Chromium

Role of Chromium

Strengthening PI3K and Akt signalling

In a nutritional model of insulin resistance in which the mice were fed a high-sucrose diet or a high-fat diet, chromium treatment reconciled the blunted Akt phosphorylation and the attenuated PI3-kinase activity, consistent with the studies by Cefalu and coworkers.

To understand whether this was a direct effect of chromium on insulin signaling, cultured adipocytes were rendered insulin resistant by chronic treatment with insulin and glucose (hyperinsulinemic, hyperglycemic conditions) resulted in a blunting of insulin-stimulated Akt phosphorylation and cellular glucose uptake. Both these effects were inhibited by pretreating the cells with the chromium complex, results consistent with a direct action of chromium on insulin signalling. (R1)

Reduction of ER stress

Oral supplementation with chromium significantly attenuated the levels of these ER-stress markers. In addition to these in-vivo studies, ER stress was induced in-vitro by treating cultured, differentiated myotubes with thapsigargin, a well known inducer of ER-stress. Pretreatment of the myotubes with chromium resulted in a concentration-dependent inhibition of thapsigargin-induced ER stress. Together, the findings suggest that chromium alleviates ER-stress in the cells, although the exact mechanism by which it does so is unclear. (R1)

Lowering inflammation

Together with reducing oxidative stress, chromium treatment inhibits the release of pro-inflammatory cytokines from monocytes exposed to hyperglycemic conditions. Chromium was found to inhibit protein glycosylation and lipid peroxidation in erythrocytes exposed to high levels of glucose.

In addition to these in-vitro studies, it has also been shown that chromium supplementation caused a lowering of proinflammatory cytokines (TNF-alpha, IL-6, CRP), oxidative stress, and lipids levels in streptozotocin-induced diabetic rats. (R1)

Lowering oxidative stress

Chromium attenuates oxidative stress in cultured monocytes and isolated human mononuclear cells subjected to hyperglycemic conditions.

Chromium supplementation in obese mice significantly attenuated the obesity-associated elevated ratio of oxidized glutathione to reduced glutathione and the formation of protein carbonyl. (R1)

AMPK activation

Cr(phe)₃ stimulated the phosphorylation of the α-catalytic subunit of AMPK at Thr172, as well the downstream targets of AMPK including acetyl-CoA carboxylase (at Ser212) and eNOS (at Ser1177). Chromium also stimulated glucose-uptake in these cells.

Both glucose uptake and AMPK phosphorylation mediated by chromium were inhibited by the AMPK inhibitor compound C. (R1)

Increase in membrane fluidity

• Chromium has direct effect on reduction of cholesterol level in the membrane, which increases membrane fluidity.
• Chromium facilitates GLUT4 transporters translocation to the inner side of the membrane.

Here, we report that trivalent chromium in the chloride (CrCl3) or picolinate (CrPic) salt forms mobilize the glucose transporter, GLUT4, to the plasma membrane in 3T3-L1 adipocytes. Concomitant with an increase in GLUT4 at the plasma membrane, insulin-stimulated glucose transport was enhanced by chromium treatment. In contrast, the chromium-mobilized pool of transporters was not active in the absence of insulin. Microscopic analysis of an exofacially Myc-tagged enhanced green fluorescent protein-GLUT4 construct revealed that the chromium-induced accumulation of GLUT4-containing vesicles occurred adjacent to the inner cell surface membrane.

With insulin these transporters physically incorporated into the plasma membrane. Regulation of GLUT4 translocation by chromium did not involve known insulin signaling proteins such as the insulin receptor, insulin receptor substrate-1, phosphatidylinositol 3-kinase, and Akt.

Consistent with a reported effect of chromium on increasing membrane fluidity, we found that chromium treatment decreased plasma membrane cholesterol.

Interestingly, cholesterol add-back to the plasma membrane prevented the beneficial effect of chromium on both GLUT4 mobilization and insulin-stimulated glucose transport. (R8)

Inhibition of ATP Synthase

We show that Cr(III) binds to ATP synthase at its beta subunit via the catalytic residues of Thr213/Glu242 and the nucleotide in the active site. Such a binding suppresses ATP synthase activity, leading to the activation of AMPK, improving glucose metabolism, and rescuing mitochondria from hyperglycaemia-induced fragmentation. (R9)

8 new proteins that bind Cr3+

A new study from 2023 (R9) has identified 8 new proteins that can bind Cr(III):

• ATP5B, ATP5L - structural parts of ATP Synthase
• TXN - Thioredoxin
• PRDX1 - Peroxiredoxin 1
• Hsp60 - Heat Shock Protein 60 (HSPD1)
• CLIC1 - Chloride intracellular channel protein 1
• COMT - Catechol-O-methyltransferase
• H3F3A - Histone H3

Deficiency

Two case-control studies (5), (6)found an inverse association of toenail chromium levels with the risk of myocardial infarction (MI). Also, laboratory studies have suggested that chromium deficiency was associated with elevated levels of fasting blood glucose, insulin, cholesterol, triglycerides and reduced levels of high density lipoprotein cholesterol (HDL) (R10)

Transport in the blood

Serum Transferrin is used to transport Chromium to the cells:

Cr(III) absorbed from dietary sources into the body becomes 80% sTf bound. It is believed to bind in the canonical modality at the metal binding site. Very recently, a crystal structure for Cr(III)-bound sTf was obtained, which shows Cr(III) coordinated at the C-site, in closed-conformation, canonical modality with malonate substituting for carbonate. (R3)

Bicarbonate increases binding rate to transferrin

The binding of Cr3+ to transferrin can be greatly accelerated by the addition of bicarbonate (Fig. 4). Equilibrium can be achieved in about 15 min at 25 mM bicarbonate, corresponding to the concentration of bicarbonate in blood. Fitting the curves requires assuming that Cr3+ binds to the two metal-binding sites of transferrin at different rates. If fact, half times for binding were ~15 s and 4 min. Thus under physiological conditions rather than ambient bicarbonate concentrations, the binding of Cr3+ to transferrin is sufficiently rapid to be physiologically relevant. (R5)

Cellular uptake

• Cr is bound to Transferrin at two sites: C-site and N-site.
• Cr-sTf taken up via endocytosis through transferrin receptor.
• Cr is released from the C-site of the sTf-TfR complex within endosome at normal pH, but not from N-site.
• Ascorbate and citrate double the rate of release from N-site
• Presence of Transferrin receptor accelerates release from sTf.
• Cr ions are fully released from the complex within 15 minutes.

The affinity of sTf for Cr(III) is many orders of magnitude lower than for Fe(III), which can serve to regulate how much it can bind to the protein. This lower affinity also makes sTf very capable of releasing Cr(III) from the endosome into cells.

It has been shown that at endosomal pH, Cr(III) is rapidly released from sTf at the C-site although retained at the N-site.

The presence of anions such as the physiologically relevant citrate and ascorbate and not relevant EDTA can double the release rate of Cr(III) from the N-site at endosomal pH.

A very compelling finding is that the presence of the TfR further accelerates Cr(III) release from sTf. The metal ion should be fully released from the Cr2-sTf-TfR complex within 15 min, making it fast enough for physiological transport of Cr(III). (R3)

Insulin

Insulin triggers incorporation of transferrin receptors to the cell membranes.

Increases in blood insulin concentrations (including increases stemming from increases in blood glucose concentration) result in increased urinary Cr loss;

this could arise from insulin triggering movement of transferrin receptor to cell membranes resulting in increased incorporation of transferrin (and associated Cr) into endosomes (vide infra). (R5)

Chromodulin

Only two naturally occurring chromium containing biomolecules are known.

The first is the well known iron transport protein transferrin that doubles as a chromium(III) transport agent.

The second is the oligopeptide chromodulin, the form in which chromium is excreted and which may be biologically active.

The oligopeptide chromodulin (also known as the low-molecular-weight chromium-binding substance, LMWCr) may function as part of a novel insulin signalling autoamplification mechanism in mammals. (R2)

Apochromodulin and response to insulin

• Chromodulin is maintained in metal-free form inside insulin dependent cells.
• Chromium is mobilized from blood in response to insulin.

Chromodulin is maintained in its metal-free apo-form inside insulin dependent cells. In response to insulin, chromium is mobilized from the blood and transported into these cells. Apochromodulin possesses a large chromic ion binding constant of 1.54 and a Hill constant of 3.47, suggesting that essentially only holochromodulin and apochromodulin coexist in solution. (R2)

Holochromodulin holds 4 Cr ions

Holochromodulin, which contains 4 equiv of chromium, binds to the insulin activated form of insulin receptor with an approximately 100 pM dissociation constant, further stimulating the tyrosine kinase activity of the receptor.

To down regulate this further stimulation, chromodulin is removed from the cells and ultimately appears in the urine. (R2)

Amplifying insulin cascade

In the holo form, chromodulin binds to the insulin-bound, activated insulin receptor and is believed to further enable the phosphorylation cascade of the insulin signalling process that leads to glucose uptake although there is some debate about the relevance of the chromodulin and insulin receptor interaction. (R3)

Presence of apochromodulin in cytosol and nucleus

Apo form is chromodulin not bound to the metal.

In 1986, Yamamoto et al., discovered chromodulin, a Cr-binding oligopeptide present inside the cytoplasm and nucleus in its apo form. (R3)

Apochromodulin takes up to 4 Cr ions from Cr2-sTf-TfR complex

Interestingly, apochromodulin was observed to take advantage of insulin stimulation of TfR recycling to sequester up to 4 Cr(III) ions from the Cr2-sTf-TfR complexes entering cells. (R3)

Insulin stimulates recycling of Chromium-Transferrin-TransferrinReceptor complexes and during that process Apochromodulin acquires up to 4 ions of Chromium.

Gene

The gene responsible for the synthesis of chromodulin hasn’t been found yet (as of 2023). However some research has been done:

A genomic search of the non-redundant database for all possible decapeptides of the reported composition yields three exact matches, EDGEECDCGE, DGEECDCGEE and CEGGCEEDDE.

The first two sequences are found in ADAM19 (A Disintegrin and Metalloproteinase domain 19) proteins in man and mouse; the last is found in a protein kinase in rice (Oryza sativa).

A broader search for pentameric sequences (and assuming a disulfide dimer) corresponding to the stoichiometric ratio E:D:G:C::2:1:1:1, within the set of human proteins and the set of proteins in, or related to, the insulin signaling pathway, yields a match at an acidic region in the α-subunit of the insulin receptor (-EECGD-, residues 175–184).

A synthetic peptide derived from this sequence binds chromium(III) and forms a metal-peptide complex that has properties matching those reported for isolated LMWCr and Cr(III)-containing peptide fractions. (R7)

Insulin receptor alpha-unit

Biochemically, the insulin receptor is encoded by a single gene INSR, from which alternate splicing during transcription results in either IR-A or IR-B isoforms. (Wikipedia)

See both isoforms in Uniprot.

Excretion

Increased excretion and absorption in diabetes type 2

Animal models and humans with type 2 diabetes have been shown to excrete more Cr in their urine; this has led to proposals that individuals with diabetes could become Cr deficient, which would exacerbate the symptoms of the diabetes. The increased urinary loss of Cr, however, has been shown to correlate with increased absorption of Cr (R5)

Good and Bad Chromium

Chromium is an essential trace element required for carbohydrate, lipid and protein metabolism4. It occurs in the environment primarily in two valence states, trivalent chromium (an essential element in humans) and hexavalent chromium (a harmful element to humans). (R10)

• Cr³⁺ is essential for humans.
• Cr⁶⁺ is toxic for humans.

Lipid metabolism

Numerous studies indicate that chromium is essential for lipid metabolism. Animal studies suggest that chromium treatment is associated with a reduction in liver triglyceride levels and lipid accumulation. Also, rats and rabbits fed a chromium deficient diet had elevated total cholesterol and aortal lipid concentrations.

In addition, increased HDL-cholesterol levels and decreased total cholesterol, LDL-cholesterol and triglycerides levels have been observed in humans after chromium supplementation.

The possible explanations are that chromium may improve the conversion of acetyl coenzyme A (acetyl-CoA) and decrease the formation of cholesterol.

Also, chromium can increase the activity of lecithin cholesterol acyltransferase (LCAT) and accelerate cholesterol esterification and excretion.

In addition, studies indicate that chromium activates glucose transporter glut4 through a cholesterol-dependent mechanism, which decreases cholesterol levels (R10)