Focal Adhesion Kinase Inactivation Reduces the Development of Acute Leukemia and Partially Rescues Hematopoietic Stem Cell Defects in Pten-Knockout Mice

Abstract 864

Phosphatase and tensin homolog on chromosome 10 (Pten) is a tumor suppressor which possesses both lipid and protein phosphatase activities. Mutations and epigenetic inactivations of the Pten gene are commonly detected in a large number of tissue malignancies, including leukemias and lymphomas. Studies using Hematopoietic Pten-knockout in adult mice (Pten^−/−) have demonstrated that Pten plays a critical role in maintaining the homeostasis of bone marrow (BM) hematopoiesis. Pten inactivation promotes the proliferation and peripheral mobilization of BM hematopoietic stem cells (HSCs). Pten^−/− mice develop myeloproliferative disorders (MPD) within days, followed by acute leukemic transformation. Most previous studies attributed such phenotypic changes observed in Pten^−/− mice to excessive activation of the PI3K/AKT/mTOR signal, a consequence of the loss of Pten's lipid phosphatase activity. However, the role of Pten's protein phosphatase activity in the regulation of HSCs and leukemogenesis is not well studied.

Focal adhesion kinase (Fak) is a critical substrate for the protein phosphatase activity of Pten. Dysregulation of Fak has been observed in many cancers, including acute myeloid leukemias (AML) and acute lymphocytic leukemias (ALL). Therefore, we postulated that Fak might play a pivotal role in the development and progression of leukemia following Pten deletion. To test this hypothesis, we generated Mx1-Cre⁺Pten^fl/flFak^fl/fl mice (an interferon-inducible Pten and Fak compound-knockout, Pten^−/−Fak^−/−) in which both the Pten and Fak genes in the hematopoietic system are deleted upon injection of polyinosinic-polycytidylic acid (pI-pC). Our results showed that the genetic inactivation of Fak can partially rescue HSC defects associated with Pten deficiency. We found that peripheral mobilization of HSCs in Pten^−/−Fak^−/− mice is significantly reduced compared to Pten^−/− mice. As a consequence, more long-term HSCs (LT-HSCs) are preserved in the BM of Pten^−/−Fak^−/− mice compared to Pten^−/− mice. Transplantation studies suggested that the hematopoietic reconstitutive capacity of Pten^−/−Fak^−/− HSCs is significantly improved compared to Pten^−/− HSCs. Although Fak deletion fails to prevent the development of MPD in Pten^−/− mice, Fak deletion does significantly reduce the frequency of AML/ALL, also significantly delays the onset of AML/ALL in comparison to Pten^−/− mice. This study suggests that Fak might be a potential target for preventing the MPD-to-AML/ALL transformation and therefore blocking the Fak activity may hold a promise for a novel anti-leukemia therapy. The molecular mechanisms underlying the phenotype restoration of Pten^−/− mice by Fak deletion in the hematopoietic system are actively being studied in our laboratory.