Insulin Translation control

Escherichia coli Easy to grow in large-scale volumes Transcriptional and translational control well known Successfully used in the manufacture of insulin, interferon and human somatotropin Difficult to achieve export of some proteins into growth medium Degradation of small proteins by proteases Unable to undertake most post-translational modifications, e.g. glycosylation Many proteins retained in the cytoplasm as insoluble aggregates... 

Kimball SR, Farrell PA, Jefferson LS. Invited review role of insulin in translational control of protein synthesis in skeletal muscle by amino acids or exercise. J Appl Physiol 2002 93 1168-1180 (review). 

A regulatory role for the cap structure in translational control in intact mammalian cells is particularly well illustrated by findings of Cordell et al. (1982). In normal pancreatic tissue, the two rat insulin genes are expressed about equally, but in tumor tissue, one of these genes is expressed ten-fold less, in spite of the fact that equivalent amounts of mRNA are produced. Insulin mRNA from the underexpressed gene is ten-fold less active in in vitro translation, but this defect can be repaired by treatment with vaccinia virus capping extract. 

Disproportionate expression of the two nonallelic rat insulin genes in a pancreatic tumor is due to translational control. Cell 31 531. 

Proud, C.G and Denton, R.M. Molecular mechanism for the control of translation by insulin ... 

Synthesis of many enzymes is repressed most of the time. The appearance of an enzyme at a particular stage in the life of an organism as well as the differing distributions of isoenzymes within differentiated tissue result from derepression. The control of enzyme synthesis may also be exerted during the splicing of transcripts and at the translational level as well. These control mechanisms are often relatively slow, with response times of hours or even days. However, effects on the synthesis of some hormones, such as insulin (Section G), may be observed within a few minutes. 

Fatty add synthetase is not controlled directly by phosphorylation however, insulin, glucagon, and thyroxine have an effect on its activity by controlling its cellular concentration. Both insulin and thyroxine increase the biosynthesis of the enzyme, whereas glucagon is inhibitory. Thyroxine and glucagon appear to regulate the biosynthesis at the transcription level, whereas insulin affects the enzyme activity at the translation level. It has no effect on cellular fatty add synthetase mRNA concentration. In summary, fatty add synthetase levels are up in the fed state and down in the fasting state. 

C. G. Proud and R. M. Denton Molecular Mechanisms for the control of translation of insulin. Biochemical Journal 328,329 (1997). 

Translational and transcriptional control are sometimes combined. For example, insulin (which regulates the synthesis of a large number of substances) and prolactin (another hormone) are required together for production of casein (milk protein) in mammary tissue. Both hormones are needed to initiate transcription but prolactin in addition, increases the lifetime of casein mRNA. 

HMG-CoA reductase is the major regulatory enzyme in cholesterol biosynthesis. HMG-CoA reductase is controlled hormonally by insulin and glucagon and transcription and translation of the enzyme can be suppressed by the presence of cholesterol in cells. 

HMG-CoA reductase is the major regulatory enzyme in cholesterol biosynthesis. HMG-CoA reductase is controlled hormonally by insulin and glucagon and transcription and translation of the enzyme can be suppressed by the presence of cholesterol in cells. Mevalonate is converted in the cytosol to the five carbon building blocks of isoprene synthesis-isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DPP)-in the reactions shown in Figure 19.19. Subsequently, IPP and DPP form famesyl pyrophosphate in the cytosol (Figure 19.20)... 

Insulin and the counterregulatory hormones exert two types of metabolic regulation (see Chapter 26). The first type of control occurs within minutes to hours of the hormone-receptor interaction and usually results from changes in the catalytic activity or kinetics of key preexisting enzymes, caused by phosphorylation or dephosphorylation of these enzymes. The second type of control involves regulation of the synthesis of key enzymes by mechanisms that stimulate or inhibit transcription and translation of mRNA. These processes are slow and require hours to days. 

Figure 4. Autoradiogram of [-methionine labeled peptides synthesized in rabbit reticulocyte lysate and immuno-precipitated by a rabbit anti-insulin serum. The RNAs used were 1 pg of Brome Mosaic Virus as a control (lane 1), no RNA (lane 2), 0.5 yg of polyA+ RNA isolated from rat pancreas (lane 3), 0.2 yg of mRNA synthesized by T7 polymerase (lane 4), 0.2 yg of polyA+ RNA combined with 1 yg of antisense mRNA synthesized by SP6 polymerase (lane 5), and 0.2 yg of synthetic mRNA combined with 1 yg of antisense mRNA. The RNAs used in lane 5 and 6 were heated at 70°C for 15 min and cooled slowly before the translation reaction.

Effects on protein synthesis at the translation level have also been suggested for other hormones. In the case of the action of insulin on rat diaphragm, insulin activates inert cell ribosomes. This effect requires intact protein synthesis. For other hormonal actions, the evidence in favour of control at the translation level has generally been a stimulation of protein synthesis in the presence of doses of actinomycin-D, which block the synthesis of RNA. 

---