Role of CD 14 Expression in the Differentiation-Apoptosis Switch in Human Monocytic Leukemia Cells Treated with 1 a , 25-Dihydroxyvitamin D 3 or Dexamethasone in the Presence of Transforming Growth Factor b 1

Transforming growth factor b (TGF-b) enhanced the growth-inhibitory activities of dexamethasone (Dex) and 1a,25-dihydroxyvitamin D3 (VD3) on human monocytoid leukemia U937 cells. TGF-b and VD3 synergistically increased the expression of differentiation-associated markers such as the CD11b and CD14 antigens, whereas TGF-b and Dex did not. On the other hand, TGF-b and Dex synergistically increased the number of Apo2.7-positive cells, which represents the early stage of apoptosis, whereas TGFb and VD3 did not, suggesting that TGF-b enhanced apoptosis with Dex and enhanced monocytic differentiation with VD3. In the presence of TGF-b, the retinoblastoma susceptibility gene product, pRb, was synergistically dephosphorylated by Dex as well as VD3. TGF similarly enhanced the expression of the p21 gene in U937 cells treated with Dex and VD3. TGF-b dose-dependently increased the expression of Bcl-2 and Bad and decreased the expression of Bcl-XL in U937 cells. Dex enhanced the down-regulation of Bcl-XL expression in TGF-b-treated cells, whereas VD3 blocked this down-regulation of Bcl-XL. However, the down-regulation of Bcl-XL by treatment with the antisense oligomer did not affect the apoptosis or differentiation of U937 cells. The apoptosis of CD14positive cells was suppressed in the VD3 plus TGF-btreated cultures. These results suggest that the expression of CD14 is involved in the survival of differentiated cells. Introduction Cellular homeostasis is regulated by proliferation, differentiation, and death. The ability of cells to exit the cell cycle, induce apoptosis, and differentiate is mediated by intricate molecular and biochemical mechanisms. Certain myeloid leukemia cells have been used to study the biochemical and molecular mechanisms that govern these processes. Human monoblastic leukemia U937 cells can be induced to differentiate toward monocytes and macrophages by treatment with VD3 in the presence of TGF-b, whereas they are induced to undergo programmed cell death (apoptosis) by treatment with a low concentration of glucocorticoid in the presence of TGF-b. This experimental model is useful for studying the regulatory mechanisms of the signaling pathway of differentiation or apoptosis (1–3). During the early phase of differentiation and apoptosis, cells exit the cell cycle and cease to proliferate (4, 5). For differentiation, cells must possess a mechanism to avoid cell death, thus allowing the development of mature cells to exhibit their functional and morphological phenotypes. The intracellular levels of proteins coded for by apoptosis suppressor and effector genes seem to regulate viability or death (6–10). Glucocorticoid-induced growth arrest is involved in the transcriptional repression of G1 cyclins and cyclin-dependent kinases (cdks) or in enhancing the transcription of cdk inhibitors by the activated glucocorticoid receptor (11). Dex induces apoptosis in lymphoid leukemia cells by suppressing the expression of Bcl-2 (12), and the monocytic differentiation of myeloid leukemia HL-60 and U937 cells is accompanied by an increased expression of Bcl-6 (13, 14). Moreover, the expression of Bcl-XL is either maintained or increased during the monocytic differentiation of human myeloid leukemia cells