Splice Isoforms of the Apoptosis Gene <i>BCL-X</i> Have Opposing Effects in Diabetic Kidney Disease: Potential Treatment Target and Prognostic Value

In recent years, the importance of alternative splicing (AS) of certain genes in the nature and progression of diabetic nephropathy (DN) has been studied. We report a novel AS event observed in the diabetic kidney-AS of the apoptosis gene BCL-X to increase the proapoptotic BCL-XS and decrease the antiapoptotic BCL-XL. This study is aimed at further investigating the role of this novel AS event in the pathogenesis of DN. To characterize important splicing events in progression of DN, human glomerular endothelial cells (GEnCs) were exposed to a diabetic environment for 1 week, and RNAseq was performed. Although several splicing events were discovered, we focused on the apoptosis gene BCL-X in the present study. In GEnCs, an upregulation of the BCL-XS/BCL-XL ratio was observed, resulting in an increase in GEnC apoptosis. An upregulation of IL-6 was also observed, and treatment with IL-6 alone induced a dose-dependent shift in BCL-X splicing to promote expression of the proapoptotic BCL-XS. Furthermore, we identified certain splicing factors, SF3B1 and PTBP1, involved in BCL-X splicing regulation. Overexpression of the antiapoptotic BCL-XL isoform rescued apoptosis, suggesting a possible therapeutic avenue. In patients, an increase in the proapoptotic BCL-XS in urinary RNA correlated with a decline in the glomerular filtration rate, whereas in blood it correlated with the level of albuminuria. There is an increase in the proapoptotic BCL-XS isoform in the diabetic glomerular endothelium, resulting in increased GEnC apoptosis. Increased BCL-XS expression correlates with markers of renal function decline, implicating this AS event as a potential biomarker for DN severity. Furthermore, a switch of isoforms from BCL-XS to BCL-XL is highlighted as a novel therapeutic strategy.