UCP4 overexpression improves fatty acid oxidation and insulin sensitivity in L6 myocytes

Obesity, which is caused by energy uptake being greater than energy expenditure, is widely prevalent today. Currently, only a limited number of efficient interventional strategies are available for the prevention of obesity. Previous studies have shown that UCP4 transcription occurs at a considerable level in mouse skeletal muscle; however, the exact functions of UCP4 remain unclear. In this study, we investigated the effect of UCP4 on mitochondrial function and insulin sensitivity in mature L6 myocytes. UCP4 overexpression in L6 myocytes induced increased mitochondrial carnitine palmitoyltransferase 1A (CPT1A) and decreased citrate synthase (CS) mRNA in the basal condition (i.e., in the absence of insulin). UCP4 overexpression significantly improved insulin sensitivity, increased tyrosine phosphorylation of IRS-1 in the presence of insulin, and significantly reduced intracellular triglyceride (TG). Additionally, intracellular ATP content and mitochondrial membrane potential were downregulated. We also observed that intracellular ROS, mitochondrial morphology, and mitochondrial mtDNA copy number were maintained upon UCP4 expression, with no change in mitochondrial fusion and fission. In summary, our findings provide evidence to show that UCP4 overexpression reduced the insulin sensitivity and mitochondrial fatty acid oxidation of L6 myocytes. These findings support the notion that UCPs are ideal targets for treatment of insulin resistance.

Monoclonal antibody to the six-transmembrane epithelial antigen of prostate 4 promotes apoptosis and inhibits proliferation and glucose uptake in human adipocytes

We previously identified the six-transmembrane epithelial antigen of prostate (STEAP) 4 as a novel plasma membrane protein that is up-regulated in obese patients and may play a significant role in the development of human obesity. In this study, a STEAP4-specific antibody was used to characterize the biological functions of the STEAP4 protein in human adipocytes. Cell viability assays (Trypan Blue exclusion), CCK-8 assays and cell cycle analysis showed that the STEAP4 antibody inhibited pre-adipocyte proliferation. Morphological observations by electron microscopy and confocal laser microscopy, annexin V-FITC labeling, caspase-3 and caspase-8 activity assays as well as data from quantitative real-time RT-PCR (qPCR) further determined that the STEAP4 antibody could promote apoptosis in pre-adipocytes. Based on quantitative Oil Red O staining and the expression profiles of specific markers, we demonstrated that the STEAP4 antibody did not affect adipogenesis, but the 2-deoxy-d-[3H]-glucose uptake tests showed that it induced the insulin-stimulated glucose uptake in mature human adipocytes. In conclusion, our results demonstrated that the STEAP4 antibody does not influence human adipocyte differentiation, but it is likely that the STEAP4 protein regulates proliferation and apoptosis and plays an important role in modulating the insulin sensitivity of human adipocytes.

Knockdown of STEAP4 inhibits insulin-stimulated glucose transport and GLUT4 translocation via attenuated phosphorylation of Akt, independent of the effects of EEA1

The aim of this study was to investigate whether the early endosome antigen 1 (EEA1) and/or PI3K pathway is involved in the molecular mechanisms underlying the effects of the six-transmembrane protein of prostate 4 (STEAP4; also called STAMP2 and TIARP) on the insulin sensitivity of human adipocytes. Our data demonstrated that siRNA-mediated STEAP4 deficiency significantly decreased insulin-stimulated glucose transport in mature human adipocytes by decreasing GLUT4 translocation to the plasma membrane through attenuated Akt phosphorylation. We further found that EEA1 may not be involved in the mechanisms underlying the effects of STEAP4 on insulin-stimulated glucose uptake and GLUT4 translocation, as indicated by the results that i) STEAP4 does not alter the effects of EEA1 on insulin-stimulated glucose uptake and GLUT4 translocation; ii) STEAP4 does not modify the expression of EEA1 protein; and iii) STEAP4 does not interact with EEA1 according to FRET analysis. In conclusion, this study revealed that the knockdown of STEAP4 inhibits insulin-stimulated glucose transport and GLUT4 translocation via the attenuated phosphorylation of Akt, independent of the effects of EEA1.

Overexpression of LYRM1 induces mitochondrial impairment in 3T3-L1 adipocytes

Homo sapiens LYR motif containing 1 (LYRM1) is a recently discovered gene involved in adipose tissue homeostasis and obesity-associated insulin resistance. The exact mechanism by which LYRM1 induces insulin resistance has not yet been fully elucidated. In this study, we demonstrated that the overexpression of LYRM1 in 3T3-L1 adipocytes resulted in reduced insulin-stimulated glucose uptake, an abnormal mitochondrial morphology, and a decrease in intracellular ATP synthesis and mitochondrial membrane potential. In addition, LYRM1 overexpression led to excessive production of intracellular of reactive oxygen species. Collectively, our results indicated that the overexpression of LYRM1 caused mitochondrial dysfunction in adipocytes, which might be responsible for the development of LYRM1-induced insulin resistance.

TNF-alpha induces mitochondrial dysfunction in 3T3-L1 adipocytes

TNF-alpha was the first proinflammatory cytokine identified linking obesity, insulin resistance and chronic inflammation. However, the mechanism of TNF-alpha in the etiology of insulin resistance is still far from clear. Because the mitochondria play an important role in energy metabolism, we investigated whether mitochondrial dysfunction is involved in pathogenesis of TNF-alpha-mediated insulin resistance. First, a fully differentiated insulin-resistant 3T3-L1 adipocyte model was established by incubating with 4 ng/ml TNF-alpha for 4 d, and then the mitochondrial morphology and functions were observed. TNF-alpha treatment induced pronounced morphological changes in the mitochondria, which became smaller and condensed, and some appeared hollow and absent of cristae. Mitochondrial dynamics changes were observed as increased mitofusion protein mfn1 and mitofission protein Drp1 levels compared with controls. No obvious effects on mitochondrial biogenesis were found. PGC-1alpha levels decreased, but no significant changes were found in mtTFA mRNA expression, NRF1mRNA expression and mitochondrial DNA (mtDNA). TNFalpha treatment also led to decreased mitochondrial membrane potential and reduced production of intracellular ATP, as well as accumulation of significant amounts of reactive oxygen species (ROS). Further research is required to determine if mitochondrial dysfunction is involved in the inflammatory mechanism of insulin resistance and may be a potential target for the treatment of insulin resistance.

Short report: Tissue-specific expression profiles of the uncoupling protein family in normal control mice and genetically ob/ob mice

Uncoupling proteins (UCPs) located in the inner mitochondrial membrane are involved in the regulation of energy balance. Thus far, 5 UCP isoforms have been identified, but controversies exist in the research focused on the function of the UCPs (except UCP1) in the pathogenesis of obesity. Because of the known cross-reactivity of the antibodies presently available for the detection of UCP proteins, this study systematically analyzed the differential tissue expression profiles of the 5 UCP isoforms in lean control mice and ob/ob mice by using real-time polymerase chain reaction (PCR) analysis. The results show that the tissue-specific expression patterns of individual isoforms in normal and ob/ob mice are considerably different; this will provide new insights into the functions of UCPs in the pathogenesis of genetic obesity.

Immunohistochemical observation on luteinizing hormone in rat testes before and after testicular capsulotomy

AIM

In the testicular capsulotomized rats, although there was a significant increase in the luteinizing hormone (LH) levels, the secretion of testosterone remained low. In order to clarify the mechanisms of this phenomenon, the binding of endogenous LH to the testes were observed before and after testicular capsulotomy.

METHODS

Peroxidase-antiperoxidase (PAP) method was used to detect the binding of LH to the testes in rats.

RESULTS

An intense positive staining of LH was found in the Leydig cells of both the normal and sham-operated control testes. However, at 40 d after operation, the LH immunoreactivity was decreased in the Leydig cells of the capsulotomized testis. By d 60, only very weak positive staining could be observed in these cells.

CONCLUSION

A progressive reduction of endogenous LH binding to the testis cccurred in the capsulotomized rat.

Effect of testicular capsulotomy on lipid droplets in the seminiferous tubules of rats

AIM

In order to reveal the histochemical alteration that might occur during the processes of the spermatogenic disruption induced by testicular capsulotomy, the location and alteration of lipid droplets in the seminiferous tubules were observed in the present study.

METHODS

Osmium tetroxide was used to demonstrate the lipid droplets in the seminiferous tubules of capsulotomized and sham-operated control testes.

RESULTS

In the seminiferous tubules of the sham-operated rat testes, many small lipid droplets were located close to the basement membrane of the seminiferous tubules. But for the capsulotomized testes, the lipid droplets in the seminiferous tubules had increased in size and number, with many lipid droplets migrated towards the lumen of the tubules.

CONCLUSION

The results indicated that a progressive fatty degeneration occurred in the seminiferous tubules after testicular capsulotomy.

Effect of testicular capsulotomy on fertility of rats

AIM

To demonstrate the effect of capsulotomy on the fertility of male rats.

METHODS

Testicular capsulotomy was carried out in immature (21 days) and adult (60 days) male rats. The fertility of them was assessed by cohabitation with proestrus females overnight and 20 days later, the females were examined for impregnation. Morphological changes at the site of the capsulotomy were observed under light microscope.

RESULTS

In rats capsulotomized at Day 60, the fertility was gradually depressed and all the rats completely lost their fertility 2 months post-operation. At that time, a partial regeneration of the capsule at the site of capsulotomy was observed. Immature rats capsulotomized at Day 21 were found to possess normal fertility at maturity. The capsulotomy site was almost completely recovered 60 days post-operation.

CONCLUSION

In male rats, testicular capsulotomy at the age of Day 60 will damage fertility. However, when capsulotomy is performed at Day 21, fertility is preserve.

Effect of testicular capsulotomy on secretion of testosterone and gonadotrophins in rats

AIM

In order to clarify further the mechanisms underlying the effect of capsulotomy on testicular function, the levels of testosterone, LH and FSH were observed.

METHODS

Intratesticular testosterone levels and LH, FSH levels in the peripheral blood of normal, sham-operated and capsulotomized rats were detected by RIA.

RESULTS

After testicular capsulotomy, there was a progressive reduction in the testosterone level in the testicular venous blood together with a progressive increase in the LH and FSH levels in the peripheral blood from approximately 30 days post-capsulotomy. Morphological changes were observed at 5-10 days after capsulotomy, i.e., far ahead of the hormonal changes.

CONCLUSION

The seminiferous tubular damage after testicular capsulotomy was not caused by the reduction in testosterone, and on the contrary, the hormonal change might be secondary to the morphological alterations. The increase in LH level most likely resulted from a negative feedback influence from the lowered testosterone level, while the increase in FSH secretion may be a feedback signal of the damaged seminiferous tubules.

Studies on relationship between testicular capsule and sperm transport in rat testis

AIM

In SD rats, histological changes in the testis were observed after bilateral capsulotomy (of the tunica albuginea) in order to investigate the physiological role of the testicular capsule on sperm transport.

METHODS

Bilateral longitudinal capsulotomy was devised to disrupt the capsular contractile function. With this technique, only the tunica vaginalis and tunica albuginea were slit open, leaving the tunica vasculosa intact to embrace the underlying testicular parenchyma. After capsulotomy, the structural changes in the seminiferous tubules, the transitional distal seminiferous segment, and the rete testis were observed.

RESULTS

In the capsulotomized testis, there was sperm retention at the transitional seminiferous segment and progressive degenerative changes in seminiferous tubules.

CONCLUSION

The results clearly indicated that an intact testicular capsule was required for normal sperm transport from the seminiferous tubules into the rete testis. This is the first attempt to study the physiological role of the testicular capsule in intact animals.

Effects of flavonoids from Semen Cuscutae on the reproductive system in male rats

AIM

To evaluate the effects of the flavonoids extracted from the Semen Cuscutae (FSC) on the reproductive and endocrine functions in male rats.

METHODS

(1) FSC were obtained from the semen of Cuscuta sinensis Lam through solvent extraction and polyamide columnar chromatography; (2) Effect of FSC on the reproductive organs was assessed in immature rats. Rats were administered FSC through gastric gavage at a dose of 300 mg/kg per day for 7 days and the weights of testis, epididymis, seminal vesicle and pituitary gland were then observed; (3) To observe the effect of FSC on the reproductive endocrine function: same dose level of FSC was given to male rats of different age groups for 7 days; on day 8, the plasma testosterone (T), estradiol (E2) and LH were determined by RIA, the specific binding of LH was estimated and the testes were weighed. (4) Effect of FSC on LH secretion was assessed in vitro on cultured adenohypophysis. (5) Effect of FSC on T secretion was assessed in vitro on Leydig cell culture.

RESULTS

FSC increased the weights of testis, epididymis and pituitary gland, and stimulated T and LH secretion both in vitro and in immature rats.

CONCLUSION

FSC invigorates the reproductive system and reproductive endocrine function in male rats.

Functional study and immunocytochemical identification of endothelin in cultured epididymal cells and intact epididymis of the rat

Endothelin, a novel potent vasoconstrictor peptide produced by vascular endothelial cells, stimulated anion secretion by a cultured secretory epithelium derived from the rat epididymis as measured by changes in short-circuit current (SCC). Stimulation of the SCC was observed when endothelin was added to the basolateral or the apical side of the epithelium. The response to basolateral application was greater than that to apical application. The EC50 values were found to be 1.3 and 3.0 nM for basolateral and apical application, respectively. These values were about one-half to one order of magnitude higher than that required for its vasoconstrictor action. The stimulation of SCC by endothelin was likely to be due to an increase to anion secretion as removal of Cl from the incubation medium markedly reduced the SCC response to endothelin. Diphenylamine-2-carboxylate (DPC, 0.1 mM), a Cl-channel blocker, added to the apical side also inhibited the endothelin-induced rise in SCC. The stimulation of SCC by endothelin was accompanied by a rise in the intracellular cyclic AMP content in epididymal monolayers. Immunofluorescence staining has shown the presence of immunoreactive endothelin-like compound in the interstitium and epithelial cells of the rat epididymis. It is speculated that endogenous endothelin plays an important role in the control of water and electrolyte transport in the epididymis.