PP2C phosphatases-terminators of suicidal thoughts

Abstract

Cell death and related signaling pathways are essential during development and in various physiological and pathological conditions. Post-translational modifications such as ubiquitination and phosphorylation play an important role in these signaling pathways. The involvement of kinases - enzymes that catalyze protein phosphorylation - in cell death signaling has been extensively studied. On the other hand, not many studies have been devoted to analyzing the role in cell death of phosphatases, enzymes involved in the removal of phosphorylated residues added to proteins by kinases. Obviously, the two opposite reactions, phosphorylation and dephosphorylation, are equally important in the regulation of protein functions and subsequently in the execution of the cell death program. Here, we have summarized recent work on the involvement of serine-threonine PP2C phosphatases in cell death pathways, senescence and autophagy, focusing in particular on the most studied phosphatase PPM1D (PP2Cδ) as an example of the regulatory role of PP2Cs in cell death. The review should help to draw attention to the importance of PP2C family phosphatases in cell death checkpoints and to discover new targets for drug development.

Fig. 1. Protein structure and list of the PP2C family members.

PP2C family proteins require Mn²⁺ and/or Mg²⁺ for activation. The PP2Cs family is composed of 20 isoforms including PPM1A, PPM1B, PPM1D, PPM1E, PPM1F, PPM1G, PPM1H, PPM1J, PPM1K, PPM1L, PPM1M, PPM1N, ILKAP, PDP1, PDP2, PHLPP1, PHLPP2, PP2D1, PPTC7 et TAB1. The structures of the family member are obtained using Neurosnap Inc. - Computational Biology Platform for Research, Wilmington, DE, 2022 [128] and designed with BioRender.com.

Fig. 2. Involvement of the PP2C family members in different types of cell death.

The most described PP2C proteins including PPM1D, PPM1A, PPM1K, PPM1F, PHLPP, PPM1L, PPM1G, PPM1B, PPM1H, ILKAP, and PHLPPL1/2 are involved in apoptosis and others cell death pathway. Actually, PPM1D, PPM1A, PPM1K, PPM1F, PHLPP, PPM1L, PPM1G, PPM1B, PPM1H, ILKAP, and PHLPPL1/2 have been reported to regulate apoptosis and PPM1G, PPM1B, PPM1D, PPM1A and PPM1K are involved in autophagy mechanisms. PPM1B play a role in PANaptosis pathway. NETosis and pyroptosis cell death are strictly regulated by PPM1D. Ferroptosis regulation involved PPM1K and PPM1F regulation. All these types of cell death are driven by several signaling pathways and PP2C protein regulate many factors shown in this scheme. N.D. annotation means that there is no data about target of the PP2C family member. Created with BioRender.com.

Fig. 3. PPM1D-mediated apoptosis and autophagy.

PPM1D is a negative regulator of apoptosis by a p53-dependant manner by its direct interaction with ATM, ATR, CHK1, Chk2 and p53. In p53-negative cells, overexpression of PPM1D regulates positively apoptosis by its inhibition of NF-κB and activation of RUNX2. Apoptosis activation leads to apoptotic bodies formation responsible for cell death. PPM1D regulated positively autophagy by its inhibitor functions on ATM and mTOR which are negative regulators of autophagy. Otherwise, PPM1D dephosphorylates ULK1 which turns to its activation leading to autophagophore formation responsible for autophagy mechanisms. PPM1D involvement in apoptosis pathway was demonstrated mainly in cancer disease otherwise PPM1D functions in autophagy mechanism were shown in atherosclerosis development. Created with BioRender.com.

Fig. 4. Schematic of the two main molecular cascades of senescence regulated by PP2C family members.

In p53-p21 axis, PPM1D regulates senescence mainly by its direct negative interaction with ATM, p53 and CHK2 turning to p21 reduction which decreases senescence induction. The p53-p21 axis can also be regulated by PHLPP2 by inhibiting AKT and by PPM1G by inhibiting PI3K, which activates AKT-p53-p21. In a p38MAPK -p16 axis, PPM1D and PPM1B act as a negative regulator through their interaction with p38MAPK leading to p16 reduction which turns to senescence decrease. Created with BioRender.com.