Mycophenolic Acid Activation of p53 Requires Ribosomal Proteins L5 and L11

Mu-Shui Dai,Xiao-Xin Sun,Hua Lu

doi:10.1074/jbc.m801387200

Abstract

Mycophenolate mofetil (MMF), a prodrug of mycophenolic acid (MPA), is widely used as an immunosuppressive agent. MPA selectively inhibits inosine monophosphate dehydrogenase (IMPDH), a rate-limiting enzyme for the de novo synthesis of guanine nucleotides, leading to depletion of the guanine nucleotide pool. Its chemotherapeutic effects have been attributed to its ability to induce cell cycle arrest and apoptosis. MPA treatment has also been shown to induce and activate p53. However, the mechanism underlying the p53 activation pathway is still unclear. Here, we show that MPA treatment results in inhibition of pre-rRNA synthesis and disruption of the nucleolus. This treatment enhances the interaction of MDM2 with L5 and L11. Interestingly, knockdown of endogenous L5 or L11 markedly impairs the induction of p53 and G(1) cell cycle arrest induced by MPA. These results suggest that MPA may trigger a nucleolar stress that induces p53 activation via inhibition of MDM2 by ribosomal proteins L5 and L11.

Highlights

Inosine monophosphate dehydrogenase (IMPDH)3 is an essential, rate-limiting enzyme for the de novo synthesis of guanine nucleotides
It has been shown that certain specific inhibitors of ribonucleotide biosynthesis, including mycophenolic acid (MPA), cause a reversible p53-dependent G1 arrest, and p53 has been proposed to serve as a sensor of ribonucleotide pool perturbation [20], MPA has been shown to inhibit DNA synthesis [21, 22]. p53 has been shown to mediate the cell cycle arrest and apoptosis in response to guanine nucleotide depletion in human neuroblastoma cell lines [17, 23]
We found that guanine nucleotide depletion by MPA treatment drastically inhibits pre-rRNA synthesis (Fig. 2B), disrupts the nucleolus resulting in massive translocation of the nucleolar protein B23 into the nucleoplasm (Fig. 2A), and induces p53 activity (Fig. 1)

Summary

Introduction

Inosine monophosphate dehydrogenase (IMPDH) is an essential, rate-limiting enzyme for the de novo synthesis of guanine nucleotides. MPA, the active metabolite of MMF, is a selective inhibitor of IMPDH [8] It can effectively induce cell-cycle arrest in late G1 phase in lymphocytes (9 –11), and results in differentiation [12,13,14] or apoptosis [15,16,17,18] in cultured cell lines depending on cell type. The induction of apoptosis in multiple myeloma cell lines occurs through both caspase-dependent [18] and caspaseindependent [19] mechanisms These signaling pathways are only the potential downstream targets; the upstream mechanisms that sense the depletion of guanine nucleotide and trigger the cell cycle arrest or apoptosis are still not very clear. These results suggest that MPA may trigger a nucleolar stress, resulting in p53 activation that requires the inhibition of MDM2 activity by ribosomal proteins L5 and L11

Methods

Results

Conclusion