NOREPINEPHRINE AS A MORPHOGEN - ITS UNIQUE INTERACTION WITH BROWN ADIPOSE-TISSUE

Norepinephrine is normally considered a neurotransmitter mediating acu te metabolic effects in target cells. However, analysis of the regulat ion of the recruitment process in brown adipose tissue has indicated t hat norepinephrine may interact with this tissue in such a way that it could be considered a morphogen for this tissue, Besides stimulating the acute thermogenic processes, norepinephrine can induce the express ion of tissue-specific proteins such as the uncoupling protein, induce expression of non-tissue specific proteins necessary of the thermogen ic process (e.g. lipoprotein lipase) and repress the expression of non -essential proteins (e.g. subunit c of the ATP-synthase). Upon chronic adrenergic stimulation, the general differentiation state of the tiss ue is advanced, indicating that the expression of factors with a more general effect on brown adipocyte differentiation is also under adrene rgic control. It may even be discussed that norepinephrine may be invo lved early in the embryonal determination process directing cell clone s into this line, The molecular basis for these effects of norepinephr ine are only poorly known at present, but adrenergic effects on the ex pression level of many transcription factors, such as C/EBP alpha, C/E BP beta and PPAR gamma 2, have been noted. These collective recruitmen t effects of norepinephrine are well suited to allow the tissue to gro w or atrophy in response to the physiological needs of the organism.