Three different monocyte stimuli (lipopolysaccharide (LPS), concanavalin-A (Con-A), and phorbol myristate acetate (TPA)) all induced production and release of IL-1beta from human monocytes in vitro. LPS demonstrated the greatest potency and induced IL-1beta release by 3 h after activation, while Con-A-stimulated release appeared at 12-24 h. IL-1beta release occurred at activator concentrations greater than or equal to those required for optimal IL-1beta production, correlated best with total IL-1beta produced, and was associated with release of the cytosolic enzyme marker lactate dehydrogenase (LDH) which followed IL- 1beta release temporally. LPS and Con-A had no effect on protein synthesis, a measure of overt toxicity, while TPA inhibited protein synthesis in a dose-dependent fashion. TPA-stimulated IL-1beta release was not blocked by the protein kinase C (PK-C) inhibitor staurosporine. While LPS and Con-A induced expression of a 33 and a 29 kDa precursor IL-1beta, only the 17 kDa form was released. These data indicate that LPS and Con-A may stimulate release of IL-1beta from human monocytes in vitro through induction of microdamage to the cell membrane by an unregulated process. TPA induces a more profound damage to cellular integrity. The involvement of microdamage to membrane integrity in this in vitro system, as determined by LDH release, casts doubt upon its physiological relevance. However, since IL-1beta and LDH both locate in the cytosol, an IL-1beta release mechanism that coincidentally transports LDH to the outside of the call cannot be ruled out at this time.
