The p53 Inhibitor MDM2 Facilitates Sonic Hedgehog-Mediated Tumorigenesis and Influences Cerebellar Foliation

Abstract

Disruption of cerebellar granular neuronal precursor (GNP) maturation can result in defects in motor coordination and learning, or in medulloblastoma, the most common childhood brain tumor. The Sonic Hedgehog (Shh) pathway is important for GNP proliferation; however, the factors regulating the extent and timing of GNP proliferation, as well as GNP differentiation and migration are poorly understood. The p53 tumor suppressor has been shown to negatively regulate the activity of the Shh effector, Gli1, in neural stem cells; however, the contribution of p53 to the regulation of Shh signaling in GNPs during cerebellar development has not been determined. Here, we exploited a hypomorphic allele of Mdm2 (Mdm2puro), which encodes a critical negative regulator of p53, to alter the level of wild-type MDM2 and p53 in vivo. We report that mice with reduced levels of MDM2 and increased levels of p53 have small cerebella with shortened folia, reminiscent of deficient Shh signaling. Indeed, Shh signaling in Mdm2-deficient GNPs is attenuated, concomitant with decreased expression of the Shh transducers, Gli1 and Gli2. We also find that Shh stimulation of GNPs promotes MDM2 accumulation and enhances phosphorylation at serine 166, a modification known to increase MDM2-p53 binding. Significantly, loss of MDM2 in Ptch1+/− mice, a model for Shh-mediated human medulloblastoma, impedes cerebellar tumorigenesis. Together, these results place MDM2 at a major nexus between the p53 and Shh signaling pathways in GNPs, with key roles in cerebellar development, GNP survival, cerebellar foliation, and MB tumorigenesis.

p63 sustains self-renewal of mammary cancer stem cells through regulation of Sonic Hedgehog signaling

“…However, the existence of interplay between p53 and the Hh pathway has been reported (35,36). p53 was shown to repress tumorigenic Hh signals by negatively regulating the activity of GLI1 (35,37).…”

“…However, the existence of interplay between p53 and the Hh pathway has been reported (35,36). p53 was shown to repress tumorigenic Hh signals by negatively regulating the activity of GLI1 (35,37). Interestingly, constitutively activated mutants of SMO, as well as elevated levels of GLI1 and GLI2, inhibit p53 accumulation by promoting binding of p53 to Mdm2 and its subsequent ubiquitylation (38).…”

p53 in the CNS: Perspectives on Development, Stem Cells, and Cancer

“…Recent work in our lab, for example, supports MDM2 as an important contributor to the inhibition of p53 in Shh-driven MB tumorigenesis. In cerebellar development, MDM2 is required to inhibit p53-mediated apoptosis in GNPs, 39 the presumed cell of origin for MB tumors of the Shh subgroup, and MDM2 deficiency potently restricts cerebellar tumorigenesis in Ptch1 +/− mice, a model of human Shh-induced MB.…”

“…Furthermore, Shh-mediated enhancement of the anti-p53 function of MDM2 is predicted to obviate the selective pressure for p53 mutations as well as for Mdm2 gene amplification events, which, although present in ~7% of human tumors with wild-type p53, 91 are rare in MB. 88,91,110 Intriguingly, when the level of MDM2 is reduced in GNPs, Shh signaling is attenuated concomitant with p53 activation, 39 suggesting a complex interplay between the p53 and Shh pathways. Recently, p53 has been shown to negatively regulate the level and subcellular localization of Gli1 in NSCs, 56 and both Gli1 and Gli2 expression is reduced in Mdm2 puro/Δ7-9 GNPs.…”

“…39,40 As the restoration of p53 function in the mature brain of adult MDM2/p53-deficient mice fails to induce neuronal apoptosis, 41 there appears to be an increased need for MDM2 to buffer p53 from endogenous p53 activating signals in the developing CNS.…”