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The role of inducible repressor proteins in the adrenergic induction of arylalkylamine-N-acetyltransferase and mitogen-activated protein kinase phosphatase-1 in rat pinealocytes.

In this study, we investigated the role of two inducible repressor proteins, inducible cAMP early repressor (ICER) and Fos-related antigen 2 (Fra-2) in the adrenergic induction of MAPK phosphatase-1 (MKP-1) as compared with their roles in the induction of arylalkylamine-N-acetyltransferase (AA-NAT) in rat pinealocytes. Treatment of pinealocytes with norepinephrine (NE) caused an increase in the mRNA and protein levels of MKP-1 and AA-NAT, as well as in the AA-NAT activity and melatonin production. NE stimulation also caused a simultaneous increase in the mRNA and protein levels of ICER and Fra-2. Transient knockdown of icer using adenovirus expressing small interfering RNA (siRNA) abolished the NE induction of icer expression but had little effect on the NE induction of mkp-1 or aa-nat expression. In contrast, pretreatment with adenovirus overexpressing icer was effective in reducing the NE induction of mkp-1 and aa-nat. The inhibitory effect of overexpressing icer was reversed by cotreatment with siRNA against icer. siRNA against fra-2 also abolished the NE-stimulated expression of fra-2 but had little effect on the NE induction of mkp-1 and aa-nat expression. Proteasomal inhibition, which reduced the NE-stimulated induction of aa-nat, caused a reduction of ICER and Fra-2. Together, these results indicate that whereas overexpression of ICER can suppress the NE induction of aa-nat and mkp-1, the amount of the repressors, ICER and Fra-2, present during NE induction appears insufficient to exert a significant effect in controlling the expression of these genes.