Dissection of a novel autocrine signaling pathway via quantitative secretome and interactome mapping.

Epidermal homeostasis is a balancing act governed by a multitude of underlying regulatory events, and several growth factors and signaling pathways have been implicated in regulation of the balance between proliferation and differentiation in keratinocytes. We show here that the signal transducer/transcription factor FIZ1 (Flt3 interacting zinc finger protein-1) is a previously unknown player in this regulatory axis, promoting an increase in proliferation of HaCaT human immortalized keratinocytes that is driven by more rapid G1/S progression and mediated by activation of the MAP/ERK kinase pathway. Utilizing quantitative SILAC-based secretome analysis, we identified the insulin growth factor binding protein IGFBP3 as the key mediating factor, demonstrating that elevated FIZ1 levels promote increased IGFBP3 expression and secretion and a concurrent increased sensitivity to IGF1 signaling, while antibody-based neutralization of IGFBP3 abrogates the FIZ1-induced growth advantage. To identify underlying protein-protein interactions likely to govern these events, we mapped the interactome of FIZ1 and found eight novel binding partners that form complexes with the protein in the cytoplasm and nucleus. These include signal transduction and transcription factors and the cell cycle regulatory NDR (Nuclear Dbf2-related) kinases. Our results provide further insight into the complex balance of epidermal homeostasis and identify FIZ1 as a novel therapeutic target.