THYROID-HORMONE RECEPTOR C-ERBA - CONTROL OF COMMITMENT AND DIFFERENTIATION IN THE NEURONAL CHROMAFFIN PROGENITOR LINE-PC12

The c-erbA proto-oncogenes encode nuclear receptors for thyroid hormon e (T3), a hormone intimately involved in mammalian brain maturation. T o study thyroid hormone receptor (TR) action on neuronal cells in vitr o, we expressed the chicken c-erbA/TRalpha-1 as well as its oncogenic variant v-erbA in the adrenal medulla progenitor cell line PC12. In th e absence of T3, exogenous TRalpha-1 inhibits NGF-induced neuronal dif ferentiation and represses neuron-specific gene expression. In contras t, TRalpha-1 allows normal differentiation and neuronal gene expressio n to occur in the presence of T3. Finally, TRalpha-1-expressing cells become NGF-responsive for proliferation when T3 is absent, but NGF-dep endent for survival in presence of T3. A similar differentiation induc tion by NGF plus T3 was observed in a central nervous system-derived n euronal cell line (E 18) expressing exogenous TRalpha-1. Together with the finding that TRalpha-1 constitutively blocked dexamethasone-induc ed differentiation of PC12 cells into the chromaffin pathway, these re sults suggest that TRalpha-1 plays an important role in regulating com mitment and maturation of neuronal progenitors. In contrast, the v-erb A oncogene, a mutated, oncogenic version of TRalpha-1, partially but c onstitutively inhibited NGF-induced neuronal differentiation of PC12 c ells and potentiated dexamethasone-induced chromaffin differentiation, giving rise to an aberrant ''interlineage'' cell phenotype.