PPM1D is directly degraded by proteasomes in a ubiquitination-independent manner through its carboxyl-terminal region.

Background: PPM1D (protein phosphatase Mg²⁺/Mn²⁺ dependent 1D) is a Ser/Thr phosphatase that negatively regulates p53 and functions as an oncogenic driver. Its gene amplification and overexpression are frequently observed in various malignancies and disruption of PPM1D degradation has also been reported as a cause of cancer progression. However, the precise mechanisms regulating PPM1D stability remain to be elucidated.

Methods: PPM1D stability and degradation pathways were examined using cycloheximide chase assays in multiple cell lines. Proteasome and lysosome inhibitors were used to determine the degradation mechanism, while ubiquitination dependency was assessed using TAK-243, an E1 ubiquitin-activating enzyme inhibitor. In vitro degradation assays with purified 20S proteasome were performed to evaluate direct proteasomal degradation. Immunoprecipitation followed by mass spectrometry was performed to identify proteasomal regulators of PPM1D, with their functional roles validated through knockdown experiments. Finally, cell viability assays were conducted to assess the therapeutic potential of combined proteasome and PPM1D inhibition.

Results: Cycloheximide chase assays demonstrated that wild-type PPM1D is a short-lived protein, whereas a C-terminal truncation mutant exhibits increased stability. PPM1D undergoes rapid, ubiquitin-independent proteasomal degradation via its C-terminal 35 amino acid residues. Additionally, the region spanning residues 450-501 is necessary for ubiquitination-mediated suppression of the ubiquitin-independent degradation pathway. We also found that PPM1D is directly degraded by the 20S proteasome, with the regulatory proteasome subunits PSMD14 and PSME3 acting as activators in this process. Proteasome inhibition resulted in PPM1D accumulation, potentially reducing therapeutic efficacy. Combined proteasome and PPM1D inhibition synergistically enhanced the antitumor effect.

Conclusions: The rapid degradation of the cancer driver PPM1D is achieved through direct recognition by the proteasome, and proteasome inhibitors may reduce therapeutic efficacy due to the accumulation of PPM1D. PPM1D may serve as a suitable model substrate for elucidating the mechanism of ubiquitin-independent proteasomal degradation and represents a potential novel therapeutic target for cancer treatment based on proteasome inhibition.