E-cigarettes have put nicotine back in the news and into the hands of a growing number of American smokers who now "vape," that is, inhale a steam of nicotine, polyethylene glucose (PEG), and flavoring generated by cigarette-shaped, battery-powered vaporizers.
E-cigarettes are being promoted as "safe" nicotine delivery systems even though the safety of nicotine, the key addictive substance in tobacco smoke, is disputed, partly because the mechanism by which it acts on the circulatory system has not been well understood. Now a nicotine researcher at Brown University, Chi-Ming Hai, presents evidence from human and rat vascular smooth muscle cells that shows a link between nicotine and atherosclerosis.
In Hai's experiments, nicotine appeared to drive the formation of a kind of cellular drill called podosome rosettes, which are members of the invadosome family, consisting of invadopodia, podosomes, and podosome rosettes. These are specialized cell surface assemblies that degrade and penetrate the tissue during cell invasion. Invasion of vascular smooth muscle cells from the middle layer of the arterial wall (media) to the inner layer of the arterial wall (intima) contributes substantially to plaque formation in atherosclerosis.
Hai subjected rat and primary human vascular smooth muscle cells to prolonged (six hours) nicotine treatment, enabling the cells to form podosome rosettes in response to Protein Kinase C (PKC) activation, which controls protein phosphorylation in signal transduction cascades. The podosome rosettes set the scene for global extracellular matrix degradation and internalization. PKC activation alone, that is, without nicotine treatment, could induce the formation of podosomes in the rat muscle cells, accompanied by focal extracellular matrix degradation.
Nicotinic acetylcholine receptors, which bind neurotransmitters, co-localized with other podosome markers (vinculin, PKC-alpha, and metalloproteinase-2) at podosomes and podosome rosettes in the rat cells. Matrigel-coated transwell experiments indicated that nicotine treatment and PKC activation worked synergistically to enhance invasiveness in the primary human vascular smooth muscle cells. Inclusion of alpha-bungarotoxin, a nicotinic acetylcholine receptor antagonist, or cycloheximide, a protein synthesis inhibitor, during nicotine treatment abolished nicotine-induced podosome rosette formation in the rat cells, suggesting that signaling through the nicotinic acetylcholine receptors and synthesis of new proteins are required for podosome rosette formation. Altogether, the data acquired from rat and primary human vascular smooth muscle cells suggest that nicotine enhances vascular smooth muscle cell invasion by activating synergistic mechanisms between the nicotinic acetylcholine receptor and PKC signaling.
According to Hai, a potential clinical implication of findings from this study is that replacing cigarette smoking by nicotine administration may not bring much benefit to lowering the risk of developing atherosclerosis. Still, Hai believes that understanding the synergistic mechanisms between nicotinic acetylcholine receptor and protein kinase C in vascular smooth muscle invasion may lead to new therapeutics for minimizing the damaging effects of nicotine on the vascular system.
This study was supported by a research grant (HL52714) from the NIH National Heart, Lung, and Blood Institute.