Signaling in Insulin-Secreting MIN6 Pseudoislets and Monolayer Cells

Cell–cell interactions are
of fundamental importance for
cellular function. In islets of Langerhans, which control blood glucose
levels by secreting insulin in response to the blood glucose concentration,
the secretory response of intact islets is higher than that of insulin-producing
beta-cells not arranged in the islet architecture. The objective was
to define mechanisms by which cellular performance is enhanced when
cells are arranged in three-dimensional space. The task was addressed
by making a comprehensive analysis based on protein expression patterns
generated from insulin-secreting MIN6 cells grown as islet-like clusters,
so-called pseudoislets, and in monolayers. After culture, glucose-stimulated
insulin secretion (GSIS) was measured from monolayers and pseudoislets.
GSIS rose 6-fold in pseudoislets but only 3-fold in monolayers when
the glucose concentration was increased from 2 to 20 mmol/L. Proteins
from pseudoislets and monolayers were extracted and analyzed by liquid-chromatography
mass spectrometry, and differentially expressed proteins were mapped
onto KEGG pathways. Protein profiling identified 1576 proteins, which
were common to pseudoislets and monolayers. When mapped onto KEGG
pathways, 11 highly enriched pathways were identified. On the basis
of differences in expression of proteins belonging to the pathways
in pseudoislets and monolayers, predictions of differential pathway
activation were performed. Mechanisms enhancing insulin secretory
capacity of the beta-cell, when situated in the islet, include pathways
regulating glucose metabolism, cell interaction, and translational
regulation.