Abstract:

SLURP-1 (secreted mammalian Ly-6/urokinase plasminogen activator receptor-related protein-1) is a novel auto/paracrine cholinergic peptide that can bind to ₇-nicotinic acetylcholine receptor (nAChR), a high Ca²⁺-permeable ion channel coupled to regulation of nuclear factor-B (NF-B) expression. Elucidation of intracellular signaling events elicited by SLURP-1 is crucial for understanding the molecular mechanism of functioning of this novel hormone-like peptide that alters vital cell functions and can protect from tumorigenic transformation. In this study, we sought to dissect out the role of ₇-nAChR in mediating the biologic effects of recombinant SLURP-1 on the immortalized line of human oral keratinocytes Het-1A. A multifold upregulation of the NF-B expression at the mRNA and protein levels by SLURP-1 was only slightly diminished due to elimination of Na⁺, whereas in Ca²⁺-free medium the effect of SLURP-1 was inhibited by >50%. Both in the absence of extracellular Ca²⁺ and in the presence of Cd²⁺ or Zn²⁺, the SLURP-1-dependent elevation of NF-B was almost completely blocked by inhibiting MEK1 activity. Downstream of ₇-nAChR, the SLURP-1 signaling coupled to upregulation of NF-B also involved Jak2 as well as Ca²⁺/calmodulin-dependent kinase II (CaMKII) and protein kinase C (PKC), whose inhibition significantly (P < 0.05) reduced the SLURP-1-induced upregulation of NF-B. The obtained results indicated that activation of ₇-nAChR by SLURP-1 leads to upregulation of the NF-B gene expression due to activation of the Raf-1/MEK1/ERK1/2 cascade that proceeds via two complementary signaling pathways. One is mediated by the Ca²⁺-entry dependent CaMKII/PKC activation and another one by Ca²⁺-independent involvement of Jak2. Thus, there exists a previously not appreciated network of noncanonical auto/paracrine ligands of nAChR of the Ly-6 protein family, which merits further investigations.