Insulin biosynthesis in HIT cells. Effects of glucose, forskolin, IBMX, and dexamethasone

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Abstract
Glucose, forskolin, 3-isobutyl-1-methylxanthine (IBMX), and dexamethasone were tested as regulators of proinsulin biosynthesis in HIT T-15 cells, which are glucose-responsive simian virus 40-transformed hamster β-cells. Rate of [3H]leucine incorporation into proinsulin was increased as glucose concentrations were raised from 0 to 20 mM. Biosynthetic rate increases were significant after 48 but not at 4 or 24 h of glucose and were greater for proinsulin than for total extractable proteins. After 48 h, glucose-stimulated proinsulin biosynthesis was unaffected by 10−6 M forskolin and/or 3 × 10−5 M IBMX but was specifically and significantly inhibited by 10−6 M dexamethasone. Four hours of exposure to dexamethasone had no effect. When cells were incubated for 24 h and then continuously labeled for an additional 24 h, cellular conversion of labeled proinsulin to insulin was increased by glucose, and this increase was reversed or inhibited by 10−6 M dexamethasone. Therefore, proinsulin biosynthesis in transformed HIT T-15 cells is regulated in several ways by metabolites and hormones in a manner that compares with biosynthetic regulation in normal β-cells.