The key findings from our study are that norepinephrine and epinephrine protect ovarian cancer cells from anoikis via a FAK-mediated signaling pathway that is initiated by ADRB2 and involves subsequent Src-associated phosphorylation of FAKY397. Norepinephrine-induced FAK activation was also found to play a significant role in the effects of chronic stress on ovarian cancer growth in vivo in an orthotopic mouse model. Additional studies of human clinical tumors showed that both depression and tumor norepinephrine content were associated with increased FAK activation and that increased FAK activation was associated with substantially accelerated mortality times. Thus, these studies identify norepinephrine- and epinephrine-induced FAK activation as a novel mechanism by which stress might accelerate the pathogenesis of ovarian cancer.
FAK signaling is critical in many biological pathways, including embryonic development (26
), cell migration and invasion (14
), proliferation (27
), and apoptosis (11
). FAK is a particularly important regulator of signaling processes between the ECM and tumor cells (30
). FAK phosphorylation at Y397 follows integrin stimulation or ligand binding by growth factor receptors (32
). The present study identified the ADRB2
/Src signaling axis as a molecular pathway for inhibiting ovarian cancer anoikis via FAKY397
phosphorylation. Avoidance of anoikis provides a selective advantage for metastatic cancer cells to allow for transit to new sites for attachment (33
). The present studies suggest that inhibition of the ADRB2/Src/FAK pathway by beta blockade or siRNA inhibition might provide a novel strategy for suppressing tumor growth and metastasis in clinical settings.
All 3 major catecholamines (norepinephrine, epinephrine, and dopamine) are known to be present in the ovary, with norepinephrine being the most abundant (34
). Ovarian norepinephrine levels are substantially higher than those in circulating blood (36
), and SNS activity can further enhance those levels to induce precystic follicles (38
). The doses of norepinephrine used for our study were selected to reflect the physiologic conditions of this hormone at the level of the tissue microenvironment. Studies suggest that within the parenchyma of the ovary, concentrations may reach as high as 10 μmol/l (39
). Although the physiological role of norepinephrine and epinephrine in the ovarian tissue environment is not yet fully known, the present data imply that these signaling molecules might contribute to disease pathogenesis in the context of an emerging tumor. Further analysis of the roles of stress hormones in normal ovarian physiology would provide valuable insights into the basis for the presently observed effects in tumor cells. Particularly important in these future studies will be defining the physiological role of norepinephrine and epinephrine in modulating cellular dependence on neighboring cells and ECM. It is possible that stress hormone–mediated activation of FAK-dependent resistance to anoikis might play a role in normal tissue remodeling or development.
The present study expands the scope of molecular pathways involved in effects of stress on cancer growth and progression (4
). In addition to potential effects of stress on immune response in cancer (4
), we and others have demonstrated that stress mediators such as norepinephrine and epinephrine from the SNS and glucocorticoids from the hypothalamic-pituitary-adrenal axis can directly regulate the function of human cancer cells in ways that promote their survival and metastasis (4
). In the case of the SNS, these effects are mediated by ADRBs expressed on ovarian, mammary, and other cancer cells (15
). Norepinephrine activation of ADRB2 has been shown to enhance tumor growth in part via induction of VEGF-dependent angiogenesis (6
). Dopamine, an important member of the catecholamine family, appears to play an opposing role against the angiogenic effects of VEGF but is present at reduced levels under chronic stress conditions (43
). Other studies have identified additional mechanisms by which stress might contribute to cancer pathogenesis, including impaired DNA repair (45
) and modulation of matrix metalloproteinases (47
). The current study identifies a new pathway by which stress biology can impact tumor growth and progression via ADRB2-dependent activation of FAK and the resulting protection of cells from anoikis. Resistance to anoikis is a hallmark of malignant transformation, affording tumor cells increased survival times in the absence of matrix attachment and facilitating migration, reattachment, and colonization of secondary sites (50
). Overexpression of oncogenes such as ras
, and src
as well as the downregulation of tumor suppressor genes such as PTEN
and p53 (TP53
) contribute to protection from anoikis (52
). This study identifies a novel neuroendocrine pathway by which behavioral stress factors can exert similar effects. These findings also imply that the neuroendocrine “macroenvironment” may play a significant role in shaping cellular activity in the tumor microenvironment in ways that ultimately facilitate cancer progression. Thus, protective interventions targeting the neuroendocrine system might simultaneously modulate multiple molecular pathways involved in tumor metastasis (e.g., anoikis, angiogenesis, and invasion).