Upregulation of nuclear factor-κB expression by SLURP-1 is mediated by α₇-nicotinic acetylcholine receptor and involves both ionic events and activation of protein kinases

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 α(7)-nicotinic acetylcholine receptor (nAChR), a high Ca(2+)-permeable ion channel coupled to regulation of nuclear factor-κB (NF-κB) 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 α(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-κB expression at the mRNA and protein levels by SLURP-1 was only slightly diminished due to elimination of Na(+), whereas in Ca(2+)-free medium the effect of SLURP-1 was inhibited by >50%. Both in the absence of extracellular Ca(2+) and in the presence of Cd(2+) or Zn(2+), the SLURP-1-dependent elevation of NF-κB was almost completely blocked by inhibiting MEK1 activity. Downstream of α(7)-nAChR, the SLURP-1 signaling coupled to upregulation of NF-κB also involved Jak2 as well as Ca(2+)/calmodulin-dependent kinase II (CaMKII) and protein kinase C (PKC), whose inhibition significantly (P < 0.05) reduced the SLURP-1-induced upregulation of NF-κB. The obtained results indicated that activation of α(7)-nAChR by SLURP-1 leads to upregulation of the NF-κB 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 Ca(2+)-entry dependent CaMKII/PKC activation and another one by Ca(2+)-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.

A Human-Specific α7-Nicotinic Acetylcholine Receptor Gene in Human Leukocytes: Identification, Regulation and the Consequences of CHRFAM7A Expression

“…The natural ligand(s) responsible for CHRFAM7A activation is presumed to include acetylcholine, but its ligand selectivity is not known compared with other nAChRs, each of which have been described extensively (68). Normally, acetylcholine is thought to be the intrinsic ligand that activates the cell surface α7nAChR homopentameric channel, but there are numerous alternative ligands including choline, nicotine and even peptide hormone-like ligands (69)(70)(71)(72)(73) that can activate α7nAChR. The discovery of an alternative α7nAChR in human leukocytes raises the possibility that heteropentamers composed of CHRNA7 and CHRFAM7A proteins could modulate the cellular immune response to proinflammatory challenges (see Figure 6).…”

α4β2 Nicotinic Receptors Partially Mediate Anti-Inflammatory Effects through Janus Kinase 2-Signal Transducer and Activator of Transcription 3 but Not Calcium or cAMP Signaling

“…Evidence that ␣4␤2 receptors mediate this effect is based on using lobeline as a "selective ␣4␤2 antagonist" to distinguish nicotine's effect between these receptor subtypes at concentrations greater than 20 times lobeline's IC 50 for human ␣7 receptors expressed in oocytes (Briggs and McKenna, 1998)). Very recent work suggests that in certain circumstances, nonphosphorylated STAT3 regulates NF-B activity and cytokine production in macrophages (Peñ a et al, 2010) and that the cholinergic peptide SLURP-1 modulates ␣7-JAK2-mediated up-regulation of NF-B in keratinocytes (Chernyavsky et al, 2010). Further work is clearly required to sort out how JAK2-STAT3 signaling can be both pro-and anti-inflammatory and both block and stimulate NF-B signaling, depending on context.…”

CHRFAM7A: a human‐specific α7‐nicotinic acetylcholine receptor gene shows differential responsiveness of human intestinal epithelial cells to LPS

“…In this capacity, the a7nAChR can act as a ligand-gated ion channel or stimulate intrinsic signal transduction and metabotropic activities (1,(26)(27)(28). In view of the significance of a7nAChRs to epithelial biology and the observation that human-specific genes are disproportionately implicated in complex disease (29), it is remarkable that little attention has been paid to the 1998 discovery that there exists a human-specific gene called CHRFAM7A (gene-encoding dup-a7-nicotinic acetylcholine receptor), which can modify a7nAChR responsiveness (30).…”