Protein kinase C delta and eta isoenzymes control the shedding of the interleukin 6 receptor alpha in myeloma cells

The soluble interleukin 6 receptor alpha is an agonistic molecule of interl eukin 6 (IL-6) and is important in the biology of multiple myeloma. More pr ecisely, it potentiates the deleterious effects of IL-6 during tumour progr ession, facilitating angiogenesis and bone resorption. Because the mechanis ms involved in the shedding of the interleukin 6 receptor alpha (IL-6R alph a) 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-6R a lpha shedding is controlled by a metalloproteinase and by protein kinase C (PKC) isoenzymes that do not require Ca2+ for their activation. We show tha t XG-6 cells express PKC-delta, -eta and -zeta isoenzymes. However, after s timulation with PMA, only PKC-delta and PKC-eta are activated, as shown by their translocation to the membrane. Treatment with PMA induces an increase in PKC-delta phosphorylation in its active loop. In addition, by using rot tlerin, a specific inhibitor of PKC-delta, we demonstrate that PKC-delta is involved in the PMA-induced shedding of IL-6R alpha. With the use of UO126 , a specific inhibitor of the mitogen-activated protein kinase (MAPK) pathw ay, we show that the PMA-induced IL-6R alpha shedding is mediated in part b y the MAPK pathway. Finally, whereas GF109203X, a general PKC inhibitor, in hibits the activation of ERK1/2 (extra cellular signal-regulated protein ki nase 1/2), rottlerin has no inhibitory effect, indicating that the Ras/MAPK activation is PKC-dependent but PKC-delta -independent. Taken together, th ese results suggest that the PMA-induced shedding of IL-6R alpha is mediate d by a PKC isoenzyme network.