Molecular Mechanisms of Cell Density-Dependent Growth in Myeloid and Lymphoid Leukemia Cell Lines.

Normal hematopoietic stem cells have been shown to be maintained through interaction with their environmental niches, such as osteoblastic and endothelial ones. The growth of leukemia cells has been shown to be stimulated by environmental niches (paracrine growth) or by cell-to-cell interaction and/or excreted factors of leukemia cells (autocrine growth). The growth of myeloid (MO7-E and HL-60) and lymphoid (Raji, U-266, Daudi and RPMI-1788) leukemia cell lines cultured at various cell densities in serum free medium (Sigma H 4281) with 1% BSA was evaluated. The cells cultured at higher cell densities (cultured cell densities of more than 10⁵/ml) showed logarithmic linear increases in cell number, whereas those at lower cell densities (cultured cell densities of less than 10⁴/ml) ceased increasing cell number. Supernatants of myeloid leukemia cells stimulated the growth of autologous clonogenic cells, but not those of lymphoid leukemia cells. Neutralizing antibodies (Abs) against various hematopoietic growth factors failed to inhibit cell growth except for anti-VEGF Ab, which significantly decreased HL-60 leukemia cell growth. In contrast, anti-TNF-α Ab significantly stimulated the growth of the HL-60 cells. To clarify the nature of the cultured cell density on the growth of leukemia cells, leukemia cells were cultured at higher cell densities (group H, cultured cell densities of 10⁶/ml) or at lower cell densities (group L, cultured cell densities of 10⁴/ml). After culture of 3-, 6-, 10-, and 24-hr, cells were serially harvested and total cellular RNA was extracted. Gene transcript levels were determined by using Real-Time PCR. Gene transcripts examined in the present study were as follows: Jagged-1, -1, Notch-1, -2, -3, Ang-1, -2, Tie-1, -2, Wnd3a, Wnd5a, β-Catenin, γ-Catenin, N-Cadherin, Cyclin D1, p16, p21, HOXA6, HOXA7, HOXA10, HOXB4, and Mef2c. At 24-hr cultures, transcripts of myeloid leukemia cell lines for Bmi-1, Wnt-3a, β-Catenin and γ-Catenin were higher, and those of lymphoid leukemia cell lines for Notch 1, 2, and 3 were higher in group H compared with group L. Transcript levels for Wnt5a were higher at 10-hr culture (HL-60 and Raji), those for HOXA7 at 30–10-hr (MO-7E, U-266 and Raji), and those for Mef2c at 3-hr (MO-7E, U-266 and Raji) in group H compared with group L. Taken together, our present results favor the conclusions that genes related to transcription factors and growth factors are sequentially and differentially expressed through cell-to-cell interaction of leukemia cells. The nature of the leukemia cell-to-cell interacrtion, which is related to the growth advantages of leukemia stem cells over normal hematopoietic stem cells, remains to be further clarified.