ization has many effects on birds, and itremains to be seen how noise ranksamong them. —S. MILIUS

Tiny LabsPolymers on silicon chipcatch, release proteins

Using polymers as tiny molecule-absorbingsponges, researchers have taken a steptoward shrinking room-size chemical labo-ratories to the size of a crumb.

Microchips full of tiny channels andmixing chambers may eventually enable

scientists to analyze minute amounts ofany solution quickly and accurately (SN:8/15/98, p. 104). Such so-called laborato-ries-on-a-chip could be use-ful for detecting the firstmolecular signs of disease ina blood sample or the pres-ence of a bioterrorism agentin the environment.

For the technology towork, however, it has tomanipulate molecules to beanalyzed on the chip. Towardthat end, researchers at San-dia National Laboratories inAlbuquerque have developeda new method for gentlygrabbing and releasing pro-teins at particular places ona silicon wafer. This systemcould be used to concentrate biological mol-ecules from dilute solutions for analysis,says Bruce C. Bunker.

He and his Sandia colleagues describetheir research in the July 18 Science.

The investigators started with a siliconwafer on which they’d applied a siliconnitride coating, says team member Dale L.

Huber. After etching a nar-row channel in the silicon,the scientists depositedthin gold lines on top of thenitride layer so that thelines formed bridges overthe silicon nitride–coveredtrench. Because the trenchacts as an insulator, eachgold bridge can be electri-cally heated. Each is essen-tially “a microtoaster,” saysBunker.

Finally, the researchersgrew a dense thicket ofpolymer molecules into athin film over the gold

lines. At 35°C, about body temperature, thepolymer switches from a water-attractingstate to a water-repelling one. When theresearchers introduce a couple of micro-liters of protein-containing solution to the

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SCIENCENEWSThis Week

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Trace amounts of cadmiumcan mimic estrogen’seffects on cells and alter

the reproductive system offemale rats, a new studyshows. The finding may expandthe rap sheet on cadmium—already fingered in lung cancerand kidney damage—to includereproductive disorders and pos-sibly hormone-related malig-nancies such as breast cancer.

Meanwhile, a separatereport reveals that cadmiumalso disrupts DNA repair insidecells, offering an explanationfor its cancer-causing effects incigarette smoke and industrialpollution. Cadmium is a whitemetal used in alloys, batteries,metal coatings, and pigments.

In the August Nature Medi-cine, molecular biologist MaryBeth Martin of GeorgetownUniversity in Washington, D.C.,and her colleagues report thatfemale rats injected with cad-mium chloride grew thickeruterine linings and largermammary glands, effects thatmirror the animals’ normalresponses to an estrogenboost. These changes aroseeven though the rats had hadtheir ovaries removed and so

were making none of theirown estrogen.

The findings validate earliercell-culture studies showingthat cadmium binds to areceptor molecule on cells thatnormally binds estrogen. Whenthe metal does so, it sets off agenetic chain reaction andgrowth processes similar tothose induced by the hormoneitself, Martin says.

The rats also showed evi-dence of extra amounts of twoproteins typically activated byestrogen—the progesteronereceptor protein and C3, animmune system protein.

Revealing another estrogen-mimicking effect, cadmium-exposed female rats with intactovaries bore female pups thatgained weight quicker thanusual and reached puberty ear-lier than normal.

The rat findings indicate thatcadmium can rev up unwantedcell growth by mimickingestrogen, says Martin. Hergroup is planning experimentsto determine whether such anendocrine disruption by cad-mium may be a cause ofbreast cancer.

In the other new study,

researchers found that lowconcentrations of cadmiumcause an extremely high rateof genetic mutation in yeast.But rather than damage DNAdirectly, the metal appears tocause mutations by inhibiting acell’s DNA-repair mechanism.

Normally in any organism,some cells die off naturally andothers multiply to take theirplaces. However, errors com-monly arise in this replicationprocess, which is why DNA hasbuilt-in repair mechanisms.Among them is DNA-mismatchrepair, a mutation-avoidancesystem that suppresses tumorformation, says study coauthorDmitry A. Gordenin, a geneti-cist at the National Institute ofEnvironmental Health Sciencesin Research Triangle Park, N.C.

However, in yeast cellsexposed to cadmium, thisDNA-repair process goes awry.Cadmium exposure pushedthe mutation rate up as muchas 2,000-fold in these cells,Gordenin says.

DNA-mismatch repair isguided by specific genes andthe proteins they encode. Inthe July Nature Genetics, Gor-denin and his colleagues say

they don’t know which pro-teins are affected by cadmiumbut that the evidence of muta-tions is unmistakable.

Cadmium is present in soiland therefore in many foods.In people, it lingers in thebody for decades, tending toaccumulate in the kidneys,liver, lungs, and prostate. Onceinside a person, it bindschemically to certain mole-cules in tissues and so isn’treadily excreted.

Cadmium is one of themajor contaminants oftobacco smoke, says Gordenin.“Smokers definitely accumu-late more cadmium in theirlungs” than nonsmokers do, hesays, and the new researchsuggests one of the mecha-nisms by which smoking leadsto lung cancer.

These studies are “bothextremely important steps in[determining] cadmium’s rolein the human body,” says JohnA. McLachlan of Tulane Uni-versity in New Orleans. Theyadd to a growing body ofknowledge about pollutantsthat mimic hormones andthus have an impact onhealth, he says. —N. SEPPA

Metal’s MayhemCadmium mimics estrogen’s effects, thwarts DNA repair

PROTEIN HOT SPOT Eachdark bar in the white area ofthis microchip is a polymer-covered gold bridge that canbe heated. Doing so canconcentrate protein from asolution applied to the chip.

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