SUN-134 Estrogen Induces Granulocytic Myeloid Derived Suppressor Cell Production to Potentially Modulate Lymphangioleiomyomatosis (LAM) Tumor Progression

Affecting almost exclusively women, lymphangioleiomyomatosis (LAM) is a rare lung disease characterized by estrogen-sensitive metastatic smooth muscle cell-like adenomas that grow slowly, resulting in cystic lung change and loss of pulmonary function. LAM tumors are caused by mutations in tuberous sclerosis complex 1 or 2 genes (TSC1 or TSC2). Mutations in TSC1 or TSC2 genes results in deficient inhibitory regulation of the mammalian target of rapamycin complex 1 (mTORC1), which in turn leads to increased mTORC1 activity and cell proliferation. There is no consensus amongst LAM researchers regarding the origin of these estrogen receptor-positive smooth muscle like LAM cells; however, we previously reported inactivation of TSC2 in the mouse uterus results in notable LAM features in the setting of primary myometrial tumors. Approximately 50% of the TSC2-null mice had metastatic myometrial tumors present in the lung, suggesting that LAM tumor cells might in fact originate from the uterus, thus explaining the female sexual dimorphism, the estrogen sensitivity, and the metastatic nature of the LAM tumors. Interestingly, flow cytometry revealed large numbers of granulocytic myeloid derived suppressors cells (G-MDSCs) in the blood and myometrial tumors of uterine-specific Tsc2 null mice. MDSCs are known to accumulate in the setting of chronic inflammation caused by trauma, infection, and various cancers. This granulocytic subtype not only has the capacity to suppress anti-tumor immune cells of the tumor microenvironment, but also directly promotes tumor cell malignant neoplasicity. We found that Tsc2-null myometrial tumors required MDSCs for normal progression, as MDSC depletion or inhibition of MDSC recruitment reduced tumor growth. We have showed that these tumors expressed estrogen receptors and were exquisitely sensitive to estrogen, while other studies demonstrate that G-MDSCs are also positively influenced by estradiol. Therefore, we hypothesized that, in addition to direct effects of estrogen on tumor cells, estrogen also stimulates tumor growth by promoting MDSC production in the bone marrow. We have developed a technique to stimulate MDSC production from mouse bone marrow. Using this strategy, we found that estradiol is indeed a potent promotor of G-MDSC production. These effects occurred in both male and female bone marrow. Employing both pharmacologic agents and bone marrow from ERα; knockout mice, we showed that ERα; is necessary for promoting a G-MDSC fate for immature myeloid cells and precursors. Thus, estradiol may have duel effects in LAM, both directly promoting tumor growth and indirectly upregulating MDSC production, which in turn promotes tumor growth. We propose that these estrogen effects on MDSC production are not limited to LAM and may be important regulators of tumor growth in many tissues.