PC12 rat phaeochromocytoma cells are widely used as a model of neuronal differentiation. They express IGF receptors and are responsive to IGFs. The main IGF-binding protein synthesized by these cells is IGFBP-6. Glucocorticoids induce differentiation of PC12 cells towards a chromaffin phenotype. The effect of dexamethasone on IGFBP-6 levels was therefore studied. Dexamethasone (500 nM) decreased IGFBP-6 protein in conditioned media and mRNA levels to 61 +/- 5% (P < 0.0001) and 34 +/- 14% (P = 0.03) of control levels respectively. Incubation of PC12 cells with IGF-II (100 ng/ml) for 72 h increased IGFBP-6 protein levels in media to 217 +/- 19% of control (P < 0.0001). IGFBP-6 mRNA levels, however, were unchanged. IGF-I had similar effects on IGFBP-6 protein and mRNA levels. IGFs increased cell number by 50-60%, but this was insufficient to explain the increases in protein levels. IGFBP-6 was not released from a cell-associated reservoir or protected from proteolysis by IGFs, excluding these post-translational mechanisms as explanations for the IGF effects on IGFBP-6 levels. The effects of IGF-II and dexamethasone on IGFBP-6 levels were independent. These results indicated that (1) dexamethasone decrease IGFBP-6 at the mRNA level, and (2) IGFs stimulate IGFBP-6 levels by a post-transcriptional mechanism.
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