ROLE OF CARNITINE PALMITOYLTRANSFERASE-I IN THE CONTROL OF KETOGENESIS IN PRIMARY CULTURES OF RAT ASTROCYTES

The role of carnitine palmitoyltransferase (CPT-I) in the control of k etogenesis was studied in primary cultures of rat astrocytes. Ketone b odies were the major product of [C-14]palmitate oxidation by cultured astrocytes, whereas CO, made a minor contribution to the total oxidati on products. Using tetradecylglycidate as a specific, cell-permeable i nhibitor of CPT-I, a flux control coefficient of 0.77 +/- 0.07 was cal culated for CPT- I over the flux of [C-14]palmitate to ketone bodies. CPT-I from astrocytes was sensitive to malonyl-CoA(IC50 = 3.4 +/- 0.8 mu M) and cross-reacted on western blots with an antibody raised again st liver CPT-I, On the other hand, astrocytes expressed significant ac etyl-CoA carboxylase (ACC) activity, and consequently they contained c onsiderable amounts of malonyl-CoA. Western blot analysis of ACC isofo rms showed that ACC in astrocytes-like in neurons, liver, and white ad ipose tissue-mostly comprised the 265-kDa isoform, whereas the 280-kDa isoform-which was highly expressed in skeletal muscle-showed much low er abundance. Forskolin was used as a tool to study the modulation of the ketogenic pathway in astrocytes. Thus, forskolin decreased in para llel ACC activity and intracellular malonyl-CoA levels, whereas it sti mulated CPT-I activity and [C-14]palmitate oxidation to both ketone bo dies and CO2. Results show that in cultured astrocytes (a) CPT-I exert s a very high degree of control over ketogenesis from palmitate, (b) t he ACC/malonyl-CoA/CPT-I system is similar to that of liver, and (c) t he ACC/malonyl-CoA/CPT-I system is subject to regulation by cyclic AMP .