Protein kinase C δ and η isoenzymes control the shedding of the interleukin 6 receptor α in myeloma cells

The soluble interleukin 6 receptor α is an agonistic molecule of interleukin 6 (IL-6) and is important in the biology of multiple myeloma. More precisely, it potentiates the deleterious effects of IL-6 during tumour progression, facilitating angiogenesis and bone resorption. Because the mechanisms involved in the shedding of the interleukin 6 receptor α (IL-6Rα) in multiple myeloma are not known, we have investigated them in the XG-6 human myeloma cell line. Here we provide evidence that PMA-induced IL-6Rα shedding is controlled by a metalloproteinase and by protein kinase C (PKC) isoenzymes that do not require Ca2+ for their activation. We show that XG-6 cells express PKC-δ, −η and −∊ isoenzymes. However, after stimulation with PMA, only PKC-δ and PKC-η are activated, as shown by their translocation to the membrane. Treatment with PMA induces an increase in PKC-δ phosphorylation in its active loop. In addition, by using rottlerin, a specific inhibitor of PKC-δ, we demonstrate that PKC-δ is involved in the PMA-induced shedding of IL-6Rα. With the use of UO126, a specific inhibitor of the mitogen-activated protein kinase (MAPK) pathway, we show that the PMA-induced IL-6Rα shedding is mediated in part by the MAPK pathway. Finally, whereas GF109203X, a general PKC inhibitor, inhibits the activation of ERK1/2 (extracellular signal-regulated protein kinase 1/2), rottlerin has no inhibitory effect, indicating that the Ras/MAPK activation is PKC-dependent but PKC-δ-independent. Taken together, these results suggest that the PMA-induced shedding of IL-6Rα is mediated by a PKC isoenzyme network.