INHIBITION OF PROLIFERATION AND TUMORIGENIC POTENTIAL OF HUMAN THYROID ANAPLASTIC CARCINOMA CELLS BY LONG-TERM EXPOSURE TO cAMP.

Biological response modifiers, such as dibutyryl-cAMP (db-cAMP) and retinoic acid (RA) have been demonstrated to induce in vitro the reappearance of a differentiated phenotype in transformed cells (i.e.: neuroblastoma, myeloid leukaemia, teratoma cells, etc.), simultaneously decreasing cell proliferation rate. Treatment for 72 hours with db-cAMP (1mM) of a highly malignant human anaplastic thyroid carcinoma cell line (ARO) induced a marked reduction (- 44%) of cell proliferation , as measured by (3H)-thymidine uptake. No effect on cell proliferation rate was observed in the presence of RA (5uM), while the combination of RA and db-cAMP treatment showed results similar to those obtained with db-cAMP alone. The db-cAMP dependent inhibition of ARO cell proliferation, was confirmed by direct assessment of the number of mitotic cells. The expression of cell cycle-related genes, such as those coding for cyclins and cyclin-dependent kinases, was not modified after ARO cell treatment with either db-cAMP or RA, as shown by indirect immunofluorescence, Northern and Western blot analyses. Colony-forming ability of ARO cells in soft agar, was dramatically reduced by db-cAMP treatment but unmodified by RA. In conclusion, prolonged exposure to db-cAMP but not to RA treatment is able to inhibit the proliferation rate of ARO cells and to greatly reduce their tumorigenic potential in vitro.