Experimental tumour induction model
The aim of this work was to evaluate, in a rat model, how liver resection (like that which may be performed in a metastasectomy) might modify metastatic growth in the remaining liver tissue. Laboratory animals are still needed for this type of research, because it is not ethically or legally possible to undertake the necessary experiments in humans (28).
It could be argued that the ideal animal model to study this phenomenon would be based on naturally-appearing primary CRC which subsequently develops CRCLM. Then, PHx could be performed to remove metastases, and the recurrence of liver metastases could be studied. On the other hand, such a model based on natural or genetic predisposition to develop primary CRC and then CRCLM has several drawbacks. Major problems that discourage use of such a model include: low rates of development of primary tumours or metastases, the longer time needed for metastatic disease to become evident, the limited number of models (syngeneic and xenograft), metastatic disease not being confined to a single location and the asynchronous development of metastatic disease, as well as greater associated costs of animal housing (29–31).
Chemical induction has also been accepted as an appropriate tool for tumour induction. Various compounds, including dimethylhydrazine and its metabolites methylnitronitrosoguanidine, N-Nitroso-N-methylurea and azoxymethane, have been shown to induce tumour and/or metastases growth in laboratory animals (32–36). These compounds have alkylating activity, which cause breakage of DNA chains, abnormal pairing of bases, and inhibition of cell division, finally, resulting in cell death. Despite these products being well known, they have several disadvantages when used to induce tumours. For example, long induction time, low rates of tumour development and undefined site of tumour development, meaning that larger numbers of animals would be required for each experimental group. Furthermore, as these chemical products are carcinogenic in humans, the risk of researcher exposure should be taken into account.
Heterotopic implantation of cancer cells is also a validated model to induce tumour development in laboratory animals, particularly in rats and mice. Cells may be implanted directly into the liver parenchyma, via subcapsular injection, and this approach achieves a high tumour development rate: up to 60–70% of the injected animals (37–39). But this model is not appropriate for our purposes because it only allows the development of single well-defined and localized tumour implants, and also it lacks the process of tumour cell dissemination through the vascular tree and subsequent extravasation to produce metastatic disease in the liver (40). Another heterotopic-implantation method to induce CRCLM development is intrasplenic injection. The success rate with this model is, however, very heterogeneous, ranging between 20–100% (41, 42).
For our experiments, we chose intrasplenic injection because it mimics, as closely as possible, the natural haematogenous dissemination pathway through the portal vein of CRC cells from primary colorectal tumours; as well as all the other natural events involved in the spreading of metastases, such as extravasation, implantation, ECM remodelling and CRCLM growth and development.
The immune barrier is another factor to be considered when performing tumour induction by cell injection. To achieve a successful model, allo- or xeno-rejection has to be avoided. This problem can be overcome by the use of cell lines that grow in syngeneic hosts, also called isotransplantation (43) or using immunocompromised host, animals which have an immune system that is suppressed or depleted and does not respond when cells or tissues from other species are implanted (43, 44).
As the immune system plays an important role in cancer, we decided to avoid interfering with it. For this reason, we chose a CRC cell line (CC531) which is syngeneic to WAG/RijHsd rats. When CC531 cells are injected into the spleen of this strain of rats, rejection seldom occurs (45) and a high rate of success is achieved. In fact, in our study, we saw no cases of rejection at all, and a 100% success rate in CRCLM development. The fact that tumour foci were observed in every liver lobe is in consonance with the natural pattern of metastasis development, which reinforces our idea of it being a good experimental model for studies on liver metastases. Nonetheless, it is also true that this model lacks the genetic variability which is observed in human tumours (46). Moreover, Robertson et al. sustain that as rat liver architecture and homeostasis are quite different from those in humans, caution should be exercised with any extrapolation of results from this model to a clinical setting (44).
Effects of 40% PHx on CRC growth
Though many experiments support the idea that GFs stimulate cancer cells, in our experiment, high concentrations of HRS clearly inhibited cell growth percentage (Fig. 3). Doubling HRS concentration (10%) reduced cell counts after 3 days by 30% (Table 1). The inhibitory effect was even stronger with 20% HRS, the cell count falling to less than 50% of that observed with 5% HRS (3.22 × 105 ± 8.24 × 104 vs. 1.27 × 105 ± 3.17 × 104; p < 0.01).
In vitro studies have taught us some interesting lessons. Though the name “growth factors” suggest they stimulate cell growth, their real effect is highly dependent on their concentration. We have seen a “window” in which they markedly increase cell proliferation rate, but when levels are higher than this window, they start to act as inhibitors (47–49). Our results are in consonance with those published by other authors.
A second important idea emerges from our experiment. When comparing low concentrations of serum from either normal or hepatectomized rats, we have seen that there is something in the latter (possibly growth factors) that makes it a stronger stimulus to cancer cell growth in vitro (50–52). This could explain the high rate of recurrence after surgery for colorectal liver metastases.
In fact, the experiments we have carried out in vivo have shown that 40% PHx strongly increases CRCLM development. Authors including Panis et al., García-Alonso et al., Krause et al. and Harun et al. have obtained similar results. Specifically, in an intraportal induction model of CRCLM (DHDK12 CRC cell line and syngeneic BDIX rats), Panis et al. demonstrated that three times more rats developed metastases among those that had undergone 70% PHx than those that had not undergone hepatectomy (62% vs. 20%, p < 0.05) (53). García-Alonso et al., working with S4MH (a rhabdomyosarcoma cell line, syngeneic for WAG rats) and performing 40% PHx, reported stimulatory effects on liver metastasis development, compared to that observed in controls (non-hepatectomised) (54). Regarding the behaviour of each liver lobe, our results are quite similar to this latter study. The percentage of the liver parenchyma occupied by tumour implants was significantly higher in the paramedian and caudate lobes in hepatectomised animals (33% vs. 24% and 47% vs. 17%, respectively) and also higher in the right lateral lobe, though the difference did not reach significance (20% vs. 17%). These findings are in accordance with our results described above. Krause et al., with CC531 cells and WAG rats, also observed a significantly higher RTSA% in the PHx group than the non-hepatectomized group (2.8 vs. 2.5, respectively) (p < 0.05) (55).
In 2007, Harun et al. (56) compared the effects of both early and late liver regeneration stages in intrasplenic-induced CRCLM after 40% and 70% PHx, performed at the same time as CRCLM induction or 6 days later. They observed that metastatic development was only affected by 70% PHx and also that it was mainly factors involved in late stages of hepatic regeneration that interfered in the progression of metastasis. This group also demonstrated that tumour growth and progression were caused by an upregulation of c-Met in the peripheral area of CRCLM (57).
In summary, the performance of our experimental model and our preliminary results are in accordance with data published by other authors, demonstrating that liver resection to remove the macroscopic metastases, though effective in improving short- and medium-term survival, carries the risk of promoting the proliferation of any tumour cells remaining in the patient. Our in vitro results suggest that the mechanism involved could well be related to GFs. If this were to be proven beyond doubt, targeting these GFs could be a reasonable approach to prevent tumour recurrence following surgical treatment for CRCLM.