Cooperation is manifested at every taxonomic level of the animal kingdom, from bacteria to social mammals. One requirement essential for the evolution of cooperative behavior is the ability for conspecifics to communicate, a tenet equally valid at the cellular level. In metazoan organisms, for example, the formation of epithelial layers relies heavily on cell-cell communication via secreted ligands. Furthermore, it has been noted that one reputed function of osteocyte-osteocyte communication may be in sensing and modulating signals that control osteoblastic and osteoclastic activity. A number of signaling modalities have evolved which facilitate cell-cell communication, including gap junctions, chemical synapses, and tunneling nanotubes. Recent studies involving irradiation of cell populations in vitro with low energy alpha-particles demonstrated cell-cell communication enhanced the magnitude of cell-kill beyond that predicted. This enhanced mortality phenomenon, labeled the “bystander effect”, has translated into anti-cancer “suicide gene therapy”. Conversely, cell-cell interactions can also promote cell survival. Naïve “recipient” cells prove less susceptible to treatment-induced damage if the recipient cells are allowed to interact with a previously-exposed (primed) subpopulation prior to treatment of the mixed population. We hypothesized (a) damaged cells can influence communal survival by communicating with unaffected neighboring cells, (b) this capability is an inherent attribute of normal cells but may be modified by neoplastic transformation, and (c) the process can be ascribed (at least in part) to “cooperative communication”.

In preliminary in vitro experiments involving cancer cells exposed to doxorubicin (Adriamycin®), an anthracycline anti-tumor agent, we were able to demonstrate enhanced survival consistent with our notion of cooperative communication. “A” in Fig. 4 represents overall cell survival (41%) when 5x10⁴ cells were primed for 30 min with 1 µM doxorubicin, and then primed and 2x10⁵ untreated recipient cells immediately exposed to 1 µM doxorubicin for 60 min, after which the 2 cultures were mixed. A prime (A’), in contrast, reflects survival (41.4%) when primed and untreated recipient cultures remained separated for 4 h prior to the 60 min doxorubicin dose. The 2 populations were then combined. B represents 1 µM doxorubicin for 60 min (single culture). The key is shown by “C”, which represents overall cell survival (56.4%) when 5x104 cells were primed for 30 min with 1 µM doxorubicin, then immediately mixed with 2x10⁵ untreated recipient cells and allowed to co-incubate (communicate) with the recipient cells for 4 h prior to the 60 min application of 1 µM doxorubicin. C is significantly greater than A’ (t = 5.1; P < 0.0001; t-test with equal variances). Additional treatments represented in Fig. 4 consist of D through H; single flasks with 1x105 cells primed with 1 µM doxorubicin for 30 min, then held for 0, 1, 2, 3 or 4 hours before application of the 60 min dose. D through H tested whether the enhanced survival apparent in C could be attributed to sub-lethal damage repair.

Impact: This study investigates how cell-cell communication promotes tissue homeostasis and whether transformation influences this process. Clinically, identification of the signaling pathway(s) responsible for cooperative communication could provide potential therapeutic targets for new anti-cancer agents. The elimination of “warning” signals emitted by cells might ensure uniform vulnerability of an entire tumor population, thus allowing for fewer chemotherapy treatments with lower doses. Conversely, induction or amplification of signals directed toward normal cells located in proximity to a tumor (or throughout the body) might reduce the level of “collateral damage” associated with systemic chemotherapy.