VITAMIN-D-3 AND CERAMIDE REDUCE THE INVASION OF TUMOR-CELLS THROUGH EXTRACELLULAR-MATRIX COMPONENTS BY ELEVATING PROTEIN PHOSPHATASE-2A

Increasing phosphorylation reactions by protein kinase A (PKA) or redu cing dephosphorylation reactions of protein phosphatase-2A (PP-2A) inc reases the invasiveness of Lewis lung carcinoma (LLC) cells, as measur ed by their capacity to traverse extracellular matrix (ECM)-coated fil ters, Metastatic LLC-LN7 variants have reduced PP-2A activity when com pared to nonmetastatic LLC-C8 variants, Immunoblotting showed that thi s reduced level of PP-2A activity was not due to reduced levels of the PP-2A catalytic (C) subunit, The cellular PP-2A activity could be sti mulated by addition of C-2-ceramide to LLC-LN7 lysates, or by incubati ng cells with either C-2-ceramide or with a noncalcemic analog of vita min D-3, which has previously been shown to stimulate the release of c eramide, These treatments to elevate PP-2A activity in metastatic LLC- LN7 cells resulted in a decline in their capacity to invade through se lect (ECM) components, particularly through vitronectin and laminin, U nderscoring the importance of PP-2A in limiting the invasiveness of tu mor cells was the demonstration that LLC-LN7 cell transfectants overex pressing the PP-2A C-alpha subunit were less invasive through ECM comp onents than the wild-type cells. Invasion by these cells was further r educed by additionally increasing PP-2A activity by incubation with C- 2-ceramide or the vitamin D-3 analog. These results suggest a role of a vitamin D-3/ceramide/PP-2A pathway in limiting the invasiveness of t umor cells through select ECM components.