Chemokine IP-10 expression in cultured human keratinocytes

IP-10, a member of the CXC family of chemokines, is considered to play an important role in inflammation via its T-cell chemotactic and adhesion-promoting properties. Elevated IP-10 levels in the epidermis of psoriasis, delayed-type hypersensitivity reactions, cutaneous T-cell lymphoma and fixed drug eruptions prompted us to study its expression in keratinocytes. IP-10 mRNA could be detected using the sensitive RT-PCR method, but not by Northern blotting in RNA preparations from unstimulated normal cultured keratinocytes, indicating a low steady-state level of IP-10 mRNA. Upon stimulation with IFN-gamma, IP-10 mRNA was found to accumulate in high amounts in a time- and dose-dependent manner. Superexpression was found with the combination of IFN-gamma and TNF-alpha or IL-1, although these latter cytokines by themselves did not induce accumulation of IP-10 mRNA. Nuclear run-on experiments performed to investigate the regulation of IP-10 mRNA expression, showed a very high constitutive transcriptional activity of the IP-10 gene in unstimulated keratinocytes, which was not affected by stimulation with IFN-gamma, TNF-alpha, or a combination of IFN-gamma and TNF-alpha. Protein kinase C (PKC) was shown to be involved in IP-10 mRNA expression since the PKC inhibitor H7 decreased IP-10 mRNA accumulation. A protein was isolated from culture supernatants of stimulated keratinocytes using HPLC techniques and, by sequence analysis, was found to be identical to IP-10. The dynamics of secretion of IP-10 protein as monitored by ELISA was shown to parallel the mRNA expression.