Defective microtubule reorganization in phorbol ester-resistant U937 variants: reconstitution of the normal cell phenotype with nocodazole treatment.

Phorbol ester-induced beta 2-integrin transport to the cell surface is defective in cloned 12-O-tetradecanoylphorbol-13-acetate (TPA)-resistant U937 cell variants. Failure of the integrin-containing vesicles to reach the plasma membrane effectively blocks development of all integrin-mediated responses and the formation of a functional oxidase complex. Several lines of evidence suggested that the underlying cause of this defect may be the loss of regulatory elements in the cytoskeleton, which mediate microtubule stability and organization. Diminished protein kinase C (PKC) beta 2 association with microtubules correlated with the loss of heat-soluble microtubule-associated PKC-binding proteins and the loss of TPA-inducible reorganization of the microtubule cytoskeleton in the resistant U937 variants. Treatment with the microtubule-disrupting drug, nocodazole, was sufficient to induce the modest increase in cd11b surface expression associated with the release of this preformed integrin. Furthermore, brief nocodazole treatment followed by TPA treatment completely restored susceptibility to phorbol ester-induced differentiation in the resistant cell lines. The combination of nocodazole and TPA treatment also restored NADPH oxidase activity in the TPA-resistant clones. Results from these studies suggest that TPA-induced microtubule reorganization is a prerequisite for integrin vesicle translocation in U937 cells and that vesicle translocation to the plasma membrane may be a prerequisite for the transcriptional activation of cd11b and cd11c integrin genes in the early stages of monocyte differentiation.