Dual roles for the phosphatase PPM1D in regulating progesterone receptor function

Area of Science:

• Molecular Biology
• Cancer Research
• Endocrinology

Background:

• Protein phosphatase magnesium-dependent 1 delta (PPM1D) is known to regulate p53 activity via p38 MAPK.
• Overexpression and amplification of PPM1D in human breast cancers suggest a role beyond its known functions.
• The role of PPM1D in steroid hormone action, particularly in nuclear receptor signaling, remains largely unexplored.

Purpose of the Study:

• To investigate the function of PPM1D in the context of steroid hormone action.
• To determine if PPM1D influences the activity of nuclear receptors, such as the progesterone receptor (PR).
• To elucidate the mechanisms by which PPM1D may affect breast cancer progression.

Main Methods:

• Assessed PPM1D's effect on nuclear receptor activity using reporter assays.
• Utilized small interfering RNA (siRNA) to inhibit PPM1D expression in MCF-7 breast cancer cells.
• Employed mammalian two-hybrid assays to study protein-protein interactions.
• Investigated the role of PPM1D phosphatase activity and its effect on coactivator function.

Main Results:

• PPM1D significantly stimulated the transcriptional activity of PR and other nuclear receptors.
• PPM1D's stimulation of PR activity exceeded that of a p38 MAPK inhibitor (SB202190).
• Inhibition of PPM1D, but not SB202190, reduced endogenous PR transcriptional activity in MCF-7 cells.
• PPM1D enhanced the intrinsic activity of p160 coactivators and promoted PR-coactivator interaction, independent of p38 MAPK.

Conclusions:

• PPM1D possesses a novel function that enhances nuclear receptor activity, including PR.
• This PPM1D-mediated enhancement of PR activity is largely independent of its known role in p38 MAPK signaling.
• PPM1D may promote breast tumor growth by inhibiting p53 and enhancing steroid hormone receptor signaling pathways.