ABSTRACT
The tumor suppressor protein p53 is a transcription factor that induces G(1) arrest of the cell cycle and/or apoptosis. The murine double-minute protein MDM2 and its homologue MDM4 (also known as MDMX) are critical regulators of p53. Altered transcripts of the human homologue of mdm2, MDM2, have been identified in human tumors, such as invasive carcinoma of the breast, lung carcinoma, and liposarcoma. MDM2 alternate forms act to negatively regulate the normal MDM2 gene product, thus activating p53. Although many reports have documented a plethora of tumor types characterized by MDM2 alternative transcripts, few have investigated the signals that might initiate alternative splicing. We have identified a novel role of these alternative MDM2 transcripts in the normal surveillance mechanism of the cell and in DNA damage response. We report that alternate forms of MDM2 are detected after UV irradiation. Furthermore, we show that mouse cells treated with UV are also characterized by alternative transcripts of mdm2, suggesting that this is an important and evolutionarily conserved mechanism for regulating the expression of MDM2/mdm2. An additional p53 regulator and mdm2 family member, MDM4, is likewise alternatively spliced following UV irradiation. By activating alternative splicing of both MDM2 and MDM4, yet another layer of p53 regulation is initiated by the cells in response to damage. A stepwise model for malignant conversion by which alternate forms of MDM2 and MDM4 place selective pressure on the cells to acquire additional alterations in the p53 pathway is herein proposed.
Subject(s)
Alternative Splicing/radiation effects , Genes, p53 , Nuclear Proteins/physiology , Proto-Oncogene Proteins c-mdm2/physiology , Proto-Oncogene Proteins/physiology , Ubiquitin-Protein Ligases/physiology , Alternative Splicing/genetics , Animals , Ataxia Telangiectasia Mutated Proteins , Caffeine/pharmacology , Cell Cycle Proteins/antagonists & inhibitors , Cell Line, Tumor/drug effects , Cell Line, Tumor/metabolism , Cell Line, Tumor/radiation effects , Cisplatin/toxicity , DNA Damage , DNA Modification Methylases/genetics , DNA Repair Enzymes/genetics , DNA, Neoplasm/drug effects , DNA, Neoplasm/radiation effects , DNA-Binding Proteins/antagonists & inhibitors , Humans , Mice , Nuclear Proteins/genetics , Protein Isoforms/genetics , Protein Isoforms/physiology , Protein Serine-Threonine Kinases/antagonists & inhibitors , Proto-Oncogene Proteins/genetics , Proto-Oncogene Proteins c-mdm2/genetics , Recombinant Fusion Proteins/physiology , Tumor Suppressor Proteins/antagonists & inhibitors , Tumor Suppressor Proteins/genetics , Ubiquitin-Protein Ligases/genetics , Ultraviolet RaysABSTRACT
Peripheral benzodiazepine receptor (PBR) expression increases in small dorsal root ganglion (DRG) sensory neurons after peripheral nerve injury. To determine the functional significance of this induction, we evaluated the effects of PBR ligands on rodent sensory axon outgrowth. In vitro, Ro5-4864, a PBR agonist, enhanced outgrowth only of small peripherin-positive DRG neurons. When DRG cells were preconditioned into an active growth state by a prior peripheral nerve injury Ro5-4864 augmented and PK 11195, a PBR antagonist, blocked the injury-induced increased outgrowth. In vivo, Ro5-4864 increased the initiation of regeneration after a sciatic nerve crush injury and the number of GAP-43-positive axons in the distal nerve while PK 11195 inhibited the enhanced growth produced by a preconditioning lesion. These results show that PBR has a role in the early regenerative response of small caliber sensory axons, the preconditioning effect, and that PBR agonists enhance sensory axon regeneration.
