These studies identify SNS activation as a novel physiologic regulator of breast cancer metastasis to distant tissue sites including lymph node and lung. SNS effects were mediated through β-adrenergic signaling, which acted to recruit alternatively activated macrophages into the primary tumor parenchyma and thereby induce a pro-metastatic gene expression signature. These effects occurred in syngeneic tumors in immunocompetent mice, as well as in mice lacking a functional T cell compartment. Thus, direct regulation of macrophage biology by the SNS appears to constitute a previously unrecognized pathway by which external conditions affecting the autonomic nervous system can activate a metastatic switch within a growing primary tumor. These findings expand our understanding of the physiological processes that regulate breast cancer metastasis, and they suggest novel strategies for inhibiting metastatic spread through targeted inhibition of SNS-regulated macrophage dynamics.
The current findings confirm previous indications that macrophage infiltration can influence breast cancer metastasis (30
), and extend those findings to identify a novel SNS / β-adrenergic signaling pathway that can drive changes in macrophage recruitment and differentiation, and thereby alter gene expression within the primary tumor. SNS regulation of pro-metastatic macrophage dynamics provides an alternative mechanism to Th2 lymphocyte regulation of tumor-associated macrophage activity (32
) and suggests that therapeutic strategies that seek to promote a tumoricidal M1 macrophage phenotype by orienting T helper cell response towards Th1 cytokine production (33
) may be insufficient to modulate metastasis in the presence of chronic SNS activation. Furthermore, SNS regulation of macrophages in the absence of a T lymphocyte population suggests an antigen-independent mechanism of tumor-associated inflammation that need not invoke exogenous triggers (eg. tumor-associated viruses) (33
), but instead occurs through direct neurotransmitter signaling to myeloid cells. SNS regulation of physiological dynamics via altered macrophage / monocyte communication would extend the influence of the peripheral nervous system beyond the distance of neurotransmitter diffusion from primary neural fibers (10–100s µm) (34
), and suggest a mechanism for transmitting the effects of autonomic nervous system activation into non- or poorly-innervated tissues such as solid epithelial tumors.
The present studies clearly indicate a role for macrophages in mediating SNS effects on metastasis, and they rule out any requirement for T lymphocytes in such dynamics, but it remains conceivable that other immune cells act upstream of macrophages to regulate their recruitment and polarization (e.g., NK cells might potentially play a role, as suggested in recent studies (35
)). Defining the tumor and microenvironmental dynamics that ultimately shape SNS modulation of tumor-associated macrophage biology represents an important topic for future research. Particularly important would be identification of the cell types and signaling pathways that regulate macrophage recruitment and the functional impact of stress-induced M2 macrophage phenotype. The current study shows that SNS signaling modulates both the extent of tumor infiltration by macrophages and associated intratumoral expression of pro-metastatic genes, including Cox2 (Ptgs2)
. Consistent with SNS regulation of pro-metastatic macrophage gene expression, SNS activation has been shown to promote peritoneal growth of metastatic ovarian tumors in nude mice through increased VEGF-mediated vascularization (2
). The present studies extend those observations into the domain of metastatic dissemination to distant tissues (including vascular-mediated colonization of target organs), and they identify macrophage modulation within the primary tumor as a central molecular mechanism for alterations in primary tumor metastatic seeding.
Neural regulation of macrophage pro-metastatic activity may provide potential cellular and molecular mechanisms for clinical observations linking chronic stress to increased breast cancer progression in humans (1
). Although these observations remain controversial, recent analyses have begun to define the circumstances in which such relationships are most likely to be observed. Few consistent relationships have been found between stress and the initial incidence
of breast cancer (38
). However, several epidemiological studies and a large meta-analysis of 131 prospective studies have linked chronic stress to increased progression
of established breast cancers (36
). Those results are consistent with data from the present experimental model in which stress-induced activation of the SNS showed no significant impact on the growth of primary tumors, but reliably enhanced metastatic spread via effects on both primary tumor seeding of metastasis and tumor cell extravasation / colonization of distant target tissues. The present findings also suggest that other physiologic or pharmacologic influences on SNS activity besides stress might potentially influence cancer progression. Such results would be consistent with epidemiologic findings linking beta-blocker usage to reduced cancer risk (6
). Two case-control studies of advanced breast cancer did not find a link between beta-blocker use and breast cancer incidence (46
) but a recent study has linked beta-blockers to reduced metastasis and breast cancer-specific mortality (48
The present data suggest that pharmacological inhibition of SNS activity (e.g., with beta antagonists or anti-NGF antibodies (49
)) could potentially constitute a novel adjunctive strategy for minimizing breast cancer metastasis. Localized targeting of tumor-associated macrophages (e.g., with GW2580) might also block adverse effects of neuroendocrine activation on macrophage recruitment into primary tumors and thereby reduce pro-metastatic gene expression. Macrophage recruitment into the primary tumor might also serve as a biomarker for early detection of disease progression and/or therapeutic impact in clinical and intervention studies.
The present results show that systemic physiologic conditions can significantly shape the primary tumor microenvironment and may alter conditions in distant tissues in ways that facilitate metastasis to distant organs. This raises the intriguing possibility that systemic interventions (e.g., that target neural or immune compartments) may provide new adjunctive strategies to complement existing anti-cancer therapies (50
). These findings highlight the importance of considering the patient’s overall physiology in the development of new therapeutic approaches to limit cancer progression and minimize metastatic rates in breast cancer.