1: Antimicrob Agents Chemother. 2009 Jun;53(6):2410-6. Epub 2009 Mar 23. Identification of the efflux transporter of the fluoroquinolone antibiotic ciprofloxacin in murine macrophages: studies with ciprofloxacin-resistant cells. Marquez B, Caceres NE, Mingeot-Leclercq MP, Tulkens PM, Van Bambeke F. Unité de Pharmacologie Cellulaire et Moléculaire, Louvain Drug Research Institute, UCL 7370 Avenue Mounier 73, B-1200 Brussels, Belgium. Ciprofloxacin, the most widely used totally synthetic antibiotic, is subject to active efflux mediated by a MRP-like transporter in wild-type murine J774 macrophages. To identify the transporter among the seven potential Mrps, we used cells made resistant to ciprofloxacin obtained by long-term exposure to increasing drug concentrations (these cells show less ciprofloxacin accumulation and provide a protected niche for ciprofloxacin-sensitive intracellular Listeria monocytogenes). In the present paper, we first show that ciprofloxacin-resistant cells display a faster efflux of ciprofloxacin which is inhibited by gemfibrozil (an unspecific MRP inhibitor). Elacridar, at a concentration known to inhibit P-glycoprotein and breast cancer resistance protein (BCRP), only slightly increased ciprofloxacin accumulation, with no difference between resistant and wild-type cells. Analysis at the mRNA (real-time PCR) and protein (Western blotting) levels revealed an overexpression of Mrp2 and Mrp4. Mrp4 transcripts, however, were overwhelmingly predominant (45% [wild-type cells] to 95% [ciprofloxacin-resistant cells] of all Mrp transcripts tested [Mrp1 to Mrp7]). Silencing of Mrp2 and Mrp4 with specific small interfering RNAs showed that only Mrp4 is involved in ciprofloxacin transport in both ciprofloxacin-resistant and wild-type cells. The study therefore identifies Mrp4 as the most likely transporter of ciprofloxacin in murine macrophages but leaves open a possible common upregulation mechanism for both Mrp4 and Mrp2 upon chronic exposure of eukaryotic cells to this widely used antibiotic. PMID: 19307362 [PubMed - in process] Related Links Active efflux of ciprofloxacin from J774 macrophages through an MRP-like transporter. [Antimicrob Agents Chemother. 2004] PMID:15215125 Cooperation between prokaryotic (Lde) and eukaryotic (MRP) efflux transporters in J774 macrophages infected with Listeria monocytogenes: studies with ciprofloxacin and moxifloxacin. [Antimicrob Agents Chemother. 2008] PMID:18573933 Cellular accumulation and activity of quinolones in ciprofloxacin-resistant J774 macrophages. [Antimicrob Agents Chemother. 2006] PMID:16641436 Influence of P-glycoprotein and MRP efflux pump inhibitors on the intracellular activity of azithromycin and ciprofloxacin in macrophages infected by Listeria monocytogenes or Staphylococcus aureus. [J Antimicrob Chemother. 2003] PMID:12697643 ATP-binding cassette C transporters in human pancreatic carcinoma cell lines. Upregulation in 5-fluorouracil-resistant cells. [Pancreatology. 2009] PMID:19077464