Mitochondrially Targeted p53 Has Tumor Suppressor Activities In vivo

Abstract

Complex proapoptotic functions are essential for the tumor suppressor activity of p53. We recently described a novel transcription-independent mechanism that involves a rapid proapoptotic action of p53 at the mitochondria and executes the shortest known circuitry of p53 death signaling. Here, we examine if this p53-dependent mitochondrial program could be exploited for tumor suppression in vivo. To test this, we engage EM-Myc transgenic mice, a well-established model of p53-dependent lymphomagenesis. We show that exclusive delivery of p53 to the outer mitochondrial membrane confers a significant growth disadvantage on EM-Myc-transformed B-cells of p53-deficient or alternate reading frame-deficient genotypes, resulting in efficient induction of apoptosis and impinged proliferation. Conversely, normal cells from thymus, spleen, and bone marrow showed poor infectivity with these viruses. This proof-of-principle experiment shows that exclusive reliance on the direct mitochondrial program exerts a significant tumor suppressor activity in vivo. Our in vivo data on the direct mitochondrial apoptotic p53 program lays the groundwork to further investigate its efficacy and safety and to address its possible therapeutic value in the future.

Mitochondrially targeted wild-type p53 suppresses growth of mutant p53 lymphomas in vivo

“…The data for this graph are tabulated in detail in Supplementary Table 1. For comparison, our previous results obtained in p53-null lymphomas using the same competitive assay are also shown (Talos et al, 2005 Mitochondrially targeted wild-type p53 G Palacios and UM Moll tumors each. p53CTB and p53CTM proteins remained detectable, consistent with residual cells (Figure 3d).…”

“…We recently showed that this direct mitochondrial p53 program also exerts a significant tumor suppressor activity in vivo. In mice with B-cell lymphomas of p53-null or ARF-null genotypes, exclusive delivery of p53 to the OMM of tumor cells caused efficient apoptosis and growth suppression within the animals (Talos et al, 2005).…”

“…p53CTB and p53CTM proteins remained detectable, consistent with residual cells (Figure 3d). When compared to p53-null lymphomas (Talos et al, 2005), mutant p53 lymphomas were somewhat less susceptible to killing mediated by mitochondrially targeted p53 (Figure 3a, black and white bars). In sum, targeting p53 to mitochondria in mutant p53 lymphomas overexpressing an altered protein induces apoptosis and a significant growth disadvantage in vitro and in vivo.…”

“…Each tumor overexpressed the corresponding mutant p53 protein, whereas p19 Arf levels were also elevated (Figure 1). To determine if mitochondrially targeted p53 exerts apoptotic and tumor suppressor activity in these tumors, we used a series of recombinant MSCV viruses (murine stem cell virus) expressing three different versions of mitochondrially targeted p53, together with green fluorescent protein (GFP) and a selectable marker (called Lp53wt, p53CTM and p53CTB) that we previously generated and characterized (Mihara et al, 2003;Talos et al, 2005). Briefly, Lp53wt is a fusion of wtp53 with the prototypical mitochondrial import leader of ornithine transcarbamylase, whereas p53CTB and p53CTM are fusions of wtp53 with the transmembrane domains of BclXL and Bcl2, respectively.…”

“…Briefly, Lp53wt is a fusion of wtp53 with the prototypical mitochondrial import leader of ornithine transcarbamylase, whereas p53CTB and p53CTM are fusions of wtp53 with the transmembrane domains of BclXL and Bcl2, respectively. All are expressed at levels comparable to that of stress-induced endogenous wtp53, localize to and act exclusively at mitochondria but not the nucleus, and are devoid of transcriptional activity in sensitive reporter assays (Mihara et al, 2003;Talos et al, 2005). Control viruses lacked p53 inserts (vector) or expressed conventional nuclear wtp53 (Nuclp53).…”