Metformin Inhibits Autophagy, Mitophagy and Antagonizes Doxorubicin-Induced Cardiomyocyte Death

The antidiabetic drug metformin has been shown to reduce cardiac injury under various pathological conditions, including anticancer drug doxorubicin (DOX)-induced cardiotoxicity, which makes metformin a prime candidate for repurposing. However, the mechanisms that mediate the cardioprotective effects of metformin remain highly controversial. In this study, we tested a prevailing hypothesis that metformin activates autophagy/mitophagy to reduce DOX cardiotoxicity. FVB/N mice and H9C2 cardiac myoblasts were treated with metformin, respectively. Autophagy/mitophagy was determined by Western blot analysis of microtubule-associated protein light chain 3, form-II (LC3-II), a well-established marker of autophagic vesicles. Although metformin had minimal effects on basal LC3-II levels, it significantly inhibited the accumulation of LC3-II levels by the lysosomal protease inhibitors pepstatin A and E64d in both total cell lysates and mitochondrial fractions. Also, dual fluorescent autophagy/mitophagy reporters demonstrated that metformin slowed the degradation rate of autophagic cargos or mitochondrial fragments in the lysosomes. These surprising results suggest that metformin inhibits rather than stimulates autophagy/mitophagy, sharply contrasting the popular belief. In addition, metformin diminished DOX-induced autophagy/mitophagy as well as cardiomyocyte death. Together, these results suggest that the cardioprotective effects of metformin against DOX cardiotoxicity may be mediated by its ability to inhibit autophagy and mitophagy, although the underlying molecular mechanisms remain to be determined.

Lysosomal dysfunction in diabetic cardiomyopathy

Diabetes is a major risk factor for a variety of cardiovascular complications, while diabetic cardiomyopathy, a disease specific to the myocardium independent of vascular lesions, is an important causative factor for increased risk of heart failure and mortality in diabetic populations. Lysosomes have long been recognized as intracellular trash bags and recycling facilities. However, recent studies have revealed that lysosomes are sophisticated signaling hubs that play remarkably diverse roles in adapting cell metabolism to an ever-changing environment. Despite advances in our understanding of the physiological roles of lysosomes, the events leading to lysosomal dysfunction and how they relate to the overall pathophysiology of the diabetic heart remain unclear and are under intense investigation. In this review, we summarize recent advances regarding lysosomal injury and its roles in diabetic cardiomyopathy.

Niacin treatment of the atherogenic lipid profile and Lp(a) in diabetes

OBJECTIVE

We tested the hypotheses that niacin is effective for the separate treatments of abnormalities of LDL particle size, HDL2 percentage and Lp(a) without potential negative effects on glycated haemoglobin.

RESEARCH DESIGN AND METHODS

The atherogenic lipid profile lipids, such as triglycerides, small, dense LDL cholesterol (LDLc) particle mass, LDL particle size, total HDLc and HDL2 percentage as well as Lp(a), were measured in 42 diabetic patients with abnormalities of LDL particle size, HDL2 percentage and/or Lp(a) levels before and after niacin treatment. LDL particle size and HDL2 were measured using polyacrylamide gradient gel electrophoreses and Lp(a) was measured by enzyme-linked immunoabsorbance assay (ELISA).

RESULTS

After niacin treatment, LDL peak particle diameter increased from 252 +/- 7 A to 263 +/- 7 (p < 0.0001), small, dense LDLc particle mass decreased from 27 +/- 11 mg/dL to 15 +/- 4 (p < 0.0001), total HDLc increased from 39 +/- 7 mg/dL to 51 +/- 11 (p < 0.0001), HDL2 as the percentage of total HDLc mass increased from 29 +/- 8% to 45 +/- 10 (p < 0.0001) and Lp(a) decreased from 43 +/- 17 mg/dL to 25 +/- 10 (p < 0.0001). Mean haemoglobin A1c level was improved during treatment from 7.6 +/- 1.8% to 6.5 +/- 1.0 (p < 0.0001) using increased oral agent and insulin treatment doses in more than 90% of the patients. A total of 21% of the patients were unable to tolerate niacin owing to reversible side-effects, and 14% were unable to adhere to the niacin dosing regimen of three times daily.

CONCLUSIONS

These data indicate that in diabetic patients, niacin (i) is effective for treating diabetic dyslipidaemias associated with both the atherogenic lipid profile and Lp(a); (ii) must be used with modern and aggressive oral hypoglycaemic agents or insulin treatment; and (iii) is an important drug to treat diabetes dyslipidaemias because of its broad spectrum of effectiveness.

Inhibition of mammary tumorigenesis in carcinogen-treated Lewis rats by suppression of prolactin secretion

One hundred sixty-eight female Lewis rats were treated intragastrically with 10 mg 7,12-dimethylbenz[a]anthracene at 56 and 63 days of age. Pituitary prolactin secretion was suppressed in one-half of these rats by daily sc administrations of 2-bromoergocryptine mesylate (CB-154; 0.4 mg/100 g body wt) from 29 to 90 days of age (series 1) and from 90 to 140 days of age (series 2). Treatment with CB-154 was initiated prior to the onset of palpable mammary carcinomas. Control rats were given injections of saline. Inguinal mammary glands were excised from 10 control and 10 CB-154-treated rats at the cessation of saline and CB-154 treatments and examined for hyperplastic nodules (HN). The remaining rats were palpated weekly for mammary carcinomas (MC) and killed at 200 days of age. Mean number of HN per rat, mean number of MC per rat, and percent of rats with MC were, respectively: series 1--controls, 0.6, 1.5, and 68; CB-154 treatment, 0.5, 1.1, and 62; series 2--controls, 10.4, 2.0, and 94; CB-154 treatment, 5.1, 1.1, and 56. The number of HN and MC was only slightly reduced in rats when prolactin was suppressed during carcinogen treatment (series 1) but markedly reduced when prolactin was suppressed after carcinogen treatment (series 2). These results provide evidence that prolactin is involved in the early development of mammary dysplasias in the carcinogen-treated female Lewis rat.