The Multitarget Ligand 3-Iodothyronamine Modulates β-Adrenergic Receptor 2 Signaling

Abstract

Background: 3-Iodothyronamine (3-T₁AM), a signaling molecule with structural similarities to thyroid hormones, induces numerous physiological responses including reversible body temperature decline. One target of 3-T₁AM is the trace amine-associated receptor 1 (TAAR1), which is a member of the rhodopsin-like family of G protein-coupled receptors (GPCRs). Interestingly, the effects of 3-T₁AM remain detectable in TAAR1 knockout mice, suggesting further targets for 3-T₁AM such as adrenergic receptors. Therefore, we evaluated whether β-adrenergic receptor 1 (ADRB1) and 2 (ADRB2) signaling is affected by 3-T₁AM in HEK293 cells and in human conjunctival epithelial cells (IOBA-NHC), where these receptors are highly expressed endogenously. Methods: A label-free EPIC system for prescreening the 3-T₁AM-induced effects on ADRB1 and ADRB2 in transfected HEK293 cells was used. In addition, ADRB1 and ADRB2 activation was analyzed using a cyclic AMP assay and a MAPK reporter gene assay. Finally, fluorescence Ca²⁺ imaging was utilized to delineate 3-T₁AM-induced Ca²⁺ signaling. Results: 3-T₁AM (10^-5- 10^-10<smlcap>M</smlcap>) enhanced isoprenaline-induced ADRB2-mediated G_s signaling but not that of ADRB1-mediated signaling. MAPK signaling remained unaffected for both receptors. In IOBA-NHC cells, norepinephrine-induced Ca²⁺ influxes were blocked by the nonselective ADRB blocker timolol (10 µ<smlcap>M</smlcap>), indicating that ADRBs are most likely linked with Ca²⁺ channels. Notably, timolol was also found to block 3-T₁AM (10^-5<smlcap>M</smlcap>)-induced Ca²⁺ influx. Conclusions: The presented data support that 3-T₁AM directly modulates β-adrenergic receptor signaling. The relationship between 3-T₁AM and β-adrenergic signaling also reveals a potential therapeutic value for suppressing Ca²⁺ channel-mediated inflammation processes, occurring in eye diseases such as conjunctivitis.