c-myc can induce expression of G0/G1 transition genes.

The human c-myc oncogene was linked to the heat shock-inducible Drosophila hsp70 promoter and used to stably transfect mouse BALB/c 3T3 cells. Heat shock of the transfectants at 42 degrees C followed by recovery at 37 degrees C resulted in the appearance of the human c-myc protein which was appropriately localized to the nuclear fraction. Two-dimensional analysis of the proteins of density-arrested cells which had been heat shock treated revealed the induction of eight protein species and the repression of five protein species. All of the induced and repressed proteins were nonabundant. cDNA clones corresponding to genes induced during the G0/G1 transition were used as probes to assay for c-myc inducibility of these genes. Two anonymous sequences previously identified as serum inducible (3CH77 and 3CH92) were induced when c-myc was expressed. In response to serum stimulation, 3CH77 and 3CH92 were expressed before c-myc mRNA levels increased. However, in response to specific induction of c-myc by heat shock of serum arrested cells, 3CH77 and 3CH92 mRNA levels increased after the rise in c-myc mRNA. Therefore, we hypothesize that abnormal expression of c-myc can induce genes involved in the proliferative response.     

Cytochrome bc(1) regulates the mitochondrial permeability transition by two distinct pathways

The mitochondrial permeability transition (MPT) pore is a calcium-sensitive channel in the mitochondrial inner membrane that plays a crucial role in cell death. Here we show that cytochrome bc(1) regulates the MPT in isolated rat liver mitochondria and in CEM and HL60 cells by two independent pathways. Glutathione depletion activated the MPT via increased production of reactive oxygen species (ROS) generated by cytochrome bc(1). The ROS producing mechanism in cytochrome bc(1) involves movement of the "Rieske" iron-sulfur protein subunit of the enzyme complex, because inhibition of cytochrome bc(1) by pharmacologically blocking iron-sulfur protein movement completely abolished ROS production, MPT activation, and cell death. The classical inhibitor of the MPT, cyclosporine A, had no protective effect against MPT activation. In contrast, the calcium-activated, cyclosporine A-regulated MPT in rat liver mitochondria was also blocked with inhibitors of cytochrome bc(1). These results indicate that electron flux through cytochrome bc(1) regulates two distinct pathways to the MPT, one unregulated and involving mitochondrial ROS and the other regulated and activated by calcium.     

Differential expression of two distinct phenylalanine ammonia-lyase genes in condensed tannin-accumulating and lignifying cells of quaking aspen

Lignins, along with condensed tannins (CTs) and salicylate-derived phenolic glycosides, constitute potentially large phenylpropanoid carbon sinks in tissues of quaking aspen (Populus tremuloides Michx.). Metabolic commitment to each of these sinks varies during development and adaptation, and depends on L-phenylalanine ammonia-lyase (PAL), an enzyme catalyzing the deamination of L-phenylalanine to initiate phenylpropanoid metabolism. In Populus spp., PAL is encoded by multiple genes whose expression has been associated with lignification in primary and secondary tissues. We now report cloning two differentially expressed PAL cDNAs that exhibit distinct spatial associations with CT and lignin biosynthesis in developing shoot and root tissues of aspen. PtPAL1 was expressed in certain CT-accumulating, non-lignifying cells of stems, leaves, and roots, and the pattern of PtPAL1 expression varied coordinately with that of CT accumulation along the primary to secondary growth transition in stems. PtPAL2 was expressed in heavily lignified structural cells of shoots, but was also expressed in non-lignifying cells of root tips. Evidence of a role for Pt4CL2, encoding 4-coumarate:coenzyme A ligase, in determining CT sink strength was gained from cellular co-expression analysis with PAL1 and CTs, and from experiments in which leaf wounding increased PAL1 and 4CL2 expression as well as the relative allocation of carbon to CT with respect to phenolic glycoside, the dominant phenolic sink in aspen leaves. Leaf wounding also increased PAL2 and lignin pathway gene expression, but to a smaller extent. The absence of PAL2 in most CT-accumulating cells provides in situ support for the idea that PAL isoforms function in specific metabolic milieus.