Interleukin-1 beta secretion. A possible multistep process that is regulated in a cell type-specific manner.

In a prior study, we found that the processed form of human interleukin-1 beta (mature IL-1 beta) is secreted to a significantly greater extent than the precursor form of the protein, indicating that the precursor domain acts in some manner to reduce the secretory potential of the protein. In view of this observation, we sought to define the sequence(s) in the IL-1 beta precursor that limit the secretion of the protein as well as the sequences in the mature protein that promote secretion. The P388D1 murine macrophage cell line and the Jurkat human T-cell line were transiently transfected with cDNA expression vectors encoding truncated forms of human precursor IL-1 beta proteins, lacking either the first 76, 94, 99, or 104 amino acids. The removal of increasing numbers of precursor amino acid residues resulted in a graded increase in the secretion of the truncated precursor IL-1 beta proteins from both cell lines. The minimal region of the precursor sequence required to inhibit the optimal secretion of IL-1 beta occurs between amino acids 100 and 104 for P388D1 cells and 95-99 for Jurkat cells. Deletion of the amino acids within these regions increased the secretion level of the truncated proteins to that of mature IL-1 beta. Mutagenesis of the mature IL-1 beta sequence revealed that a region of basic amino acids may play an important role in the optimal secretion of mature IL-1 beta in P388D1 cells, but not in Jurkat cells. Based on the differences in the structural requirements for IL-1 beta secretion in P388D1 and Jurkat cell lines, it is likely that the secretion of IL-1 beta may be subject to multiple levels of regulation that are differentially operative in different cell types.