Li et al. Nanoscale Res Lett 2011, 6:29 http://www.nanoscalereslett.com/content/6/1/29 PLA/Au nanocomposites, significant increase in the cell death could be detected. Considering the good biocom- patibility of PLA and Au nanomaterials, these observa- tions suggest that the apparent increase in cancer cell death should be attributed to the synergistic function derived from the combination of DNR with PLA/Au nanocomposites. Especially, the PLA/Au nanocompo- sites–DNR complexes can remarkably facilitate the accumulation of the DNR molecules in the drug- resistant cancer cells and apparently reserve the MDR of K562/AO2. With the good fluorescence characteristics, DNR could also be utilized as the fluorescence probe to track its location and concentration inside the cells. As shown in Figure 4, the synergistic effect for the uptake of DNR on K562 and K562/AO2 could be obviously observed by the laser confocal fluorescence micro- scopy. Figure 4 showed the typical images of the con- focal fluorescence microscopy of different leukemia cancer cells. It appeared that the relatively weak drug uptake was observed when the cancer cells were only treated with DNR. While the intracellular fluorescence was slightly strengthened after relevant cells incubated by DNR together with PLA nanofibers. In comparison, when PLA/Au nanocomposites conjugated with DNR were incorporated into the target system, PLA/Au nanocomposites have an apparent synergistic effect on the drug uptake of DNR in the respective cancer cells, where the intracellular fluorescence intensity was remarkably enhanced upon application of the PLA/Au nanocomposites together with DNR, as shown in Figure 4e and 4f. Since the PLA/Au nanocomposites themselves have no fluorescence, the intracellular fluorescence was only generated by the anticancer drug DNR. Our previous study indicates that the presence of bare Au nanoparticles alone could just slightly enhance the uptake of DNR by K562 cells [8]. Hence, these results indicated that much more DNR molecules could be efficiently accumulated in the cancer cells upon application of DNR together with PLA/Au nanocomposites. Additionally, our observations demonstrate that for drug-resistant K52/AO2 cells treated with DNR alone, scarce cancer cells with weak intracellular fluorescence could be observed. The very weak intracellular fluores- cence of drug-resistant leukemia cells treated with DNR alone may be attributed to the over-expression of P-gp protein on the cell membrane of the drug-resis- tant cancer cells [34,35], which could readily pump DNR molecules out of the relevant cancer cells so that much lower intracellular DNR fluorescence could be observed for K562/AO2 cells than that of K562 cells, as shown in Figure 4a and 4b. Interestingly, Page 6 of 8 the presence of PLA/Au nanocomposites could effi- ciently facilitate the drug uptake of DNR into the drug-resistant leukemia cells and lead to the much higher intracellular drug concentration in the target cells, resulting in the remarkable enhancement of the intracellular fluorescence of drug-resistant leukemia cells (Shown in Figure 4b and 4f). This result suggests that the PLA/Au nanocomposites may affect the activ- ity of P-gp protein and efficiently prevent the drug efflux from the drug-resistant leukemia cells. Thus, much more DNR could be readily permeated and accumulated into the relative cancer cells because the relevant synergistic effect of the PLA/Au nanocompo- sites could apparently inhibit the drug resistance of K562/AO2. In view of these observations, it appears that the PLA/Au nanocomposites may play as potential inhibitor of multidrug resistance (MRD) and thus effi- ciently promote the cellular uptake of the drug into the relevant drug-resistant cancer cells. Cytotoxicity Study by MTT Assay The cell viability of leukemia cancer cells in the pre- sence of PLA/Au nanocomposites loaded with DNR has been explored by MTT assay. As shown in Figure 5, the results demonstrate that the combination of the PLA/ Au nanocomposites with DNR could more effectively inhibit the growth of these two different kinds of leuke- mia cells than that treated with DNR alone. It is evident that the biocompatible PLA/Au nanocomposites have a synergistic effect to facilitate the drug uptake into human leukemia cells, increase the relative intracellular drug concentration and hence enhance the cytotoxicity of anticancer agents. Meanwhile, it is observed that the inhibition effect for drug-resistant K562/AO2 cancer cells was relatively significant than that for drug- sensitive K562 cancer cells when treated with DNR con- jugated with PLA/Au nanocomposites. As shown in Table 1, our results revealed that the resistant factor of the reversal index to resistant leukemia cells was 70.14 for K562/A02 in the control group, while the resistant factor in the presence of the PLA/Au nanocomposites significantly decreased to 38.88; therefore, the reversal index to K562/A02 was 1.8. This suggests that the pre- sence of PLA/Au nanocomposites can reinforce the accumulation of DNR in drug-resistant K562/A02 cells and lead to a great extent decreasing of the resisting fac- tors. Thus, the interaction of PLA/Au nanocomposites with bioactive molecules on the cell membrane could provide a new strategy to overcome the multidrug resis- tance (MDR) of K562/AO2 cells by improving the effi- ciency of drug delivery. These observations were coherent with the above results of confocal fluorescence studies.