Evidence for a direct correlation between c-Jun NH2 terminal kinase 1 activation, cyclin D2 expression, and G(1)/S phase transition in the murine hybridoma 7TD1 cells

In this study we show that the addition of fresh culture medium to high-den sity growth-arrested 7TD1 cells induces a strong and transient stimulation of the c-Jun NH2 terminal kinase activity (Jun kinase/JNK), a marked increa se in cyclin D2 expression, the phosphorylation of pRb, and the transition from G(1) to S phase. The stimulation of cyclin D2 expression and the induc tion of JNK activity appear to be the consequences of the alkalinization of the extracellular medium. Indeed both parameters (i) can be induced, regar dless of cell dilution, by the addition of a weak base such as triethylamin e, and (ii) are together inhibited by (N-ethyl-N-isopropyl)amiloride, a spe cific inhibitor of the Na+/H+ exchanger. We provide a strong argument indic ating the existence of a direct correlation between JNK1 activation and cyc lin D2 stimulation. Indeed, we demonstrate that cyclin D2 expression is blo cked by SE 202190, an agent known to inhibit both JNK and p38(MAPK), but no t by SE 203580, a specific inhibitor of p38(MAPK). Furthermore, we also obs erved that DIMSO and forskolin, two agents that inhibit the proliferation o f 7TD1 cells, inhibit in parallel cyclin D2 and JNK1. Altogether our result s suggest that (i) JNK1 participates in the signaling pathway which control s the expression of cyclin D2 and (ii) that the inhibition of JNK1 by DMSO and forskolin could explain, at least in part, the antiproliferative action of these drugs in 7TD1 cells. (C) 2000 Academic Press.