Insulin receptor isoform switching in intestinal stem cells, progenitors, differentiated lineages and tumors: evidence that IR-B limits proliferation

Abstract

Despite evidence for impact of insulin on intestinal epithelial physiology and pathophysiology, the expression patterns, roles, and regulation of insulin receptor (IR) and IR isoforms in the intestinal epithelium are not well characterized. IR-A is thought to mediate proliferative effects of insulin or IGFs in fetal or cancer cells. IR-B is considered the metabolic receptor for insulin in specialized tissues. This study employed a novel Sox9-EGFP reporter mouse that permits isolation of intestinal epithelial stem cells (IESC), progenitors, enteroendocrine cells, and differentiated lineages, the ApcMin/+ mouse model of precancerous adenoma, and normal human intestinal and colorectal cancer cell (CRC) lines. We tested the hypothesis that there is differential expression of IR-A or IR-B in stem and tumor cells versus differentiated intestinal epithelial cells (IEC) and that IR-B impacts cell proliferation. Our findings provide novel evidence that IR-B expression is significantly lower in highly proliferative IESC and progenitor cells versus post-mitotic, differentiated IEC and in subconfluent/undifferentiated versus differentiated Caco-2 cells. IR-B is also reduced in ApcMin/+ tumors and highly tumorigenic CRC cells. These differences in IR-B were accompanied by altered levels of mRNAs encoding muscleblind-like 2, a known regulator of IR alternative splicing. Forced IR-B expression in subconfluent/undifferentiated Caco-2 cells reduced proliferation and increased biomarkers of differentiation. Our findings indicate that impact of insulin on different cell types in the intestinal epithelium may differ depending on relative IR-B∶ IR-A expression levels and provide new evidence for roles of IR-B to limit proliferation of CRC.