peptide hormones as tumor markers

PEPTIDE HORMONES AS TUMOR MARKERS JUDITH L. VAITUKAITIS, MD”

Ectopic production and secretion of hormones by a wide variety of tumors were initially recognized by signs and symptoms of excess circulating biologically active hormone. With the development of more sophisticated and sensitive techniques, it has become apparent that not all tumors secrete biologically active hormones. Some altered forms of polypeptide hormones may be in very high concentrations immunologically but be inactive biologically. On the other hand, polypeptide hormones may circulate at concentrations too low to induce clinical signs and symptoms. Consequently, new ectopic humoral syndromes have been recognized and the incidence of previously described syndromes has become considerably greater than previously recognized. Every hormone known to be normally secreted by endocrine Grgans or the placenta has been documented to be secreted ectopically by a wide variety of tumors. Moreover, several of those hormones may be used as biochemical markers of malignancy for both screening and monitoring of patients with documented or suspected tumors.

C U W ~ 37:567-572, 1976.

VER THE PAST SEVERAL YEARS, INCREASINGLY 0 more syndromes of ectopic hormone production have been described. Initially, the clinical syndromes were recognized because of the excess secretion of biologically active hormones by the nonendocrine tumors. With development of more sophisticated techniques for detection and analysis, it has become apparent that some varieties of excess circulating biologically active hormones may result in no clinical manifestations, or, on the other hand, an excess of hormones may circulate but be biologically inactive. The hormone may be incorrectly or partially synthesized and, consequently, exert no biologic effect, yet be immunologically active. Moreover, some ectopic hormones circulate in the form of prohor- mones, possessing very little intrinsic biologic activity in that form.

ACTH and ADH have marked effects on water and electrolyte balance when present in supraphysiologic concentrations. Conse- quently, tumors secreting those substances will

Prcscntcd a t thc ACS-NCI National Confcrence on Advanccs i n Cancer Management, Part 11, Dctection and Diagnosis, Denver, CO, May 1-3, 1975.

From thc Thorntlike Memorial Laboratory Section of Endocrinology and Metabolism, Boston City Hos- pital, Division of Mcdicine, Boston University School of Met1 i ci ne.

Ashociatc Professor of Medicinc. Address for reprints: Judith L. Vaitukaitis, MD.

Section of Endocrinology and Mctabolism. Boston City Hospital, 818 Harrison Avenue, Boston, M.4 02118.

result in marked aberrations in salt and water metabolism. In contrast, some tumors may ectopically secrete large quantities of growth hormone or human chorionic gonadotropin, yet there may be no clinical manifestations of the excess circulating hormone.

T h e development of sensitive immunologic techniques has permitted large scale screening of sera for specific hormones from patients with documented tumors of nonendocrine origin. Since every known glycoprotein or polypeptide hormone normally secreted has been documented in these syndromes, criteria have been set up to distinguish between normal physiologic levels of those hormones secreted by normal endocrine tissues and aber- rant secretion by tissues of nonendocrine origin. Several criteria have been established and are enumerated in Table 1. If any criteria listed in Table 1 are met, then the term “ectopic” may be applied to characterize the hormonal secretion by the tumor. For some unknown reasons, some hormones are more likely to be secreted by some tumor types and not others. For instance, ectopic ACTH secretion more commonly occurs with tumors of the lung and ectopic hCG secretion with tumors of the gastrointestinal tract.

Hormonal tumor markers may serve as an aid to early cancer detection, locating of a neoplasm by demonstrating concentration gradients of a given hormone, as well as to detection of recurrent tumors. In some cases,

567

568 CANCER January

TABLE 1. Criteria to Distinguish between Normal and Ectopic Abnormal Hormonal Secretion Levels

1. Greater concentration of hormone in the tumor than in surrounding normal tissue

2. Immunologic or histochemical localization of hormone in tissue

3. Disappearance of hormone after removal of tiiinor 4. Arterial-venous concentration difference across the

tumor bed 5 . Production of hormone bv tumor in vitro

multiple hormones may be ectopically secreted by a tumor. For example, adenocarcinoma oE the stomach has been associated with ectopic calcitonin, parathormone, and free alpha subunit secretion40 and lung tumors with ectopic ACTH, MSH, gastrin, parathormone, and ADH secretion.32

ECTOPIC HORMONAL SYNDROhlES

Ectopic ACTH Almost 50 years ago, Brown described pa-

tients with adrenocortical overactivity associated with tumors of nonendocrine origin.” However, not until recently has it been documented that a variety of tumors synthesize and secrete a form of ACTH very similar to, if not identical with, pituitary ACTH.lgJOJ3J8 In addition, N-terminal and C-terminal fragments of ACTH have been isolated from lung tumor extracts of patients without clinical evidence of Cushing’s syndrome.33 Antisera have been developed against the N- and C- terminals of ACTH.33 Unlike normal ACTH, ectopic hormone has a greater reactivity with the C-terminal directed antibody.33 A large number of lung tumors contain a “big” form of ACTH.1*s5* T h e biologic activity of “big” ACTH is only 1/25 of its immunologic activity, but can be converted to a more biologically active smaller form with trypsin treatment.’*

When a biologically active form of ACTH is secreted by tumors of nonendocrine origin, high plasma levels of cortisol, in excess of 40 pg/IOO ml, result. Plasma ACTH levels in excess of 200 pg/ml in a patient with Cush- ing’s syndrome are strongly suggestive of ectopic tumor secretion of ACTH. Clinically, marked hypokalemic alkalosis may be present with plasma potassium concentrations less than 3 mEq/L. Hypertension, edema, and hyperglycemia are usually present as well. T h e metabolic manifestations of the syndrome may

Supplement 1976 VOl. 37

be corrected by treating the patient with agents that block adrenocortical steroid biosynthesis. For rapid alteration in steroidogenesis, elipten with or without metyrapone may be given: 0, p’-DDD is also effective, but requires several weeks to alter steroidogenesis.

Carcinomas of the lung are among those most commonly associated with ectopic ACTH production. Tumors of the pancreas and thymomas are next most commonly associated. Tumors ectopically secreting ACTH also secrete both normal and abnormal melano- cyte-stimulating hormone (MSH) which may be responsible for the increased pigmentation noted in 10 to 33% of patients with ectopic ACTH syndromes.

Ectopic Syndrome Associated with H y percalcemia

Hypercalcemia is perhaps the most common biochemical aberration associated with a wide variety of cancers. T h e hypercalcemia usually results from bone metastases. However, in some cases, no radiologic evidence of bone metastases is present; moreover, hypercalcemia is accompanied by low or normal concentrations of phosphorus, especially when associated with ectopic tumor production. Ex- tracts of non-endocrine tumors contain ma- terial indistinguishable from parathormone (PTH).2Rs31+39,42 Hypercalcemia results from increased bone resorption, induced by a parathormone-like polypeptide secreted by some tumors.

Some patients with nonparathyroid tumors have associated hypercalcemia and hypophosphatemia, but without excessive circulating levels of parathormone.36 Moreover, the hypercalcemia and hypophosphatemia remitted when specific tumor therapy was Using a variety of antisera to parathormone, no gradient of P T H was demonstrated across the tumor bed and no immunoreactive P T H was detected in the tumor extracts.:36 However, the extracts did contain a substance which exhibited parathormone-like activity in in vi tro bi0assays.3~

With development of radioimmunoassays for parathormone, it has become apparent that several different fragments or even pro- hormone forms of the native polypeptide hormone, containing 84 amino acid residues, circulate. Consequently, exact quantitation of the various forms of parathormone has become more complex than any other polypeptide hormone. It would appear that the N-

No. 1 PEPTIDE HORMONE TUMOR MARKERS - Vaitukai t is 569

terminal fragment of parathormone possesses most of the intrinsic biologic activity of the hormone and that the smaller circulating species of parathormone represent degrada- tion productions of parathormone.

Patients with hypercalcemia may present with anorexia, constipation, polymia, and drowsiness. If the hypercalcemia is sufficiently high and of sufficiently long duration, coma may result. Since the hypercalcemia is a life- threatening situation, therapy should be in- stituted immediately. Initial efforts should be directed to hydrating the patient. Infusion with isotonic saline, increasing delivery of sodium to the proximal tubule, will promote calcium excretion.131’7 Glucocorticoids may be used, especially in patients who cannot tol- erate an excessive fluid load, to lower plasma calcium concentrations. Inorganic phosphates may be used orally or intravenously.2’ How- ever, if renal function is impaired, intravenous inorganic phosphates should be avoided since ectopic calcification may result. In some cases mithramycin, a cytotoxic antibiotic, or calcitonin may be used to correct clinically significant hypercalcemia. Mithramycin appar- ently inhibits bone absorption, however. Thrombocytopenia as well as renal or hepatic damage may limit the theapeutic usefulness of mithramycin. On the other hand, the salmon form of calcitonin (Armour) has recently become available for clinical use but no extensive clinical experience has accrued using that agent for treating forms of hyper- ralcemia associated with ectopic parathormone or parathormone-like substances. The fore- going are short-term therapeutic maneuvers. Effective antitumor therapy is the treatment of choice.

Ectopic ADH and Neurophysin Secretion T h e first description of hyponatremia and

high urinary sodium excretion in a patient with a lung tumor appeared more than 30 years ago.57 However, only within the last 10 years has inappropriate secretion of ADH been documented by bioassay or radioimmunoassay of serum obtained from patients with tumors of the lung or pancreas.’~8~”J In sotne cases tumor tissue extracts have contained concentrations of biologically or immunologically active ADH significantly greater than those of surrounding normal tissues or pituitary.zQ.51 Moreover, in vitro synthesis of vasopressin has been demonstrated in lung tumor slices using radioactive

substrate incorporation.17 T h e “-vasopressin behaved physically on Sephadex G-25 and on radioimmunoassay as arginine vasopressin (AVP) . I7

Immunoreactive AVP behaved as a larger molecular species on gel chromatography, suggesting that the peptide was bound to another protein, possibly neuropliysin.~4 Sig- nificant levels of neurophysin have been found by specific radioimmunoassay of tumor tissue extracts from patients with documented syndromes of inappropriate ADH.24

Patients with tumors ectopically secreting ADH may have a spectrum of clinical symptoms ranging from no symptoms to a comatose state, depending on the level of tumor secretion of ADH. Syndromes of mild water in- toxication, characterized by lethargy, weak- ness, and confusion, may intervene. Water restriction will alleviate the patients’ symptoms. However, more effective therapy should be directed toward the tumor in an effort to reduce its synthesis and secretion of ADH.

Ectopic Human Chorionic Gonadotropin W G )

Unlike most other hormonal tumor markers, hCG is not normally present in peripheral blood except during pregnancy. I ts presence under any other circumstances strongly sug- gests the presence of a tumor. Patients with tumors ectopically secreting gonadotropin were initially recognized by accompanying clinical syndromes attributed to ectopic gonadotropin secretion.5.15926J8 Precocious puberty in boys and gynecomastia in men have been the only syndromes associated with ectopic gonadotropin secretion. However, levels of hCG in excess of levels found in the first trimester of pregnancy have been documented in patients without any clinical manifestations of excess circulating hCG.4*

Human luteinizing hormone (hLH), normally secreted by the pituitary, and hCG are both glycoproteins, possessing extensive struc- tural homology.14 T h e two hormones have indistinguishable biologic and similar immunologic activities. A radioimmunoassay has been developed which selectively measures hCG in serum or plasma samples containing both hCG and hLH.50 Using that assay system, a large number of serum or plasma samples from patients with documented tumors has been assayed for the presence of immunoreactive hCG.Q.1Q-49 Adenocarcinomas of the stomach, ovary, pancreas, and hepatomas are

570 CANCER January Supplement 1976 VOl. 37

the tumors most commonly associated with ectopic hCG secretion.gJ0 2 ~ 4 1 . 4 9 The incidence of ectopic hCG secretion ranged between 17 and 40y0 in those selected tumor types.

Heterogeneity has been observed among the hormonal peptides secreted by tumors.33~48 Human chorionic gonadotropin is composed of two dissimilar, noncovalently linked subunits. In a variety of tumor tissue extracts, samples of serum or urine contained not only hCG, but either or both subunits of hCG.48 A few rare tumors have had only free alpha or free beta subunits pre~ent.4035~

Human chorionic gonadotropin alters lymphocyte function in vitro.1 Supraphysiologic concentrations of hCG inhibit the response of lymphocytes to phytohemagglutinin. Whether or not hCG, encountered during normal pregnancy or in patients with tumors ectopically secreting hCG, exerts any physiologic effect on the immune system is unknown. If hCG were to exert a significant immuno- suppressive effect, then one might expect to see a significant difference in the clinical course of patients with tumors ectopically secreting hCG as compared to patients with comparable tumors not ectopically secreting hCG.

Ectopic Growth Hormone Growth hormone (GH) secretion is one of

the less frequently documented tumor-associated hormones. Under normal physiologic circumstances, growth hormone undergoes wide serum fluctuations. Consequently, it be- comes exceedingly difficult to differentiate between low levels of G H reflecting secretion by tumors as compared to non-specifically elevated levels of GH associated with stress, physical activity, diet, etc.

Steiner et al. documented elevated circulating levels of immunoreactive GH in a patient with a lung tumor; after removing most of the tumor tissue, plasma GH levels decreased from 38 to 3 ng/m1.45 Two other patients with lung tumors have been studied in more detail. Tumor extracts from both have contained concentrations of G H significantly different from that of normal lung tissue, consistent with ectopic growth hormone synthesis and secretion by the tumors studied.12.44 The patients from whom the tissue was obtained had digital clubbing.12>44 It has been suggested that hypertrophic pulmonary osteoarthropathy noted in patients with lung tumors may be related to

ectopic growth hormone secretion by their tumors since alleviation of the syndrome had been noted in patients after the tumor had been resecte~l.'~ Beck and Burger reported that tumor extracts from 7 of 18 patients with bronchogenic tumors contained significant concentrations of immunoreactive GH; three of the seven patients had hypertrophic pulmonary osteoarthropathy.* On the other hand, tumor extracts from five of the eight patients with gastric tumors contained significant immunoreactive GH in that study; no mention was made of hypertrophic pulmonary osteoarthropathy in those patients.

Ectopic Human Chorionic Somatomamrnotropin (hCS)

Up to a few years ago, hCS was named human placental lactogen, hPL. As is true for hCG, hCS is a trophoblastic hormone and should normally be present only during pregnancy. In 1968, Grumbach et al. reported clearly detectable hCS in the serum of a man with gynecomastia and an anaplastic large-cell carcinoma of the lung.?3 That patient's tumor had been previously studied and reported to contain gonadotropin, determined by bioassay.15 In a study of 128 sera of patients with nontrophoblastic tumors, ten contained immunoreactive hCS.53 Of 64 patients with lung tumors in that study, seven had immunoreactive hCS in sera; of the seven, five had gynecomastia and increased estradiol secretory rates. Moreover, four of the five patients with gynecomastia also had presumptive evidence of hCG secretion by their tumors.53 T h e rela- tionship between ectopic hCS or hCG secretion and gynecomastia is not clear, to say the least.

Other Ectopic Hormones Calcitonin, enteroglucagon, prolactin, and

thyroid-stimulating hormone have been ex- tracted from a variety of nonendocrine tumors. Only a few cases, at best, have been described with each hormone. Table 2 sum- marizes associated clinical syndromes and tumor source in the limited number of cases described to date.

Postulated Mechanisms for Ectopic Hormone Synthesis

A variety of hypotheses have been offered to explain nonendocrine tumor tissue synthesis of a variety of polypeptide hormones.

No. 1 PEPTIDE HORMONE TUMOR MARKERS - Vaitukai t is 57 1

TABLE 2. Ectopic Humoral Syndromes .

Tumor most commonly Ectopic hormone Associated syndrome assoriated Reference

Calcitonin None Lung, Stomach 43, 55

Thyroid stimiilating hormone Hyperthyroidism Lunp, Breast 3

Enteroglucagon Malabsorption Kidney 6 , 20 Prolactin Galac torrhea Lung, Kidney 37, 46

No one hypothesis explains all the described syndromes.

Nonspecific trapping of serum hormones by tumors has been postulated,l7 but there has been no substantiation of that mechanism. Random and nonrandom derepression of the genome has been advocated.7slfi.56 However, that specific hormones are more likely to be associated with specific tumor types, e.g., ACTH antl lung tumors, argues against this hypothesis. Moreover, the hormones or por- tions of hormones synthesized are indistinguishable from those normally synthesized and secreted by normal endocrine tissues.

The APUD (amine and precursor uptake

and decarboxylation) cell concept34.3j capnot account for ectopic production of a wide variety of polypeptide hormones. Tumors arising from APUD cells produce serotin and catecholamines as well as ACTH and AVP. Ectopic production of hCG, PTH ancl hCS cannot be readily explained on the basis of this hypothesis.

Finally, cell hybridization has been postulated as a mechanism for ectopic hormone secretion by nonendocrine tumors.52 This hypothesis can account for polypeptide hormone secretion by a wide variety of tumor types, but whether cell hybridization occurs in vivo is unknown.

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peptide hormones as tumor markers

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