Multilocular fat cells in WAT of CL-316243-treated rats derive directly from white adipocytes

Multilocular, mitochondria-rich adipocytes appear in white adipose tissue ( WAT) of rats treated with the beta 3-adrenoceptor agonist, CL-316243 (CL). Objectives were to determine whether these multilocular adipocytes derived from cells that already existed in the WAT or from proliferation of precurs or cells and whether new mitochondria contained in them were typical brown adipocyte mitochondria. Use of 5-bromodeoxyuridine to identify cells that h ad undergone mitosis during the CL treatment showed that most multilocular cells derived from cells already present in the WAT. Morphological techniqu es showed that at least a subpopulation of unilocular adipocytes underwent conversion to multilocular mitochondria-rich adipocytes. A small proportion of multilocular adipocytes (similar to 8%) was positive for UCP1 by immuno histochemistry. Biochemical techniques showed that mitochondrial protein re covered from WAT increased 10-fold and protein isolated from brown adipose tissue (BAT) doubled in CL-treated rats. Stained gels showed a different pr otein composition of new mitochondria isolated from WAT from that of mitoch ondria isolated from BAT. Western blotting showed new mitochondria in WAT t o contain both UCP1, but at a much lower concentration than in BAT mitochon dria, and UCP3, at a higher concentration than that in BAT mitochondria. We hypothesize that multilocular adipocytes present at 7 days of CL treatment have two origins. First, most come from convertible unilocular adipocytes that become multilocular and make many mitochondria that contain UCP3. Seco nd, some come from a cell that gives rise to more typical brown adipocytes that express UCP1.