GDP binding to brown-adipose-tissue mitochondria was decreased in obese Zucker rats. Adrenalectomy restored both GDP binding and serum tri-iodothyronine of obese rats to values observed in lean rats. The effects of adrenalectomy on GDP binding and serum tri-iodothyronine were reversed by corticosterone. Decreasing food intake had no effect on brown-adipose-tissue GDP binding in obese rats. Young (5-week-old) obese rats showed a normal increase in brown-adipose-tissue mitochondrial GDP binding after housing at 4 degrees C for 7 days, but this response was attenuated in 10-week-old obese rats. Overfeeding with sucrose increased brown-adipose-tissue thermogenesis in lean, but not in obese, rats. After adrenalectomy, overfeeding with sucrose enhanced brown-adipose-tissue mitochondrial GDP binding in obese rats.

No abstract

Amphiregulin (AR), a heparin-binding, epidermal growth factor (EGF) receptor ligand has homology with EGF but exhibits a lower affinity for the EGF receptor than EGF. As the mature form of AR is truncated at the C terminus and lacks a conserved leucine residue known to be essential for high affinity binding of EGF to the EGF receptor, wild-type AR (AR1-84), a C-terminally extended AR construct incorporating six residues from the predicted coding sequence of AR (AR1-90) and a similarly extended construct with a Met86 to Leu substitution (AR1-90(leu86)) were expressed as recombinant proteins in yeast, purified by heparin affinity and C18 reverse phase chromatography and their relative biological activities determined. The growth factors were tested in mitogenesis and EGF receptor autophosphorylation assays and their relative order of potencies was found to be leu86> met86 > wt. The AR1-90(leu86) construct was found to be 50- to 100-fold more active than wild type AR1-84 consistent with previously reported studies of the role of the equivalent C-terminal leucine in EGF or TGF alpha. Significantly, the C-terminally extended form of AR, AR1-90, which utilized six residues from the predicted coding sequence, was 10-times more active than wild type AR1-84. This difference in activity of the C-terminally extended form of AR may be of biological significance since differential proteolytic processing of the AR precursor in vivo could result in production of multiple forms of the growth factor with differing affinities for the EGF receptor and hence differing biological potencies.

GDP binding to brown-adipose-tissue mitochondria of young obese Zucker rats (fa/fa) was significantly lower than in lean control rats, as a result of a decrease in the number of binding sites. Adrenalectomy of fa/fa rats restored GDP binding to control values. Corticosterone replacement suppressed GDP binding in adrenalectomized obese rats.

The concentration of the ‘uncoupling protein’ in brown adipose tissue mitochondria has been measured in lean and obese (ob/ob) mice and Zucker (fa/fa) rats at different ages using a specific radioimmunoassay. During the suckling period the concentration of the protein was similar in normal and mutant animals of both types, despite the decrease in mitochondrial GDP binding observed in the obese. The concentration of uncoupling protein was, however, decreased in adult ob/ob mice and adult Zucker rats compared with their respective lean siblings, in parallel with the decrease in GDP binding. It is concluded that there is a ‘masked’, or inactive, form of uncoupling protein in young ob/ob mice and fa/fa rats.