Identification of a potent serum factor that causes desensitization of the receptor for C-type natriuretic peptide

Background: Guanylyl cyclase-B (GC-B; NPR-B), the receptor for C-type natriuretic peptide (CNP) is rapidly and effectively desensitized by a factor(s) in serum. Given the potential importance of this receptor in remodeling after tissue injury, identification of the serum factor(s) is of significant medical importance. Results: Partial purification of desensitization activity in serum by DEAE-Sepharose and reverse phase C18 chromatography, followed by mass spectroscopy, identified peptide sequences identical to those of apolipoprotein A2 (Apo A2), a known component of high density lipoprotein (HDL). Apo A2, however, could be eliminated as the active desensitization factor. Never the less, substantial desensitization activity was associated with purified preparations of bovine or human HDL. Since HDL is a well-known transporter of various lipids and phospholipids, we extracted either HDL or partially purified serum preparations with butanol and all activity extracted into the solvent. Of various lipophilic signaling molecules known to be associated with HDL, a prominent component is sphingosine-1-phosphate (SIP). We therefore tested authentic SIP as well as other known components of HDL (sphingosylphosphorylcholine; platelet activating factor) for activity; only SIP caused desensitization of GC-B. SIP was relatively potent, causing one-half maximal desensitization of GC-B at concentrations of 5-10 nM. These effects were seen within a few minutes after addition. Lysophosphatidic acid, another component of serum capable of desensitizing GC-B, was only effective at Micromolar concentrations. The pathway by which serum or SIP desensitizes GC-B seems unique in that pertussis toxin failed to inhibit GC-B desensitization, and yet blocked serum or SIP activation of extracellular signal-regulated kinase (ERK) or Akt/protein kinase B (Akt/PKB). Conclusion: Since the concentrations of SIP that desensitize GC-B are well within serum physiological ranges, this mitogenic signaling molecule likely functions as a strong adversary of the CNP/GC-B signaling pathway in the regulation of cell proliferation and other growth factor-induced phenotypes. The mechanism by which SIP desensitizes GC-B appears different than the known SIP signaling pathways. © 2003 Chrisman et al; licensee BioMed Central Ltd.