Fostriecin is an antitumor drug in phase I clinical trials. We have recently shown that it is a potent inhibitor of protein phosphatases 1 and 2A in vitro, a property not previously described for an antitumor drug. We have investigated its effects on protein phosphorylation in baby hamster kidney cells. Fostriecin strongly stimulated the phosphorylation of a single protein, which we identified as the intermediate filament vimentin. Fostriecin also caused rounding of the cells and a reorganization of the vimentin filaments. These effects are similar to those of the known protein phosphatase 1 and 2A inhibitors okadaic acid and calyculin A, which are also tumor promoters. Fostriecin induced vimentin hyperphosphorylation mostly at two sites, which were sensitive to staurosporine and could be phosphorylated by protein kinase C in vitro. Fostriecin-induced vimentin hyperphosphorylation also occurred in cells that lack p34cdc2 kinase activity. These results suggest that protein kinase C plays a direct or indirect role in vimentin hyperphosphorylation during exposure to fostriecin. The results also provide strong evidence that fostriecin inhibits protein phosphatases 1 and 2A in vivo and raise the possibility that it may have tumor-promoting activity.

No abstract

Hyperphosphorylation of the microtubule‐associated protein τ is a characteristic of Alzheimer brain tissue. Recent in vitro data suggest that mitogen‐activated protein kinase (MAPK), a proline‐directed protein kinase, phosphorylates the sites on τ common to Alzheimer's disease. Using an okadaic acid‐induced τ hyperphosphorylation model, we have tested the requirement for MAPK activity, using a specific inhibitor {PD098059 [2‐(2′‐amino‐3′‐methoxyphenyl)oxanaphthalen‐4‐one]} of the MAPK activator Mek1. Mobility shift, phosphoepitope analysis, and direct measurement of kinase activity indicated that the Mek1 inhibitor dose‐dependently blocked basal and okadaic acid‐induced MAPK activation. Despite a block of MAPK activation by this inhibitor, robust τ hyperphosphorylation was observed in response to okadaic acid. In addition, activation of MAPK by phorbol 12‐myristate 13‐acetate did not result in τ phosphorylation, indicating that in primary cultures of cortical neurons elevated MAPK activity is not sufficient to induce τ hyperphosphorylation.

No abstract