HL-60 cells degrade α-actinin to produce a fragment that promotes monocyte/macrophage maturation

An amino-terminal fragment of α-actinin can promote monocyte/macrophage maturation. This fragment was initially isolated from media of HL-60 myeloid leukemia cells cultured on extracellular bone marrow matrix. To determine the source of this fragment in this culture system, we investigated whether HL-60 cells grown on bone marrow stroma have increased intracellular levels of α-actinin that may be released into the media during cell apoptosis. HL-60 cells grown on matrix showed no evidence of increased cellular α-actinin compared to cells grown on plastic substrata as measured by flow cytometry. In addition, there was no evidence of increased apoptosis as determined by DNA fragmentation assays or flow cytometry. However, 100 kD α-actinin was found in the extracellular matrix of bone marrow stroma by Western blot analysis and immunofluorescence microscopy. The α-actinin content in the stroma was markedly decreased after exposure to HL-60 cells. Furthermore, lysates of HL-60 cells or of peripheral blood monocytes can degrade exogenous α-actinin to produce a 31 kD fragment, which promotes monocyte/macrophage maturation. We conclude that when α-actinin is present in the extracellular matrix, it can be modified by HL-60 cells to produce a maturation promoting 31 kD fragment.