Biphasic effect of a primary tumor on the growth of secondary tumor implants

Abstract

Background: The phenomenon of hormesis is characterized by a biphasic dose-response, exhibiting opposite effects in the low- and high-dose zones. In this study, we explored the possibility that the hormesis concept may describe the interactions between two tumors implanted in a single mouse, such that the resulting tumors are of different sizes.

Materials and methods: We used two murine tumors of spontaneous origin and undetectable immunogenicity growing in BALB/c mice. A measure of cell proliferation was obtained by immunostaining for Ki-67 protein and by using the [(3)H] thymidine uptake assay. For serum fractionation, we utilized dialysis and chromatography on Sephadex G-15.

Results: The larger primary tumor induced inhibitory or stimulatory effects on the growth of the smaller secondary one, depending on the ratio between the mass of the larger tumor relative to that of the smaller one, with high ratios rendering inhibition and low ratios inducing stimulation of the secondary tumor.

Conclusion: Since metastases can be considered as natural secondary tumor implants in a tumor-bearing host and that they constitute the main problem in cancer pathology, the use of the concept of hormesis to describe those biphasic effects might have significant clinical implications. In effect, if the tumor-bearing host were placed in the inhibitory window, tumor extirpation could enhance the growth of distant metastases and, reciprocally, if placed in the stimulatory window, tumor extirpation would result not only in a reduction or elimination of primary tumor load but also in a slower growth or inhibition of metastases.

Fig. 1

Growth of s.c. LB tumor initiated with an inoculum of 1 × 10⁶ LB cells in the right flank. Each point represents the mean ± SE of 12 mice

Fig. 2

Growth of a secondary LB tumor implant in primary LB tumor-bearing mice. The primary LB tumor was initiated with 1 × 10⁶ tumor cells inoculated in the right flank on day 0. A secondary tumor was implanted (1 × 10⁵ LB tumor cells) in the left flank on days 0 (a), 1 (b), 2 (c), 3 (d), 4 (e), 6 (f) or 9 (g) of primary tumor growth (n = 6 mice per group). Controls were six mice receiving only the inoculum of 1 × 10⁵ LB cells in the left flank. *p < 0.05, **p < 0.01, ***p < 0.001

Fig. 3

Hormetic curve summarizing the stimulatory and inhibitory effects induced by a primary LB tumor implanted in the right flank (1 × 10⁶ tumor cells) on the growth of secondary LB tumor implanted (1 × 10⁵ tumor cells) in the left flank, on selected days of primary tumor growth. Dotted line represents the tumor volume in control mice only receiving the tumor implant in the left flank. *p < 0.05, **p < 0.01, ***p < 0.001

Fig. 4

A total of 23 mice received an s.c. implant of 1 × 10⁶ LB cells in the right flank on day 0. Six days later (day 6), mice were s.c. challenged with 1 × 10⁵ (n = 6) (a); 1 × 10⁶ (n = 6) (b) or 1 × 10⁷ (n = 11) (c) LB cells in the left flank. Controls only received 1 × 10⁵ (n = 6), 1 × 10⁶ (n = 6) or 1 × 10⁷ (n = 15) LB cells in the left flank. *p < 0.05, **p < 0.01, ***p < 0.001

Fig. 5

(a–b) Experimental group: 11 mice were inoculated s.c. on day 0 with 1 × 10⁶ LB tumor cells in the right flank. On day 6, they were challenged with 1 × 10⁷ LB tumor cells in the left flank. Four days later, the underlying secondary enhanced tumor from the left flank was examined. a Tumor cells with numerous mitotic figures (arrows), ×1,000. b Presence of abundant tumor cells displaying intense Ki-67 nuclear expression, ×400. (c–d): Control group: 15 mice received on day 6, 1 × 10⁷ LB tumor cells only in the left flank. Four days later, the underlying tumor was processed as in the experimental group. c Tumor cells with few mitotic figures (arrows), ×1,000. d The presence of relatively few cells exhibiting slight Ki-67 nuclear expression, ×400

Fig. 6

Growth of s.c. CEI tumor initiated with an inoculum of 1 × 10⁶ LB cells in the right flank. Each point represents the mean ± SE of 12 mice

Fig. 7

Growth of a secondary CEI tumor implant in primary CEI tumor-bearing mice. The primary CEI tumor was initiated with 1 × 10⁶ tumor cells inoculated in the right flank on day 0. A secondary tumor was implanted (3 × 10⁵ CEI tumor cells) in the left flank on days 2 (n = 6) (a), 6 (n = 5) (b), 10 (n = 4) (c), 16 (n = 6) (d), 22 (n = 15) (e) or 29 (n = 7) (f) of primary tumor growth. Controls were 12 mice receiving only the inoculum of 3 × 10⁵ CEI cells in the left flank. *p < 0.02, **p < 0.01, ***p < 0.001

Fig. 8

Hormetic curve summarizing the stimulatory and inhibitory effects induced by a primary CEI tumor implanted in the right flank (1 × 10⁶ tumor cells) on the growth of secondary CEI tumors implanted (3 × 10⁵ tumor cells) in the left flank, at selected days of primary tumor growth. Dotted line represents the tumor volume in control mice only receiving the tumor implant in the left flank. *p < 0.02, **p < 0.01, ***p < 0.001

Fig. 9

Tumor stimulatory and inhibitory activities of serum from a LB tumor-bearing mice (LB serum) or b CEI tumor-bearing mice (CEI serum) depending on the day of tumor growth on which the serum was collected. Values represent the mean ± SE of [³H] thymidine uptake by LB tumor cells cultured with 25% of LB serum (n = 6–8 assays per day) or by CEI tumor cells cultured with 25% of CEI serum (n = 5). Dotted line represents the [³H] thymidine uptake by LB (a) or CEI (b) tumor cells cultured with 25% of normal serum. *p < 0.02, **p < 0.01, ***p < 0.001