Protein phosphorylation in multidrug resistant Chinese hamster cells.

Protein phosphorylation is altered in multidrug resistant, reverse transformed Chinese hamster cells selected for resistance to vincristine (DC-3F/VCRd-5L) or actinomycin D (DC-3F/AD X), as compared to drug-sensitive parental DC-3F cells. Evidence for this was obtained by gel electrophoretic analysis of proteins phosphorylated in vitro in the presence of [gamma -32P]ATP. In general, the level of incorporation of 32P into resistant cell proteins was higher than into proteins of sensitive cells, when reactions were carried out in either the presence or absence of exogenous protein kinase modulators. Phosphorylation of P-glycoprotein a multidrug resistance-related protein, and of sorcin, a 22 kDa calcium-binding protein overproduced in many multidrug resistant cells including DC-3F/VCRd-5L, was demonstrated. Analysis of proteins metabolically labeled with [32P]-orthophosphate suggests that protein phosphorylation differences in cell-free extracts are representative of events in the intact cells. Data support the probability that a variety of kinase and/or phosphatase activities were altered in the multidrug resistant cells. These may be associated with resistance development, P-glycoprotein function, reverse transformation, state of differentiation, inhibition of cellular proliferation, or all of these components.