Introduction:
Renal ischemia-reperfusion injury (IRI) is one of the major causes of acute kidney injury (AKI), which increases the risk of developing chronic kidney disease (CKD). An essential role for TGFβ family members called activins has emerged in both acute injury in IRI as well as in kidney fibrosis in CKD. Activins are formed as homo- or heterodimers of inhibin β subunits, with dimerization required for activin function. Activins A and B have both been implicated in IRI and as contributors to the fibrotic process in CKD of varying etiology. However, there is little knowledge on the role of activin C. Recently, proteome-wide Mendelian randomization studies showed that a lifetime of higher genetically determined blood concentration of the subunit of activin C (inhibin βC) is associated with worse kidney function. Further, the signaling receptor for activin C has recently been identified to be activin receptor-like kinase 7 (ALK7). The expression of activin C and ALK7 in kidney, and their role in kidney injury and fibrosis, however, are as yet unknown and are investigated here.
Methods:
Immunohistochemistry, immunofluorescence, and immunoblotting were used to analyze expression patterns and localization of ALK7 and inhibin βC in normal kidney and in models of AKI and CKD. Immortalized proximal tubular epithelial cells (HK2) and human primary proximal tubular cells were used for in vitro studies.
Results:
Immunofluorescence showed expression of inhibin βC in normal kidney, particularly in proximal tubules as colocalized with their marker megalin. ALK7 expression was also seen localized particularly to proximal tubules. Expression of both inhibin βC and ALK7 was seen in tubules of the cortex as well as the outer strip of the medulla. Western blotting analysis confirmed expression of inhibin βC in HK2 and primary human tubular cells. Nondenaturing gel analysis showed cellular and secreted activin C in the latter. AKI was induced by bilateral IRI (30 minutes) which increased the expression of inhibin βC and ALK7 in tubules. In vitro, cobalt (II) chloride to induce chemical hypoxia (100µM, 24h) in HK2 cells increased expression of ALK7. This was associated with an increase in Smad3 phosphorylation, supporting increased signaling through ALK7. In both the unilateral ureteral obstruction and the 5/6 nephrectomy model of fibrosis/CKD in mice, inhibin βC and ALK7 were increased in tubules.
Conclusions:
Inhibin βC and ALK7 are expressed in normal kidney and increased after both acute injury and in CKD models. In vitro, hypoxia induces their expression and likely augments their signaling. These data support a potential pathogenic role of inhibin βC in kidney injury. Future studies will provide more mechanistic insight into their function in kidney disease.
I have no potential conflict of interest to disclose.
I did not use generative AI and AI-assisted technologies in the writing process.