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.

Reactive oxygen species (ROS) are critical determinants of the fate of hematopoietic stem cells (HSCs) and hematopoiesis. Thioredoxin-interacting protein (TXNIP), which is induced by oxidative stress, is a known regulator of intracellular ROS. Txnip(-/-) old mice exhibited elevated ROS levels in hematopoietic cells and showed a reduction in hematopoietic cell population. Loss of TXNIP led to a dramatic reduction of mouse survival under oxidative stress. TXNIP directly regulated p53 protein by interfering with p53- mouse double minute 2 (MDM2) interactions and increasing p53 transcriptional activity. Txnip(-/-) mice showed downregulation of the antioxidant genes induced by p53. Introduction of TXNIP or p53 into Txnip(-/-) bone marrow cells rescued the HSC frequency and greatly increased survival in mice following oxidative stress. Overall, these data indicate that TXNIP is a regulator of p53 and plays a pivotal role in the maintenance of the hematopoietic cells by regulating intracellular ROS during oxidative stress.

Transforming growth factor-␤1 (TGF-␤1) is a multifunctional cytokine that signals through the interaction of type I (T␤RI) and type II (T␤RII) receptors to activate distinct intracellular pathways. TAK1 is a serine/threonine kinase that is rapidly activated by TGF-␤1. However, the molecular mechanism of TAK1 activation is incompletely understood. Here, we propose a mechanism whereby TAK1 is activated by TGF-␤1 in primary mouse mesangial cells. Under unstimulated conditions, endogenous TAK1 is stably associated with T␤RI. TGF-␤1 stimulation causes rapid dissociation from the receptor and induces TAK1 phosphorylation. Deletion mutant analysis indicates that the juxtamembrane region including the GS domain of T␤RI is crucial for its interaction with TAK1. Both T␤RI-mediated TAK1 phosphorylation and TGF-␤1-induced TAK1 phosphorylation do not require kinase activity of T␤RI. Moreover, T␤RI-mediated TAK1 phosphorylation correlates with the degree of its association with T␤RI and requires kinase activity of TAK1. TAB1 does not interact with TGF-␤ receptors, but TAB1 is indispensable for TGF-␤1-induced TAK1 activation. We also show that TRAF6 and TAB2 are required for the interaction of TAK1 with T␤RI and TGF-␤1-induced TAK1 activation in mouse mesangial cells. Taken together, our data indicate that TGF-␤1-induced interaction of T␤RI and T␤RII triggers dissociation of TAK1 from T␤RI, and subsequently TAK1 is phosphorylated through TAB1-mediated autophosphorylation and not by the receptor kinase activity of T␤RI.