INHIBITION OF TUMOR INVASIVENESS BY 1-ALPHA,25-DIHYDROXY-VITAMIN-D-3 COUPLED TO A DECLINE IN PROTEIN-KINASE-A ACTIVITY AND AN INCREASE IN CYTOSKELETAL ORGANIZATION

The capacity of cloned metastatic Lewis lung carcinoma cells (LLC-LN7) to invade through reconstituted basement membrane-coated filters was reduced after incubation with 1 alpha,25-dihydroxyvitamin D-3 [1,25(OH )(2)D-3]. This was observed at doses as low as 10(-10) M 1,25(OH)(2)D- 3. The 1,25(OH)(2)D-3-treated cells also had reduced levels of protein kinase A (PKA) activity and an increase in the level of polymerized a ctin, properties that have previously been demonstrated for less metas tatic LLC variants, In addition, levels of the intermediate filament p rotein vimentin increased in 1,25(OH)(2)D-3-treated LLC-LN7 tumor cell s, In contrast, the levels and distribution of tubulin were not affect ed by 1,25(OH)(2)D-3. The possibility that the decline in PKA activity was involved in the 1,25(OH)(2)D-3 modulation of the cytoskeletal com ponents was evaluated, To accomplish this, LLC-7 transfectants whose P KA levels were blocked due to expression of a mutated PKA R(1 alpha) s ubunit (LN7-REV) were incubated with 1,25(OH)(2)D-3 and their levels o f F-actin were measured, In the absence of 1,25(OH)(2)D-3 treatment, t he PKA-defective LN7-REV cells had an increased level of polymerized a ctin as compared to the wild-type LLC-LN7 cells, This level of F-actin was minimally affected by 1,25(OH)(2)D-3, suggesting that PKA activit y is required for 1,25(OH),D, modulation of actin polymerization, Thes e studies show that 1,25(OH)(2)D-3 can reduce PKA activity in tumor ce lls, and that this reduction in PKA may be an intermediate signal thro ugh which 1,25(OH)(2)D-3 affects the cytoskeleton and diminishes tumor invasiveness.