Bone Marrow-Derived Adipocytes Promote Differentiation, Proliferation and Survival of Acute Monoblastic Leukemia Cells

Abstract 1501

Acute monoblastic leukemia occurs most commonly in young individuals, whereas acute monocytic leukemia is common in adults (WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues, 2008). Adipocytes are the prevalent stromal cell type in adult bone marrow (BM) and play an important role in the leukemic bone marrow microenvironment (Tabe et al., Blood 2004 103: 1815–22). BM stromal cells (MSCs) from elderly subjects have a reduced capacity to differentiate into osteoblasts and an increased capacity to differentiate into adipocytes, which leads to progressive accumulation of fat in the BM space with increasing age. In this study, we examined whether BM-derived adipocytes participate in the molecular events associated with proliferation, differentiation and apoptosis of monoblastic leukemia cells. To this end, we performed gene expression microarray analysis of 339 genes associated with lipid metabolism (GeneSQUARE, Kurabo, Japan) using RNA from monoblastic leukemia cell line U937 co-cultured with MSC or MSC-derived adipocytes. Five genes were upregulated (>2.0 fold) and 2 genes down-regulated in U937 cells co-cultured with adipocytes compared to U937 cultured alone. The up-regulated genes included monocyte/macrophage differentiation specific genes, such as scavenger receptor CD36, adipsin or fatty acid binding protein 4 (FABP4). CD36 and FABP4 are classical target genes of PPARγ which was also upregulated in U937 co-cultured with adipocytes. Upregulation of the inflammation-related haptoglobin gene was observed in U937 cells co-cultured with either adipocytes or MSC. These findings were confirmed by quantitative RT-PCR assay with concordant results. Down-regulated genes included monocyte chemoattractant protein-1 (MCP-1) (0.1 fold) and glycolytic enzyme hexokinase 2 (0.4 fold). In turn, U937 cells co-cultured with premature or mature adipocytes accumulated in G₂ cell cycle phase compared to U937 cultured alone or co-cultured with MSC (% G₂/M-phase fraction; U937 cultured alone, 14.6, co-culture with MSC 25.1, co-culture with preadipocyte 32.0, with mature adipocyte 31.9) and were protected from spontaneous cell death under serum-starved conditions (% subG1 fraction: U937 cultured alone 23.9%, co-cultured with MSC 15.7%, with preadipocytes 3.9% and with mature adipocytes 11.6%). In summary, results suggest that BM adipocytes, abundant in adult BM space, promote monocytic differentiation, proliferation and survival of monoblastic leukemia cells. We propose that these cells represent an essential component of the BM microenvironment that contributes to malignant phenotypes of adults with monocytic leukemia.