Beta Cell Proinsulin Response to Amino Acid Availability

Through fasting and feeding (both in vivo and in vitro), nutrient availability fluctuates over time. Pancreatic β-cells must respond to nutrients not only by regulated insulin secretion but also by insulin synthesis, which depends upon proinsulin. β-cells respond biosynthetically to glucose, but little is known about amino acids as regulators of proinsulin levels. We find that in INS1E (rat) β-cells, isolated murine islets, and isolated human islets-even in the presence of abundant glucose, proinsulin levels remain sensitive to reduced availability of other nutrients contained in β-cell/islet bathing medium. A low proinsulin level upon in vitro fasting is accompanied by diminished ribosomal phospho-S6 and phospho-eIF4E-BP1 plus increased phospho-eIF2α, reversed upon re-feeding. Even as extracellular glucose levels remained steady, INS1E cells consumed the majority of extracellular glutamine and serine (measured by mass spectrometry); concurrently, six nonessential amino acids (most notably proline and alanine, plus glutamic and aspartic acid) were actually exported from cells to media. Proinsulin suppression in response to amino acid limitation appears to be β-cell-autonomous, is linked to proinsulin biosynthesis, and is ameliorated upon GCN2 inhibition in INS1E cells. Upon re-feeding amino acid-containing medium, the mTORC1 inhibitor rapamycin fully suppressed phospho-S6 but did not block the recovery of proinsulin levels, whereas Torin1 inhibition of mTOR inhibited the amino acid-dependent increase in proinsulin biosynthesis. Altogether, these data indicate that inadequate amino acid-triggered β-cell signaling can blunt the proinsulin biosynthetic response, contributing to insulin deficiency-a finding that has potential relevance to the pathogenesis of malnutrition-related diabetes.