Caspase-1 promotes monocyte-macrophage differentiation by repressing PPARγ

Monocyte-to-macrophage differentiation is tightly controlled in vivo, as disruption of the normal differentiation program can lead to diverse disorders. Caspase-1, the first identified member of the caspase family, regulates differentiation in various cell types such as Th17 cells and adipocytes. However, the contribution of caspase-1 in monocyte-macrophage differentiation remains elusive. Here we report that caspase-1 is significantly downregulated in leukemia cells from patients with acute monocytic leukemia. By using the phorbol 12-myristate 13-acetate-induced cell differentiation model, we found that caspase-1 activation was required for the differentiation of human monocytes to macrophages. Further analysis of peroxisome proliferator-activated receptor γ (PPARγ) protein levels revealed that the monocyte-macrophage differentiation program could be divided into two stages. Caspase-1-mediated downregulation of PPARγ was important in the late stage of monocyte-macrophage differentiation; however, PPARγ protein levels had little effect on the early stage differentiation. Accumulation of PPARγ protein by troglitazone treatment potently suppressed the late stage of macrophage differentiation, which might be linked to inhibition of nuclear factor-κB activity. The data provide a plausible mechanistic basis by which caspase-1 promotes the differentiation of macrophages from monocytes.