Multidrug resistance-associated protein 2 (Mrp2, Abcc2), an organic anion transporter present in the apical membrane of hepatocytes, renal epithelial cells, and enterocytes, is postulated to undergo post-transcriptional regulation. We hypothesized that Mrp2 protein undergoes altered rates of protein synthesis or degradation consistent with different Mrp2 protein expression. We analyzed Mrp2 synthesis, expression, and degradation in control female, 19-and 20-day pregnant, and pregnenolone-16␣-carbonitrile (PCN)-treated rats using in vivo metabolic-labeling studies with [35 S]cysteine/methionine or [ 14 C]NaHCO 3 , polysomal distribution analyses and ribonuclease protection assays (RPA). Mrp2 protein was significantly increased in rats treated with PCN for 2 days but significantly decreased in 19-day pregnant rats relative to controls; no significant differences were observed in Mrp2 mRNA expression among these groups. The measured half-lives of 14 C-labeled Mrp2 in control, pregnant, and PCN-treated rats were 27, 36, and 22 h, respectively, and were not significantly different. The rate of incorporation of 35 S into Mrp2 was highest in PCNtreated rats. Polysomal distribution analysis of Mrp2 mRNA was consistent with increased Mrp2 protein synthesis after PCN treatment. The major transcription-initiation site for rat liver determined by RPA was Ϫ98 nucleotides (nt), with other start sites observed at Ϫ213, Ϫ163, Ϫ132, and Ϫ71 nt; use of transcription sites did not differ among the groups. Differences in the degradation of Mrp2 protein cannot explain the posttranscriptional regulation of Mrp2 in control, pregnant, and PCN-treated rats. Rather, the observed difference in protein synthesis suggests an intrinsic role for the translational regulation of rat Mrp2 protein.

Epidermal growth factor receptor (EGFR) inhibitors are highly effective in treating non-small cell lung cancers (NSCLC) expressing activated EGFR, particularly those harboring EGFR mutations. However, most patients who benefit from EGFR inhibitors achieve only partial responses or stable disease, facilitating the emergence of resistance. Thus, progression-free survival advantages in responding patients are modest. Combination therapy, preferably using agents with synergistic activity, could both improve responses and reduce acquired resistance rates. We hypothesized that combining MEK inhibitors with EGFR inhibitors could result in such a benefit. The MAPK pathway lies downstream of EGFR and transduces both proliferative and survival signals in a variety of cancer types. Inhibitors of this pathway are currently in clinical trials, but little evidence exists supporting the use of these agents as monotherapy in EGFR-dependent NSCLC. In this study, we find EGFR-dependent NSCLC cell lines are moderately sensitive to loss of ERK1/2 activity, either by small molecule inhibition or by siRNA knockdown. The consequence of inhibition is dependent upon the trophic content of the culture media, primarily anti-proliferative in serum-rich conditions and pro-apoptotic in serum-poor conditions. However, when MEK inhibition was combined with EGFR inhibitors, cytotoxic synergy was observed for all EGFR-dependent cell lines tested in serum-containing media. Enhanced cytotoxicity is demonstrated in cell lines with and without EGFR mutations, including those harboring the T790M escape mutation. These findings support future clinical studies that combine EGFR-and MEK1/2-targeted agents to investigate whether improved outcomes can be achieved in clinically screened EGFR-dependent NSCLC.

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Organic anion transporting polypeptides (Oatp) mediate the transport of a wide variety of amphipathic organic substrates. Rat Oatp1b2 and human OATP1B3 are members of a liverspecific subfamily of Oatps/OATPs. We investigated whether prolactin (PRL) and growth hormone (GH) regulated Oatp1b2 and OATP1B3 gene expression via signal transducers and activators of transcription 5 (Stat5). Binding sites for Stat5 transcription factors were located in the promoters of Oatp1b2 and OATP1B3 at Ϫ209 to Ϫ201 (5Ј-TTCTGGGAA-3Ј) and Ϫ170 to Ϫ162 (5Ј-TTCTGAGAA-3Ј), respectively. In primary hepatocytes from female and male rats treated with PRL or GH, Oatp1b2 mRNA measured by real-time polymerase chain reaction was significantly induced 2-fold. HepG2 cells were transiently transfected with expression vectors containing Oatp1b2 or OATP1B3 promoter fragments, cDNAs for Stat5a, and the receptors for PRL (PRLR L ) or GH (GHR), and treated with PRL or GH. PRL and GH induction of Oatp1b2 and OATP1B3 promoter activity required cotransfection of Stat5a and PRLR L or GHR. Mutation of the Stat5 binding site in both promoters eliminated hormonal induction. In DNA binding assays, HepG2 cells transfected with cDNAs for Stat5a and PRLR L were treated with PRL, and nuclear extracts were probed with a 32 P-labeled oligomer corresponding to Ϫ177 to Ϫ157 of the OATP1B3 promoter. PRL enhanced the binding of Stat5a to the OATP1B3 promoter and DNA-protein binding was inhibited in competition assays by excess OATP1B3 and Stat5 consensus oligomers but not by mutant Stat5 oligomers. These findings indicate that PRL and GH can regulate Oatp1b2 and OATP1B3 gene expression via the Stat5 signal-transduction pathway.