Role of protein phosphatases in insulin-like growth factor II (IGF II)-stimulated mannose 6-phosphate/IGF II receptor redistribution.

The regulated expression of mannose 6-phosphate/insulin-like growth factor II (M6P/IGF II) receptors in plasma membranes has previously been shown to be accompanied by marked changes in the phosphorylation state of the receptors (Corvera, S., Folander, K., Clairmont, K. B., and Czech, M. P. (1988) Proc. Natl. Acad. Sci. U. S. A. 85, 7567-7571). In the present study we show that protein phosphatase 2A dephosphorylates the human M6P/IGF II receptor in vitro. Incubation of human fibroblasts with okadaic acid, a specific inhibitor of this phosphatase, resulted in a depletion of M6P/IGF II receptors at the cell surface without affecting their internalization kinetics. The phosphorylation state of the remaining cell surface receptors was 3-fold increased. Thus, the endocytosis rate of M6P/IGF II receptors appears to be unaltered by increased phosphorylation. While the decreased cell surface expression of receptors was reversible upon removal of okadaic acid the IGF II-induced redistribution of M6P/IGF II receptors to the plasma membrane (Braulke, T., Tippmer, S., Neher, E., and von Figura, K. (1989) EMBO J. 8, 681-686) was irreversibly inhibited by the phosphatase inhibitor. Receptor redistribution in response to protein kinase C activation was not affected by okadaic acid. These results suggest that the cell surface expression of M6P/IGF II receptor can be regulated by phosphatase-dependent and -independent pathways. In addition, the phosphorylation state and the steady-state cell surface number of transferrin receptors were not affected by okadaic acid, whereas it impaired the IGF II-stimulated receptor redistribution similarly as for M6P/IGF II receptors. The data indicate that okadaic acid-sensitive protein phosphatases may play a general role in terms of IGF II-modulated receptor recycling.