Acute myelogenous leukemia (AML) is characterized by a differentiation block at an early stage of hematopoiesis that results in the accumulation of immature myeloid cells and an impaired production of blood cells. This parallels findings in solid tumors that demonstrate differentiation stage is often associated with tumor behavior and generally an immature tumor is more aggressive than the more differentiated counterpart. Thus, new

research designed to more fully understand the mechanisms of cellular differentiation may improve our understanding of neoplastic transformation and identify new therapeutic targets. Recently our laboratory has determined that increased expression and activation of the interferon-inducible, dsRNA-activated kinase (PKR) is associated with shorter remission duration and worse survival for patients with AML. PKR is a key mediator of the cellular response to inflammatory stress that has been found to be upregulated by chronic inflammation and aging. Since we have previously reported that mice expressing a PKR transgene (TgPKR) in hematopoietic cells have a reduced number of bone marrow (BM) Lin-, Sca1+, cKit+ hematopoietic stem/progenitor cells (HSPCs) and develop a dysplastic BM with an increased frequency of BM blasts, we investigated whether PKR may affect HSPC differentiation. Results indicate that BM of TgPKR mice contains a significantly reduced number of multipotent progenitor cells (MPPs) that are more frequently arrested in G0/G1 than wild type (WT) mice. In contrast, BM of PKR knockout mice (PKRKO) have an increased frequency of proliferating MPPs (S+G2/M) but a decreased frequency of both Long-term (LT) and Short-term (ST) HSCs compared to either WT or TgPKR mice. Although LT-HSCs of TgPKR mice are more frequently quiescent in vivo they remain functional in vitro as demonstrated by nearly double the colony forming capacity compared to BM of PKRKO mice after >4 weeks in long term culture initiating cell (LTC-IC) assays. In addition, when differentiation of the myeloid cell line K562 is induced by PMA treatment, cells with reduced PKR expression by siRNA knockdown display an increased rate and extent of megakaryocyte differentiation compared to control cells. These findings suggest that increased PKR expression has a previously unrecognized function to inhibit hematopoietic cell differentiation. This may account for why TgPKR mice accumulate quiescent LT-HSCs and develop BM dysplasia with age. Furthermore, these results suggest that increased PKR expression/activation as observed in high-risk MDS, AML and other hematologic malignancies may contribute to leukemogenesis, at least in part, by blocking hematopoietic cell differentiation. In addition, these new findings may be applicable to solid tumors since increased PKR signaling has also been reported to be associated with a more rapid and aggressive course of tumor progression in breast cancer, melanoma and colon cancer.

Citation Format: Richard L. Bennett, Xiaodong Cheng, Michael T. Byrne, Amalin Asous, Mohini Patel, Zeel Patel, W. Stratford May. Increased PKR expression inhibits differentiation of hematopoietic stem cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 194.