P-GLYCOPROTEIN IS HYPERPHOSPHORYLATED IN MULTIDRUG-RESISTANT HOB1 LYMPHOMA-CELLS TREATED WITH OVERDOSE OF VINCRISTINE

Two proteins with M(r) values of 170 kDa and 200 kDa, respectively, were identified in HOB1 lymphoma cells resistant to 1.0 mu M vincristine (designated HOB1/VCR1.0) by Western blot. Using anti-P-glycoprotein monoclonal antibody to immunoprecipitate the protein from the P-32-labeled extract of HOB1/VCR1.0 cells, the major form of the protein was in the area of 200 kDa. When the cells were cultured in 0.5 mu M vincristine for 72 h, the major form shifted to the area of 170 kDa. Northern blot analysis of the mdr transcription showed the gene had been overexpressed to a maximum in the cells resistant to 0.5 mu M vincristine. [H-3]Vincristine uptake study showed the cells with hyperphosphorylated P-glycoprotein accumulated only half the amount of the agent after 60 min of incubation as compared with those with hypophosphorylated protein. The current study suggests hyperphosphorylated P-glycoprotein is a form by which the cells can effectively exploit the protein when it can not be induced any more.