Diverse physiological actions of growth hormone (GH) are mediated by changes in gene transcription. Transcription can be regulated at several levels, including post-translational modification of transcription factors, and formation of multiprotein complexes involving transcription factors, co-regulators and additional nuclear proteins; these serve as targets for regulation by hormones and signaling pathways. Evidence that GH regulates transcription at multiple levels is exemplified by analysis of the proto-oncogene c-fos. Among the GH-regulated transcription factors on c-fos, C/EBPβ appears to be key, since depletion of C/EBPβ by RNA interference blocks the stimulation of c-fos by GH. The phosphorylation state of C/EBPβ and its ability to activate transcription are regulated by GH through MAPK and PI3K/Akt-mediated signaling cascades. The acetylation of C/EBPβ also contributes to its ability to activate c-fos transcription. These and other post-translational modifications of C/EBPβ appear to be integrated for regulation of transcription by GH. The formation of nuclear proteins into complexes associated with DNA-bound transcription factors is also regulated by GH. Both C/EBPβ and the co-activator p300 are recruited to c-fos in response to GH, altering c-fos promoter activation. In addition, GH rapidly induces spatio-temporal relocalization of C/EBPβ within the nucleus. Thus, GH-regulated gene transcription mediated by C/ EBPβ reflects the integration of diverse mechanisms including post-translational modifications, modulation of protein complexes associated with DNA and relocalization of gene regulatory proteins. Similar integration involving other transcription factors, including Stats, appears to be a feature of regulation by GH of other gene targets.

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Transcription factor function can be modulated by posttranslational modifications. Because the transcription factor CCAAT/enhancer-binding protein (C/EBP) ␤ associates with the nuclear coactivator p300, which contains acetyltransferase activity, acetylation of C/EBP␤ was examined to understand its regulation and function. C/EBP␤ is acetylated by acetyltransferases p300 and p300/CREB-binding protein associated factor. Endogenous C/EBP␤ in 3T3-F442A preadipocytes is also recognized by an acetyl-lysine-specific antibody. Analysis of truncations of C/EBP␤ and peptides based on C/EBP␤ sequences identified multiple lysines within C/EBP␤ that can be acetylated. Among these, a novel acetylation site at lysine 39 of C/EBP␤ was identified. Mutation of Lys-39 to arginine or alanine impairs its acetylation and the ability of C/EBP␤ to activate transcription at the promoters for C/EBP␣ and c-fos. Different C/EBP␤-responsive promoters require different patterns of acetylated lysines in C/EBP␤ for transcription activation. Furthermore, C/EBP␤ acetylation was increased by growth hormone, and mutation of Lys-39 impaired growth hormone-stimulated c-fos promoter activation. These data suggest that acetylation of Lys-39 of C/EBP␤, alone or in combination with acetylation at other lysines, may play a role in C/EBP␤-mediated transcriptional activation.

The hippocampus plays an important role in spatial learning and memory. However, the biochemical alterations that subserve this function remain to be fully elucidated. In this study, rats were subjected to a single-trial contextual fear conditioning (CFC) paradigm; the activation of different protein kinase C (PKC) subtypes and the levels and phosphorylation of the plasticity-associated protein GAP-43 were assayed in the hippocampus at varying times after training. We observed a rapid activation of hippocampal PKC (15 min through 24 h), with differential translocation of the PKC isotypes studied. At early times after CFC (15-90 min), PKCalpha and PKCgamma translocated to the membrane, while PKCbetaII and PKCepsilon moved more transiently (15 to 30 min) to the cytosol. These PKC isotypes returned to the membrane at later time points after CFC. Correlating with these changes in PKC translocation and activity, there was an early decrease in GAP-43 phosphorylation followed by a more sustained increase from 1.5-72 h. GAP-43 protein levels were also increased after 3 h, and these levels remained elevated for at least 72 h. These changes in PKC and GAP-43 were specific to the CFC trained animals and no changes were seen in animals exposed to the same stimuli in a non-associative fashion. Comparison of translocation of different PKC isotypes with the changes in GAP-43 phosphorylation suggested that PKCbetaII and PKCepsilon may mediate both the early changes in the phosphorylation of this protein and the increases in GAP-43 expression at later times after CFC.

ΔFosB protein accumulates in the striatum in response to chronic administration of drugs of abuse, L-DOPA, or stress, triggering long lasting neural and behavioral changes that underlie aspects of drug addiction, abnormal involuntary movements (dyskinesia), and depression. ΔFosB binds AP-1 DNA consensus sequences found in promoters of many genes and can both repress or activate gene transcription. In the striatum, ΔFosB is thought to dimerize with JunD to form a functional transcription factor, though strikingly JunD does not accumulate in parallel. One explanation is that ΔFosB can recruit different partners, including itself, depending on the neuron type in which it is induced and the chronic stimulus, generating protein complexes with different effects on gene transcription. To develop chemical probes to study ΔFosB, a high-throughput screen was carried out to identify small molecules that modulate ΔFosB function. Two compounds with low micromolar activity, termed C2 and C6, disrupt the binding of ΔFosB to DNA via different mechanisms, and in in vitro assays stimulate ΔFosB-mediated transcription. In cocaine-treated mice, C2 significantly elevates mRNA levels of the AMPA glutamate receptor GluR2 subunit with specificity, a known target gene of ΔFosB that plays a role in drug addiction and endogenous resilience mechanisms. C2 and C6 show different activities against ΔFosB homodimers compared to ΔFosB/JunD heterodimers, suggesting that these compounds can be used as probes to study the contribution of different ΔFosB-containing complexes on the regulation of gene transcription in biological systems and to assess the utility of ΔFosB as a therapeutic target.