Overexpression of PTHrP in beta cells reveals key effects on endoplasmic reticulum stress

Administration of parathyroid hormone-related protein (PTHrP) has been shown to increase insulin content and secretion in mice. PTHrP also increases beta-cell mass and proliferation. However, the mechanisms for these effects are unknown, and investigations have yet to examine PTHrP in the transcriptome. In this study, we transiently transfected a mouse beta-cell line (MIN6) with either full-length PTHrP or DSred vector. Insulin content and glucose-stimulated insulin secretion were measured, and RNA from the cells after incubating in 20mM glucose was collected. The results showed that PTHrP overexpression increased insulin content and the ratio of insulin secretion between low and high glucose (stimulation index). RNA-sequencing showed that PTHrP overexpression downregulated many genes associated with responses to endoplasmic reticulum (ER) stress such as Hspa40, Dnajc3 and Xbp1. Among enriched KEGG pathways, the ER stress gene pathway was the most strongly downregulated by far, and the most upregulated pathway was for biosynthesis of amino acids required for protein synthesis. These pathways suggest increased rates of protein biosynthesis. qPCR supported RNA-sequencing results for several ER stress genes (Xbp1, Bax, Bip). MIN6 cells transfected with PTHrP also had lower proinsulin-to-insulin ratio, indicting that PTHrP enhanced insulin processing in the ER. Our working hypothesis is that PTHrP augments insulin production and ER efficiency, which is consistent with observations of increased insulin content, decreased proinsulin-to-insulin ratio and reduced ER stress markers in MIN6 cells. In conclusion, our findings suggest a previously unknown role for PTHrP in beta-cell endoplasmic reticulum, which may have therapeutic implications for enhancing insulin production.