# The Intrabody Against Murine Double Minute 2 via a p53-Dependent Pathway Induces Apoptosis of Cancer Cell

## TL;DR

A new antibody targeting MDM2 helps activate p53, leading to cancer cell death and reduced tumor growth.

## Contribution

A human-derived intrabody (VH-HT3) was developed to inhibit MDM2 and activate p53, showing tumor-suppressive effects.

## Key findings

- VH-HT3 inhibits MDM2 and activates p53, leading to apoptosis in cancer cells.
- VH-HT3 induces cell cycle arrest and reduces tumor growth in xenograft models.
- VH-HT3 increases HIPK2 levels and activates p53 at the Ser46 site.

## Abstract

Murine double minute 2 (MDM2) is involved in various cancers and is an attractive target. The RING domain of MDM2 has been discussed as an alternative target to stabilize p53. Designing drugs to target the RING domain of MDM2 is an alternative approach to preventing MDM2-mediated deactivation of p53. In this study, we obtained a human VH single-domain antibody and revealed its regulatory effects and mechanisms. The RING domain of MDM2 was synthesized using a chemical synthesis method, and antibodies against the MDM2 RING domain were screened from a human VH single-domain antibody library and expressed intracellularly. A nuclear localization sequence was designed to ensure intrabody efficiency. The binding activity of the individually cloned antibodies was detected using ELISA. MTT and flow cytometry assays were used to detect the reactions related to intrabody in vitro. The combination and its influence on MDM2 were detected using immunoprecipitation assays, confocal microscopy, and Western blotting. The effects on apoptosis-related mitochondrial pathways downstream of p53 were examined using Western blotting. The influence on cell cycle distribution and cyclin-related proteins was detected using flow cytometry and Western blotting. A549 cell xenografts were constructed to assess the effect of intrabodies on growth in vivo. The molecular mechanisms of MDM2 and p53 were studied using Western blotting. Eight individual cloned antibodies were positive compared to the signals on the BSA-coated plates, especially intrabodies VH-HT3. In A549 and MCF-7 cell lines, VH-HT3 exhibited significant inhibitory effects on cell proliferation and apoptosis. VH-HT3 co-localized with MDM2 in the nucleus and cytoplasm. The specific combination of VH-HT3 triggered no significant effect on MDM2 activity for p53 degradation but upregulated the levels of factors downstream of p53, especially those in the mitochondrial apoptosis pathway. Moreover, VH-HT3 induced cell cycle arrest, and the expression of cyclin-related proteins was consistent with this observation. VH-HT3 also retarded the growth of A549 xenografts in vivo. Further tests suggested that VH-HT3 inhibited MDM2 function by increasing HIPK2 levels and activating p53 at the Ser46 site. VH-HT3, prepared from a human VH single-domain antibody library, inhibited p53 activity and produced a tumor-suppressive effect. The intrabody VH-HT3 is a candidate for the development of novel MDM2 inhibitors.