Niche-based screening identifies small-molecule inhibitors of leukemia stem cells.

Efforts to develop more effective therapies for acute leukemia may benefit from high-throughput screening systems that reflect the complex physiology of the disease, including leukemia stem cells (LSCs) and supportive interactions with the bone marrow microenvironment. The therapeutic targeting of LSCs is challenging because LSCs are highly similar to normal hematopoietic stem and progenitor cells (HSPCs) and are protected by stromal cells in vivo. We screened 14,718 compounds in a leukemia-stroma co-culture system for inhibition of cobblestone formation, a cellular behavior associated with stem-cell function. Among those compounds that inhibited malignant cells but spared HSPCs was the cholesterol-lowering drug lovastatin. Lovastatin showed anti-LSC activity in vitro and in an in vivo bone marrow transplantation model. Mechanistic studies demonstrated that the effect was on target, via inhibition of HMG-CoA reductase. These results illustrate the power of merging physiologically relevant models with high-throughput screening.

Immortalization and transformation of primary human airway epithelial cells by gene transfer.

One critical step in the development of a cancerous cell is its acquisition of an unlimited replicative lifespan, the process termed immortalization. Experimental model systems designed to study cellular transformation ex vivo have relied to date on the in vitro selection of a subpopulation of cells that have become immortalized through treatment with chemical or physical mutagens and the selection of rare clonal variants. In this study, we describe the direct immortalization of primary human airway epithelial cells through the successive introduction of the Simian Virus 40 Early Region and the telomerase catalytic subunit hTERT. Cells immortalized in this way are now responsive to malignant transformation by an introduced H-ras or K-ras oncogene. These immortalized human airway epithelial cells, which have been created through the stepwise introduction of genetic alterations, provide a novel experimental model system with which to study further the biology of the airway epithelial cell and to dissect the molecular basis of lung cancer pathogenesis.

Niche-Based Screening in Multiple Myeloma Identifies a Kinesin-5 Inhibitor with Improved Selectivity over Hematopoietic Progenitors.

Novel therapeutic approaches are urgently required for multiple myeloma (MM). We used a phenotypic screening approach using co-cultures of MM cells with bone marrow stromal cells to identify compounds that overcome stromal resistance. One such compound, BRD9876, displayed selectivity over normal hematopoietic progenitors and was discovered to be an unusual ATP non-competitive kinesin-5 (Eg5) inhibitor. A novel mutation caused resistance, suggesting a binding site distinct from known Eg5 inhibitors, and BRD9876 inhibited only microtubule-bound Eg5. Eg5 phosphorylation, which increases microtubule binding, uniquely enhanced BRD9876 activity. MM cells have greater phosphorylated Eg5 than hematopoietic cells, consistent with increased vulnerability specifically to BRD9876's mode of action. Thus, differences in Eg5-microtubule binding between malignant and normal blood cells may be exploited to treat multiple myeloma. Additional steps are required for further therapeutic development, but our results indicate that unbiased chemical biology approaches can identify therapeutic strategies unanticipated by prior knowledge of protein targets.

Csnk1a1 inhibition has p53-dependent therapeutic efficacy in acute myeloid leukemia.

Despite extensive insights into the underlying genetics and biology of acute myeloid leukemia (AML), overall survival remains poor and new therapies are needed. We found that casein kinase 1 α (Csnk1a1), a serine-threonine kinase, is essential for AML cell survival in vivo. Normal hematopoietic stem and progenitor cells (HSPCs) were relatively less affected by shRNA-mediated knockdown of Csnk1a1. To identify downstream mediators of Csnk1a1 critical for leukemia cells, we performed an in vivo pooled shRNA screen and gene expression profiling. We found that Csnk1a1 knockdown results in decreased Rps6 phosphorylation, increased p53 activity, and myeloid differentiation. Consistent with these observations, p53-null leukemias were insensitive to Csnk1a1 knockdown. We further evaluated whether D4476, a casein kinase 1 inhibitor, would exhibit selective antileukemic effects. Treatment of leukemia stem cells (LSCs) with D4476 showed highly selective killing of LSCs over normal HSPCs. In summary, these findings demonstrate that Csnk1a1 inhibition causes reduced Rps6 phosphorylation and activation of p53, resulting in selective elimination of leukemia cells, revealing Csnk1a1 as a potential therapeutic target for the treatment of AML.

In Vivo RNAi screening identifies a leukemia-specific dependence on integrin beta 3 signaling.

We used an in vivo small hairpin RNA (shRNA) screening approach to identify genes that are essential for MLL-AF9 acute myeloid leukemia (AML). We found that Integrin Beta 3 (Itgb3) is essential for murine leukemia cells in vivo and for human leukemia cells in xenotransplantation studies. In leukemia cells, Itgb3 knockdown impaired homing, downregulated LSC transcriptional programs, and induced differentiation via the intracellular kinase Syk. In contrast, loss of Itgb3 in normal hematopoietic stem and progenitor cells did not affect engraftment, reconstitution, or differentiation. Finally, using an Itgb3 knockout mouse model, we confirmed that Itgb3 is dispensable for normal hematopoiesis but is required for leukemogenesis. Our results establish the significance of the Itgb3 signaling pathway as a potential therapeutic target in AML.

The Spemann organizer gene, Goosecoid, promotes tumor metastasis.

The process of invasion and metastasis during tumor progression is often reminiscent of cell migration events occurring during embryonic development. We hypothesized that genes controlling cellular changes in the Spemann organizer at gastrulation might be reactivated in tumors. The Goosecoid homeobox transcription factor is a known executer of cell migration from the Spemann organizer. We found that indeed Goosecoid is overexpressed in a majority of human breast tumors. Ectopic expression of Goosecoid in human breast cells generated invasion-associated cellular changes, including an epithelial-mesenchymal transition. TGF-beta signaling, known to promote metastasis, induced Goosecoid expression in human breast cells. Moreover, Goosecoid significantly enhanced the ability of breast cancer cells to form pulmonary metastases in mice. These results demonstrate that Goosecoid promotes tumor cell malignancy and suggest that other conserved organizer genes may function similarly in human cancer.