Methionine adenosyltransferase:adrenergic-cAMP mechanism regulates a daily rhythm in pineal expression.
(S)-adenosylmethionine (SAM) is a critical element of melatonin synthesis as the methyl donor in the last step of the pathway, the O-methylation of N-acetyl 5-hydroxytryptamine by hydroxyindole-O-methyltransferase. The activity of the enzyme that synthesizes SAM, methionine adenosyltransferase (MAT), increases 2.5-fold at night in the pineal gland. In this study, we found that pineal MAT2A mRNA and the protein it encodes, MAT II, also increase at night, suggesting that the increase in MAT activity is caused by an increase in MAT II gene products. The night levels of MAT2A mRNA in the pineal gland were severalfold higher than in other neural and non-neural tissues examined, consistent with the requirement for SAM in melatonin synthesis. Related studies indicate that the nocturnal increase in MAT2A mRNA is caused by activation of a well described neural pathway that mediates photoneural-circadian regulation of the pineal gland. MAT2A mRNA and MAT II protein were increased in organ culture by treatment with norepinephrine (NE), the sympathetic neurotransmitter that stimulates the pineal gland at night. NE is known to markedly elevate pineal cAMP, and here it was found that cAMP agonists elevate MAT2A mRNA levels by increasing MAT2A mRNA synthesis and that drugs that block cAMP activation of cAMP dependent protein kinase block effects of NE. Therefore, the NE-cAMP dependent increase in pineal MAT activity seems to reflect an increase in MAT II protein, which occurs in response to cAMP-->protein kinase-dependent increased MAT2A expression. The existence of this MAT regulatory system underscores the importance that MAT plays in melatonin biogenesis. These studies also point to the possibility that SAM production in other tissues might be regulated through cAMP.
Kim
Jong-So
JS
Section on Neuroendocrinology, Laboratory of Developmental Neurobiology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892, USA.
Coon
Steven L
SL
Blackshaw
Seth
S
Cepko
Constance L
CL
Møller
Morten
M
Mukda
Sujira
S
Zhao
Wan-Qian
WQ
Charlton
Clivel G
CG
Klein
David C
DC
D000818:Animals; D002940:Circadian Rhythm; D000242:Cyclic AMP / Q000378:metabolism*; D015971:Gene Expression Regulation, Enzymologic; D008297:Male; D008716:Methionine Adenosyltransferase / Q000235:genetics / Q000378:metabolism*; D009928:Organ Specificity; D010870:Pineal Gland / Q000201:enzymology* / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley
D000242:Cyclic AMP; D008716:Methionine Adenosyltransferase