Background Despite intensive research, cancer remains a major health problem. The difficulties in treating cancer reflect the complex nature of this disease, including high levels of heterogeneity within tumours. Intra-tumour heterogeneity creates the conditions for inter-clonal competition and selection, which could result in selective sweeps and a reduction in levels of heterogeneity. However, in addition to competing, cancer clones can also cooperate with each other, and the positive effects of these interactions on the fitness of clones could actually contribute to maintaining the heterogeneity of tumours. Consequently, understanding the evolutionary mechanisms and pathways involved in such activities is of great significance for cancer treatment. This is particularly relevant for metastasis (i.e., tumor cell migration, invasion, dispersal and dissemination), which is the most lethal phase during cancer progression. To explore if and how genetically distant clones can cooperate during migration and invasion, this study used three distinct cancer cell lines with different metastatic potentials. Results We found that (i) the conditioned media from two invasive lines (breast and lung) increased the migration and invasion potential of a poorly metastatic line (breast), and (ii) this inter-clonal cooperative interaction involved the TGF-β1 signalling pathway. Furthermore, when the less aggressive line was co-cultured with the highly metastatic breast line, the invasive potential of both lines was enhanced, and this outcome was dependent on the co-option (through TGF-β1 autocrine-paracrine signalling) of the weakly metastatic clone into expressing an enhanced malignant phenotype that benefited both clones (i.e., a “help me help you” strategy). Conclusions Based on our findings, we propose a model in which crosstalk, co-option, and co-dependency can facilitate the evolution of synergistic cooperative interactions between genetically distant clones. Specifically, we suggest that synergistic cooperative interactions can easily emerge, regardless of the degree of overall genetic/genealogical relatedness, via crosstalk involving metastatic clones able to constitutively secrete molecules that induce and maintain their own malignant state (producer-responder clones) and clones that have the ability to respond to those signals (responder clones) and express a synergistic metastatic behaviour. Taking into account the lack of therapies that directly affect the metastatic process, interfering with such cooperative interactions during the early steps in the metastatic cascade could provide additional strategies to increase patient survival.

Background Despite intensive research, cancer remains a major health problem. The difficulties in treating cancer are due to the complex nature of this disease, including high levels of heterogeneity within tumours. Intra-tumour heterogeneity creates the conditions for inter-clonal competition and selection, which should result in selective sweeps and a reduction in levels of heterogeneity. However, in addition to competing, cancer clones could also cooperate with each other, and the positive effects of these interactions on the fitness of clones can actually contribute to maintaining the heterogeneity of tumours. Consequently, understanding the evolutionary mechanisms and pathways involved in such behaviours is of great significance for cancer treatment. This is particularly relevant for metastasis, which is the most lethal phase during cancer progression. To explore if and how genetically distant clones can cooperate during invasion, this study used three genetically distant cancer cell lines with different metastatic potentials. Results We found that (i) the conditioned media from the invasive lines increased the migration and invasion potential of the poorly metastatic line, and (ii) this inter-clonal interaction involved the TGF-β1 signalling pathway. Furthermore, when a highly and poorly metastatic lines were co-cultured, the invasive potential of both lines was enhanced, and this outcome was dependent on the co-option of the less aggressive clone into expressing a malignant phenotype. Based on our findings, we propose a two-tier model whereby highly metastatic clones can co-opt (through autocrine-paracrine crosstalk) weakly metastatic clones into expressing an invasive phenotype, which in turn augments the invasion ability of the former (i.e., a “help me help you” strategy). Conclusions We suggest that such synergistic cooperative interactions can easily emerge via crosstalk involving metastatic clones able to constitutively secrete molecules that induce and maintain their own malignant state (producer-responder clones) and clones that have the ability to respond to those signals (responder clones) and express a synergistic metastatic behaviour, regardless of the degree of overall genetic/genealogical relatedness. Taking into account the lack of therapies that directly affect the metastatic process, interfering with such cooperative behaviours that tumour cells engage in during the early steps in the metastatic cascade could provide additional strategies to increase patient survival.

Intratumour heterogeneity is often associated with poor response to treatment and bad prognosis. In addition to constitutive genetic/epigenetic sources, phenotypic and functional heterogeneity can reflect cell plasticity due to changes in gene expression patterns induced by signals from the tumour microenvironment or other cells. Positive interactions between cancer clones can increase their fitness and contribute to tumour growth, resistance to drugs and metastasis. Consequently, understanding the pathways involved in such behaviours is of great significance for cancer treatment. To explore if and how genetically distant clones can synergistically enhance each other's metastatic potential, this study used three (two breast and one lung) cancer cell lines with different aggressiveness levels. We found that (i) the conditioned media from the breast and lung aggressive lines induce mesenchymal features and increase the migration and invasion potential of the poorly metastatic breast line, and (ii) in both cases, this interclonal communication is based on the same soluble factor – namely, the tumour growth factor TGF-β1. Furthermore, when the two breast lines are mixed and co-cultured, the invasive potential of both lines is enhanced, and this outcome is dependent on the recruitment of the less aggressive clone into expressing a malignant phenotype. Based on our findings, we propose a two-tier model whereby highly metastatic clones can recruit weakly metastatic clones into acquiring an invasive phenotype, which in turn augments the invasion ability of the former (i.e., a “help me help you” strategy) through shared proteases and/or ECM remodelling. We suggest that such synergistic cooperation can easily emerge via cross-talk involving metastatic clones able to constitutively secrete signalling molecules that induce and maintain their own malignant state (i.e., autocrine/cell-autonomous signalling) and clones that have the ability to respond to those signals (i.e., paracrine/non-cell-autonomous signalling) and express a metastatic phenotype. Taking into account the lack of therapies to directly affect the metastatic process, interfering with such cooperative behaviours that tumour cells engage in during the early steps in the metastatic cascade could provide an additional strategy to increase patient survival.

Intratumour heterogeneity is often associated with poor response to treatment and bad prognosis. In addition to genetic and epigenetic sources, phenotypic heterogeneity can also reflect plastic responses to signals from other cells. The latter can be mediated by various cell-cell interactions, from antagonistic (i.e., competition) to commensalistic or cooperative (mutually beneficial or altruistic). Positive exchanges can increase the fitness of clones and contribute to tumour growth, resistance to drugs and metastasis. Consequently, understanding the pathways involved in such interactions is of great significance for cancer treatment. This study used two breast cancer cell lines with different aggressiveness levels and very different secretome profiles (i.e., MDA-MB-231 and MCF7) to address the nature and mechanistic basis of interclonal crosstalk through paracrine signalling involving soluble factors during the early stages of metastasis. Our data show that MDA-MB-231 is able to recruit MCF7, through TGFβ1-mediated paracrine signalling, into expressing mesenchymal features and increased migration. On the other hand, MCF7 has no effect on the migration of MDA, suggesting a passive/commensalistic interaction. However, we found that the invasive potentials of both lines are enhanced when they are co-cultured, indicating that the two lines act synergistically and that such interclonal interactions can be mutually beneficial in vivo. Taking into account the negative impact that metastasis has on cancer prognosis and the lack of therapies to directly affect this process, interfering with the specific cooperative behaviours that tumour cells engage in during tumour progression should provide an additional strategy to increase patient survival.