Recombinant Human Insulin in Cell Culture

Human insulin is a small protein of 51 amino acids arranged as two chains -- an A chain of 21 residues and a B chain of 30 -- held together by two interchain disulphide bonds, with a third disulphide within the A chain. Its molecular weight is approximately 5.8 kDa (5808 Da) and its isoelectric point is around pH 5.3-5.4, a fact that governs how it must be handled.

In cell culture, insulin is one of the small number of supplements that turn a basal medium into something cells will actually grow in without serum. Its functions are:

• Driving nutrient uptake. Insulin promotes glucose transport and amino acid uptake, which for a rapidly dividing culture is a direct determinant of growth rate and of the biomass and product a bioreactor can support.
• Anabolic signalling. It stimulates protein synthesis, nucleic acid synthesis and lipogenesis, and suppresses catabolic pathways.
• Survival signalling. Insulin receptor engagement activates the PI3K-Akt pathway, which is anti-apoptotic. In serum-free medium, where the survival signals normally supplied by serum growth factors are absent, this is a substantial part of what insulin is doing.
• Intracellular transport and general metabolic support, which is why it appears in nearly every serum-free formulation regardless of cell type.

The point most people miss: in culture, insulin is largely an IGF-1 receptor agonist. Physiological circulating insulin is in the nanomolar range. The concentration used in culture -- 5-10 ug/mL, roughly 1-2 uM -- is around a thousandfold higher. Insulin binds its own receptor with high affinity and the closely related IGF-1 receptor with much lower affinity, but at micromolar concentrations that low-affinity interaction becomes significant. The mitogenic effect of insulin in culture is therefore substantially mediated through IGF-1 receptor signalling rather than through classical metabolic insulin signalling.

This is not a technicality. It explains why culture concentrations are so far above physiological, why IGF-1 analogues can replace insulin, and why insulin-supplemented culture is a poor model for studying physiological insulin signalling. If you are researching insulin action itself, the standard supplement concentration is far outside the physiological range and will saturate both receptors.

Recombinant, not animal-derived. Cell culture insulin was historically extracted from bovine or porcine pancreas. Recombinant human insulin, expressed in E. coli or in yeast such as Saccharomyces cerevisiae or Pichia pastoris, has displaced it for the same reasons that apply across the sector: it is animal-origin-free, removing an adventitious agent pathway and the associated documentation burden; it is the human sequence, avoiding the small species differences present in bovine and porcine insulin; and it is far more consistent between lots than a tissue extract.

For any animal-origin-free or xeno-free process, recombinant insulin is not optional. It is also worth checking, when assembling a defined formulation from individual components, that the insulin in a supplement is genuinely recombinant -- a medium described as serum-free is not necessarily animal-origin-free.

The zinc form. Insulin is normally supplied as a zinc-stabilised crystalline powder. Zinc ions coordinate insulin monomers into hexamers -- two zinc ions per hexamer -- and the hexameric crystalline form is markedly more stable than monomeric insulin, both in storage and against aggregation. This is why commercial cell culture insulin is described as zinc insulin or as containing a specified zinc content.

Practical consequences:

• The zinc content is part of the specification and is worth noting, because zinc is itself a trace element with biological activity. At the amounts delivered alongside insulin it is generally negligible relative to other medium components, but for zinc-sensitive work it should be accounted for.
• The hexamer dissociates on dilution. At the concentrations used in medium, insulin is present as monomers and dimers, so the zinc form affects storage stability rather than biological activity.
• Zinc-free and specific-zinc-content preparations exist for applications where the zinc matters.

Formats. Dry powder is the most common format for bulk use and has the longest shelf life. Ready-made sterile solutions avoid the dissolution step described below, which is the main practical hazard in handling insulin, and are worth the premium for smaller-scale work.

The standard is 5-10 ug/mL, and 10 ug/mL is the concentration delivered by a 1X ITS supplement. This suits the large majority of cell types and is a sound default for a new formulation.

The practical range is wider, roughly 0.1-20 ug/mL depending on the cell type and what the culture is being asked to do:

• Many CHO and other production processes run at 5-20 mg/L (equivalently 5-20 ug/mL), sometimes at the upper end for high-density fed-batch culture.
• Sensitive primary cells and some stem cell systems use considerably less, in the region of 0.1-5 ug/mL.
• Some neural and specialised cultures deliberately exclude insulin. Insulin-free formulations exist precisely because insulin drives IGF-1 receptor signalling that can interfere with differentiation, survival assays or metabolic experiments. If you are running such an experiment, check whether your supplement contains insulin -- this is easy to overlook when a supplement is added by habit.

Optimise rather than inherit. Titrate across the range on your own cells, measuring growth rate, peak viable density, viability and product titre or functional endpoint. Because insulin is one of the more expensive components of a defined medium at scale, and because more is not always better, the optimum is worth establishing.

Stability in medium is a real constraint. Insulin degrades in complete medium at 37 C over a period of hours to a day or so, more quickly in the presence of proteases and at elevated temperature. In a short batch culture this is unimportant. In a fed-batch or perfusion process running for a week or more, the insulin added at the start is long gone by the end, and processes that depend on it need supplementation through the feed. This degradation is one of the main reasons the field developed more stable alternatives.

Alternatives to insulin. LongR3 IGF-1 is an engineered analogue of insulin-like growth factor 1 with an extended N-terminus and a substitution that greatly reduces binding by IGF binding proteins. It is considerably more potent and more stable than insulin, so it is used at far lower concentrations -- typically in the tens of nanograms per millilitre rather than micrograms -- and it is widely used in bioprocess media as a direct replacement. Given that insulin in culture is largely acting through the IGF-1 receptor anyway, using an optimised IGF-1 analogue is a logical substitution. It is more expensive per gram but used at a fraction of the mass. Native IGF-1 is also used, but is less stable and is sequestered by binding proteins.

This is where insulin most often goes wrong, and the reason is straightforward chemistry.

Insulin is poorly soluble near neutral pH. Its isoelectric point is around 5.3-5.4, and at pH values approaching neutrality it has minimal net charge and low solubility. Adding insulin powder directly to culture medium at pH 7.2-7.4 does not work -- it disperses as an undissolved suspension that looks partially dissolved, then is largely removed by the sterilising filter. The medium then contains far less insulin than intended, and the culture underperforms for a reason that is invisible.

Dissolve in dilute acid first:

1. Dissolve the powder in dilute hydrochloric acid, typically 0.01 M (0.01 N), or in dilute acetic acid, at a stock concentration of around 4-10 mg/mL. Use the minimum acid volume needed.
2. Mix gently until fully dissolved. Insulin is a protein and will denature at interfaces -- do not vortex vigorously and do not shake to the point of foaming. The solution should become completely clear; if it does not, it is not dissolved.
3. Neutralise or dilute promptly. Prolonged storage at low pH promotes deamidation and other chemical degradation.
4. Sterile filter through 0.22 um. Use a low-protein-binding membrane -- insulin is small and adsorbs appreciably to some filter materials, and at these low concentrations adsorptive loss is a genuine source of error.
5. Dilute into medium to the final working concentration. The small volume of dilute acid carried across has a negligible effect on the pH of a properly buffered medium, but check the pH of the finished medium if you are adding a large volume of stock.

Storage. Store dry powder desiccated at 2-8 C or -20 C as specified by the supplier, protected from moisture. Store acidic stock solutions frozen in single-use aliquots and avoid repeated freeze-thaw. Do not keep insulin stock solutions at room temperature.

The simplest way to avoid all of this is to buy a ready-made sterile insulin solution or to use an ITS supplement, which delivers correctly formulated insulin alongside the other components it usually accompanies. For most laboratory-scale work this is the better choice; dissolving powder becomes worthwhile at manufacturing scale where the cost difference is material.

Insulin is rarely used alone. The standard combination is ITS -- insulin, transferrin and selenium -- supplied as a 100X concentrate. The composition is consistent across suppliers:

The three components address different problems, which is why they travel together:

• Insulin supplies the growth and survival signal and drives nutrient uptake.
• Transferrin is the iron carrier. Iron is essential for cell proliferation -- ribonucleotide reductase requires it -- but free iron catalyses Fenton chemistry and generates hydroxyl radicals. Transferrin delivers iron in a bound, controlled form, and it also helps reduce the burden of oxygen radicals and peroxide. The delivery mechanism matters as much as the amount.
• Selenium, as sodium selenite, is a cofactor for glutathione peroxidase and other selenoproteins, and functions as part of the cell's antioxidant defence. Note the units: the working concentration is nanograms per millilitre, three orders of magnitude below the other two. Selenium is toxic at higher concentrations and this is not a component to approximate.

Common variants:

• ITS-G -- the basic combination, typically with recombinant human insulin and human transferrin, available in animal-origin-free form.
• ITS-A -- ITS with sodium pyruvate added, an additional energy substrate and a scavenger of hydrogen peroxide.
• ITS-X -- ITS with ethanolamine added, a precursor for phosphatidylethanolamine synthesis, useful for cells with high membrane turnover and often beneficial in low-protein and hybridoma media.

Choosing between them is largely empirical. Start with ITS-G unless you have a reason to think your cells are limited by phospholipid precursor availability, in which case ITS-X is worth testing, or by energy substrate, in which case ITS-A is.

Check the animal-origin status. ITS supplements exist in both animal-origin-free and conventional forms. A formulation containing bovine transferrin or pancreas-derived insulin is not animal-origin-free regardless of what the rest of the medium contains. For xeno-free and clinical-track work, verify each component, not just the product name.

Insulin-free supplements exist for good reasons. Formulations supplied explicitly without insulin -- including insulin-free versions of common neural supplements -- are made for experiments where insulin and IGF-1 receptor signalling would confound the readout. If your experiment concerns growth factor signalling, metabolism or differentiation, using an insulin-free supplement and adding back a defined amount is often the cleaner design.

Adsorptive losses matter at these concentrations. Insulin is a small protein used at micrograms per millilitre, and it adsorbs to glass and plastic surfaces. In small volumes, in tubing and in low-protein media the loss is not negligible. Albumin in the medium reduces this substantially by competing for surface sites, which is one of several reasons albumin and insulin are often used together. Use low-binding tubes for stock handling, and do not prepare highly dilute intermediate solutions that will sit before use.

Do not autoclave. Sterilise by 0.22 um filtration only. Heat destroys insulin.

Watch for the silent failure. The characteristic insulin failure mode is a formulation that simply underperforms without any obvious cause, because undissolved insulin was removed at the filter and the medium never contained what the recipe said. If a defined medium is not working and insulin was prepared from powder, verify the dissolution step before investigating anything else.

Record the concentration and the source in your methods. Insulin concentration varies by an order of magnitude across published formulations, and the difference is biologically meaningful. "Serum-free medium with ITS" leaves out information a reader needs to reproduce the work.

Account for insulin when interpreting results. Because standard culture insulin concentrations saturate both the insulin and IGF-1 receptors, any experiment concerning growth factor signalling, PI3K-Akt activity, metabolic regulation or differentiation is being run against a strong, constant agonist background. This is not a reason to avoid insulin, but it is a reason to state it, to consider insulin-free controls, and to be cautious about attributing an effect to a treatment when the culture is already receiving a saturating growth signal.