Biological responses to electromagnetic fields

Electrification in developed countries has progressively increased the mean level of extremely low-frequency electromagnetic fields (ELF-EMFs) to which populations are exposed; these humanmade fields are substantially above the naturally occurring ambient electric and magnetic fields of approximately 10(-4) Vm(-1) and approximately 10(-13) T, respectively. Several epidemiological studies have concluded that ELF-EMFs may be linked to an increased risk of cancer, particularly childhood leukemia. These observations have been reinforced by cellular studies reporting EMF-induced effects on biological systems, most notably on the activity of components of the pathways that regulate cell proliferation. However, the limited number of attempts to directly replicate these experimental findings have been almost uniformly unsuccessful, and no EMF-induced biological response has yet been replicated in independent laboratories. Many of the most well-defined effects have come from gene expression studies; several attempts have been made recently to repeat these key findings. This review analyses these studies and summarizes other reports of major cellular responses to EMFs and the published attempts at replication. The opening sections discuss quantitative aspects of exposure to EMFs and the incidence of cancers that have been correlated with such fields. The concluding section considers the problems that confront research in this area and suggests feasible strategies.

Loss of a consensus heparin binding site by alternative splicing of latent transforming growth factor-beta binding protein-1

Latent transforming growth factor-beta binding protein-1 (LTBP-1), plays an important role in controlling localisation and activation of transforming growth factor-beta (TGF-beta). We show that alternative splicing generates a form of mRNA which lacks bases 1277-1435 (termed LTBP-1delta53). The 53 amino acids encoded by these bases include the eighth cysteine of the first cysteine repeat and a consensus heparin binding sequence. Sequencing of genomic clones showed that alternative splicing resulted from the use of an intra-exonic 3' splice acceptor site. The loss of the heparin binding site implies that LTBP-1delta53 will bind to the extracellular matrix less efficiently than LTBP-1.

Transforming growth factor beta is sequestered into an inactive pool by lipoproteins

Elevated plasma concentrations of low density lipoprotein (LDL) and very-low density lipoprotein (VLDL) have been correlated with the development of atherosclerosis. These lipoproteins may promote atherogenesis by direct deposition of lipid in the vessel wall. In addition, previous data suggested that there was an inverse correlation between serum LDL-cholesterol concentration and the proportion of transforming growth factor beta (TGF-beta) in an active form (Grainger et al. 1995. Nature Med. 1:74). Here we have investigated whether lipoproteins can affect the activity of TGF-beta1 in plasma and show that TGF-beta can associate with the lipoprotein fraction. In the plasma of healthy males, 16 +/- 5% (mean +/- standard deviation; n = 57) of the total plasma TGF-beta1 was associated with the lipoprotein fraction, with the major proportion (64 +/- 15%) in the HDL-3 subfraction. However, in ten diabetic subjects with moderately poor glucose control (Hb alc > 8.0), the proportion of total plasma TGF-beta in the lipoprotein fraction was 68 +/- 21%. This large increase in TGF-beta1 associated with the lipoprotein fraction was mainly due to association with VLDL, chylomicrons, and LDL. The lipoprotein fraction inhibits TGF-beta1 binding to the type II TGF-beta receptor extracellular domain in an ELISA and inhibits TGF-beta1 activity in the mink lung cell bioassay. We propose that sequestration of TGF-beta into lipoproteins represents a novel mechanism by which TGF-beta activity in circulation may be regulated. Lipoprotein sequestration of TGF-beta may therefore contribute to the severe depression of TGF-beta activity in advanced atherosclerosis.

Tamoxifen decreases cholesterol sevenfold and abolishes lipid lesion development in apolipoprotein E knockout mice

BACKGROUND

Apolipoprotein E (apo E) knockout mice develop severe vascular lipid lesions resembling human atherosclerotic plaques, irrespective of the fat content of their diet.

METHODS AND RESULTS

Oral tamoxifen (TMX) at a dose of 1.9 mg.kg body wt-1.d-1 abolished lipid lesion development, assayed by oil red O staining, whether the mice were fed a normal diet or a diet with high fat content. The TMX-treated mice showed a sevenfold decrease in total cholesterol. However, the proportion of plasma cholesterol present in VLDL remained unchanged, whereas the proportion in LDL decreased by 37%, and that in HDL increased by 64%. Consistent with the shift from LDL to HDL cholesterol, there was a 62% decrease in total triglycerides. The concentrations of active and acid-activatable latent plus active TGF-beta in the aorta were substantially elevated by TMX (87% and 24% increase, respectively).

CONCLUSIONS

Although the mechanism of cardiovascular protection by TMX in apo E knockout mice is unknown, the inhibition of lipid lesion formation may be attributable to the changes in lipoprotein profile and the elevated levels of TGF-beta, both of which are thought to be protective against atherosclerosis in humans and animal models.

Feedback mechanism of focal vascular lesion formation in transgenic apolipoprotein(a) mice

Apolipoprotein(a) (apo(a)), the distinguishing protein of atherogenic lipoprotein(a), directs accumulation of the lipoprotein(a) particle to sites in the arterial wall where atherosclerotic lipid lesions develop in man and in transgenic mice expressing human apo(a). It has been proposed that focal apo(a) accumulation in the transgenic mouse vessel wall causes the observed severe local inhibition of transforming growth factor-beta (TGF-beta) activity and the consequent activation of the smooth muscle cells, which subsequently accumulate lipid to form lesions if the mice are fed a high fat diet. We show that blocking formation of these vascular lesions by two independent mechanisms, tamoxifen treatment and increasing high density lipoprotein, also abolishes apo(a) accumulation, inhibition of TGF-beta activity, and activation of smooth muscle cells. The data are consistent with a feedback mechanism in which an initial accumulation of apo(a) inhibits local TGF-beta activity, leading to further accumulation of apo(a). Breaking the feedback loop prevents smooth muscle cell activation and therefore lipid lesion development.

Carbonic anhydrase and cardiac pH regulation

Membrane-bound carbonic anhydrase (CA) has recently been identified in mammalian cardiac tissue. In this study, we have investigated the histochemical location and functional role of CA in the ferret heart. Heart sections stained by a modified Hansson's technique showed CA to be located on capillary endothelial membranes as well as on sarcolemmal membranes. In the Langendorff-perfused heart, washout of CO2 brought about by switching perfusion between 25 mM HCO3(-)-5% CO2-buffered solution and nominally HCO3(-)-CO2-free solution caused a transient rise in intracellular pH (pHi) measured by the chemical shift of 2-deoxy-D-glucose 6-phosphate with 31P nuclear magnetic resonance spectroscopy. The initial rate of change of pHi, measured over the first 60-75 s of CO2 efflux, was significantly reduced from 0.41 +/- 0.03 pH units/min (n = 9) in control hearts to 0.28 +/- 0.02 pH units/min (n = 5) in the presence of the membrane-permeable CA inhibitor 6-ethoxzolamide (P < 0.05 compared with control) and to 0.22 +/- 0.04 pH units/min (n = 5) in the presence of the membrane-impermeable CA inhibitor CL-11,366 (P < 0.01 compared with control). After reperfusion of the ischemic myocardium, both CA inhibitors caused a significant slowing of initial rate of change in pH (and initial rate of recovery of contractile function) compared with control hearts. These results suggest that CA, by facilitating the hydration-dehydration of CO2-H2CO3, alters the relative concentrations of CO2 inside and outside the cells, thus enhancing the rate of CO2 transfer from the intracellular to extracellular compartments, which contributes significantly to pHi recovery after reperfusion of the ischemic myocardium.

Optimization of immunofluorescence methods by quantitative image analysis

There is a growing trend towards the objective quantification of immunohistochemical staining. However, quantification has not been used previously to optimize the original published immunohistochemical methods. We present a quantitative method for analyzing immunofluorescence staining employing the Applied Imaging MAGISCAN image analysis system, which has then been used to optimize major aspects of the standard immunofluorescent staining protocols. The optimization process resulted in a method that increased specific staining up to fivefold over typical published protocols, with no increase in nonspecific staining. The method is extremely reproducible. For slides stained by a single experimenter in one batch on one day, the coefficient of variation between replicate means is 1.2%. The image analysis protocol gave a linear response with increasing antigen concentration, as determined by using purified antigen dried onto slides. The revisions to the standard protocol presented here can also be applied to nonquantitative staining. It will help users of immunofluorescence to maximize their staining and may enable the detection of previously undetected antigens.

Angiotensin II stimulates sodium-dependent proton extrusion in perfused ferret heart

The Na+/H+ antiport and Na(+)-HCO3- coinflux carrier contribute to recovery from intracellular acidosis in cardiac tissue. The effects of angiotensin II (10(-12)-10(-6) M) on H+ fluxes after intracellular acid loading and during reperfusion after myocardial ischemia have been investigated in the isovolumic, Langendorff-perfused ferret heart. Intracellular pH (pHi) was estimated using 31P nuclear magnetic resonance (NMR) spectroscopy from the chemical shift of intracellular deoxyglucose-6-phosphate or inorganic phosphate. Angiotensin II produced concentration-dependent stimulation (maximum at 10(-6) M: 67%) of 5-(N-ethyl-N-isopropyl)amiloride (EIPA)-sensitive Na(+)-dependent of H+ efflux consistent with stimulation of the Na+/H+ antiport. Half-maximal stimulation of H+ efflux occurred at approximately 10(-9) M, which is close to the dissociation constant of the cardiac angiotensin AT1 receptor. Stimulation via this receptor was confirmed with the nonpeptide AT1 receptor blocker, GR-117289. Angiotensin II had less pronounced effects on HCO3(-)-dependent pHi recovery after acid loading with no effect on pHi recovery after intracellular alkalosis. During reperfusion, angiotensin II significantly increased H+ extrusion but impaired contractile recovery. The results support the hypothesis that angiotensin II facilitates H+ extrusion in the heart. This may help maintain physiological homeostasis, but the hypothesized obligated Na+ influx could exacerbate cellular dysfunction during reperfusion.

Cultures of proliferating vascular smooth muscle cells from adult human aorta

Abnormal proliferation of human vascular smooth muscle cells (hVSMCs) is a central event in the development of atherosclerosis (1-3) As a result, there is considerable interest in the establishment of hVSMC cultures as a mode1 of this disease process However, it has been noted in the past (4,5) that hVSMCs, especially when cultured by the enzyme-dispersal technique (hVSMC,(ED)), grow poorly in culture compared to VSMCs from other species (e.g., rat) This has limited their use for cell-culture studies. We have recently reported that the reduced proliferative capacity of hVSMC(ED) from adult aorta can be attributed to the endogenous production of active TGF-β(6,7). We (6-8) and others (9-11) have shown that TGF-β; is a potent inhibtior of smooth muscle cell proliferation. Moreover, recent studies in animal models of atherosclerosis (12) have suggested that TGF-P plays a pivotal role in regulation of vessel wall architecture (13). We have also shown that hVSMCs derived by the alternative method of explanting (hVSMC(EX)) have a greater proliferative capacity than the hVSMC(ED) (14) In accordance with our hypotheses, the cells grown from explanted tissue did not produce TGF-β; (14).

No effect of 60 Hz electromagnetic fields on MYC or beta-actin expression in human leukemic cells

Epidemiological studies have shown weak correlations between exposure to extremely low-frequency electromagnetic fields (ELF EMFs) and the incidence of several cancers, particularly childhood leukemias, although negative studies have also been reported. These observations have prompted a broad range of in vitro cellular studies in which effects of ELF EMFs have been observed. However, no reported response has been replicated widely in independent laboratories. One potentially important response is the rapid activation of proto-oncogenes and other genes in human leukemic (HL60) cells and a wide variety of other eukaryotic cells, because of the role of these genes in cell proliferation. We describe quantitative Northern analysis of MYC and beta-actin mRNAs from HL60 cells exposed to fields under conditions very similar to those reported previously to activate these genes, namely 60 Hz sinusoidal magnetic fields of 0.57, 5.7 or 57 microT for 20 min. In addition we have used a new design of field-exposure system and introduced a number of other modifications to the protocol to optimize any response. We have also developed a novel method providing enhanced accuracy for the quantitative measurement of mRNA. No significant effect of ELF EMFs on gene expression was observed using any of these systems and analytical methods.

Tamoxifen elevates transforming growth factor-beta and suppresses diet-induced formation of lipid lesions in mouse aorta

When C57B16 male mice are fed a high-fat diet, they develop significant fatty streak lesions in the aorta. Addition of tamoxifen (TMX) to a high-fat diet, equivalent to a dose of approximately 1 mg TMX per kg body weight per day, suppressed the diet-induced increase in the area of lipid staining in the aortic sinus of the mice by 88% and in the average number of lesions by 86%. The TMX-treated mice had 11% +/- 5% less total plasma cholesterol, with most of the reduction in the high density lipoprotein fraction, whereas plasma triglycerides were significantly elevated, and circulating concentrations of 17 beta-estradiol and testosterone were unaffected. Both circulating and aortic concentrations of active and latent transforming growth factor-beta (TGF-beta) were substantially elevated by TMX. The inhibition of lesion formation may be due, at least in part, to cardiovascular protection by TGF-beta.

Cloning and analysis of the promoter region of the rat SM22 alpha gene

We have cloned and sequenced a 1.9 kb fragment of the 5'-upstream sequence of the smooth-muscle-specific gene SM22 alpha. The region cloned consisted of the SM22 alpha promoter, a 65 bp exon containing most of the 5'-untranslated region and 307 bp of the first intron. A 1.5 kb fragment at the 5' end of this sequence was able to drive the expression of a reporter chloramphenicol acetyltransferase (CAT) gene in both vascular smooth-muscle cells and Rat-1 fibroblasts. This promoter region did not contain a consensus TATAA box but contained the sequence TTTAAA 25 bp from the major start site identified by primer extension. Deletion analysis showed that a fragment of the promoter from +65 to -303 was more active in both cell types than the 1.5 kb fragment suggesting that there are silencer sequences in the region 5' to the core promoter. CAT activity was also observed with fragments containing bases +65 to -193 and +65 to -117 in smooth-muscle cells. In contrast with the smooth-muscle cells, no CAT activity was detected in Rat-1 fibroblasts with the smallest two fragments. The residual promoter activity in the smallest fragment of the SM22 alpha promoter tested suggested that, unlike the smooth-muscle alpha-actin promoter, transcription from the SM22 alpha promoter can occur in smooth-muscle cells in the absence of factors binding to CC(A/Trich)6GG (CArG box) or CANNTG (E box) motifs.

Release and activation of platelet latent TGF-beta in blood clots during dissolution with plasmin

Transforming growth factor beta 1 (TGF-beta 1) is a platelet-derived cytokine involved in both normal wound healing and scarring. We show that human platelets contain two pools of latent TGF-beta 1, which constitute more than 95% of the total TGF-beta assayed in whole platelets. During clotting, one pool, the large latent TGF-beta complex consisting of latent TGF-beta binding protein (LTBP), the latency-associated peptide (LAP) and the 25-kD mature TGF-beta 1 dimer is released into the serum. A second pool, which contains LAP but not LTBP, is retained in the clot, but can be released by RGD peptide. When the clot is dissolved by plasmin this bound TGF-beta 1 is gradually activated and released. If similar mechanisms operate in vivo, the clot will act as a slow-release capsule of TGF-beta 1 activity during wound healing.

ID--a dominant negative regulator of skeletal muscle differentiation--is not involved in maturation or differentiation of vascular smooth muscle cells

Rat aortic vascular smooth muscle cells (VSMCs) expressing the MyoD gene formed myotubes at a low frequency in the presence of serum but at a high frequency in the absence of serum. Expression of an antisense Id gene increased myotube formation in the presence of serum indicating that a reduction in Id levels is a major mechanism by which serum withdrawal promotes myotube formation. The role of Id in the development of VSMCs was investigated by expressing an antisense Id gene in neonatal VSMCs. No evidence was found for the conversion of neonatal VSMCs to adult VSMCs in the presence of the antisense Id gene. Similarly reduction in Id by serum withdrawal also failed to cause conversion of the neonatal VSMCs to the adult phenotype. These data suggest that the maturation of neonatal smooth muscle cells is not controlled by a VSMC homologue of the skeletal muscle basic-helix-loop-helix proteins.

Proliferation of human aortic vascular smooth muscle cells in culture is modulated by active TGF beta

OBJECTIVE

Human vascular smooth muscle cells (VSMC) prepared by enzyme dispersal have been reported to proliferate more slowly and remain more differentiated than VSMC derived by explant culture. The aim of this study was to investigate whether these differences are attributable to the production of transforming growth factor beta (TGF beta), a major modulator of VSMC properties in culture, by enzyme dispersal, but not explant, cultures.

METHODS

Human aortic VSMC cultures were prepared by enzymatic dissociation of aortic media or by outgrowth of cells from explants derived from the vessel media. Properties, including rate of proliferation and differentiation, of the two different types of culture obtained from 10 donors (age 3-54 years, of either sex) were compared. The role of TGF beta as mediator of these differences was investigated.

RESULTS

VSMC from enzyme dispersal cultures proliferated with a long doubling time in 20% fetal calf serum of 68(SEM 2) h, reaching a low saturation density with no "hills and valleys", and the cells were predominantly of stellate morphology. VSMC from explant cultures proliferated with a shorter doubling time of 35(2) h, reached a higher saturation density with "hills and valleys", and showed a spindle shaped morphology. The enzyme dispersed cells, but not the explant cells, released TGF beta into the medium. Addition of exogenous TGF beta to the explant cells extended the doubling time, while addition of a neutralising antibody to the enzyme dispersed cells decreased the doubling time. Tamoxifen, an anti-oestrogenic drug, decreased the rate of proliferation of the explant cells (ED50 = 50 nM; n = 3) but not enzyme dispersed cells by stimulating production of TGF beta. The explant and enzyme dispersal cultures also differed in response to growth factors. The explant cultures stimulated DNA synthesis in response to platelet derived growth factor (PDGF) AA and BB. Enzyme dispersal cultures stimulated DNA synthesis in response to PDGF-BB but response to AA was weaker and also variable.

CONCLUSIONS

Human explant derived VSMC systematically differ from enzyme dispersed VSMC obtained from the same donor tissue. Wherever the potential role of TGF beta was investigated, the differences between enzyme dispersal and explant cultures were due to autocrine production of TGF beta by the enzyme dispersal, but not explant cultures.

Transforming growth factor-beta: the key to understanding lipoprotein(a)?

Most large studies of the blood parameters that act as risk factors for myocardial infarction, stroke and atherosclerosis have identified elevated circulating levels of lipoprotein(a) as an important risk factor. Lipoprotein(a) consists of an LDL particle that is covalently bound to the distinguishing protein component apolipoprotein(a). Ever since apolipoprotein(a) was cloned in 1987 and the marked sequence homology to plasminogen was noted, mechanisms for the atherogenic activity of lipoprotein(a), based on the competitive inhibition of plasminogen activity, have been proposed. However, with the availability of transgenic mice expressing both human apolipoprotein(a) and lipoprotein(a), recent studies have demonstrated that lipoprotein(a) acts to inhibit plasminogen activation in vivo. One consequence of this is reduced activation of the cytokine transforming growth factor-beta, an important regulator of vessel wall structure.

Active and acid-activatable TGF-beta in human sera, platelets and plasma

Assays which measure active and latent forms of transforming growth factor beta (TGF-beta) separately in human serum and plasma are required to investigate the biological role of TGF-beta in a variety of human diseases. We have developed an enzyme-linked immunosorbent assay (ELISA) using two polyclonal antibodies against TGF-beta which rapidly determines the amount of active plus acid-activatable, latent TGF-beta forms ((a+l)TGF-beta) present in human serum and plasma in the range 4 pmol/l to 2000 pmol/l. To measure active TGF-beta alone, we have developed a second ELISA using the extracellular domain of the TGF-beta type II receptor as the capture reagent which detects active TGF-beta in serum and plasma samples in the range 20 pmol/l to 4000 pmol/l. Both assays detect TGF-beta 1 and TGF-beta 3 with similar sensitivity, are > 10-fold less sensitive to TGF-beta 2 and are not affected by a range of other peptide growth factors. The mean (a+l)TGF-beta present in human serum was 330 pmol/l but the range was very large (< 4 pmol/l to 1400 pmol/l). The mean active TGF-beta present was 230 pmol/l (range < 20 pmol/l to 1400 pmol/l) and the proportion of the (a+l)TGF-beta present which was active [a/(a+l)] varied from < 10% to 100%. The concentration of (a+l)TGF-beta and the proportion of TGF-beta which was active were very similar in the serum and platelet-poor plasma prepared from the same whole blood sample. The clot formed during serum preparation retained all of the TGF-beta which was detected by the (a+l)TGF-beta ELISA in the corresponding platelet releasate, although the PDGF in platelets was released into the serum. In contrast, platelet-poor plasma contained no detectable PDGF demonstrating that the (a+l)TGF-beta assayed in the plasma was not due to platelet degranulation after bleeding. Serum active TGF-beta and (a+l)TGF-beta concentrations therefore provide a reliable estimate of these forms of TGF-beta present in plasma.

The serum concentration of active transforming growth factor-beta is severely depressed in advanced atherosclerosis

Recent evidence has led us to propose that transforming growth factor-beta (TGF-beta) is a key inhibitor of atherosclerosis. We show here that a population of patients with advanced atherosclerosis all have less active TGF-beta in their sera than patients with normal coronary arteries, with a fivefold difference in average concentration between the two groups. This correlation with atherosclerosis is much stronger than for other known major risk factors and it may therefore have important diagnostic and prognostic significance. Aspirin medication correlates with an increase in active TGF-beta concentration, indicating that therapeutic interventions for TGF-beta are possible.

Activation of transforming growth factor-beta is inhibited in transgenic apolipoprotein(a) mice

A HIGH concentration of serum lipoprotein(a) is a risk factor for atherosclerosis. Lipoprotein(a) consists of low-density lipoprotein with the additional protein component, apolipoprotein(a), a homologue of plasminogen. Lipoprotein(a) and apolipoprotein(a) enhance proliferation of human vascular smooth muscle cells (hVSMCs) in culture by inhibiting activation of plasminogen to plasmin, thus blocking the proteolytic activation of transforming growth factor-beta (TGF-beta), an autocrine inhibitor of hVSMC proliferation. The hypothesis that this pathway is a key step in atherogenesis is tested on transgenic mice expressing the human apolipoprotein(a) gene. We show here that the activation of TGF-beta is inhibited in the aortic wall and serum of mice expressing apolipoprotein(a), as a consequence of apolipoprotein(a) inhibition of plasminogen activation. These effects are closely correlated with VSMC activation.