Contractility of the uterus during labour is under the control of prostaglandins regulated by the enzyme cyclooxygenase-2 (COX-2). Preterm labour is responsible for 70 % of neonatal deaths and one of its leading causes is intrauterine infection (Challis et al. 2002). The onset of both term and preterm labour has been likened to an inflammatory reaction. COX-2 expression is rapidly induced in response to a number of growth factors, tumour promoters and inflammatory cytokines including TNF-α and IL-1β (Smith et al. 1996). The exact mechanisms involved in COX-2 regulation by inflammatory cytokines remain unclear. We have previously demonstrated a role for p38 mitogen activated protein kinase (MAPK) and protein kinase C (PKC) in IL-1β-induced COX-2 expression in human myometrial smooth muscle cells (HMSMCs). Recent work has focused on the precise isoforms of PKC involved and possible interaction with NF-κB-regulated COX-2 expression.

HMSMCs were isolated from fresh biopsies of the lower segment myometrium taken during elective caesarean section after informed consent. Cultured cells were used to examine COX-2 expression by western blotting after 5 h of treatment with 10 ng ml^-1 IL-β. Pre-incubation of cells with a range of doses of two PKC inhibitors, GF109203X and LY379196, suggests a role for atypical isoforms z{special} and ¸{special} with complete inhibition of COX-2 expression seen at 10 µM GF109203X (the IC₅₀ for z{special} = 5.8 µM). Inhibition of COX-2 expression was not observed with LY379196 at 10 µM, a concentration known to inhibit all isoforms except the atypical (IC₅₀ for z{special} = 48 µM). Furthermore, experiments with pseudosubstrate inhibitors to α/β and z{special} isoforms showed inhibition of COX-2 expression only with the z{special} inhibitor. Long-term treatment of HMSMCs with 100 nM phorbol 12-myristate 13-acetate (PMA) for 48 h down-regulated expression of the conventional and novel PKC isoforms but not the atypical. IL-1β induced an increase in COX-2 expression after this long-term PMA treatment that was inhibited by GF109203X, demonstrating that atypical isoforms are the only PKCs required for IL-1β-induced COX-2 expression in HMSMCs. Further experiments using NF-κB gel shift assays or transfection of HMSMCs with an NF-κB promoter-luciferase reporter construct suggested that atypical PKC isoforms regulate COX-2 expression in these cells by interacting with NF-κB, via a mechanism that involves transactivating the bound NF-κB subunit in the promoter rather than assisting translocation of p50/p65 to the nucleus through degradation of IκB.

This work was supported by the Wellcome Trust.