Evidence of angiogenesis and microvascular regression in autosomal-dominant polycystic kidney disease kidneys: A corrosion cast study

Inhibition of tubule-cell proliferation to prevent cyst formation: a new avenue to treat ADPKD?

Tubule-cell hyperproliferation precedes cyst development in autosomal dominant polycystic kidney disease (ADPKD). Parker et al. report that insulin-like growth factor-1 stimulates ADPKD cell proliferation by activating Ras and Raf; inhibition of Ras or Raf abolished this effect. Inhibiting tubule-cell proliferation could halt cyst formation and prolong survival of functional tubules, offering a new treatment for ADPKD patients.

Involvement of hypoxia-inducible transcription factors in polycystic kidney disease

In polycystic kidney disease (PKD), erythropoietin (EPO) production and interstitial vascularization are increased compared with other kidney diseases. EPO and several angiogenic factors are controlled by hypoxia-inducible transcription factors (HIFs), which are composed of a constitutive beta-subunit and two alternative alpha-subunits (HIF-1alpha, HIF-2alpha). We hypothesized that cyst expansion may result in pericystic hypoxia and consecutive up-regulation of HIF and thus examined the expression of HIF-alpha and HIF target genes in human PKD and in a rodent PKD model. HIF-1alpha and HIF-2alpha were found to be up-regulated in cyst epithelium and cells of cyst walls, respectively. The distinct expression pattern of the HIF-alpha isoforms closely resembles the respective pattern in normal kidneys under systemic hypoxia. Pimonidazole staining, a marker for tissue hypoxia, confirmed the existence of regional hypoxia in polycystic kidneys. Immunohistochemistry for selected target genes implicated a role for HIF-1alpha in vascular endothelial growth factor and Glut-1 activation and HIF-2alpha in endoglin and EPO stimulation. Polycystin-deficient cells showed physiological, oxygen-dependent HIF-alpha modulation, excluding a direct influence of polycystin deficiency on HIF-alpha regulation. In conclusion, HIF accumulation in human and rat PKD seems to be responsible for increased EPO production and pericystic hypervascularity and may have an impact on progression of PKD.

Hypoxia-Inducible Transcription Factors and Their Role in Renal Disease

The 2 hypoxia inducible factors (HIF)-1alpha and HIF-2alpha are key mediators of cellular adaptation to hypoxia. They show a specific distribution pattern and possibly have complementary transcriptional targets in the kidney: HIF-1alpha is found mainly in tubular and HIF-2alpha in peritubular interstitial, endothelial, and glomerular cells. Both isoforms are regulated by oxygen-dependent hydroxylation of specific amino acid residues, which determines protein stability and transcriptional activity. Small molecule inhibitors of HIF hydroxylases act as pharmacologic inducers of HIF. HIF target genes are involved in cellular mechanisms that increase hypoxia tolerance or improve oxygen supply at the systemic or regional level, but also have been implicated in cellular apoptosis and profibrotic mechanisms. In experimental acute kidney injury the up-regulation of HIF either through endogenous hypoxia-sensing or after pharmacologic HIF stabilization confers tissue protection. Thus, HIF stabilization offers a promising novel and clinically feasible approach for nephroprotection. On the other hand, continuous activation of the HIF system occurs in kidney cancer and potentially promotes tumor growth. HIF therefore also is explored as a target for anticancer therapy.