OKADAIC ACID INDUCES CELLULAR HYPERTROPHY IN AKR-2B FIBROBLASTS - INVOLVEMENT OF THE P70(S6) KINASE IN THE ONSET OF PROTEIN AND RIBOSOMAL-RNA SYNTHESIS

At low concentrations (50 nM), okadaic acid (OA), an inhibitor of phos phatases 1 and 2A, inhibits platelet-derived growth factor-induced cel l proliferation in late G(1) (A, Simm et at, Exp, Cell Res., 210: 160- 165, 1994), This inhibition is caused by the interference of OA in the induction and activation of the cell division protein kinases cdk1 an d cdk2, OA alone has no effect on cell number, but induces a pronounce d increase in cell size, The OA-induced hypertrophy can be divided int o two phases, The first phase is characterized by a swelling of tile c ells. This increase in cellular volume is not accompanied by a change in the level of cellular macromolecules, i.e., protein and RNA, Inhibi tor studies indicated a possible role of the Na+/H+ antiporter and Cl- channels in this process. In the second phase, an increase in the cel lular protein and RNA content was observed along with a minor change i n cell volume. To delineate a possible signaling pathway, the involvem ent of numerous protein kinases was analyzed, Low concentrations of OA lead to pronounced and sustained activation of the p70(S6) kinase, Th ere was little or no effect on various other kinases that can be activ ated by extracellular signals, e,g., mitogen-activated kinase, ribosom al S6 kinase, or other S6 peptide kinases, Likewise, at these concentr ations, OA did not activate the genes for fos, myc, or ornithine decar boxylase. At very low concentrations (ED(50), 0.5 nM), rapamycin, a sp ecific inhibitor of the activation of p70(S6) kinase, reversed the act ivation of the p70(S6) kinase and the enhancement of RNA synthesis and partially the increase in cell volume and protein synthesis, The OA-i nduced hypertrophy of AKR-2B fibroblasts may serve as a model system f or investigations aimed at the identification of signaling pathways le ading to hypertrophy of differentiated nonproliferating cells.