The goal of controlling ovarian cancer metastasis formation has elicited considerable interest in identifying the tissue microenvironments involved in cancer cell colonization of the omentum. Omental adipose is a site of prodigious metastasis in both ovarian cancer models and clinical disease. This tissue is unusual for its milky spots, comprised of immune cells, stromal cells, and structural elements surrounding glomerulus-like capillary beds. The present study shows the novel finding that milky spots and adipocytes play distinct and complementary roles in omental metastatic colonization. In vivo assays showed that ID8, CaOV3, HeyA8, and SKOV3ip.1 cancer cells preferentially lodge and grow within omental and splenoportal fat, which contain milky spots, rather than in peritoneal fat depots. Similarly, medium conditioned by milky spot-containing adipose tissue caused 75% more cell migration than did medium conditioned by milky spot-deficient adipose. Studies with immunodeficient mice showed that the mouse genetic background does not alter omental milky spot number and size, nor does it affect ovarian cancer colonization. Finally, consistent with the role of lipids as an energy source for cancer cell growth, in vivo time-course studies revealed an inverse relationship between metastatic burden and omental adipocyte content. Our findings support a two-step model in which both milky spots and adipose have specific roles in colonization of the omentum by ovarian cancer cells.
Introduction: In ovarian cancer, attachment of cells to the omentum represents a rate-limiting step in metastasis formation. Thus, there is significant interest in identifying the tissue microenvironments involved in cancer cell colonization of this preferred metastatic site. The omentum is composed of translucent membranes and adipose, which contains immune structures known as milky spots. Two general models of omental colonization can be discerned from the published literature. In the “milky spot-driven” model, cancer cell lodging is due to factors produced by milky spots which serve as a colonization niche1. In contrast, the “adipose-driven” model proposes that adipocyte-derived factors are solely responsible for cancer cell lodging2. By taking advantage of peritoneal adipose deposits which either contain or lack milky spots, we set forth to test the hypothesis that these unusual immune structures are required for cancer cell colonization of adipose-rich tissues such as the omentum. Methods: Ovarian cancer cell migration, lodging and growth was assessed in a panel of peritoneal fat depots which either contain milky spots (omentum, splenoportal fat) or lack them (uterine fat, mesentery, and gonadal fat). The presence or absence of milky spots was confirmed by histology and immunohistochemistry (IHC) for CD45. Ovarian cancer cell lodging was evaluated via experimental metastasis assays using SKOV3ip.1 (human) or ID8 (mouse) ovarian cancer cells injected into Athymic Nude or C57BL/6 mice, respectively. At 7 days post injection (dpi) peritoneal fat depots were harvested and evaluated for the presence of cancer cells using histology and pan-cytokeratin IHC. The relative chemoattractant ability of tissue-conditioned media was evaluated by transwell migration assays using SKOV3ip.1 and ID8 cells. Results: The presence of milky spots in omental and splenoportal fat and their absence in uterine, gonadal, and mesenteric fat was confirmed by histology and IHC. In metastasis assays, cancer foci were observed in omental and splenoportal milky spots at 7 dpi. In contrast, cancer cells were never observed in adipose that lacked milky spots; even at 63 dpi. Consistent with this, conditioned media from milky-spot containing tissues had a significantly increased ability to promote ovarian cancer cells migration . Conclusions: In sum, our findings show that milky spots are an absolute requirement for cancer cell lodging on peritoneal adipose. Consistent with this, migration assays show that the presence of milky spots causes a significant enhancement in the ability of peritoneal adipose to promote directed migration. Taken together, these finding suggest that while adipose may secrete a general chemoattrative signal, additional contributions from the milky spots are required for localization and invasion. Current experiments include investigating the chemoattractant(s) used by the milky spots to attract cancer cells and the cell type(s) responsible for their secretion. Competing interests: The authors declare no competing interests. References: 1. Sorensen et al. 2009. Immunol Res.; 2. Nieman et al. 2011. Nat Med. Work was supported by grants from the DOD (W81XWH-09-1-0127), NCI/NIH (2RO1CA089569), Elsa U. Pardee Foundation; A Rivkin Center for Ovarian Cancer Research Pilot Study Award, and Section of Urology funds (The University of Chicago) Citation Format: Robert Clark, Venkatesh Krishnan, Michael Schoof, Irving Rodriguez, Marina Chekmareva, Betty Theriault, Carrie Rinker-Schaeffer. Milky spots are required for ovarian cancer metastatic colonization of peritoneal adipose depots. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr B65.
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