Doxycycline Dc is of special interest because
Doxycycline (Dc) is of special interest because it is the most potent MMP inhibitor of the tetracycline-class antibiotics. Its mechanism of action involves Zn chelation, a property distinct from the antibacterial action . Many studies have demonstrated the beneficial use of tetracycline for several inflammatory diseases in which MMPs play a pathological role , , . In the kidney, Dc treatment attenuates IRI by reducing oxidative stress, inflammation, and apoptosis as well as facilitating repair , , , , , . To the best of our knowledge, other than the molecular benefits of Dc, the outcomes of renal function protection by I/R have not been demonstrated. Another issue involves the appropriate Dc dose needed to improve renal function without causing bacterial resistance. We hypothesized that intraperitoneal treatment with low doses of Dc prevents glomerular filtration rate (GFR) decrease and the disruption in tubular Na+ handling. To test this hypothesis, we used a model of bilateral kidney ischemia for 30min followed by 24h reperfusion. The protection provided by treatment with low doses of Dc 2h before induction of ischemia was evaluated by measuring GFR and the activity of the primary Na+ transporters, as well as the Protease Inhibitor Cocktail levels of fibronectin, TGFβ, and MMPs.
Materials and methods
> Discussion We provide evidence that intraperitoneal administration of low-dose Dc (3mg/kg) protects kidney function (glomerular filtration and epithelial Na+ transport) from IRI. To our knowledge, there are no previous studies comparing different low doses of Dc on the renal function impairment provoked by I/R, although its beneficial effects on kidney structure and signaling pathways associated with kidney injury have been reported , , , , , . In our study, the usual Dc dose was 10mg/kg, administered either orally or by intraperitoneal injection. We have demonstrated that treatment with 3mg/kg Dc attenuates total MMP activities, restores MMP-2 and -9 protein contents, fibronectin deposition, and tubular TGF-β expression. All of these changes appear to be associated with preservation of GFR and tubular Na+ transport. Notably, Dc 3mg/kg, the dose with beneficial effects in renal function in I/R rats, did not affect kidney function and structure when administered to sham-operated rats. The transient ischemic episode (30min) followed by 24h reperfusion induced polyuria and proteinuria, reduced GFR, increased urea-nitrogen accumulation in the blood, and diminished FENa. Moreover, the alteration in kidney morphology (score of 2 in the cortex and 2.5 in the medulla) corresponded to a mild tubular injury which is similar to that found in a previous rat model of AKI , , , , , . This disrupted renal function in an early stage of IRI can be related to rapid alterations in the renal cortex structures, interstitial fibronectin accumulation, and augmented tubular TGFβ1 immunoexpression. Although fibrosis has been generally accepted as a marker of chronic kidney injury, it was also demonstrated in IRI. Yang et al.,  linked fibrosis to cell cycle arrest in five different models of acute tubular injury. The authors elegantly demonstrated that the development of fibrosis in each AKI model correlated with the arrest of proximal tubule cells cycle in G2/M. Injured tubular cells drive damage and inflammation by releasing profibrotic factors, in particular TGF-β, identifying proximal tubule cells as a major player in injury and fibrosis . In the kidney cortex, we observed interstitial fibronectin accumulation and increase in tubular TGF-β1immunoexpression. Treatment with 3mg/kg Dc hinders these events, thus we propose that Dc may impede acute kidney injury from progressing to chronic kidney disease. The immunoexpression of TGF-β in the glomeruli of the control rat could be related to the regulation of the extracellular matrix mediated by mesangial cells, since these cells express and respond to TGF-β1 signaling , . The same profile was observed in patients with established structural kidney injury that presented a significant increase in serum creatinine and decreased GFR .