Proteasomal Degradation of the Nuclear Targeting Growth Factor Midkine

Abstract

It is widely held that growth factor signaling is terminated by lysosomal degradation of its activated receptor and the endocytosed growth factor is transported to lysosomes. Nuclear targeting is another important pathway through which signals of growth factors are mediated. However, mechanisms underlying desensitization of nuclear targeting growth factors are poorly understood. Here we report that the nuclear targeting pathway is down-regulated by the proteasome system. Degradation of endocytosed midkine, a heparin-binding growth factor, was suppressed by both proteasome and lysosome inhibitors to similar extents. By contrast, a proteasome inhibitor, but not lysosome ones, accelerated the nuclear accumulation of midkine. An expression vector of signal sequence-less midkine, which is produced in the cytosol, was constructed because endocytosed midkine may be translocated to the cytosol from cellular compartments before entering the nucleus. The cytosol-produced midkine underwent proteasomal degradation and accumulated in the nucleus as did the endocytosed midkine. It was polyubiquitinated, and its nuclear accumulation was enhanced by a proteasome inhibitor. We further dissected the midkine molecule to investigate roles in degradation and trafficking. The Nterminal half-domain of midkine was significantly more susceptible to proteasomal degradation, whereas the Cterminal half-domain was sufficient for nuclear localization. Together, these data highlight the desensitization of nuclear targeting by growth factors and indicate a critical role of the proteasome system in it.Coordinated regulation of "on" and "off" of signaling is essential for life. Upon ligation of a growth factor with its receptor(s), receptor-mediated intracellular signaling starts from the cell surface and/or endosomes. Desensitization (termination of signaling) then takes place via degradation of the activated receptor. Degradation of the activated receptor occurs essentially through the following two mechanisms. First, the intracellular domain of the activated receptor is degraded by the proteasome system as revealed for the ␤c chain, a shared receptor for interleukin-5 and -3 and granulocyte-macrophage colony-stimulating factor (1). Second, endocytosed receptors are transported from endosomes to lysosomes where they are degraded.Because the proteasome system plays a critical role in endosome-to-lysosome trafficking (1-7), it may indirectly regulate the lysosomal degradation of receptors. In addition to the classical desensitization described above, an additional route, namely nuclear targeting by growth factors, should be considered. In this case, signaling is mediated directly by the growth factor and not via the cell surface receptor. Therefore, desensitization occurs only when the growth factor is degraded.Mounting evidence indicates that nuclear targeting by growth factors plays an indispensable role in their biological activities. For example, the nuclear localization of fibroblast growth factor 1 and Schwannoma-derived growth factor is n...

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