An ectopic pregnancy is bad news _ for mother and child alike.
When a fertilized ovum starts growing in an anatomical site outsidethe uterus, as in the cervix or fallopian tubes, even the ovary itself,the off-base embryo almost always dies. All too often, so does thechild-bearing mother.
About one pregnancy in 300 is ectopic. But ectopia also affects manyother cells in the human body, especially hormones.
Besides the specialized glands that make them, notably the brain'spituitary and suprarenals (above the kidneys), many malignant tumorschurn out hormones. For example, insulinomas, tumors of thepancreatic islets of Langerhans, secrete insulin in pathogenicquantities.
Conversely, autoimmune lymphocytes (T cells) programmed todestroy the insulin molecule cause Type 1, insulin-dependent,diabetes mellitus. (See BioWorld Today, April 15, 1996, p. 1.)
At the Joslin Diabetes Center, in Boston, physician and researchimmunologist Myra Lipes wondered whether those crazed T cellswith a grudge against insulin would be as deadly if that hormonewere made by some other cell instead of the islets.
Logically, she turned to the pituitary gland, a wholesale pharmacy ofhormone production. This small but complex cluster of tissues belowthe base of the brain turns out, on demand, thyrotropin,adrenocorticotropin, gonadotropins and growth hormone, to namejust a few, as well as prolactin, oxytocin and vasopressin, amongothers.
A large molecule with a long name, proopiomelanocortin (muchbetter known as POMC), serves as a precursor to theadrenocorticotropin hormone (ACTH), as well as to its breakdownpeptides.
Lipes is lead author of a paper in the current Proceedings of theNational Academy of Sciences (PNAS), dated Aug. 6, 1996, titled:"Insulin-secreting non-islet cells are resistant to autoimmunedestruction." Moreover, that pituitary source of ectopic insulinrescued transgenic diabetic mice from the severe hyperglycemia oftheir disease.
"The pituitary cells to which we targeted insulin," Lipes toldBioWorld Today, "are professional secretory cells, just like the islets'beta cells, which make insulin. Originally," she continued, "we justwanted to see if pituitary-made insulin would be targeted by theautoimmune cascade that destroys the hormone in Type 1, insulin-dependent human diabetes."
That disease in young people has a close mimic in the NOD mouse.NOD stands for "non-obese diabetic," which sets this breed of rodentapart from the OB/OB obese mouse model.
"The beauty of this system," Lipes pointed out, "is that NOD micedevelop spontaneous autoimmunity, at their adolescence or earlyadulthood, around three months of age. Just like humans, theydevelop polyuria (frequent urination), severe thirst, weight loss."
And just like humans, these mice go through a lengthy prodromalphase, in which at four weeks of age the autoimmunity starts, undercover, unseen, to ravage their insulin-secreting cells.
Switching From Pancreas To Pituitary
To endow NOD mice with a pituitary source of insulin, Lipes' teamharnessed the POMC promoter to the structural region of a mousepreproinsulin gene, which expresses the hormone's precursor peptidechain. They then microinjected this construct directly into thepronuclei of one-cell NOD embryos.
These transgenic mice were born with pituitary glands that secreted"an abundant 550-base-pair insulin transcript in the pituitary,identical in size to the endogenous pancreatic insulin transcript."
That was the up-side. But that ectopic cellular environment had aserious down-side too. "Because pituitary cells lack key elements ofthe glucose-sensing apparatus . . . insulin secreted from the pituitarywould not be expected to respond to ambient glucose levels," whichis insulin's main job.
Nonetheless, when the NOD transgenics underwent a 48-hour stretchof experimental starvation, their serum insulin levels "were markedlyelevated," demonstrating that "the ectopically produced insulin wassecreted into the circulation."
So too were breakdown peptides of ACTH, for which POMC is alsoa promoter. "The two proteins are synthesized independently of eachother," Lipes said. "And these ACTH breakdown productsaccompanying the insulin don't have any adverse metabolic effects,which ACTH would have."
To confirm that their ectopic insulin is proof against autoimmuneattack, the team transplanted transgenic pituitaries under one kidneycapsule of non-transgenic NOD mice with florid diabetes. Under theopposite-side capsule they placed control islets.
Two weeks later, all the beta cells had been wiped out, but theectopic-secreting cells remained intact.
Finally, to show therapeutic effect, the Joslin workers transplantedfour transgenic pituitaries under the kidney capsule of NOD mice,which "resulted in a significant gain in body weight and in thecomplete remission from diabetic symptoms [and] progressive returnto near normoglycemia."
Diabetic control animals, which received non-transgenic pituitaries,were dead of their disease within three weeks.
Tackling Glucose-Sensing Limitation
Still, that ectopic insulin, pouring non-stop into the circulation fromthe pituitary, doesn't perform the constant glucose-monitoring andregulating task assigned to insulin. "It's a limitation," Lipes observed,"so obviously we now want to engineer glucose-sensing capabilitiesinto those cells," she added, "and we've already done quite a bit ofwork in that direction.
"What we're trying to do is reconstitute piece by piece these cells thatconfer glucose sensing. It's a non-trivial challenge."
As for "possible futuristic application to humans," she concluded:""It's a very appealing strategy to prevent recurrence ofautoimmunity without using immunosuppressive drugs, as in islettransplantation, by targeting insulin expression to non-islet cells."
In her PNAS paper, Myra Lipes cites the work of biochemistChristopher Newgard on glucose-stimulated insulin secretion inpituitary cells
Commenting on her report, Newgard told BioWorld Today: "I haveconcerns about the metabolic control data, but I believe the veryexciting and intriguing thing that Myra has uncovered is thisapparently preferential ability of these cells to survive in what wouldotherwise be a hostile environment for transplanted islet cells."
Newgard regards the "growing interest in genetically engineered cellsas a vehicle for insulin replacement, or indeed, any hormonereplacement," as "an increasingly interesting and perhaps tenableconcept." While noting that several laboratories besides the Joslin'sand his own are working in this area, he observed, "It's by no meanstime to abandon primary-cell transplantation."
A professor of internal medicine at the University of TexasSouthwestern Medical Center, at Dallas, Newgard is foundingscientist and principal consultant of four-year-old BetaGene Inc., inDallas. "Our primary focus," he said, "is based on the use ofengineered cells for insulin replacement in diabetes," but added, "Asour technology base grows, one can begin to think about deliveringother products as well." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.