Phlpp1 regulates endochondral ossification via the chondrocyte phospho-proteome

Appendicular growth and bone mass acquisition are controlled by a variety of growth factors, hormones, and mechanical forces in a dynamic process called endochondral ossification. Chondrocytes in the growth plate must proliferate and undergo hypertrophy to drive growth plate expansion and lay the template for bone. Pleckstrin homology (PH) domain and leucine rich repeat phosphatase 1 and 2 (Phlpp1 and Phlpp2) are protein phosphatases that regulate intracellular signaling cascades through posttranslational modification of AKT, PKC, and S6K, among others. Phlpp1 controls chondrocyte proliferation and survival and germline deletion of Phlpp1 suppresses bone lengthening. Here, we demonstrate that Phlpp2 does not regulate endochondral ossification. Phlpp2-/- mice are phenotypically indistinguishable from their wildtype (WT) littermates, with similar bone length, bone mass, and growth plate dynamics. By contrast, Phlpp1/2-/- mice are phenotypically indistinguishable from Phlpp1-/- mice. Deletion of Phlpp1 or Phlpp2 had moderate effects on the chondrocyte transcriptome and proteome compared to WT control cells. By contrast, Phlpp1-/- and Phlpp1/2-/- chondrocytes had significantly altered phospho-proteomes compared to WT and Phlpp2-/- chondrocytes. Data integration via multiomics analysis revealed that RAF-MEK-ERK signaling was altered in cells lacking Phlpp1. In conclusion, these data demonstrate that Phlpp1, but not Phlpp2, regulates endochondral ossification via the chondrocyte phospho-proteome.