The Roles Of PI3K/Akt Signaling Pathway In Egakaryocyte/Platelet Mediated Pathological Processes

PI3K/Akt (Phosphoinositide3-kinases/protein kinase B) pathway, as a classicsignaling pathway, involves in cell proliferation, differentiation, survival andmigration, and plays an important role in many diseases therefore serving as one ofthe major targets for drug development.PDK1(phosphoinositide-dependent protein kinase-1), as a master kinase inPI3K/Akt pathway, its functions on platelet formation and activation are unknown.Accordingly, platelet-specific PDK1deficient mice were characterized to elucidatethe platelet-related function(s) of PDK1. We found that PDK1deficiency caused amild thrombocytopenia. Also, the aggregation of PDK1/platelets was diminished inresponse to low levels of thrombin, U46619and ADP, respectively. Further resultsdemonstrated that PDK1regulates thrombin-induced platelet activation by affectingαIIbβ3-mediated outside-in signaling. This result provided an explanation for thediminished spreading of PDK1/platelets on immobilized fibrinogen (Fg) and thedecreased rate of clot retraction in platelet rich plasma containing PDK1/platelets.PDK1deficiency diminished agonist-induced Akt Ser473phosphorylation, andthoroughly abolished Akt Thr308and Gsk3β Ser9phosphorylation in response toagonist treatment, and platelet spreading, respectively. A Gsk3β inhibitor fullyrestored the aggregation of PDK1/platelets in response to low levels of thrombin,the normal spreading of PDK1/platelets on Fg, and normal clot retraction in PRPcontaining PDK1/platelets. Those results indicated that Gsk3β is one of the majordownstream effectors of PDK1in thrombin-induced platelet activation andαIIbβ3-mediated outside-in signaling. In addition, the in vivo data demonstrated thatPDK1is an important regulator in arterial thrombosis formation.PTEN (phosphatase and tensin homolog) dephosphorylates PtdIns(3,4,5)P3toPtdIns(4,5)P2, thereby negatively regulating PI3K/Akt signaling pathway. Long-term observation of the megakaryocyte specific PTEN knockout mice, we found there aremore than90%of megakaryocyte-specific PTEN deficient mice finally progress tolymphoma among total65mice. Immunohistochemistry and flow cytometery resultsrevealed these mice developed a follicular B lymphoma. Although the results frommagnetic resonance images and survival curves indicated these mice develop a type ofrestrictive indolent lymphoma, some older mice ware gradually transform into a moreaggressive lymphoma. Further flow cytometery analysis of B cell subpopulation inbone marrow, peripheral lymph nodes and spleen revealed that earlier progenitor Bcell development aberrantly in bone marrow is responsible for follicularlymphomagenesis. Platelet mRNA microarray data indicated megakaryocytic PTENdeficiency caused the abnormal cytokines expression in megakaryocytes resulting inchanges in the microenvironment for Pre/Pro B cells differentiation and apoptosis inbone marrow. These B cells with chromosomal instability eventually evolved into thetumor cells at peripheral lymph node after aberrant somatic hypermutation. Lowexpression levels of PTEN in follicular lymphoma patients’ platelet suggest thathuman follicular lymphomagenesis may undergo a similar process. These resultspresented here reveale an important role of megakaryocytes in follicularlymphomagenesis. Our findings not only provide a mouse model for follicularlymphoma, but also supply some clues for the diagnosis and treatment of follicularlymphoma.In summary, the data presented here demonstrated that PI3K/Akt pathway positiveregulator PDK1regulated platelet activation and arterial thrombosis; however,PI3K/Akt pathway negative regulator PTEN involved in megakaryocyte-mediatedfollicular B lymphomagenesis.