Nuclear translocation of cell-surface transmembrane growth factor HB-EGF

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Heparin-binding EGF-like growth factor (HB-EGF) is synthesized as a type I plasma membrane protein (proHB-EGF) containing of the extracellular EGF-like domain, transmembrane segment and cytoplasmic short tail. Cleavage of proHB-EGF via metalloprotease activation (ectodomain shedding) yields a soluble ligand of EGF receptor and transmembrane-cytoplasmic fragment We previously showed that the cytoplasmic domain of HB-EGF (HB-EGF-cyto) interacts with a transcriptional repressor and effects on its activity. Here we attempt to reveal the transcriptional regulation by and spatio-temporal information of HB-EGF-cyto after shedding stimulation.We found that another transcriptional repressor, Bcl6 associates with HB-EGF-cyto. A luciferase assay showed that shedding of proHB-EGF reversed the Bcl6-mediated transcriptional repression. Moreover chromatin immunoprecipitation indicated that upon shedding stimuli HB-EGF-cyto abolished Bcl6 binding on cyclinlin D2 promoter which is repressed by BcI6 at a steady-state. Consistently the level of endogenous cyclin D2 protein increased in parallel with the shedding of proHB-EGF. These observations indicated that HB-EGF-cyto interacts with Bcl6 and abolishes its repression activity. Next we focus on the localization of HB-EGF-cyto. Immunofluorescent microscopy demonstrated that HB-EGF-cyto targeted to the nuclear envelope after shedding stimulation. A 14 amino acids region in HB-EGF-cyto showed a nuclear envelope targeting activity. Digitonin permeabilized cells suggested that HB-EGF-cyto targeted inside the nucleus, i.e., the inner nuclear membrane. Furthermore over expressed Rab11 suppressed the nuclear envelope targeting of HB-EGF-cyto, suggesting the involvement of recycling endosome for the relocalization of HB-EGF-cyto. Collectively these data point to a novel transcriptional regulation via the nuclear envelope targeting of a plasma membrane growth factor.