Background: Adiponectin (ADPN) is predominantly produced by adipose tissue, and high ADPN levels have been detected in patients affected by proteinuric glomerulonephritis. In this study we investigate whether human tubular epithelial cells express and secrete ADPN.
Methods: In human proximal tubular epithelial cells, HK-2, ADPN mRNA was evaluated by real-time PCR assay, while protein expression levels were measured by Western blot analysis and immunofluorescence assay. Moreover, renal ADPN distribution was assessed by immunohistochemical analysis of kidney biopsy samples from healthy patients. Finally, by ELISA, we measured ADPN concentrations in culture media of HK-2 cells treated with 10 µg/mL lipopolysaccharide (LPS).
Results: Our analyses revealed that HK-2 cells express ADPN both in terms of mRNA and protein. These results were confirmed by the observed cytoplasmatic HK-2 intense immunoreactivity for ADPN antibody and by immunohistochemical analysis showing a diffuse ADPN distribution in normal kidney tissue. Furthermore, we observed that tubular cells secrete ADPN in the basal condition and, more interestingly, that this secretion significantly increases (p<0.05) upon LPS treatment in a time-dependent manner. Finally, immunohistochemical analysis of kidney biopsy samples obtained from patients affected by membranous and rapidly progressive glomerulonephritis showed a similar pattern of ADPN staining to that observed in healthy controls.
Conclusions: Our study demonstrates for the first time that renal tubular cells express and secrete ADPN, and their concentration increases upon inflammatory stimulus. These results suggest that in renal inflammatory diseases, tubular cells may contribute to the increase in circulating ADPN levels, triggering a feedback response in order to self-mitigate the inflammatory process.