Differences in agonist-independent and -dependent 5-hydroxytryptamine2C receptor-mediated cell division

Area of Science:

• Molecular Biology
• Cell Biology
• Pharmacology

Background:

• Agonist activation of the 5-hydroxytryptamineC (5-HT2C) receptor in NIH-3T3 fibroblasts induces a transformed phenotype.
• Recent findings indicate constitutive activity of the 5-HT2C receptor, prompting investigation into its role in basal cell division.

Purpose of the Study:

• To investigate the contribution of agonist-independent (constitutive) 5-HT2C receptor activity to basal cell division in NIH-3T3 fibroblasts.
• To characterize the effects of various 5-HT2C receptor ligands on cell proliferation and signaling pathways.

Main Methods:

• NIH-3T3 fibroblasts were transfected with 5-HT2C receptor cDNA.
• Effects of 5-HT2C receptor ligands (agonists, inverse agonists, antagonists) on [3H]thymidine incorporation, DNA content, and cell number were assessed.
• Phosphoinositide hydrolysis and effects of pertussis toxin were examined to elucidate signaling pathways.

Main Results:

• 5-HT2C receptor ligands modulated basal [3H]thymidine incorporation in transfected cells; agonists increased it, inverse agonists decreased it, and antagonists had minimal effect alone but blocked others.
• Constitutive 5-HT2C receptor activity, unlike agonist-induced activation, appears to involve different or additional G proteins.
• Some ligands classified as inverse agonists based on phosphoinositide hydrolysis did not similarly affect cell division, suggesting pathway divergence.

Conclusions:

• Constitutively active 5-HT2C receptors stimulate cell division in transfected fibroblasts independently of agonist binding.
• The 5-HT2C receptor may employ multiple signaling pathways to mediate its diverse cellular effects, including proliferation.