The cyclooxygenase system participates in functional mdr1b overexpression in primary rat hepatocyte cultures

Overexpression of mdr1-type P-glycoproteins (P-gps) is thought to contribute to primary chemotherapy resistance of untreated hepatocellular carcinoma. However, mechanisms of endogenous multidrug resistance 1 (mdr1) gene activation still remain unclear. Because recent studies have demonstrated overexpression of cyclooxygenase-2 (COX-2) in hepatocytes during early stages of hepatocarcinogenesis, we investigated whether the COX system, which catalyzes the rate-limiting step in prostaglandin synthesis, participates in mdr1 gene regulation. In the present study, primary rat hepatocyte cultures, exhibiting time-dependent mdr1b overexpression, demonstrated basal COX-2 and COX-1 mRNA expression and liberation of prostaglandin E(2) (PGE(2)), indicative of an active COX-dependent arachidonic acid metabolism. PGE(2) accumulation in culture supernatants was further enhanced by arachidonic acid (1mumol/L) and epidermal growth factor (EGF) (16 nmol/L). PGE(2) and prostaglandin F(2alpha) (PGF(2)alpha) (3-6mug/mL), added directly to the culture medium, significantly up-regulated intrinsic mdr1b mRNA overexpression and mdr1-dependent transport activity. Up-regulation was maximal after 3 days of culture. Like prostaglandins, the COX substrate, arachidonic acid, also induced mdr1b gene expression. Apart from this, structurally different COX inhibitors (indomethacin, meloxicam, NS-398) mediated significant inhibition of time-dependent and EGF-induced mdr1b mRNA overexpression, resulting in enhanced intracellular accumulation of the mdr1 substrate, rhodamine 123 (Rho123). Thus, the present data support the conclusion that the release of prostaglandins through activation of the COX system participates in endogenous mdr1b gene regulation. COX-2 inhibition might constitute a new strategy to counteract primary mdr1-dependent chemotherapy resistance.