The Regulation of Growth and Metabolism by Insulin Signaling Pathways
Summary: Morris Birnbaum studies a phylogenetically conserved pathway that mediates the effects of extracellular signals on organismal size and anabolic metabolism.
The survival of all organisms depends on the recognition of nutritional abundance versus depletion. In single-cell organisms, nutrients themselves provide this information to the interior of the cell, where such signaling networks as those centered on the target of rapamycin protein (TOR) and AMP-activated protein kinase (AMPK) recognize the substrates. In metazoans, there is also a need for communication among tissues regarding the state of energy availability, and this function is provided by the hormone insulin and its closely related paralogs. Thus, it appears that most complex organisms utilize insulin to signal times of nutritional plenty, but what differs is the response of the class of organism to the recognition of the presence of abundant nourishment. For example, nutrient deprivation in the fruit fly Drosophila melanogaster leads to a decrease in fly size, whereas an abundance of food leads insulin to signal enhanced growth. In mammals, specialized tissues such as liver and adipose tissue have evolved to store energy in long-term forms like glycogen and triglyceride, respectively, and after a meal insulin promotes the redistribution of simple substrates into these tissues. A related protein, insulin-like growth factor, has assumed the role of primary regulator of growth in mammals. Strikingly, the intracellular signaling pathways that transmit insulin's signal for growth or nutrient storage have also been conserved over vast evolutionary distances. Hormone-dependent growth and metabolism both appear to depend on the proto-oncogene Akt, also known as protein kinase B.
Akt, a protein kinase specific for serine and threonine residues, has been implicated in a variety of biological functions. A dramatic illustration of the importance of Akt to growth and metabolism is provided by the phenotypes of mice deficient for each of the three mammalian isoforms of Akt. Akt1-null mice demonstrate normal metabolism but are reduced in size at birth and remain so throughout life. Mice lacking Akt2 are of normal or slightly reduced size but display insulin resistance, a state during which insulin is incapable of producing its normal effects in adipocytes, muscle, and liver. It is generally believed that insulin resistance is the first stage in a sequence of events leading to type 2 diabetes mellitus, and thus the Akt2-null mice recapitulate many of the features of early diabetes. Akt3-null mice are normal in terms of metabolism and body size but display a 25 percent reduction in the mass of their brains.
We have been devoting considerable attention to understanding the molecular basis of metabolic and growth control by Akt, initially emphasizing the nature of isoform specificity. The knockout data described above suggest that each Akt isoform functions in a discrete pathway. However, when Akt1-null mice are made haploinsufficient for Akt2, they demonstrate a marked enhancement of the reduction in body size but preserve normal glucose metabolism. The converse is also true. Akt2-null mice with 50 percent of the normal expression of Akt1 are diabetic, with a disease considerably more severe than that of mice lacking Akt2 alone. Thus, while each isoform prefers to signal to a given biological response, it is capable of providing some functional rescue when the need arises. This observation emphasizes the similarity in functional capabilities of the two Akt isoforms, their common evolutionary origins of regulation of growth and metabolism, and probably some shared biochemical mechanisms. The challenge at this point is to define Akt downstream pathways, distinguishing those that control metabolism from those that regulate growth. (This work is supported in part by grants from the National Institutes of Health.)
Regulation of Metabolism by Insulin
In muscle and adipose tissue, the rate-limiting and most insulin-regulated step in the pathway by which glucose is withdrawn from the bloodstream and converted to triglyceride and glycogen is the uptake of sugar into the cell. These tissues express a specialized form of the facilitated glucose transport protein, Glut4, which is sequestered inside the cell in the absence of insulin but rapidly redistributes to the cell surface after exposure to hormone. For many years, attention has focused on the signaling pathways that mediate insulin's action on glucose transport, as there is substantial evidence that this process is dysfunctional in type 2 diabetes mellitus. Insulin stimulation of Glut4 translocation depends on activation of the lipid kinase phosphatidylinositol 3-kinase and subsequent stimulation of Akt. In adipocytes derived from mice lacking Akt2, the normal increase in glucose uptake in response to insulin is blunted by about half. Akt1-deficient adipocytes respond normally to insulin, but a 50 percent reduction in Akt1 significantly potentiates the defect in fat cells lacking Akt2. This is consistent with the in vivo data demonstrating a genetic interaction between Akt1 and Akt2, but shows also that Akt1 provides functional rescue in a cell-autonomous manner.
There are at least two plausible explanations for the observation that adipocytes are preferentially dependent on Akt2 for transmitting the signal for Glut4 translocation: Akt2 is expressed at substantially higher levels in insulin-responsive tissues, or this isoform contains specific properties within its structure lacking in Akt1. We have addressed this question by introducing into adipocytes Akt isoforms expressed at the same level and assessing their impact on insulin-stimulated glucose transport and Glut4 translocation. Reintroduction of Akt2 into fat cells from Akt2-null mice reestablished insulin-stimulated hexose uptake to normal. In contrast, overexpression of Akt1 to approximately the same level was incapable of restoring maximal hormone-stimulated glucose uptake. These data indicate that there is something intrinsic to the Akt2 protein that confers to it the ability to signal efficiently to glucose transport.
Insulin also regulates carbohydrate and lipid metabolism in liver, by a mechanism different from that in the adipocyte. Glut4 is not expressed in liver, and therefore glucose uptake is not hormone-responsive. Nonetheless, insulin profoundly reduces the rate of hepatic glucose production by antagonizing both the breakdown of glycogen (glycogenolysis) and the de novo synthesis of glucose from circulating substrates (gluconeogenesis). A deficiency in Akt2 but not Akt1 leads to an impairment of insulin's ability to suppress hepatic glucose output, and this correlates with a number of biochemical correlates of insulin action in liver. In liver, however, about 85 percent of the Akt expressed is the Akt2 isoform; thus, the marked dependency on Akt2 can be explained in terms of its preferential expression. Nonetheless, precise mechanisms by which Akt2 controls glucose output are not all known and this is an area of intense study. (This work is supported in part by grants from the National Institutes of Health.)
Lipid abnormalities in type 2 diabetes mellitus contribute significantly to the cardiovascular complications that account for much of the morbidity and mortality in the disease. Yet, little is known about the pathways by which insulin regulates hepatic lipid metabolism, nor how these pathways are preserved in the presence of substantial insulin resistance. In investigations regarding this process, we were surprised to learn that insulin's induction of genes critical for lipid synthesis as well as the suppression of those important to fatty acid oxidation is absolutely dependent on the presence of Akt2. Moreover, we have identified a novel pathway by which insulin coordinately regulates lipid and carbohydrate metabolism via Akt2-dependent phosphorylation and inhibition of a critical liver coactivator protein. We remain hopeful that elucidating these fundamental metabolic pathways will lead to the identification of potential targets for therapy of type 2 diabetes mellitus.
Akt and the Regulation of Growth
As noted above, Akt1-null mice demonstrate normal metabolism but are about 20 percent smaller than wild-type mice. Recently, we have shown that at least one of the functions of Drosophila Akt protein is to increase the size of cells independent of proliferation. This reflects Akt's position as an intermediate in insulin signaling, as other members of this pathway produce similar effects. Moreover, we have confirmed that this process also applies to mammalian cells, as expression of a constitutively active Akt in transgenic mice leads to a substantial increase in cell size. Work from other laboratories has, however, indicated that overexpression of Akt in mammalian cells is also capable of driving them through the cell cycle. Thus, an important question we have addressed recently is whether the proportional reduction in the size of all organs in the Akt1-deficent mice results from a decrease in cell size or number. Surprisingly, we have found that the answer to this question depends on the organ. For example, liver and brain are about 20 percent smaller exclusively due to a reduction in cell number, while heart cells are diminished in size to the same degree. The regulation of organ size is also isoform-specific: Akt1-deficient brains have a decreased number of cells, whereas brains lacking Akt3 have fewer and smaller cells.
Last updated December 20, 2005