Background: Autophagy is a process that cells use to degrade and recycle cellular proteins, however, the role of autophagy in kidney fibrosis remains largely unknown. Results: Autophagy is responsible for the intracellular degradation of type I collagen. Conclusion: Autophagy negatively regulates and prevents excess collagen accumulation in the kidney. Significance: Our findings implicate a novel role of autophagy as a cytoprotective mechanism against renal fibrosis.

Autophagy can lead to cell death in response to stress, but it can also act as a protective mechanism for cell survival. We show that TGF-␤1 induces autophagy and protects glomerular mesangial cells from undergoing apoptosis during serum deprivation. Serum withdrawal rapidly induced autophagy within 1 h in mouse mesangial cells (MMC) as determined by increased microtubule-associated protein 1 light chain 3 (LC3) levels and punctate distribution of the autophagic vesicle-associated-form LC3-II. We demonstrate that after 1 h there was a time-dependent decrease in LC3 levels that was accompanied by induction of apoptosis, evidenced by increases in cleaved caspase 3. However, treatment with TGF-␤1 resulted in induction of the autophagy protein LC3 while suppressing caspase 3 activation. TGF-␤1 failed to rescue MMC from serum deprivation-induced apoptosis upon knockdown of LC3 by siRNA and in MMC from LC3 null (LC3 ؊/؊ ) mice. We show that TGF-␤1 induced autophagy through TAK1 and Akt activation, and inhibition of PI3K-Akt pathway by LY294002 or dominant-negative Akt suppressed LC3 levels and enhanced caspase 3 activation. TGF-␤1 also up-regulated cyclin D1 and E protein levels while down-regulating p27, thus stimulating cell cycle progression. Bafilomycin A1, but not MG132, blocked TGF-␤1 down-regulation of p27, suggesting that p27 levels were regulated through autophagy. Taken together, our data indicate that TGF-␤1 rescues MMC from serum deprivationinduced apoptosis via induction of autophagy through activation of the Akt pathway. The autophagic process may constitute an adaptive mechanism to glomerular injury by inhibiting apoptosis and promoting mesangial cell survival.

Renal fibrosis is the hallmark of virtually all progressive kidney diseases and strongly correlates with the deterioration of kidney function. The renin-angiotensin-aldosterone system blockade is central to the current treatment of patients with chronic kidney disease (CKD) for the renoprotective effects aimed to prevent or slow progression to end-stage renal disease (ESRD). However, the incidence of CKD is still increasing, and there is a critical need for new therapeutics. Here, we review novel strategies targeting various components implicated in the fibrogenic pathway to inhibit or retard the loss of kidney function. We focus, in particular, on anti-fibrotic approaches that target transforming growth factor (TGF)-β1, a key mediator of kidney fibrosis, and exciting new data on the role of autophagy. Bone morphogenetic protein (BMP)-7 and connective tissue growth factor (CTGF) are highlighted as modulators of pro-fibrotic TGF-β activity. BMP-7 has a protective role against TGF-β1 in kidney fibrosis, whereas CTGF enhances TGF-β-mediated fibrosis. We also discuss recent advances in the development of additional strategies for anti-fibrotic therapy. These include strategies targeting chemokine pathways via CC chemokine receptor 1 and 2 to modulate the inflammatory response, inhibition of phosphodiesterase to restore nitric oxide (NO)-cyclic 3′,5′ guanosine monophosphate (cGMP) function, inhibition of NADPH oxidase 1 (Nox1) and 4 (Nox4) to suppress reactive oxygen species production, as well as inhibition of endothelin-1 or tumor necrosis factor-α to ameliorate progressive renal fibrosis. Furthermore, a brief overview of some of the biomarkers of kidney fibrosis currently being explored that may improve the ability to monitor anti-fibrotic therapies. It is hoped that evidence based on the preclinical and clinical data discussed in this review leads to novel anti-fibrotic therapies effective in patients with CKD to prevent or delay progression to ESRD.

In progressive kidney diseases, fibrosis represents the common pathway to end-stage kidney failure. Transforming growth factor-β1 (TGF-β1) is a pleiotropic cytokine that has been established as a central mediator of kidney fibrosis. Emerging evidence demonstrates a complex scheme of signaling networks that enable multifunctionality of TGF-β1 actions. Specific targeting of TGF-β signaling pathway is seemingly critical and attractive molecular therapeutic strategy. TGF-β1 signals through the interaction of type I (TβRI) and type II (TβRII) receptors to activate distinct intracellular pathways involving the Smad and the non-Smad. The Smad signaling axis is known as the canonical pathway induced by TGF-β1. Importantly, recent investigations show that TGF-β1 also induces various non-Smad signaling pathways. In this review, we focus on current insights into the mechanism and function of Smad-independent signaling pathway via TGF-β-activated kinase 1 (TAK1) and its role in mediating the profibrotic effects of TGF-β1.