The AMPK-Malonyl-CoA-CPT1 Axis in the Control of Hypothalamic Neuronal Function—Reply

Abstract

Dear Editor,We would like to address the comments by Zammit and Arduini about their alternative perspective on our recently published paper in Cell Metabolism (Lopez et al., 2008xLopez, M., Lage, R., Saha, A.K., Perez-Tilve, D., Vazquez, M.J., Varela, L., Sangiao-Alvarellos, S., Tovar, S., Raghay, K., Rodriguez-Cuenca, S. et al. Cell Metab. 2008; 7: 389–399Abstract | Full Text | Full Text PDF | PubMed | Scopus (252)See all ReferencesLopez et al., 2008). We consider their suggestion about FAS downregulation an interesting and attractive hypothesis, but we are not that sure it could be directly implied from our results. If this was indeed the case, then it would be expected recovery of [malonyl-CoA] to normal values independently of the fasting time. Our data do not support this, since hypothalamic [malonyl-CoA] is not totally recovered in 48 hr refed rats previously fasted by 48 hr, despite the fact that fasting-induced hyperphagia is restored to normal feeding (Lopez et al., 2006xLopez, M., Lelliott, C.J., Tovar, S., Kimber, W., Gallego, R., Virtue, S., Blount, M., Vazquez, M.J., Finer, N., Powles, T. et al. Diabetes. 2006; 55: 1327–1336Crossref | PubMed | Scopus (110)See all ReferencesLopez et al., 2006). In our opinion it is more likely that the downregulation of FAS expression observed during fasting or ghrelin treatment acts as an adaptive mechanism that prevents excessive reduction of [malonyl-CoA] secondary to fasting-induced inactivation of ACC by AMPK. Supporting this concept are our data showing that the reduction in FAS levels is not further decreased by prolonged fasting over a period of 48 hr (Lopez et al., 2006xLopez, M., Lelliott, C.J., Tovar, S., Kimber, W., Gallego, R., Virtue, S., Blount, M., Vazquez, M.J., Finer, N., Powles, T. et al. Diabetes. 2006; 55: 1327–1336Crossref | PubMed | Scopus (110)See all ReferencesLopez et al., 2006). This suggests a tightly controlled low FAS threshold in hypothalamic neurons. Since malonyl-CoA plays a critical regulatory role in this system by acting both as a substrate for fatty acid biosynthesis and an inhibitor of CPT1 (Obici et al., 2003xObici, S., Feng, Z., Arduini, A., Conti, R., and Rossetti, L. Nat. Med. 2003; 9: 756–761Crossref | PubMed | Scopus (338)See all References, Wolfgang et al., 2006xWolfgang, M.J., Kurama, T., Dai, Y., Suwa, A., Asaumi, M., Matsumoto, S., Cha, S.H., Shimokawa, T., and Lane, M.D. Proc. Natl. Acad. Sci. USA. 2006; 103: 7282–7287Crossref | PubMed | Scopus (127)See all References, Lopez et al., 2008xLopez, M., Lage, R., Saha, A.K., Perez-Tilve, D., Vazquez, M.J., Varela, L., Sangiao-Alvarellos, S., Tovar, S., Raghay, K., Rodriguez-Cuenca, S. et al. Cell Metab. 2008; 7: 389–399Abstract | Full Text | Full Text PDF | PubMed | Scopus (252)See all References), we hypothesized that extremely low levels of [malonyl-CoA] may compromise neuronal viability, by preventing lipid biosynthesis and by allowing potentially harmful neuronal fatty acid oxidation during fasting, a situation of low energy surplus.With respect to the CPT1 effects on feeding, we showed that ghrelin increases hypothalamic CPT1 activity. To determine whether the effect of ghrelin on food intake involved the activation of CPT1, we investigated the effect on ghrelin's orexigenic action of etomoxir (10 μg), an inhibitor of CPT1. Using this selected dose, we were able to show that while etomoxir rapidly inhibited hypothalamic CPT1 activity, short-term feeding (2, 4, and 6 hr) was not affected; etomoxir exerted its anorectic effect 12 hr after administration (unpublished data). It is likely that by using a higher dose of etomoxir, we would have observed a faster anorectic action; however, the rationale of using the dose selected was to show that opposite effects of this drug and ghrelin occurred through the same downstream pathway.We assayed [malonyl-CoA] and CPT1 activity from whole hypothalamic samples and using similar dilutions; thus, an infinitesimal dilution of [malonyl-CoA] was unlikely. Supporting these data, we have recently obtained new results showing that under other experimental settings, our assays show a perfect [malonyl-CoA]:CPT1 activity correlation (unpublished data). We agree that our method is not specific for CPT1c. However, our interest in CPT1c stemmed from the recent evidence demonstrating that CPT1cKO mice have decreased feeding and body weight (Wolfgang et al., 2006xWolfgang, M.J., Kurama, T., Dai, Y., Suwa, A., Asaumi, M., Matsumoto, S., Cha, S.H., Shimokawa, T., and Lane, M.D. Proc. Natl. Acad. Sci. USA. 2006; 103: 7282–7287Crossref | PubMed | Scopus (127)See all ReferencesWolfgang et al., 2006). We also have data showing that CPT1a or CPT1b expression does not change after ghrelin treatment at any time evaluated (unpublished data). Specific activity assays for each CPT1 isoform, ideally associated to laser-capture microdissection of hypothalamic nuclei, will be required to better characterize the hypothalamic malonyl-CoA-CPT1 axis.