CHANGE IN ENVIRONMENTAL GLUCOSE CONCENTRATION ALTERS ASTROCYTIC INSULIN AND INSULIN RECEPTOR LEVELS

Abstract

BACKGROUNDThe actions of insulin in the brain include effects on glucose homeostasis, cell proliferation, cell differentiation, cognition, and memory. Cellular constituents of the neurovascular unit, including the astrocyte, contribute to insulin responses in the brain. Astrocytes are an ideal target cell type for investigation because of their dynamic character, multitude of functions, and their important role in brain homeostasis. We hypothesize that astrocytes respond to environmental glucose changes and could be a source of insulin supply to the microenvironment.OBJECTIVETo investigate astrocyte insulin and insulin receptor response to environmental glucose changes and to determine insulin secretion levels.METHODSNeonatal rat astrocytes in culture were exposed to changes in environmental glucose levels (LOW‐ 0mM, NORMAL‐5.5mM, and HIGH‐25mM) over three passages. Astrocyte whole cell lysates were used to determine message (qPCR) and protein levels (Western blot) for insulin and insulin receptor. Insulin Ultrasensitive ELISA was used to measure NORMAL astrocytic insulin secretion in response to glucose (25mM) and potassium (30mM) stimuli.RESULTSAstrocytes grown in LOW, NORMAL, and HIGH glucose levels resulted in equal cell counts and morphological appearances. Increasing glucose level from LOW, to NORMAL, or to HIGH in astrocytic culture media significantly increased insulin‐1 (exon 1–2) and insulin‐2 (exon 1–2) mRNA levels (One‐way ANOVA, n=4, p<0.05). Increased glucose level also resulted in a positive correlation with increased protein levels of insulin and insulin receptor in astrocytic cell lysate (Pearson Correlation, r=>0.85, p<0.01, n=3, R2=>0.73). Astrocytes secreted measurable levels of insulin (0.02–0.05μg/L) but did not increase significantly with glucose or potassium stimuli.CONCLUSIONSAstrocytes respond to changes in environmental glucose levels with increased insulin and insulin receptor message and protein levels. Astrocytes secrete measurable amounts of insulin but do not respond to stimuli as other insulin secreting cells. We speculate that such insulin responses to glucose levels from the astrocyte may be adaptive for its own homeostasis rather than to affect the microenvironment.Support or Funding InformationFunding from NIH (NHLBI) R01 HL033833 and HL105997 and the Veterans Administration Research Career Scientist Award