afimoxifene and Lymphoma

MdmX, also known as Mdm4, is a critical negative regulator of p53, and its overexpression serves to block p53 tumor suppressor function in many cancers. Consequently, inhibiting MdmX has emerged as an attractive approach to restoring p53 function in those cancers that retain functional p53. However, the consequences of acute systemic MdmX inhibition in normal adult tissues remain unknown. To determine directly the effects of systemic MdmX inhibition in normal tissues and in tumors, we crossed mdmX(-/-) mice into the p53ER(TAM) knockin background. In place of wild-type p53, p53ER(TAM) knockin mice express a variant of p53, p53ER(TAM), that is completely dependent on 4-hydroxy-tamoxifen for its activity. MdmX inhibition was then modeled by restoring p53 function in these MdmX-deficient mice. We show that MdmX is continuously required to buffer p53 activity in adult normal tissues and their stem cells. Importantly, the effects of transient p53 restoration in the absence of MdmX are nonlethal and reversible, unlike transient p53 restoration in the absence of Mdm2, which is ineluctably lethal. We also show that the therapeutic impact of restoring p53 in a tumor model is enhanced in the absence of MdmX, affording a significant extension of life span over p53 restoration in the presence of MdmX. Hence, systemic inhibition of MdmX is both a feasible therapeutic strategy for restoring p53 function in tumors that retain wild-type p53 and likely to be significantly safer than inhibition of Mdm2.

Topics: Animals; Antineoplastic Agents, Hormonal; Apoptosis; Apoptosis Regulatory Proteins; Bone Marrow; Cyclin-Dependent Kinase Inhibitor p21; Embryo, Mammalian; Female; Fibroblasts; Gene Expression; Immunoblotting; Kaplan-Meier Estimate; Liver; Lymphoma; Male; Mice; Mice, Knockout; Mice, Transgenic; Mutation; Proto-Oncogene Proteins; Proto-Oncogene Proteins c-mdm2; Proto-Oncogene Proteins c-myc; Tamoxifen; Tumor Cells, Cultured; Tumor Suppressor Protein p53; Tumor Suppressor Proteins; Ubiquitin-Protein Ligases

To study the effects of the Myc oncoprotein in a regulatable in vivo system, we generated lines of transgenic mice in which a tamoxifen inducible Myc fusion protein (c-mycER) is expressed under the control of the CD2 locus control region. Activation of the Myc oncoprotein resulted in both proliferation and apoptosis in vivo. Lines with a high transgene copy number developed spontaneous lymphomas at low frequency, but the tumour incidence was significantly increased with tamoxifen treatment. Surprisingly, we found that cellular sensitivity to Myc-induced apoptosis was retained in tumours from these mice and in most lymphoma cell lines, even when null for p53. Resistance to Myc-induced apoptosis could be conferred on these cells by co-expression of Bcl-2. However, acquired resistance is clearly not an obligatory progression event as sensitivity to apoptosis was retained in transplanted tumours in athymic mice. In conclusion, lymphomas arising in CD2-mycER mice retain the capacity to undergo apoptosis in response to Myc activation and show no phenotypic evidence of the presence of an active dominant inhibitor.

Topics: Animals; Apoptosis; CD2 Antigens; Cell Cycle; Cell Division; Female; Gene Expression Regulation, Neoplastic; Genes, bcl-2; Genes, myc; Genes, Synthetic; Humans; In Situ Nick-End Labeling; Lymphoma; Mice; Mice, Nude; Mice, Transgenic; Neoplasm Transplantation; Proto-Oncogene Proteins c-myc; Receptors, Estrogen; Recombinant Fusion Proteins; Tamoxifen; Thymus Neoplasms