No abstract

These results demonstrate the effectiveness of liposome-encapsulated doxorubicin in overcoming resistance in the multidrug-resistant phenotype of HL-60/VCR cells by direct interaction with P-glycoprotein. Furthermore, they indicate that liposome-encapsulated doxorubicin may be an effective treatment for human cancers.

Recent studies suggest that multidrug resistance of HL60/ADR cells is related to an overexpression of the MRP (multidrug resistance associated protein) gene which encodes a 190-kDa ATP-binding membrane glycoprotein. In the present study we have further characterized P190 and have examined phosphorylation properties of the protein. The results demonstrate that P190 is highly phosphorylated and that the phosphate groups are metabolically active and undergo cycles of phosphorylation and dephosphorylation in the cell. Serine is the single amino acid phosphorylated in P190 and the phosphate groups are contained in nine tryptic peptides. Experiments have also been conducted to analyze the effect of various protein kinase inhibitors on phosphorylation levels of P190. The results show that H-7, staurosporine, and chelerythrine can reduce the phosphorylation of this protein. In the presence of both H-7 (200 microM) and staurosporine (200 nM) the phosphorylation of P190 is completely blocked. It has also been found that in the presence of these agents there is a major increase in drug accumulation and concomitant inhibition in drug efflux of resistant cells. These results therefore suggest the possibility that certain phosphate groups of protein P190 play an important role in modulating drug accumulation in resistant cells.

Studies have been carried out to elucidate the in vivo function of gene 2 in T7 DNA synthesis. In gene 2-infected cells the rate of incorporation of (3-H)thymidine into acid-insoluble material is about 60% that of cells infected with T7 wild type. Gene 2 mutants do not however produce viable phage after infection of the nonpermissive host. In T7 wild type-infected cells, a major portion of the newly alkaline sucrose gradients. The concatemers serve as precursors for the formation of mature T7 DNA as demonstrated in pulse-chase experiments. In similar studies carried out with gene 2-infected cells, concatemers are not detected when the intracellular DNA is analyzed at several different times during the infection process. The DNA made during a gene 2 infection is present as duplex structures with a sedimentation rate close to mature T7 DNA.