PROTEIN PHOSPHATASE-2A ASSOCIATES WITH THE CYTOSKELETON TO MAINTAIN CELL SPREADING AND REDUCED MOTILITY OF NONMETASTATIC LEWIS LUNG-CARCINOMA CELLS - THE LOSS OF THIS REGULATORY CONTROL IN METASTATIC CELLS

Metastatic Lewis lung carcinoma (LLC-LN7) variants have previously bee n shown to have reduced levels of protein phosphatase-2A (PP-2A) activ ity as compared to the nonmetastatic LLC-C8 cells. The present study s howed that inhibition of PP-2A in the nonmetastatic LLC-C8 cells cause d a rapid change from a spread to a rounded morphology and increased t heir in vitro invasiveness through laminin, In contrast, the metastati c LLC-LN7 cells were rounded and invasive, which was not affected by i nhibition of PP-2A, To determine whether these differences could be at tributed to alterations in PP-2A association with the cytoskeleton, th e extent of PP-2A colocalization with microtubules was tested. Immunos taining for tubulin showed prominent filamentous fibers in nonmetastat ic LLC-C8 cells and small foci of PP-2A immunostaining along these mic rotubules. In contrast, the tubulin staining was diffuse throughout th e metastatic LLC-LN7 cells and there was little evidence of associatio n with PP-2A, Western blot analyses showed that this reduced level of PP-2A association with microtubules in metastatic LLC-LN7 cells was no t due to differences in levels of the PP-2A subunits, Instead, it may be due to the reduced association of the subunits into the heterotrime ric form of the PP-2A holoenzyme. These studies show the importance of PP-2A in maintaining a spread morphology and in restricting invasiven ess, and a loss of this regulatory control in metastatic cells. This l oss of PP-2A regulatory control in metastatic cells may be due to a re duction in the trimeric form of the PP-2A holoenzyme.