Purpose: To evaluate therapeutic activity of PAK inhibition in ATLL and to characterize the role of PAK isoforms in cell proliferation, survival, and adhesion of ATLL cells in preclinical models. Experimental Design: Frequency and prognostic impact of PAK2 amplification were evaluated in an ATLL cohort of 370 cases. Novel long-term cultures and in vivo xenograft models were developed using primary ATLL cells from North American patients. Two PAK inhibitors were used to block PAK kinase activity pharmacologically. siRNA-based gene silencing approach was used to genetically knockdown (KD) PAK1 and PAK2 in ATLL cell lines. Results: PAK1/2/4 are the three most abundantly expressed PAK family members in ATLL. PAK2 amplifications are seen in 24%of ATLLsandare associated withworse prognosis in a large patient cohort. The pan-PAK inhibitor PF-3758309 (PF) has strong in vitro and in vivo activity in a variety of ATLL preclinical models. These activities of PF are likely attributed to its ability to target several PAK isoforms simultaneously because genetic silencing of either PAK1 or PAK2 produced more modest effects. PAK2 plays a major role in CADM1-mediated stromal interaction, which is an important step in systemic dissemination of the disease. This finding is consistent with the observation that PAK2 amplification is more frequent in aggressive ATLLs and correlates with inferior outcome. Conclusions: PAK2, a gene frequently amplified in ATLL, facilitates CADM1-mediated stromal interaction and promotes survival of ATLL cells. Taken together, PAK inhibition may hold significant promise as a targeted therapy for aggressive ATLLs.
Bibliographical noteFunding Information:
We are grateful to Drs. Thomas Waldmann and Michael Petrus (NIH/NCI) and Prof. Naomichi Arima (Kagoshima University, Japan) for the gift of the Japanese ATLL cell lines; Prof. Naomichi Arima for valuable advice on establishing ATLL long-term cultures; and Drs. John Chan (City of Hope) and Giorgio Inghirami (Weill Cornell Medical College) for the gift of HH, MAC1, and FePD1 cell lines. We also thank Dr. Jonathan Chernoff (Fox Chase Cancer Center, Philadelphia) for insightful discussion and Dr. Robert Stanley (Albert Einstein College of Medicine) for assisting with the design of PAK inhibitor sensitivity tests. Flow cytometry analyses were performed at the Albert Einstein College of Medicine Flow Cytometry facility, which is supported by an NCI Cancer Center Support Grant (P30CA013330). FACS sorting was performed at the Einstein Human Stem Cell FACS and Xenotransplantation Facility, which is supported by a grant from the New York State Department of Health (NYSTEM Program) for the shared facility (C029154). This work was supported by research grants from the Leukemia & Lymphoma Society Translational Research Project (No. 6471-15); Dr. Louis Skarlow Memorial Trust (B.H. Ye); and Aids Malignancy Consortium grant UM1CA121947 (M. Janakiram). E.Y. Chung and Y. Mai are supported by the Harry Eagle Scholarship from the Department of Cell Biology, Albert Einstein College of Medicine, and U.A. Shah was supported by the hematology/oncology fellowship program, Montefiore Medical Center and Jacobi Medical Center.