Caveolae and the organization of carbohydrate metabolism in vascular smooth muscle

Quantitative analysis of the murine lipid droplet-associated proteome during diet-induced hepatic steatosis

Hepatic steatosis is characterized by the accumulation of lipid droplets (LDs), which are composed of a neutral lipid core surrounded by a phospholipid monolayer embedded with many proteins. Although the LD-associated proteome has been investigated in multiple tissues and organisms, the dynamic changes in the murine LD-associated proteome in response to obesity and hepatic steatosis have not been studied. We characterized the hepatic LD-associated proteome of C57BL/6J male mouse livers following high-fat feeding using isobaric tagging for relative and absolute quantification. Of the 1,520 proteins identified with a 5% local false discovery rate, we report a total of 48 proteins that were increased and 52 proteins that were decreased on LDs in response to high-fat feeding. Most notably, ribosomal and endoplasmic reticulum proteins were increased and extracellular and cytosolic proteins were decreased in response to high-fat feeding. Additionally, many proteins involved in fatty acid catabolism or xenobiotic metabolism were enriched in the LD fraction following high-fat feeding. In contrast, proteins involved in glucose metabolism and liver X receptor or retinoid X receptor activation were decreased on LDs of high-fat-fed mice. This study provides insights into unique biological functions of hepatic LDs under normal and steatotic conditions.

Functional expression of sodium-glucose transporters in cancer

Glucose is a major metabolic substrate required for cancer cell survival and growth. It is mainly imported into cells by facilitated glucose transporters (GLUTs). Here we demonstrate the importance of another glucose import system, the sodium-dependent glucose transporters (SGLTs), in pancreatic and prostate adenocarcinomas, and investigate their role in cancer cell survival. Three experimental approaches were used: (i) immunohistochemical mapping of SGLT1 and SGLT2 distribution in tumors; (ii) measurement of glucose uptake in fresh isolated tumors using an SGLT-specific radioactive glucose analog, α-methyl-4-deoxy-4-[(18)F]fluoro-D-glucopyranoside (Me4FDG), which is not transported by GLUTs; and (iii) measurement of in vivo SGLT activity in mouse models of pancreatic and prostate cancer using Me4FDG-PET imaging. We found that SGLT2 is functionally expressed in pancreatic and prostate adenocarcinomas, and provide evidence that SGLT2 inhibitors block glucose uptake and reduce tumor growth and survival in a xenograft model of pancreatic cancer. We suggest that Me4FDG-PET imaging may be used to diagnose and stage pancreatic and prostate cancers, and that SGLT2 inhibitors, currently in use for treating diabetes, may be useful for cancer therapy.

Role of endothelial cell metabolism in vessel sprouting

Endothelial cells (ECs) are quiescent for years but can plastically switch to angiogenesis. Vascular sprouting relies on the coordinated activity of migrating tip cells at the forefront and proliferating stalk cells that elongate the sprout. Past studies have identified genetic signals that control vascular branching. Prominent are VEGF, activating tip cells, and Notch, which stimulates stalk cells. After the branch is formed and perfused, ECs become quiescent phalanx cells. Now, emerging evidence has accumulated indicating that ECs not only adapt their metabolism when switching from quiescence to sprouting but also that metabolism regulates vascular sprouting in parallel to the control by genetic signals.

Compartmentation of GAPDH

The concept of the cytosol as a space that contains discrete zones of metabolites is discussed relative to the contribution of GAPDH. GAPDH is directed to very specific cell compartments. This chapter describes the utilization of GAPDH's enzymatic function for focal demands (i.e. ATP/ADP and NAD(+)/NADH), and offers a speculative role for GAPDH as perhaps moderating local concentrations of inorganic phosphate and hydrogen ions (i.e. co-substrate and co-product of the glycolytic reaction, respectively). Where known, the structural features of the binding between GAPDH and the compartment components are discussed. The nuances, which are associated with the intracellular distribution of GAPDH, appear to be specific to the cell-type, particularly with regards to the various plasma membrane proteins to which GAPDH binds. The chapter includes discussion on the curious observation of GAPDH being localized to the external surface of the plasma membrane in a human cell type. The default perspective has been that GAPDH localization is synonymous with compartmentation of glycolytic energy. The chapter discusses GAPDH translocation to the nucleus and to non-nuclear cellular structures, emphasizing its glycolytic function. Nevertheless, it is becoming clear that alternate functions of GAPDH play a role in compartmentation, particularly in the translocation to the nucleus.

Role of glycolytically generated ATP for CaMKII-mediated regulation of intracellular Ca2+ signaling in bovine vascular endothelial cells

The role of glycolytically generated ATP in Ca(2+)/calmodulin-dependent kinase II (CaMKII)-mediated regulation of intracellular Ca(2+) signaling was examined in cultured calf pulmonary artery endothelial (CPAE) cells. Exposure of cells (extracellular Ca(2+) concentration = 2 mM) to glycolytic inhibitors 2-deoxy-D-glucose (2-DG), pyruvate (pyr) + beta-hydroxybutyrate (beta-HB), or iodoacetic acid (IAA) caused an increase of intracellular Ca(2+) concentration ([Ca(2+)](i)). CaMKII inhibitors (KN-93, W-7) triggered a similar increase of [Ca(2+)](i). The rise of [Ca(2+)](i) was characterized by a transient spike followed by a small sustained plateau of elevated [Ca(2+)](i). In the absence of extracellular Ca(2+) 2-DG caused an increase in [Ca(2+)](i), suggesting that inhibition of glycolysis directly triggered release of Ca(2+) from intracellular endoplasmic reticulum (ER) Ca(2+) stores. The inositol-1,4,5-trisphosphate receptor (IP(3)R) inhibitor 2-aminoethoxydiphenyl borate abolished the KN-93- and 2-DG-induced Ca(2+) response. Ca(2+) release was initiated in peripheral cytoplasmic processes from which activation propagated as a [Ca(2+)](i) wave toward the central region of the cell. Focal application of 2-DG resulted in spatially confined elevations of [Ca(2+)](i). Propagating [Ca(2+)](i) waves were preceded by [Ca(2+)](i) oscillations and small, highly localized elevations of [Ca(2+)](i) (Ca(2+) puffs). Inhibition of glycolysis with 2-DG reduced the KN-93-induced Ca(2+) response, and vice versa during inhibition of CaMKII 2-DG-induced Ca(2+) release was attenuated. Similar results were obtained with pyr + beta-HB and W-7. Furthermore, 2-DG and IAA caused a rapid increase of intracellular Mg(2+) concentration, indicating a concomitant drop of cellular ATP levels. In conclusion, CaMKII exerts a profound inhibition of ER Ca(2+) release in CPAE cells, which is mediated by glycolytically generated ATP, possibly through ATP-dependent phosphorylation of the IP(3)R.

Caveolins in vascular smooth muscle: form organizing function

Caveolae are becoming increasingly recognized as an important organizational structure for a variety of signal and energy-transducing systems in vascular smooth muscle (VSM). In this review, we discuss the emerging role of the caveolins in organizing and modulating the basic functions of smooth muscle: contraction, growth/proliferation, and the energetic support systems that support these functions. With clear alterations in cell metabolism and function in VSM with altered caveolin-1 (Cav-1) protein expression and with cardiovascular abnormalities associated with Cav-1 null mice, the caveolin family of proteins may play an important role in the function and dysfunction of VSM.

Overexpression of caveolin-1 results in increased plasma membrane targeting of glycolytic enzymes: The structural basis for a membrane associated metabolic compartment

Although membrane-associated glycolysis has been observed in a variety of cell types, the mechanism of localization of glycolytic enzymes to the plasma membrane is not known. We hypothesized that caveolin-1 (CAV-1) serves as a scaffolding protein for glycolytic enzymes and may play a role in the organization of cell metabolism. To test this hypothesis, we over-expressed CAV-1 in cultured A7r5 (rat aorta vascular smooth muscle; VSM) cells. Confocal immunofluorescence microscopy was used to study the distribution of phosphofructokinase (PFK) and CAV-1 in the transfected cells. Areas of interest (AOI) were analyzed in a central Z-plane across the cell transversing the perinuclear region. To quantify any shift in PFK localization resulting from CAV-1 over-expression, we calculated a periphery to center (PC) index by taking the average of the two outer AOIs from each membrane region and dividing by the central one or two AOIs. We found the PC index to be 1.92 +/- 0.57 (mean +/- SEM, N = 8) for transfected cells and 0.59 +/- 0.05 (mean +/- SEM, N = 11) for control cells. Colocalization analysis demonstrated that the percentage of PFK associated with CAV-1 increased in transfected cells compared to control cells. The localization of aldolase (ALD) was also shifted towards the plasma membrane (and colocalized with PFK) in CAV-1 over-expressing cells. These results demonstrate that CAV-1 creates binding sites for PFK and ALD that may be of higher affinity than those binding sites localized in the cytoplasm. We conclude that CAV-1 functions as a scaffolding protein for PFK, ALD and perhaps other glycolytic enzymes, either through direct interaction or accessory proteins, thus contributing to compartmented metabolism in vascular smooth muscle.

Caveolin-1 functions as a scaffolding protein for phosphofructokinase in the metabolic organization of vascular smooth muscle

Using confocal microscopy, we have demonstrated a similar distribution of phosphofructokinase (PFK) with caveolin-1 (CAV-1) mainly at the periphery (membrane) in freshly isolated vascular smooth muscle (VSM) cells and in cultured A7r5 VSM cells. Co-immunoprecipitation analysis validated the interaction between the proteins. To further test the hypothesis that PFK and CAV-1 are colocalized, we used small interfering RNA (siRNA) to downregulate CAV-1 expression and disrupt the protein-protein interactions between PFK and CAV-1. Transfection of cultured A7r5 cells with CAV-1 siRNA resulted in a decreased level of immunoreactive CAV-1 and a consequent shift in the distribution of PFK with less localization of PFK to the periphery of the cells and increased immunoreactivity at the perinuclear region as compared to control. Analysis of the average PFK intensity across cultured A7r5 cells demonstrated a higher central:peripheral intensity ratio (CPI ratio) in siRNA-treated cells than in the control. These results validate the possible role of CAV-1 as a scaffolding protein for PFK as evidenced by the significant redistribution of PFK after CAV-1 downregulation. We therefore conclude that CAV-1 may function as a scaffolding protein for PFK and that this contributes to the compartmentation of glycolysis from other metabolic pathways in VSM.

Expression of caveolin-1 in lymphocytes induces caveolae formation and recruitment of phosphofructokinase to the plasma membrane

Compartmentation of carbohydrate metabolism has been shown in a wide range of tissues including reports of one compartment of glycolysis associated with the plasma membrane of cells. However, only in the erythrocyte has the physical basis for plasma membrane-associated glycolytic pathway been established. We have previously found that phosphofructokinase (PFK) appeared to colocalize with the fairly ubiquitous plasma membrane protein caveolin-1 (CAV-1), consistent with a role for CAV-1 as an anchor for glycolysis to the plasma membrane. To test the hypothesis that CAV-1 functions as a scaffolding protein for PFK, we transfected human lymphocytes (a cell without CAV-1 expression) with human CAV-1 cDNA. We demonstrate that expression of CAV-1 in lymphocytes results in the formation of caveolae at the plasma membrane and affects the subcellular localization of PFK by recruiting PFK to the plasma membrane. Targeting of PFK by CAV-1 also was validated by the significant colocalization between the proteins after transfection, which resulted in a correlation of 0.97 +/- 0.004 between the two fluorophores. This finding is significant in as much as it illustrates the CAV-1 feasibility of generating binding sites for glycolytic enzymes on the plasma membrane. We therefore conclude that CAV-1 functions as a scaffolding protein for PFK and that this may contribute to the elucidation of the basis for carbohydrate compartmentation to the plasma membrane in a wide variety of cell types.

Caveolae-associated signalling in smooth muscle

Caveolae are flask-shaped invaginations in the membrane that depend on the contents of cholesterol and on the structural protein caveolin. The organisation of caveolae in parallel strands between dense bands in smooth muscle is arguably unique. It is increasingly recognised, bolstered in large part by recent studies in caveolae deficient animals, that caveolae sequester and regulate a variety of signalling intermediaries. The role of caveolae in smooth muscle signal transduction, as inferred from studies on transgenic animals and in vitro approaches, is the topic of the current review. Both G-protein coupled receptors and tyrosine kinase receptors are believed to cluster in caveolae, and the exciting possibility that caveolae provide a platform for interactions between the sarcoplasmic reticulum and plasmalemmal ion channels is emerging. Moreover, messengers involved in Ca2+ sensitization of myosin phosphorylation and contraction may depend on caveolae or caveolin. Caveolae thus appear to constitute an important signalling domain that plays a role not only in regulation of smooth muscle tone, but also in proliferation, such as seen in neointima formation and atherosclerosis.

Glycogen debranching enzyme is associated with rat skeletal muscle sarcoplasmic reticulum

AIMS

Gel electrophoresis revealed a band of molecular weight approximately 160 000 Da associated with the skeletal muscle sarcoplasmic reticulum (SR) vesicle preparations. This investigation sought to examine glycogen debranching enzyme associated with skeletal muscle SR.

METHODS

Sarcoplasmic reticulum samples were also taken from muscle whose glycogen content had been reduced either via stimulation of the sciatic nerve or alpha-amylase treatment of muscle homogenates.

RESULTS

The stimulation protocol reduced whole muscle glycogen by 86% (7.4 +/- 0.4 vs. 1.0 +/- 0.3 microg mg(-1) wet mass, P < or = 0.05). Glycogen associated with the SR was reduced by 82% in the stimulation protocol (533 +/- 82 vs. 96 +/- 7 microg mg(-1) protein) and by 94% in alpha-amylase treatment (493 +/- 11 vs. 29 +/- 2 microg mg(-1) protein), respectively. Gel electrophoresis and Western blots revealed that the content of glycogen debranching enzyme was reduced by approximately 53% as a result of muscle stimulation and by approximately 46% in alpha-amylase treatment (P < or = 0.05). In addition, glycogen debranching enzyme activity was reduced by 61% in stimulated samples compared with control (20.3 +/- 1.0 vs. 8.0 +/- 1.2 nmol mg(-1) min(-1), respectively), a value consistent with reductions observed from gel electrophoresis and Western blots.

CONCLUSION

These results confirm that similar to glycogen phosphorylase, glycogen debranching enzyme is associated with the skeletal muscle SR and is dissociated under exercise conditions.

The evolving role of lipid rafts and caveolae in G protein-coupled receptor signaling: implications for molecular pharmacology

The many components of G-protein-coupled receptor (GPCR) signal transduction provide cells with numerous combinations with which to customize their responses to hormones, neurotransmitters, and pharmacologic agonists. GPCRs function as guanine nucleotide exchange factors for heterotrimeric (alpha, beta, gamma) G proteins, thereby promoting exchange of GTP for GDP and, in turn, the activation of 'downstream' signaling components. Recent data indicate that individual cells express mRNA for perhaps over 100 different GPCRs (out of a total of nearly a thousand GPCR genes), several different combinations of G-protein subunits, multiple regulators of G-protein signaling proteins (which function as GTPase activating proteins), and various isoforms of downstream effector molecules. The differential expression of such protein combinations allows for modulation of signals that are customized for a specific cell type, perhaps at different states of maturation or differentiation. In addition, in the linear arrangement of molecular interactions involved in a given GPCR-G-protein-effector pathway, one needs to consider the localization of receptors and post-receptor components in subcellular compartments, microdomains, and molecular complexes, and to understand the movement of proteins between these compartments. Co-localization of signaling components, many of which are expressed at low overall concentrations, allows cells to tailor their responses by arranging, or spatially organizing in unique and kinetically favorable ways, the molecules involved in GPCR signal transduction. This review focuses on the role of lipid rafts and a subpopulation of such rafts, caveolae, as a key spatial compartment enriched in components of GPCR signal transduction. Recent data suggest cell-specific patterns for expression of those components in lipid rafts and caveolae. Such domains likely define functionally important, cell-specific regions of signaling by GPCRs and drugs active at those GPCRs.

Changes in membrane cholesterol affect caveolin-1 localization and ICC-pacing in mouse jejunum

Pacing of mouse is dependent on the spontaneous activity of interstitial cells of Cajal in the myenteric plexus (ICC-MP). These ICC, as well as intestinal smooth muscle, contain small membrane invaginations called caveolae. Caveolae are signaling centers formed by insertions of caveolin proteins in the inner aspect of the plasma membrane. Caveolins bind signaling proteins and thereby negatively modulate their signaling. We disrupted caveolae by treating intestinal segments with methyl beta-clodextrin (CD) to remove cholesterol or with water-soluble cholesterol (WSC) to load cholesterol. Both of these treatments reduced pacing frequencies, and these effects were reversed by the other agent. These treatments also inhibited paced contractions, but complete reversal was not observed. To evaluate the specificity of the effects of CD and WSC, additional studies were made of their effects on responses to carbamoyl choline and to stimulation of cholinergic nerves. Neither of these treatments affected these sets of responses compared with their respective time controls. Immunochemical and ultrastructural studies showed that caveolin 1 was present in smooth muscle membranes and ICC-MP. CD depleted both caveolin 1 and caveolae, whereas WSC increased the amount of caveolin 1 immunoreactivity and altered its distribution but failed to increase the number of caveolae. The effects of each agent were reversed in major part by the other. We conclude that signaling through caveolae may play a role in pacing by ICC but does not affect responses to acetylcholine from nerves or when added exogenously.

Metabolic organization in vascular smooth muscle: distribution and localization of caveolin-1 and phosphofructokinase

We have shown that a compartmentation of glycolysis and gluconeogenesis exists in vascular smooth muscle (VSM) and that an intact plasma membrane is essential for compartmentation. Previously, we observed that disruption of the caveolae inhibited glycolysis but stimulated gluconeogenesis, suggesting a link between caveolae and glycolysis. We hypothesized that glycolytic enzymes specifically localize to caveolae. We used confocal microscopy to determine the localization of caveolin-1 (CAV-1) and phosphofructokinase (PFK) in freshly isolated VSM cells and cultured A7r5 cells. Freshly isolated cells exhibited a peripheral (membrane) localization of CAV-1 with 85.3% overlap with PFK. However, only 59.9% of PFK was localized with CAV-1, indicating a wider distribution of PFK than CAV-1. A7r5 cells exhibited compartmentation of glycolysis and gluconeogenesis and displayed two apparent phenotypes distinguishable by shape (spindle and ovoid shaped). In both phenotypes, CAV-1 fluorescence overlapped with PFK fluorescence (83.1 and 81.5%, respectively). However, the overlap of PFK with CAV-1 was lower in the ovoid-shaped (35.9%) than the spindle-shaped cells (53.7%). There was also a progressive shift in pattern of colocalization from primarily the membrane in spindle-shaped cells (both freshly isolated and cultured cells) to primarily the cytoplasm in ovoid-shaped cells. Overall, cellular colocalization of PFK with CAV-1 was significant in all cell types (0.68 > or = R2 < or = 0.77). Coimmunoprecipitation of PFK with CAV-1 further validated the possible interaction between the proteins. We conclude that a similar distribution of one pool of PFK with CAV-1 contributes to the compartmentation of glycolysis from gluconeogenesis.

Estrogen increases endothelial nitric oxide synthase via estrogen receptors in rat cerebral blood vessels: effect preserved after concurrent treatment with medroxyprogesterone acetate or progesterone

BACKGROUND AND PURPOSE

In vivo and in vitro rat models of hormone therapy were used to test the following hypotheses: (1) estrogen acts directly on cerebrovascular estrogen receptors to increase endothelial nitric oxide synthase (eNOS); (2) increased protein correlates with higher NOS activity; and (3) effects of estrogen on eNOS are altered by concurrent treatment with either medroxyprogesterone acetate (MPA) or progesterone.

METHODS

Blood vessels were isolated from brains of ovariectomized female rats; some were treated for 1 month with estrogen, estrogen and progesterone, or estrogen and MPA. Isolated cerebral vessels were also treated in vitro with estrogen in the absence and presence of progesterone, MPA, tamoxifen, and the estrogen receptor antagonist ICI 182 780. Levels of eNOS were measured by Western blot, and NOS activity was measured by [14C]arginine-[14C]citrulline conversion.

RESULTS

Chronic hormone treatment in vivo resulted in plasma levels of 17beta-estradiol, progesterone, and MPA in the range of values found in humans. Estrogen treatment resulted in higher levels of cerebrovascular NOS activity that paralleled increases in eNOS protein. In vitro estrogen treatment for 18 hours also resulted in a concentration-dependent increase in eNOS protein (EC50 approximately 300 pmol/L) that was completely prevented by estrogen receptor antagonists tamoxifen or ICI 182 780. However, cotreatment with progesterone or MPA, either in vivo or in vitro, did not alter the effect of estrogen on eNOS protein.

CONCLUSIONS

Estrogen receptor activation in cerebrovascular tissue results in increased eNOS activity and protein levels. The latter effect persists in the presence of either progesterone or MPA. Thus, increased NO production by eNOS may contribute to the neuroprotective effects of estrogen.