This work reports the generation and characterization of PTP-1B-deficient (Ex1−/−) mice. These mice are leaner than WT mice and have dramatically decreased white adipose stores. Moreover, their energy expenditure is increased substantially, and thus they are less metabolically efficient, accounting for their resistance to diet-induced obesity. PTP-1B-deficient mice display enhanced insulin sensitivity in hyperinsulinemic-euglycemic clamp studies, as manifested by significant increases in rates of whole-body glucose disposal, glycolysis, and nonoxidative glucose metabolism. Interestingly, insulin sensitivity in PTP-1B−/− mice is elevated specifically in skeletal muscle, not in white adipose tissue. Our work indicates that PTP-1B plays a crucial role in regulating energy balance, the accumulation of body fat stores, and insulin sensitivity in some, but not all, tissues.
Elchebly et al. (28
) reported that PTP-1B Ex5/6−/−
mice are resistant to body weight gain on a high-fat diet, but the physiological basis for their observations was unclear. Here, we show that the leanness of PTP-1B-deficient mice results from increases in both BMR and the non-BMR-related component of energy expenditure. Removal of PTP-1B results in significant changes in body composition relative to WT mice, as evidenced by a marked reduction in the mass of white fat depots and body lipid content and a smaller reduction in fat free dry mass (Fig. ; Tables and ). This reflects a 61% decrease in adipocyte volume (micrograms of lipid per cell), without alteration of fat cell number. Our results identify PTP-1B as an important new regulator of energy expenditure and body composition.
Absence of PTP-1B in mice results in increased insulin sensitivity in skeletal muscle, leading to enhanced glucose tolerance and an 80% increase in insulin-stimulated whole-body glucose disposal. The enhanced insulin sensitivity in skeletal muscle of PTP-1B Ex1−/−
mice correlates with abnormal (hyper- and/or sustained) tyrosyl phosphorylation of IR/IRS-1 in this tissue (28
) and thus with previous suggestions that PTP-1B is a physiologically relevant IR phosphatase (see the introduction). The reason why PTP-1B apparently is a less important regulator of insulin-stimulated glucose uptake in adipose tissue is unclear. Conceivably, other PTPs play a more important role in adipose tissue. Alternatively, IR trafficking in adipocytes could differ from that in skeletal muscle. PTP-1B is located on intracellular membranes, and thus its ability to access tyrosyl phosphorylated IRs may vary between cell types.
There are some differences between our findings and those published previously (28
). First, we observed a significantly lower body weight for both PTP-1B Ex1−/−
and heterozygotic Ex1+/−
mice (compared to WT mice) on a chow diet, whereas Elchebly et al. (28
) reported no significant difference in weight gain between WT, heterozygotic, and homozygotic chow-fed mice. The most likely explanation for this discrepancy is that their chow diet contained nearly half as much fat as ours (M. L. Tremblay, personal communication). Second, in our study, chow-fed female PTP-1B Ex1−/−
mice were less susceptible than male mice to the effects of PTP-1B deficiency. In contrast, Elchebly et al. (28
) did not report any gender-specific differences. These findings might also be attributed to the different chow diets or to differences in genetic background, since our mice were analyzed on the C57BL6/J × 129/SvJ background whereas mice in their study were C57BL6/J × BALB/c.
Interestingly, on a chow diet, body weights (Fig. ) and serum insulin levels (fasted and fed) (Table ) of our heterozygotic PTP-1B Ex1+/− animals were similar to those of Ex1−/− mice, whereas on the high-fat diet, Ex1+/− mouse weights and serum insulin levels were more similar to those of WT controls. This suggests that on the high-fat, but not chow, diet, one copy of the PTP-1B gene is sufficient to mediate its effects on energy expenditure and insulin action. Conceivably, PTP-1B expression might be regulated by diet. Indeed, we have found recently that the expression of PTP-1B mRNA is increased significantly in skeletal muscle (+70%, P = 0.016), but not in white adipose tissue (+1%, P = 0.99), of mice fed a high-fat diet (O. Boss and L. D. Klaman, unpublished data). These results may explain, at least in part, the influence of the chow and high-fat diets on the body weight and insulin sensitivity of the heterozygotic Ex1+/− mice.
The most interesting and important question raised by this study concerns the relationship between the enhanced insulin sensitivity and the increased energy expenditure in the PTP-1B-deficient mice. There are three general classes of explanation. First, since insulin sensitivity is usually increased in lean individuals (25
), the increased energy expenditure in PTP-1B-deficient mice might result in decreased adipose stores, which in turn would lead to enhanced insulin sensitivity. In this model, the primary role of PTP-1B would be to control a pathway(s) that regulates energy expenditure, with any effect on insulin signaling being secondary. We do not favor this explanation for two reasons. Although PTP-1B Ex1−/−
mice are both insulin sensitive and lean under all conditions tested, Elchebly et al. (28
) reported significantly enhanced insulin sensitivity in PTP-1B Ex5/6−/−
mice under conditions (their chow diet) in which they detected no alteration in body weight. Moreover, several studies suggest that PTP-1B directly dephosphorylates the IR (see the introduction) and the IR is hyperphosphorylated in PTP-1B-deficient mice (28
A second possibility is that the increased insulin sensitivity in the skeletal muscle of PTP-1B−/−
mice leads to a higher metabolic rate, resulting in decreased white adipose mass in these mice. Precisely how increased insulin sensitivity might lead to decreased metabolic efficiency in PTP-1B−/−
mice remains to be determined, but increased mitochondrial proton leaks through UCPs or other mechanisms and/or increased activity of futile cycles could mediate this enhanced energy dissipation (50
). Notably, our data show similar mRNA expression of UCPs in skeletal muscle of WT and PTP-1B−/−
mice (Fig. ). This suggests that skeletal muscle UCPs are unlikely mediators of the increased metabolic rate in PTP-1B−/−
mice. However, we cannot exclude the possibility that UCP activity might somehow be increased. Insulin may also influence metabolic rate by acting on the central nervous system. For example, insulin stimulation of hypothalamic neurons increases sympathetic stimulation of BAT, leading to an increase in energy expenditure (57
). The major mechanism by which this occurs in rodents is by increasing brown fat mass and UCP expression (64
). UCP mRNA levels in BAT of PTP-1B−/−
mice were similar overall to those of WT mice. There was a small increase in UCP3 mRNA expression in BAT (Fig. ); the physiological significance of this increase is questionable. Moreover, there was no increase in BAT mass in PTP-1B−/−
animals, suggesting that PTP-1B is unlikely to regulate the sympathetic activity in BAT. In addition, insulin action in the central nervous system results in decreased food intake (55
), contrasting with the slightly increased food intake that we observe in our PTP-1B Ex1−/−
Finally, the increased energy expenditure and increased insulin sensitivity could reflect regulation of distinct signaling pathways by PTP-1B. In this model, the increased insulin sensitivity observed in the absence of PTP-1B would result from failure to appropriately dephosphorylate the IR (and possibly IRS proteins), whereas the enhanced energy expenditure would reflect altered regulation of as yet unidentified targets that regulate metabolic rate. Our work, together with that of Elchebly et al. (28
), identifies PTP-1B as a promising new target for intervention in obesity and insulin resistance.