Forskolin Increases Neuregulin Receptors in Human Schwann Cells Without Increasing Receptor MRNA

Forskolin and heregulin synergistically drive human Schwann cell (HSC) proliferation in vitro, but the role of forskolin is not completely understood. To learn how forskolin might affect receptor levels in HSC cultured from adult nerve roots, we first studied expression and localization of HER2 and HER3 in intact roots, using Western blotting and light and electron microscopic immunocytochemistry. We then determined the effect of forskolin and heregulin on receptor expression in HSC cultured from nerve roots using Western blotting and RNase protection assays. HER2 and HER3 were expressed in nonmyelinating Schwann cells in roots and in cultured HSCs before exposure to forskolin. HER2, but not HER3, was also expressed in endoneurial fibroblasts and in cultured nerve root-derived fibroblasts. Treatment with forskolin for 24 h consistently increased HER2 and HER3 protein levels in HSCs but did not alter HER2 and HER3 mRNA levels. In addition, 24-h treatment with heregulin alone decreased HER2 and HER3 protein levels, an effect not previously described. When both heregulin and forskolin were present, HER2 and HER3 protein levels were similar to initial control values. The effect of forskolin on receptor levels was mimicked by dibutyryl-cAMP and receptor levels in both untreated and forskolin treated HSCs were decreased by treatment with the protein kinase A inhibitor H-89. Following pretreatment of HSCs with forskolin, increased receptor levels were correlated with increased rates of thymidine incorporation into HSCs. These results suggest that forskolin/heregulin synergy might derive, at least in part, from post-transcriptional effects leading to increased steady-state receptor levels.