Role for Megakaryocyte Phosphatidylinositol Transfer Proteins-Alpha and -Beta in TGF Beta-Mediated Regulation of Hematopoietic Homeostasis

There are still unknowns regarding homeostatic regulation of hematopoietic stem (HSC) and progenitor (HPC) cells. Deciphering these processes are important for understanding and treating hematopoietic diseases. Phosphatidylinositol is a rare membrane structure lipid, but is critical for cellular signaling upon phosphorylation by lipid kinases to generate phosphoinositide. While phosphoinositide pathways contribute to events linked to the cytoskeleton, little is known of these pathways in regulating hematopoiesis. Critical to this pathway are phosphatidylinositol transfer proteins (PITPs) that in vitro enhance transfer of aqueous insoluble phosphatidylinositol from one membrane to another. Class I PITP proteins PITP α and β are highly conserved, small, and ubiquitously expressed in mammalian cells. To test the hypothesis that phosphatidylinositol signaling contributes to hematopoiesis, we generated conditional knock out mice that lack either PITPα single isoform (PITPα^fl/fl PF4Cre⁺) or both PITPα and PITPβ (PITPα^fl/fl β^fl/fl PF4Cre⁺) specifically in their platelets and megakaryocytes, and observed a bone marrow (BM) HSC/HPC phenotype. BM from these mice and their littermate controls were evaluated for absolute numbers of nucleated cells, HSC, and HPC. Cells were analyzed by rigorous phenotyping for long-term (LT)-HSC, short-term (ST)-HSC, multipotential (MPP), common myeloid (CMP), and granulocyte macrophage (GMP) progenitors. They were also assessed for functional HPC by colony assays in vitro for multi-cytokine (Epo, GM-CSF, IL-3, SCF, hemin) stimulated granulocyte macrophage (CFU-GM), erythroid (BFU-E), and multipotential (CFU-GEMM) progenitors, and for their cycling status using a high specific activity tritiated thymidine kill assay. PITPα-/-, and to a greater extent PITPα/β-/-, progenitor cells demonstrated significant decreases in LT-HSC and ST-HSC per femur. While there were no significant changes in numbers of MPP, CMP, and GMP in the PITPα and PITPα/β-/- BM compared to controls, there were significant decreases of approximately 50% in numbers of CFU-GM, BFU-E, and CFU-GEMM per femur. PITC-/- HPC were in a slow or non-cycling state compared to the rapid cell cycle (40-57% in S-phase) of control HPC. Thus PITPα-/- and PITPα/β -/- BM cells were associated with decreased HSC and functional HPC numbers. To evaluate mechanisms for this phenotype, we focused on BM megakaryocytes, as they have been implicated in microenvironmental regulation of hematopoiesis, and PITPα and PITPα/β activities are associated with megakaryocyte/platelet function. BM derived TPO-culture expanded megakaryocytes were allowed to condition medium for 48 hours, and conditioned medium (CM) from PITPα-/-, PITPα/β-/-, and control BM megakaryocytes were assayed for effects on colony formation by multicytokine stimulated BM cells derived from normal mice. CM from PITPα-/- and PITPα/β-/- megakaryocytes, but not from control mice, significantly suppressed colony formation by CFU-GM, BFU-E and CFU-GEMM (by ~50%). Limiting dilution analysis of the CM demonstrated that PITPα/β-/- cells had more potent suppressor activity than PITPα-/- cells. Bioplex analysis of the CM from PITPα -/- and PITPα/β -/- megakaryocytes demonstrated significantly higher levels of cytokines/chemokines with known myelosuppressive activities (including: TNF-α, VEGF, LIF, IP-10, ENA-78, MDC, MIG, and MIP-1α). However, ELISA analysis of TGF-β1, demonstrated minimal protein in BM flushes from control mice, but large amounts of TGF-β (>350 pg/ml) in BM flushes from the PITPα/β -/- mice. CM from PITPα and α/β-/- megakaryocytes also contained highly elevated TGF-β protein. Thus, we hypothesized that the effect of PITP -/- on the suppression of HPC colony formation was mediated by TGF-β. The myelosuppressive CM derived from PITPα and PITPα/β -/- megakaryocytes was completely neutralized by a monoclonal TGF-β antibody. This demonstrates that PITPα and PITP α/β-/- megakaryocytes produce elevated TGF-β that at least in part, and possibly in synergy with other myelosuppressive cytokines/chemokines, decreases numbers of HSC and functional HPC. Our studies demonstrate a link between PITPα and α/β and TGF-β levels with significant effects on HSCs and HPCs, thus demonstrating involvement of the phosphoinositide pathway in homeostatic regulation of hematopoiesis.