Interleukin-4 (IL-4) is a major cytokine regulating IgE production and IgE response. Neuroendocrine growth factors, such as growth hormone and insulin-like growth factor (IGF-1), also modulate IgE production by human B cells. We report that in both human primary immune cells and established B cell lines, IGF-1 increased expression of the IL-4-induced type II IgE receptor (Fc(epsilon)RII/CD23). This effect was also seen at the mRNA level. IGF-1 increased the activity of signal transducer and activator of transcription 6 (STAT6) in response to IL-4, and activated NF-(kappa)B. STAT6 and NF-(kappa)B are major transcription factors controlling CD23 expression. The tyrosine kinase inhibitor, tyrphostin, abolished both IL-4- and IL-4 plus IGF-1-mediated induction of STAT6 and the subsequent CD23 expression. In contrast, pyrrolidine dithiocarbamate (PDTC), an NF-(kappa)B inhibitor, only suppressed the enhancing effect of IGF-1 on IL-4-induced CD23 expression. Our data suggest that IGF-1 modulates CD23 gene expression by affecting the STAT6 and NF-(kappa)B pathways. Regulation of CD23 by IGF-1 may have an important implication for the immunomodulatory potential of IGF-1, and may provide a new therapeutic target for allergy and other hypersensitivity reactions involving an excessive IgE response.