p21ras and protein kinase C function in distinct and interdependent signaling pathways in C3H 10T1/2 fibroblasts

Identification of a ras-activated enhancer in the mouse osteopontin promoter and its interaction with a putative ETS-related transcription factor whose activity correlates with the metastatic potential of the cell

The role of RAS in transducing signals from an activated receptor into altered gene expression is becoming clear, though some links in the chain are still missing. Cells possessing activated RAS express higher levels of osteopontin (OPN), an alpha v beta 3 integrin-binding secreted phosphoprotein implicated in a number of developmental, physiological, and pathological processes. We report that in T24 H-ras-transformed NIH 3T3 cells enhanced transcription contributes to the increased expression of OPN. Transient transfection studies, DNA-protein binding assays, and methylation protection experiments have identified a novel ras-activated enhancer, distinct from known ras response elements, that appears responsible for part of the increase in OPN transcription in cells with an activated RAS. In electrophoretic mobility shift assays, the protein-binding motif GGAGGCAGG was found to be essential for the formation of several complexes, one of which (complex A) was generated at elevated levels by cell lines that are metastatic. Southwestern blotting and UV light cross-linking studies indicated the presence of several proteins able to interact with this sequence. The proteins that form these complexes have molecular masses estimated at approximately 16, 28, 32, 45, 80, and 100 kDa. Because the approximately 16-kDa protein was responsible for complex A formation, we have designated it MATF for metastasis-associated transcription factor. The GGANNNAGG motif is also found in some other promoters, suggesting that they may be similarly controlled by MATF.

Mammalian mitogen-activated protein kinase kinase kinase (MEKK) can function in a yeast mitogen-activated protein kinase pathway downstream of protein kinase C

Mitogen-activated protein kinase cascades are conserved in fungal, plant, and metazoan species. We expressed murine MAP kinase kinase kinase (MEKK) in the yeast Saccharomyces cerevisiae to determine whether this kinase functions as a general or specific activator of genetically and physiologically distinct MAP-kinase-dependent signaling pathways and to investigate how MEKK is regulated. Expression of MEKK failed to correct the mating deficiency of a ste11 delta mutant that lacks an MEKK homolog required for mating. MEKK expression also failed to induce expression of a reporter gene controlled by the HOG1 gene product (Hog1p), a yeast MAP kinase homolog involved in response to osmotic stress. Expression of MEKK did correct the cell lysis defect of a bck1 delta mutant that lacks an MEKK homolog required for cell-wall assembly. MEKK required the downstream MAP kinase homolog in the BCK1-dependent pathway, demonstrating that it functionally replaces the BCK1 gene product (Bck1p) rather than bypassing the pathway. MEKK therefore selectively activates one of three distinct MAP-kinase-dependent pathways. Possible explanations for this selectivity are discussed. Expression of the MEKK catalytic domain, but not the full-length molecule, corrected the cell-lysis defect of a pkc1 delta mutant that lacks a protein kinase C homolog that functions upstream of Bck1p. MEKK therefore functions downstream of the PKC1 gene product (Pkc1p). The N-terminal noncatalytic domain of MEKK, which contains several consensus protein kinase C phosphorylation sites, may, therefore, function as a negative regulatory domain. Protein kinase C phosphorylation may provide one mechanism for activating MEKK.