Oral administration of muscle derived small molecules inhibits tumor spread while promoting normal cell growth in mice

In vitro and in vivo studies on the inhibitory effects of myocardial cell culture medium on growth of a human lung adenocarcinoma cell line, A549

BACKGROUND

Although the heart is one of the body's vital organs, with an abundant blood supply, metastasis to the heart is considered rare. In a previous study, we found that the myocardial microenvironment might contain a low molecular weight natural tumour suppressor. The present study was designed to investigate the inhibitory effect of cardiac myocyte-conditioned medium (cmcm) on the growth of A549 human lung adenocarcinoma cells in vitro and in vivo.

METHODS

An mtt assay was used to detect the inhibition ratio with respect to A549 proliferation. Human lung adenocarcinoma cells (A549 cell strain) were transplanted subcutaneously into nude mice to produce tumours. The xenograft tumour growth in mice was observed after selected drug administration.

RESULTS

After treatment with cmcm and cisplatin (Cis), A549 cell viability significantly declined (p < 0.001). The cell viability in the cmcm and Cis groups were 53.42% ± 3.45% and 58.45% ± 6.39% respectively. Growth of implanted tumour cells in vivo was significantly inhibited in the cmcm group, the group treated with recombinant human adenovirus-p53, and the Cis-treated group compared with a control group. The inhibition rates were 41.44% in the cmcm group, 41.34% in the p53 group, and 64.50% in the Cis group. Lung metastasis capacity was significantly reduced in the presence of cmcm (p < 0.05). Lung metastasis inhibition rates in mice were 56.52% in the cmcm group, 47.83% in the p53 group, and 82.61% in the Cis group. With cmcm, the lives of A549-tumour-bearing mice could be significantly prolonged without any effect on weight loss.

CONCLUSIONS

Use of cmcm has the effect of reducing A549 cell viability, tumour volume, and lung metastasis rate, while prolonging survival duration without severe toxicity.

Inhibition of primary colon carcinoma growth and liver metastasis by the A3 adenosine receptor agonist CF101

Adenosine is a purine nucleoside that acts as a regulatory molecule by binding to specific G-protein-coupled A1, A(2A), A(2B), and A3 cell surface receptors. We have recently demonstrated that adenosine inhibits tumour cell growth and concomitantly stimulates bone marrow cell proliferation via activation of the A3 adenosine receptor (A3AR). In the present study, we show that a synthetic agonist to the A3AR, CF101, at the low nanomolar concentration range, inhibits HCT-116 human colon carcinoma cell growth. This effect was reversed by the selective A3AR antagonist MRS1523, demonstrating the specificity of the response. CF101 (given orally) was efficacious in inhibiting the development of primary tumours in xenograft and syngeneic models in which mice were inoculated subcutaneously with human HCT-116 or murine CT-26 colon carcinoma cells, respectively. Moreover, CF101 suppressed (50%, P<0.01) colon cancer liver metastases in syngeneic mice inoculated to the spleen with CT-26 cells. The mechanism of action entailed upregulation of interleukin-12 production in the CF101-treated groups and potentiation of NK cell activity. In the HCT-116 xenograft model in which a combined therapy of CF101 and 5-fluorouracyl (5-FU) was examined, an additive antitumour effect was demonstrated. Moreover, CF101 prevented the 5-FU-induced myelotoxicity, resulting in normal values of white blood cell and neutrophil counts. We conclude that the A3AR agonist CF101, a small orally bioavailable molecule, exerts systemic anticancer, antimetastatic, and myeloprotective effects in colon carcinoma-bearing mice, and may serve as an adjuvant treatment to enhance the chemotherapeutic index and prevent myelotoxicity.

A3 adenosine receptor as a target for cancer therapy

Targeting the A3 adenosine receptor (A3AR) by adenosine or a synthetic agonist to this receptor (IB-MECA and Cl-IB-MECA) results in a differential effect on tumor and on normal cells. Both the adenosine and the agonists inhibit the growth of various tumor cell types such as melanoma, colon or prostate carcinoma and lymphoma. This effect is specific and is exerted on tumor cells only. Moreover, exposure of peripheral blood mononuclear cells to adenosine or the agonists leads to the induction of granulocyte colony stimulating factor (G-CSF) production. When given orally to mice, the agonists suppress the growth of melanoma, colon and prostate carcinoma in these animals, while inducing a myeloprotective effect via the induction of G-CSF production. The de-regulation of the Wnt signaling pathway was found to be involved in the anticancer effect. Receptor activation induces inhibition of adenylyl cyclase with a subsequent decrease in the level of protein kinase A and protein kinase B/Akt leading to activation of glycogen synthase kinase-3beta, a key element in the Wnt pathway. The oral bioavailability of the synthetic A3AR agonists, and their induced systemic anticancer and myeloprotective effect, renders them potentially useful in three different modes of treatment: as a stand-alone anticancer treatment, in combination with chemotherapy to enhance its therapeutic index and myelprotection. It is evident that use of the A3AR agonist for increasing the therapeutic index of chemotherapy may also invariably give rise to myeloprotection and vice versa. The A3AR agonists are thus a promising new class of agents for cancer therapy.

Myogenic cell proliferation and generation of a reversible tumorigenic phenotype are triggered by preirradiation of the recipient site

Environmental influences have profound yet reversible effects on the behavior of resident cells. Earlier data have indicated that the amount of muscle formed from implanted myogenic cells is greatly augmented by prior irradiation (18 Gy) of the host mouse muscle. Here we confirm this phenomenon, showing that it varies between host mouse strains. However, it is unclear whether it is due to secretion of proliferative factors or reduction of antiproliferative agents. To investigate this further, we have exploited the observation that the immortal myogenic C2 C12 cell line forms tumors far more rapidly in irradiated than in nonirradiated host muscle. We show that the effect of preirradiation on tumor formation is persistent and dose dependent. However, C2 C12 cells are not irreversibly compelled to form undifferentiated tumor cells by the irradiated muscle environment and are still capable of forming large amounts of muscle when reimplanted into a nonirradiated muscle. In a clonal analysis of this effect, we discovered that C2 C12 cells have a bimodal propensity to form tumors; some clones form no tumors even after extensive periods in irradiated graft sites, whereas others rapidly form extensive tumors. This illustrates the subtle interplay between the phenotype of implanted cells and the factors in the muscle environment.

Resistance of muscle to tumor metastases: a role for a3 adenosine receptor agonists

Tumor metastases are extremely rare in striated muscles. Lately, we have found that muscle cell conditioned medium (MCM) inhibits the proliferation of various tumor cells while maintaining the growth of normal murine bone marrow cells. This dual activity was confirmed in vivo when the MCM was administered orally, i.e., it inhibited the development of tumor growth in mice and prevented the myelotoxic effects of chemotherapy. Adenosine was found to be one of the active components of MCM, inhibiting tumor cell growth while maintaining bone marrow cell proliferation in vitro. Adenosine is known to act as an important regulatory molecule through its binding to specific G-protein-associated A1, A(2a), A(2b) and A3 cell surface receptors. In distinction from MCM, adenosine did not suppress tumor development in mice and was not active as a chemoprotective agent when administered orally or intravenously. Thus, the in vivo activity of MCM could not be attributed to adenosine. In this study, MCM from which adenosine was enzymatically removed still retained its dual activity that was also found to be mediated through the A3 adenosine receptor (A3AR). This result led to the conclusion that natural agonists to A3AR were responsible for the activity of MCM. We further tested synthetic agonist to the A3AR and demonstrated that it possessed the same in vitro and in vivo activity profile as MCM. Taken together, muscle cells, in addition to adenosine, secrete natural agonists to A3AR. These agonists are stable nondegradable molecules and may contribute to the systemic anticancer and chemoprotective activity exerted by MCM. This group of molecules may account for the rarity of tumor metastases in muscle.

Resistance of muscle to tumor metastases: a role for a3 adenosine receptor agonists

Tumor metastases are extremely rare in striated muscles. Lately, we have found that muscle cell conditioned medium (MCM) inhibits the proliferation of various tumor cells while maintaining the growth of normal murine bone marrow cells. This dual activity was confirmed in vivo when the MCM was administered orally, i.e., it inhibited the development of tumor growth in mice and prevented the myelotoxic effects of chemotherapy. Adenosine was found to be one of the active components of MCM, inhibiting tumor cell growth while maintaining bone marrow cell proliferation in vitro. Adenosine is known to act as an important regulatory molecule through its binding to specific G-protein-associated A1, A(2a), A(2b) and A3 cell surface receptors. In distinction from MCM, adenosine did not suppress tumor development in mice and was not active as a chemoprotective agent when administered orally or intravenously. Thus, the in vivo activity of MCM could not be attributed to adenosine. In this study, MCM from which adenosine was enzymatically removed still retained its dual activity that was also found to be mediated through the A3 adenosine receptor (A3AR). This result led to the conclusion that natural agonists to A3AR were responsible for the activity of MCM. We further tested synthetic agonist to the A3AR and demonstrated that it possessed the same in vitro and in vivo activity profile as MCM. Taken together, muscle cells, in addition to adenosine, secrete natural agonists to A3AR. These agonists are stable nondegradable molecules and may contribute to the systemic anticancer and chemoprotective activity exerted by MCM. This group of molecules may account for the rarity of tumor metastases in muscle.