Adrenergic control of proteolysis in brown adipocytes
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Brown adipose tissue (BAT) is a sympathetically innervated tissue involved in control of thermoregulation and energy balance. BAT growth and/or atrophy occur in response to the need for energy dissipation. Fasting, deacclimation, and lactation result in tissue atrophy by loss of cells, mitochondrial proteins, and uncoupling protein 1 (UCP1), the molecular basis for thermogenesis. The overall objective is to gain a better understanding of the control and the mechanisms underlying BAT atrophy. Specific objectives are: (1) whether in vivo patterns of BAT atrophy can be reproduced in culture. (2) whether the loss of mitochondrial proteins is due to lysosomal proteolysis or proteolysis within mitochondria, under adrenergic influence. Mouse pre-adipocytes in culture differentiated to brown adipocytes. Increased expression of UCP1 was induced by norepinephrine (NE). NE was then removed and the pattern of change in thermogenic capacity evaluated. The number of cells and their protein content did not change, whereas the cell UCP1 content was decreased. Expression of cathepsin D (a lysosomal protease) was differentiation-dependent, but was not affected by NE. Direct estimates of turnover rates of UCP1 as well as mitochondrial translation products indicated that NE reduces the degradation of mitochondrial proteins as a whole. Addition of autophagic blockers reduced the loss of UCP1 upon NE removal. Therefore, in vitro cultures of brown adipocytes mimic some aspects of brown fat atrophy seen in vivo. Brown pre-adipocytes gain a large capacity for lysosomal protein degradation during differentiation. The activity of lysosomal proteases seems under the inhibitory control of NE, as its removal causes net loss of UCP1 preventable by inhibitors of autophagy.