Growth of Sinclair Swine Melanoma as a Function of Age, Histopathological Staging, and Gonadal Status1

We have analyzed the association between host hormonal status and growth rate of congenital exophytic melanomas of Sinclair swine. The growth of multiple exophytic lesions during the first year of life of intact males, orchiectomized males, intact females, and ovariectomized females was quantitated using a proliferativiindex which assigned a numerical value to fixed increments of gains or losses in tumor volume. The proliferativiindex from 6 wk (gonadectomy at 6 wk) to 52 wk of age of each treatment group was statistically increased from ()(/'< 0.01) except that of gonadectomized females. The proliferai iviindices from lesions in gonadectomized females were significantly lower than those from intact males, intact females, and gonadectomized males. A total of 93 exophytic tumors from 63 swine were biopsied and histopathologically staged according to the degree of progression or regression and analyzed as a function of animal age. There were no Stage I and only two Stage II lesions at the time biopsies were taken. Twenty-six of 32 (81.2%) Stage III tumors were found in swine of both sexes less than 26 wk of age of which 71.8% were found <10 wk of age, while 20 of 29 (68.9%) were Stage IV, and only 3 of 32 (9%) were Stage V lesions present in this age group (P < 0.001, x2)Only 20.7% of Stage IV tumors were present prior to 6 wk of age. Preliminary results suggest that castration of either sex also altered tumor histopathology. Our data suggest that a reduction in gonadal steroid secretion was associated with a decrease in exophytic tumor growth rate and regression in animals of both sexes during the first year of life in Sinclair swine. The effect in female swine was due to significant reduction in the proliferativiindex over the first 6 mo of age. Five histopathologically distinct stages (I to V) of tumor development and regression have been described (5). Stages I to III are associated with progressive lesions and have previ ously been reported to be present during the first 6 mo of life. Latter stage tumors (IV and V) are initially associated with an invasion of pigment-laden macrophages and a decline of tumor cells (Stage IV) followed by an increase in keratinocytes and fibroblasts which is often associated with depigmentation (Stage V). The regression in Sinclair swine melanoma which has been reported to be accompanied by significant decreases in tumor volume apparently occurs in close proximity to puberty (5), although the hormonal events which characterize puberty in these animals are just beginning to be described.5 Observations made on the Sinclair swine herd maintained at Texas A&M University indicate that, if metastasis occurs, it does so just prior to or immediately after weaning at 6 wk of age. These significant changes in tumor dynamics, potentially coincident with major changes in the hormonal status of the developing animal, led us to consider the hypothesis that gonadal steroid hormones may influence the natural history of SSCM. Results from the present series of experiments suggest an association between swine age, histopathological stage, and gonadal steroids in the natural history of exophytic malignant melanoma of Sinclair swine. INTRODUCTION SSCM3 presents a unique model for investigating melanoma growth, metastasis, and the phenomenon of spontaneous regression. Initial reports describing the biology of these tumors suggested an overall tumor incidence of 20 to 85% (1-3) with no sex or site preference (4). A significant number of swine present with multiple, congenital, primary lesions which exhibit significantly different growth rates (4). At various stages of postpartum development, these lesions are histopathologically similar to human junctional and compound nevi and malignant cutaneous melanoma (S, 6). A significant number of swine exhibit metastasis to regional lymph nodes, liver, and lungs (Footnote 4; Ref. 6). Approximately 35% of all primary lesions have been reported to regress following puberty, which can be accompanied by the appearance of a halo nevus and a general ized vitÃ-ligo(5, 6). Received 4/20/87; revised 8/17/87, 12/2/87; accepted 12/15/87. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 'Supported in part by NIH-NCI CA 33764, CA 31046, and the Texas Agricultural Experiment Station. 2To whom requests for reprint should be addressed, at Department of Veter inary Physiology and Pharmacology, Texas A&M University, College Station, TX 77843-4466. 3The abbreviations used are; SSCM, Sinclair swine cutaneous malignant melanoma; PI, proliferative index; l'I .M. pigment-laden macrophages; IM, intact males; IF, intact females; GDXM, gonadectomized males; GDXF, gonadecto mized females; A2, regression coefficient; SMS-TAMU, Sinclair melanoma swineTexas A&M University herd. ' Das Gupta et al. Comparative histopathology of primar) cutaneous mela noma of Sinclair swine, submitted for publication. MATERIALS AND METHODS Black male and female Sinclair swine with a history of exophytic melanomas were mated. All animals were maintained on 14% hog chow (Producers Coop, Bryan, TX) and water ad libitum. Standard vaccina tions and routine veterinary health care were provided. At 6 wk of age (weaning), piglets of both sexes were either bilaterally gonadectomized or sham gonadectomized and placed in groups of ten animals each. In these miniature swine, there was no difference in growth rate between intact and gonadectomized animals. Tumor Volume. Immediately after farrowing, all piglets were checked for the presence of exophytic lesions and width, length, and depth measured on individual tumors. All tumors of each animal placed in a treatment group were measured at 2-wk intervals for 52 wk, and tumor volumes were calculated (7). The wide variation in tumor volumes within and between animals, irrespective of sex, which has confronted tumor biologists working with Sinclair swine, was solved by assigning a numerical value to fixed increments of gains or losses in individual tumor volumes. This value was termed the PI (Table 1). Tumor volume obtained at 6 wk of age (weaning or gonadectomy when appropriate) was used as the initial or base-line value. The subsequent volume of individual tumors measured at biweekly intervals was expressed as a percentage of the 6-wk (initial) volume and an appropriate PI assigned. Biweekly Pis were then analyzed as a function of swine chronological age, and regression coefficients were calculated for each group within specific time frames over the first year of life. Histopathological Staging. Histopathological staging as a function of age was achieved by analyzing biopsy specimens of additional tumors that were not incorporated into growth studies. These tumors were ' Amoss fi al. Plasma levels of gonadal steroid hormones during development of Sinclair miniature swine, submitted for publication.