Transition of peritoneal mesothelial cells to a mesenchymal phenotype plays an integral role in the angiogenic and fibrotic changes seen in the peritoneum of patients receiving long-term peritoneal dialysis. While signaling by transforming growth factor (TGF)-beta through Smad proteins likely causes these changes, it is possible that non-Smad pathways may also play a role. Here, we found that Smad3-deficient mice were protected from peritoneal fibrosis and angiogenesis caused by adenovirus-mediated gene transfer of active TGF-beta1 to mesothelial cells; however, mesothelial transition occurred in this setting, suggesting involvement of non-Smad mechanisms. The phosphatidyl inositol 3 kinase (PI3K) target, Akt, was upregulated in both Smad-deficient and wild-type mice after exposure to TGF-beta1. In vivo inhibition of the mammalian target of rapamycin (mTOR) by rapamycin completely abrogated the transition response in Smad3-deficient but not in wild-type mice. Rapamycin blocked nuclear localization of beta-catenin independent of glycogen synthase kinase 3beta activity. Further, in Smad3-deficient mice rapamycin reduced the expression of alpha-smooth muscle actin, which is an epithelial-to-mesenchymal transition-associated gene. Hence, we conclude that TGF-beta1 causes peritoneal injury through Smad-dependent and Smad-independent pathways; the latter involves redundant mechanisms inhibited by rapamycin, suggesting that suppression of both pathways may be necessary to abrogate mesothelial transition.

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Patients with end-stage kidney disease on peritoneal dialysis often develop progressive scarring of the peritoneal tissues. This manifests as submesothelial thickening and is associated with increased vascularization that leads to ultrafiltration dysfunction. Hypoxia induces a characteristic series of responses including angiogenesis and fibrosis. We investigated the role of hypoxia in peritoneal membrane damage. An adenovirus expressing transforming growth factor (TGF) β was used to induce peritoneal fibrosis. We evaluated the effect of the mTOR inhibitor rapamycin, which has been previously shown to block hypoxia-inducible factor (HIF) 1α. We also assessed the effect of HIF1α independently using an adenovirus expressing active HIF1α. To identify the TGFβ1-independent effects of HIF1α, we expressed HIF1α in the peritoneum of mice lacking the TGFβ signalling molecule Smad3. We demonstrate that TGFβ-induced fibroproliferative tissue is hypoxic. Rapamycin did not affect the early angiogenic response, but inhibited angiogenesis and submesothelial thickening 21 days after induction of fibrosis. In primary mesothelial cell culture, rapamycin had no effect on TGFβ-induced vascular endothelial growth factor (VEGF) but did suppress hypoxia-induced VEGF. HIF1α induced submesothelial thickening and angiogenesis in peritoneal tissue. The fibrogenic effects of HIF1α were Smad3 dependent. In summary, submesothelial hypoxia may be an important secondary factor, which augments TGFβ-induced peritoneal injury. The hypoxic response is mediated partly through HIF1α and the mTOR inhibitor rapamycin blocks the hypoxic-induced angiogenic effects but does not affect the direct TGFβ-mediated fibrosis and angiogenesis.

Sensitivity of white sturgeon (Acipenser transmontanus) to copper (Cu) or cadmium (Cd) has been shown to significantly differ as a function of life-stage. This study investigated oxidative stress, metal homeostasis, and associated compensatory responses as potential mechanisms of this sensitivity pattern in three early life-stages. Sturgeon were most sensitive to Cu at 15 days post hatch (dph), which was accompanied by a significant increase in lipid peroxidation (LPO). Genes involved with amelioration of oxidative stress were significantly less inducible at this stage than in older, less sensitive fry. At 48 dph, acute lethality of sturgeon exposed to Cd was greatest and body LPO was significantly induced by 3.5-fold at 5 μg Cd/L. Moreover, there was a small but significant increase in antioxidative responses. At 139 dph, sturgeon were most tolerant to Cu and Cd and accumulation of these metals was least. Also, expression of metallothionein (MT) and apoptotic genes were greatest while expression of metal transporters was reduced and concentration of LPO was not different from controls. Our results suggest that life-stage specific sensitivity of white sturgeon to metals is complex, encompassing differences in the ability to mount compensatory responses important for metal homeostasis and combating oxidative stress and concomitant damages.

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