ACC knockout mice

Abu-Elheiga et al.
              Biochemistry 6853
              David Bruce



    The purpose of the research described in this paper was to characterize the roles of ACC1 and ACC2 with respect to control of fatty acid oxidation in animals.  Acety-CoA carboxylases 1 and 2 (ACCs) catalyze the synthesis of malonyl-CoA and thus are critically important in fatty acid biosynthesis and oxidation.  Malonyl-CoA is known to exert an inhibitory effect on the carnitine shuttle system of fatty acyl units into the mitochondria for oxidation.  In this paper, the authors describe a series of experiments in which they created knockout mice that did not express ACC2 and then compared functioning in these mice to functioning in wild-type mice.
    Control of fatty acid oxidation is apparently more complex than what was previously thought to be the case.  This research presents evidence for the existence of two separate pools of malonyl-CoA, one synthesized by ACC1 and localized to the cytosol, the other synthesized by ACC2 and localized to the mitochondria.  Although most cells express both ACCs, the level of expression differs drastically: in liver and adipose tissue, ACC1 predominates, while in heart and skeletal muscle, ACC2 is more highly expressed.  The authors found in the course of their research that ACC2 knockout mice demonstrated some important physiological and biochemical differences as compared to wild-type mice.
    ACC2 knockout mice and wild-type mice had similar levels of ACC activity in liver tissue; however ACC activity was much lower in knockout muscle and heart tissue than in the wild-type mice, which is consistent with the observation that ACC2 is more highly expressed in muscle and heart tissue.  Corresponding malonyl-CoA levels were also lower in knockout muscle and heart tissue.  Interestingly, when the authors measured the fat content of the mouse livers, they discovered that the knockout mice had a lower fat content.  They speculate that this is due to the absence of ACC2 allowing for uncontrolled fatty acid oxidation in the knockout mice.
    It would appear from this study that mitochondrial ACC2 is more important in regulating fatty acid oxidation than cytosolic ACC1.  Malonyl-CoA negatively inhibits the carnitine shuttle system.  The interesting question of exactly how and where the mitochondrial malonyl-CoA inhibits oxidation is not addressed in this paper.  Cytosolic malonyl-CoA can inhibit carnitine acyltransferase 1; does mitochondrial malonyl-CoA inhibit carnitine acyltransferase 2? 
    In addition to this crucial question, this study leaves many other questions open.  For instance, the authors reveal that between ACC2 knockouts and wild type mice, ACC2 knockouts had (1) lower serum glucose levels (2) lower serum fatty acid levels (3) higher serum triglyceride levels
and (4) reduced glycogen stores in liver tissue.  In the cases of (2,) and (3), the authors speculate that this is due to increased mobilization of adipose and liver triglycerides and fatty acids for heart and muscle oxidation.  With respect to decreased glucose levels and decreased liver glycogen in knockouts, the authors interpret the data as evidence of increased utilization of glycogen and thus glucose for fatty acid synthesis as compared to wild-type mice.  It would be very important to clarify the reasons behind the discrepancies between knockouts and wild-types; for instance, are there situations that could arise which would result in severe hypoglycemia?  The authors note that ACC2 knockouts seemed resistant to insulin stimulation, which in normal mice leads to a reduction in fatty acid oxidation.  If, as the authors hope, the discovery of ACC2's importance in regulating oxidation of fatty acids is someday harnessed by the development of selective ACC2 inhibitors, it would seem imperative to elucidate the mechanisms behind its effect on glucose levels and insulin resistance.
    The develoment of an ACC1 knockout mouse and comparison of its biochemical and physiological paramaters with what is seen in wild type and ACC2 knockouts is the most obvious experiment which is suggested by this research.  Among other things, it would help to clarify the roles of the ACCs in regulating oxidation through malonyl-CoA synthesis.  An unexpected finding from the ACC2 knockouts was the apparent role of ACC2 catalyzed mitochondrial malonyl-CoA in regulating fatty oxidation.  Perhaps the development of an ACC1 knockout would reveal more unexpected findings.  What pattern of fatty acid oxidation is seen here?  This may provide clues as to the relative importance of ACC1 pools and ACC2 pools in regulating oxidation. And what of a knockout strain that did not express either ACC1 or ACC2?  This would seem to be a fatal mutation, as such a strain would be incapable of initiating fatty acid biosynthesis, but it might be interesting to attempt.
    A gene therapy involving ACC2 RNA interference might be interesting, but would probably want to ensure that the interference of ACC2 RNA wasn’t also impacting ACC1 RNA since the proteins are share such similarity.  However, since ACC2 is localized to the mitochondria, this would seem to be the best avenue of approach as it is difficult to see how a direct ACC2 inhibitor could be transported into the mitochondria.