Quantitative Proteomics Reveals Phlorizin Protection in Diabetic Cardiomyopathy

Phlorizin may prevent the development of diabetic cardiomyopathy by regulating the expression of key proteins involved, suggests a proteomic study.¹ Phlorizin (phloretin-2′-O-glucoside), a dihydrochalcone derived from apple peels, is known for its cardio-protective and antioxidant benefits.² However, not much is known about phlorizin vis-à-vis cardiac damage in type 2 diabetes mellitus (T2DM).

Individuals with diabetic cardiomyopathy develop heart failure without developing hypertension or coronary artery disease. Diabetic cardiomyopathy is associated with structural and functional changes in the heart, including increased left ventricular (LV) mass, myocardial fibrosis, and abnormal diastolic function. The underlying mechanisms remain unclear, although  studies suggest myocardial hypertrophy, elevated wall-thickness-to-chamber ratios, and increased stiffness of the LV wall are contributing factors.³

Researchers led by Hai-qing Gao, PhD, professor of geriatrics at the Qilu Hospital of Shandong University, therefore sought to investigate the role of phlorizin in preventing diabetic cardiomyopathy in db/db mice showing symptoms such as hyperglycemia, obesity, insulin resistance and renal damage, which occur after 10–20 weeks of sustained hyperglycemia. The investigators also used a quantitative proteomic assay, isobaric tag for relative and absolute quantitation (iTRAQ), and liquid chromatography–tandem mass spectrometry (LC-MS/MS) to unravel the protein profiles of phlorizin-treated and untreated db/db mice.¹

The db/db mice in the phlorizin-treated diabetic group were treated with 20 mg/kg phlorizin intragastrically from week 8 to week 18 without hypoglycemic therapy, while the control group was administered normal saline solution. Overnight fasting blood glucose (FBG), blood triglycerides (TG) and blood total cholesterol (TC) levels at the end of the study, as well as fluorescence determinations (440 nm emissions after excitation at 370 nm) of serum advanced glycation end products (AGEs), were performed.

The researchers used a micro-LC system and an LTQ Velos ion trap mass spectrometer (Thermo Scientific) for mass spectrometric analysis following the iTRAQ proteomic analysis to tag peptides for multiplexed protein quantification after histology studies involving LV myocardium.

Using the Ingenuity Pathway Analysis (IPA, Ingenuity Systems, http://www.ingenuity.com), the differentially expressed genes were analysed and the corresponding proteins identified and mapped. Additional western blotting analysis was also carried out.

The findings suggest that phlorizin significantly decreased body weight gain and the levels of serum FBG, TG, TC and AGEs. As well, phlorizin treatment preserved the normal myocardial structure.

The iTRAQ proteomics profiling of myocardial protein revealed a total of 1,627 proteins in the db/db mice treated with phlorizin. Increased levels of 29 of the 113 differentially expressed proteins were seen in the T2DM group compared with the control group. Phlorizin therapy decreased the levels of these elevated proteins. Phlorizin treatment also normalized the decreased levels of an additional 84 proteins in the T2DM group, unlike the control group.

Phlorizin treatment specifically upregulated the microsomal triglyceride transfer protein (MTP), nicotinamide phosphoribosyltransferase (NAMPT), tyrosine-protein phosphatase non-receptor type 11 (PTPN11), low-density lipoprotein receptor (LDLr), protein-tyrosine phosphatase-like member B (PTPLB), and sorbin and SH3 domain-containing protein 1 (Sorbs1). The phlorizin-treated diabetic group showed downregulated glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPI-HBP1) — an HDL-binding protein involved in reversing cholesterol transport — compared with mice on normal saline. The ability of phlorizin to modulate cardiac lipid metabolism by lowering the level of circulating fatty acids and attenuating cardiac lipid accumulation is linked to the upregulation of MTP and NAMPT.

Treatment also led to increased levels of 5′-AMP-activated protein kinase catalytic subunit alpha-2 (PRKAA2), endonuclease G (EndoG), and NADH dehydrogenase (ubiquinone) iron-sulfur protein 6 (NDUFS6) in the myocardial mitochondria. However, phlorizin treatment significantly downregulated proteins such as apoptosis-inducing factor 2 (ATPAF22), ATP synthase mitochondrial F1 complex assembly factor 2 (AIFM2), and lipoic acid synthase (LIAS).

Phlorizin treatment reversed the abnormal decrease in several proteins seen in cardiomyopathy — for example, the cytoskeletal protein titin (TTN) and death-associated protein kinase 3 (DAPK3 or ZIPK) associated with cardiac contraction and diastolic function.

Phlorizin also improved expression of genes coding for desmin (DES), integrin-linked protein kinase (ILK), myosin regulatory light chain 2 (My12), dystrophin (DMD), gelsolin (GSN), lamin A/C (LMNA), and laminin subunit α-2 (LAMA2). Individuals with cardiomyopathy have poor expression of these proteins due to single nucleotide gene mutations.

The researchers also report upregulation of differentially expressed proteins such as adenomatous polyposis coli (APC), calnexin (CANX), myomesin-1 (MYOM1), and voltage-dependent L-type calcium channel subunit beta-2 (CACNB2), as well as glutaredoxin-3 (GLRX3) and collagen alpha-2(I) chain (COL1A2). These proteins were altered in mice with cardiac hypertrophy and heart disease in general.

The investigators successfully validated the iTRAQ data of differentially expressed proteins using western blot analysis.

The cardioprotective role for phlorizin may be mediated via the regulation of genes modulating cardiac contraction based on the results showing that it upregulated the DAPK3 (ZIPK) protein, which has a key role in the regulation of cardiac contractility. As well, by upregulating titin levels, phlorizin alters myocardial diastolic stiffness and modulates diastolic function.

Consistent with its antioxidant ability to decrease reactive oxygen species (ROS) generation in T2DM, the investigators found that phlorizin treatment significantly lowered plasma AGE levels in db/db mice.

Indeed, the study reveals that the cardioprotective effect of phlorizin is related to several proteins involved in mitochondrial energy production. The study establishes, for the first time, a quantitative iTRAQ profile of global cardiac proteins using db/db diabetic mice treated with or without phlorizin. The findings provide key data underlying the mechanism of diabetic cardiomyopathy and suggest that phlorizin may be a novel therapeutic intervention in the treatment of diabetic cardiomyopathy.

References

1.Cai, Q., et al. (2013) “Investigation of the Protective Effects of Phlorizin on Diabetic Cardiomyopathy in db/db Mice by Quantitative Proteomics,” Journal of Diabetes Research, 2013:263845, doi: 10.1155/2013/263845.

2. Wojdyło, A., et al. (2008) “Polyphenolic compounds and antioxidant activity of new and old apple varieties,” Journal of Agricultural and Food Chemistry, 56(15) (pp. 6520-6530), doi: 10.1021/jf800510j.

3. Battiprolu, P.K., et al. (2010) “Diabetic cardiomyopathy: mechanisms and therapeutic targets,” Drug Discovery Today, Discovery Mechanisms, 7(2) (pp. e135–e143), doi: 10.1016/j.ddmec.2010.08.001.