The Basic Biology of PP2A in Hematologic Cells and Malignancies

Abstract

Reversible protein phosphorylation plays a crucial role in regulating cell signaling. In normal cells, phosphoregulation is tightly controlled by a network of protein kinases counterbalanced by several protein phosphatases. Deregulation of this delicate balance is widely recognized as a central mechanism by which cells escape external and internal self-limiting signals, eventually resulting in malignant transformation. A large fraction of hematologic malignancies is characterized by constitutive or unrestrained activation of oncogenic kinases. This is in part achieved by activating mutations, chromosomal rearrangements, or constitutive activation of upstream kinase regulators, in part by inactivation of their anti-oncogenic phosphatase counterparts. Protein phosphatase 2A (PP2A) represents a large family of cellular serine/threonine phosphatases with suspected tumor suppressive functions. In this review, we highlight our current knowledge about the complex structure and biology of these phosphatases in hematologic cells, thereby providing the rationale behind their diverse signaling functions. Eventually, this basic knowledge is a key to truly understand the tumor suppressive role of PP2A in leukemogenesis and to allow further rational development of therapeutic strategies targeting PP2A.

Keywords: PP2A; PP2A-activating drugs; inhibitor; subunit; tumor suppressor reactivation therapy.

Figure 1

Structure of PP2A holoenzymes. The majority of PP2A enzymes have a heterotrimeric structure and consist of one catalytic C subunit, one scaffolding A subunit, and one regulatory B-type subunit. Owing to the existence of various isoforms of each of these subunits – in human tissues, two C (encoded by PPP2CA and PPP2CB), two A (encoded by PPP2R1A and PPP2R1B), and 23 B-type isoforms (encoded by 15 different genes) – 92 different PP2A trimeric complexes can be assembled, each characterized by its own catalytic properties, substrate specificities, tissue or cell-specific expression, and subcellular localization. In addition, about one-third of PP2A occurs as a dimer of one A and one C subunit (four holoenzymes).

Figure 2

Microarray expression profiles of PP2A subunit encoding genes in mouse tissues. Spleen, thymus, bone marrow, brain cortex, and heart were hand-dissected from 10- to 12-week-old C57Bl6 mice. Total RNA was extracted, labeled, and hybridized to the Affymetrix mouse MOE 430 2.0 array (44). Scanning, quality control, data processing, and statistical analysis of the data were as described (44). Shown is the mean mRNA expression signal ±SD of three (spleen, thymus, brain, and heart) or four (bone marrow) biological replicate experiments. (A) Expression the PP2A core subunit encoding genes. (B) Expression of the genes encoding PP2A regulatory B-type subunits. Expression of Ppp2r3d could not be analyzed because it was not present on the array used.

Figure 3

Regulators of PP2A holoenzyme biogenesis and assembly. (A) Simplified schematic of the roles of PME-1, PTPA, and LCMT1 in the biogenesis of active PP2A trimers. The PP2A methylesterase PME-1 serves to stabilize the inactive PP2A C subunit in a complex with the A subunit, at the same time preventing PP2A C methylation. With ATP/Mg²⁺ as necessary cofactors, PTPA promotes folding of PP2A C in an active conformation, and thereby, indirectly, PP2A C carboxymethylation by LCMT1. The latter modification is absolutely required for binding of B subunits, facilitates interaction of all B′ subunits but the δ isoform, is of no apparent importance for binding of B′δ and the B″ subunits, and is disliked by the striatin subunits. α4 (not depicted here) is another regulator that stabilizes PP2A C in a latent form. It is currently unclear if and how this inactive α4–C complex might become activated by similar mechanisms (47). (B) Expression of PP2A biogenesis regulators in hematologic tissues, brain, and heart. The mean mRNA expression signal ±SD for Ppme1 (PME-1), Lcmt1 (LCMT1), Igbp1 (α4), and Ppp2r4 (PTPA) is shown of three (spleen, thymus, brain, and heart) or four (bone marrow) biological replicate experiments.

Figure 4

Cellular PP2A inhibitors. (A) Schematic representation of known cellular PP2A inhibitors, highlighting their potential regulation by (yellow) or dependence on (blue) phosphorylation, their holoenzyme specificity (if known), and the PP2A substrates they affect. Best characterized so far, in terms of phosphorylation dependence and holoenzyme specificity, are the mitotic inhibitors ENSA, ARPP-19, and Bod1. Both I1 (ANP32a, e) and I2 (SET) are established phosphoproteins, but depending on the specific site of modification, these phosphorylations may increase as well as decrease their PP2A inhibitory abilities. Phosphoregulation of CIP2A or TIPRL has not yet been described. I1, I2, and TIPRL are thought to interact with PP2A complexes through the C subunit, while holoenzyme specificity of CIP2A-mediated inhibition remains undefined. PBD: polo-box domain. (B) Expression of cellular PP2A inhibitors in hematologic tissues, brain, and heart. The mean mRNA expression signal ±SD for the eight indicated cellular PP2A inhibitory proteins is shown of three (spleen, thymus, brain, and heart) or four (bone marrow) biological replicate experiments.