidative stress. TGF-beta and Smad3 mediate beta-HB-induced cell cycle-dependent growth inhibition while Smad3 mediate beta-HB-induced collagen production and p21WAF1/p27kip1 protein expression in HK-2 cells. Moreover, beta-HB increased p21WAF1/p27kip1 protein expression by increasing p21WAF1/p27kip1 protein stability.

Cellular and molecular parameters in human renal allograft rejection

Acute rejection of human renal allografts is frequent postransplantation complication. In addition, it is a risk factor for chronic rejection, the most common cause of failure of long-term allografts. Renal allografts are rejected as a result of an immune response directed against alloantigens on the graft that are absent from the host, and the most important of these are the HLA antigens. The application of molecular diagnostic methods has revealed a differential intra-renal gene expression of cytokines, chemokines and their receptors, and cytotoxic attack molecules in acute and chronic rejection processes. Differential expression of T cell costimulatory molecules B7 and CD40/CD40L, and endothelium adhesion molecules ICAM-1 and VCAM-1 has also been reported during acute rejection. These molecules play an important role in mediating the recruitment of lymphocytes into rejecting allografts and costimulation of T cell activation. Based on experimental data, it seems that it is likely that the blockade of T cell costimulatory pathways can be used in human in the future to selectively prevent transplant rejection without generally suppressing the immune system.