IGF-2 is the most abundant growth factor in human bone (8
) and, unlike in mouse bone, is 9 times more abundant than IGF-1 (11
). Our previous study revealed the role of IGFs in prostate cancer growth in human bone, as shown by the suppressive effect of a neutralizing antibody (KM1468), which cross-reacts with human IGF-1 and IGF-2 and mouse IGF-2, on the growth of MDA PCa 2b cells in implanted HAB (6
). Moreover, another study of ours indicated that tail vein-injected prostate cancer cells preferentially metastasized to implanted HAB, rather than to implanted mouse bone or native bone (5
). These findings suggested that IGF-2 may support human prostate cancer more efficiently than IGF-1 in human bone environment, in accordance with the “seed and soil” theory by Paget (16
) for metastasis. Therefore, we became interested in specifically targeting IGF-2 as a potential therapy for bone metastasis from prostate cancer.
Although clinical studies have suggested that IGF-2 contributes to the development and progression of several cancers including prostate cancer (17
), only a few studies by others and us have reported an in vivo
antitumor effect of targeting IGF-2. O’Gorman et al. reported that the overexpression of the IGF-2 receptor, which is a clearance receptor for IGF-2, on choriocarcinoma cells reduced the cell growth in vitro
and in vivo
, indicating that inhibiting IGF-2 is a potential target for cancer therapy (20
). In an another strategy using neutralizing antibodies, Miyamoto et al. reported that an IGF-2 neutralizing antibody partly suppressed the development of liver metastasis induced by the intrasplenic injection of colon cancer cells, whereas IGF-1 neutralizing antibody significantly suppressed the development of liver metastasis (21
). Because IGF-2 production in rodents becomes attenuated in most tissues soon after birth unlike IGF-1 production (22
), the effect of IGF-2 as an endocrine and/or paracrine on tumor growth may be underestimated in models using mouse-native organs.
The present study demonstrated that inhibiting IGF-2 by m610 effectively suppresses the growth of prostate cancer cells in a human bone environment. The following results support this conclusion: (a) m610 significantly suppressed the growth of bone tumors from MDA PCa2b cells in implanted HAB; it even decreased the number of mice developing tumors from 8 to 6 although larger number of mice are needed to confirm this effect; (b) Ki-67 immunostaining revealed that the proliferative status of m610-treated bone tumor was apparently suppressed; (c) in the absence of HAB, m610 had no effect on the growth of tumors from MDA PCa 2b cells, indicating that the suppressive effect of m610 on the tumor cells was restricted to those within HAB; and (d) in vitro assays confirmed that m610 prevents the exogenous IGF-2-induced proliferation of MDA PCa 2b cells. These results provide clear evidence of the important role of IGF-2 for tumor growth in the HAB model and of an in vivo antitumor effect of m610 on metastatic bone tumor from prostate cancer through a mechanism involving the inhibition of IGF-2. They also underscore the notion that IGF-2 levels in local tissue may be more relevant in tumor promotion than its plasma levels, and that a paracrine mechanism of IGF-2 may play a critical role in tumor growth.
The potency of m610 on the growth inhibition of MDA PCa 2b cells in the HAB model is 65% whereas that of the previously published antibody, KM1468 is 97%, compared to the respective controls: the antitumor effect of inhibiting IGF-2 alone is lower than that of inhibiting both IGF-1 and IGF-2 in the HAB model. Despite the lower antitumor effect of m610 in the HAB model, targeting IGF-2 by m610 might provide certain clinical benefits in cancer therapy for the following reasons. a) Growth hormone (GH) feedback is not known for IGF-2, but IGF-1 is regulated by this feedback. Lowering IGF-1 concentration triggers feedback upregulation of the GH; the GH compensates for the reduced IGF-1 levels. Thus, targeting IGF-1 might require high concentrations of anti-IGF-1 antibodies. It should be noted that KM1468 is not reactive with mouse IGF-1 and therefore its use in our HAB model does not trigger the GH feedback on the IGF-1 and the tumor growth. b) Because m610 is a fully human antibody, its clinical use is less likely to induce immune reactions compared to murine antibodies.
Targeting IGF-2 might provide additional therapeutic benefit in combination with other treatments. IGF-1R activation by IGF-1 and IGF-2 has been shown to stimulate the growth of a wide range of cancer cells (23
). Currently, potent mAbs against the IGF-1R are being tested in clinical trials against multiple tumor types including prostate, breast and colon cancers, and Ewing’s sarcoma (25
). Importantly, it is becoming increasingly evident that IR activation by IGF-2 enhances the growth of Ewing sarcoma and breast cancer in addition to the IGF-1R activation: cotargeting IGF-1R and IR is likely to be more effective than targeting the IGF-1R alone (26
). Recently reported immunohistological examinations of primary human prostate cancer show that IGF-1R as well as IR are both commonly expressed on the tissues (29
). Our results from the immunohistological examinations () and Western blot analyses () suggest that IR activation by IGF-2 plays an important role in the prostate cancer cell growth in bone in addition to the IGF-1R activation. Based on these findings, IGF-2 could be a promising candidate target in therapeutic strategies for cotargeting IGF-1R and IR.
If m610 were capable of suppressing the growth of prostate cancer in bone without any adverse reactions, m610 therapy might considerably improve the quality of life of patients with bone metastases of prostate cancer. In the present study, the administration of m610 did not affect the body weights of mice during the 4-week treatment period (Supplemental. fig. 3
), and no adverse findings were observed in the histological examination for the mouse organs.
In conclusion, the present study demonstrated that an IGF-2-specific antibody, m610, can sufficiently suppress the growth of bone tumors from MDA PCa 2b cells in a human bone environment and that this effect is caused by the suppression of the tumor cells’ proliferative status. These results suggest that the targeting of bone-derived IGF-2 using a neutralizing antibody offer a new therapeutic strategy for bone metastasis from prostate cancer.