Cbfa1/RUNX2 directs specific expression of the sclerosteosis gene (SOST).

Loss-of-function mutations in the sclerosteosis gene (SOST) cause a rare sclerosing bone dysplasia characterized by skeletal overgrowth. Cbfa1/RUNX2 is a key transcriptional regulator of osteoblast function. Here we link these two pathways by demonstrating, via gel shift and transient transfection analyses, that Cbfa1 binding to the proximal SOST promoter contributes to differential SOST expression in two osteosarcoma cell lines.

A novel activation function for NAB proteins in EGR-dependent transcription of the luteinizing hormone beta gene.

The EGR1/NGFI-A transcription factor directly activates the luteinizing hormone beta (LHbeta) subunit promoter, and female mice lacking EGR1 are infertile due to LHbeta deficiency. The NGFI-A-binding proteins NAB1 and NAB2 are corepressors of EGR1/NGFI-A and of the related proteins EGR2/Krox20 and EGR3. Here we report that at certain promoters, including LHbeta, NAB proteins display a novel ability to stimulate EGR-directed transcription.

Novel mutants of NAB corepressors enhance activation by Egr transactivators.

The NGFI-A binding corepressors NAB1 and NAB2 interact with a conserved domain (R1 domain) within the Egr1/NGFI-A and Egr2/Krox20 transactivators, and repress the transcription of Egr target promoters. Using a novel adaptation of the yeast two-hybrid screen, we have identified several point mutations in NAB corepressors that interfere with their ability to bind to the Egr1 R1 domain. Surprisingly, NAB proteins bearing some of these mutations increased Egr1 activity dramatically.

Nab1, a corepressor of NGFI-A (Egr-1), contains an active transcriptional repression domain.

Nab proteins constitute an evolutionarily conserved family of corepressors that specifically interact with and repress transcription mediated by three members of the NGFI-A (Egr-1, Krox24, zif/268) family of immediate-early gene transcription factors, which includes NGFI-C, Krox20, and Egr3. We explored the mechanism of Nab1 repression and identified structural domains required for Nab1 function. Nab1 does not act by blocking DNA binding or nuclear localization of NGFI-A.

NAB2, a corepressor of NGFI-A (Egr-1) and Krox20, is induced by proliferative and differentiative stimuli.

Previous work had identified a corepressor, NAB1, which represses transcriptional activation mediated by NGFI-A (also known as Egr-1, zif268, and Krox24) and Krox20. These zinc finger transcription factors are encoded by immediate-early genes and have been implicated in a wide variety of proliferative and differentiative processes. We have isolated and characterized another corepressor, NAB2, which is highly related to NAB1 within two discrete domains.

Identification of NAB1, a repressor of NGFI-A- and Krox20-mediated transcription.

NGFI-A (also called Egr1, Zif268, or Krox24) and the closely related proteins Krox20, NGFI-C, and Egr3 are zinc-finger transcription factors encoded by immediate-early genes which are induced by a wide variety of extracellular stimuli. NGFI-A has been implicated in cell proliferation, macrophage differentiation, synaptic activation, and long-term potentiation, whereas Krox20 is critical for proper hindbrain segmentation and peripheral nerve myelination. In previous work, a structure/function analysis of NGFI-A revealed a 34-aa inhibitory domain that was hypothesized to be the target of a cellular factor that represses NGFI-A transcriptional activity.

Increasing cAMP attenuates activation of mitogen-activated protein kinase.

Activation of the mitogen-activated protein kinase (MAP kinase) isoforms ERK1 and ERK2 was investigated in rat adipocytes. Kinase activities were measured by using myelin basic protein as substrate after the isoforms were resolved by Mono Q chromatography or by immunoprecipitation with specific antibodies. Insulin increased the activity of both isoforms by 3- to 4-fold.