Downregulation of phorbol 12-myristate 13-acetate-induced tumor necrosis factor-alpha and interleukin-1 beta production and gene expression in human monocytic cells by human alpha-fetoprotein.

We previously identified a specific receptor of alpha-fetoprotein (AFP) on human monocytes. Although AFP alters many immune cell functions, the effect of AFP on monocyte cytokine production is unknown. Because tumor necrosis factor--alpha (TNF-alpha) and interleukin-1 beta (IL-1 beta) are important cytokines in immunoregulation, we investigated whether AFP could modulate TNF-alpha and IL-1 beta production in U937, a human monocytic cell line. Our results showed that U937 cells secreted TNF-alpha and IL-1 beta in response to either phorbyl 12-myristate 13-acetate (PMA) or IFN-gamma + LPS. In contrast, AFP significantly suppressed PMA-induced TNF-alpha and IL-1 beta production by U937 cells in a time and dose dependent fashion. Pretreatment of U937 cells with AFP resulted in maximal inhibition of PMA-stimulated TNF-alpha and IL-1 beta production by 58% and 67% respectively. AFP also inhibited interferon-gamma plus lipopolysaccharide (IFN-gamma + LPS)-induced TNF-alpha and IL-1 beta production. Furthermore, Northern blot analysis showed that AFP suppressed PMA-mediated TNF-alpha and IL-1 beta messenger RNA (mRNA) expression. PMA-induced prostaglandin E2 (PGE2) production by U937 cells was enhanced by AFP. Pretreatment with indomethacin, a cyclooxygenase inhibitor, reversed AFP-inhibited TNF-alpha production by 78%. Thus, we conclude that AFP downregulates TNF-alpha and IL-1 beta production via a PGE2-dependent mechanism.