Abstract B51: Stress management: LKB1 controls growth and survival of dormant epithelial ovarian cancer spheroid cells.
Authors: Shepherd Trevor G. , Valdes Yudith Ramos , Peart Teresa , Shah Meera , DiMattia Gabriel E. 
Published: May 2016

Aberrant cell metabolism is rapidly establishing itself as a critical hallmark of human malignancies. Cancer cells are faced with huge metabolic demands to support rapid tumor growth, yet are commonly starved for nutrients. In response, cancer cells hijack alternative signaling pathways during these times of energy and metabolic stress to sustain viability. Metastatic epithelial ovarian cancer (EOC) cells are faced with additional stressors during transcoelomic spread, such as detachment from a matrix substratum and inhospitable conditions in peritoneal fluid. However, EOC cells have a natural ability to aggregate when in suspension to form multicellular aggregates, or spheroids, which supports a survival advantage for cells when transiting the peritoneal space during metastatic progression. Therefore, we utilize an in vitro spheroid culture model system to investigate signaling pathways altered in EOC cells that may be implicated in ovarian cancer pathobiology and promote metastasis. For example, we have shown that spheroid formation induces cellular quiescence and autophagy, two disparate processes which promote EOC cell survival and resistance to platinum-based chemotherapeutics. In addition, we discovered that EOC spheroids have significantly reduced mitochondrial activity and ATP levels compared with matched proliferating adherent cells. Liver kinase B1 (LKB1) acts as a chief responder to intracellular stress due to reduced energy and nutrients by eliciting general growth suppression during these starvation-like conditions. Thus, we hypothesized that LKB1 activity is increased in EOC spheroids to promote tumor cell dormancy and cell survival. Although the STK11 gene encoding LKB1 is heterozygously deleted in 84% of serous ovarian tumors, we demonstrate that almost all ovarian tumor cells and established EOC cell lines retain intact LKB1 expression. In fact, LKB1 protein expression increases when EOC cells form spheroids and this coincides with induced serine-428 phosphorylation, a modification that is required for LKB1 growth suppressive activity. To address the potential functional requirement of LKB1 in EOC spheroids, we first performed transient knockdown of STK11 to block LKB1 expression. Indeed, reduced LKB1 renders spheroids susceptible to cell death and increases sensitivity to carboplatin. Next, we utilized Cas9-mediated genome editing of the STK11 locus to completely ablate LKB1 expression in HEYA8 and OVCAR8 cells. STK11-knockout HEYA8 and OVCAR8 cells yielded significantly decreased spheroid number and viability compared with parental cell lines. In a reciprocal fashion, forced re-expression of LKB1 in CaOV3 and TOV21G cells—two EOC cell lines which harbor inactivating STK11 mutations—reduces cell growth and colony formation in proliferating adherent culture. Proliferation and dispersion of CaOV3-LKB1 and TOV21G-LKB1 cells from re-attached spheroids are also reduced. Taken together, LKB1 has growth suppressive effects in EOC cells, yet it serves the additional purpose to promote cell survival in spheroids. The canonical target of LKB1 is AMP-activated protein kinase (AMPK), which is thought to elicit the majority of LKB1 growth suppressive effects during stress metabolism signaling. As expected, AMPK phosphorylation at threonine-172 is also induced upon spheroid formation. Using STK11-knockout EOC cells, however, we demonstrate that LKB1 is not required to phosphorylate AMPK in spheroids; more importantly, knockdown of PRKAA1 encoding the catalytic alpha-subunit of AMPK has no effect on spheroid cell viability. Thus, our results suggest that LKB1 utilizes alternative mechanisms to regulate the dormancy phenotype in EOC spheroids. Future work will entail direct experiments testing whether LKB1-mediated stress metabolism signaling has the potential to promote EOC metastasis and recurrence of chemo-resistant disease.

Citation Format: Trevor G. Shepherd, Yudith Ramos Valdes, Teresa Peart, Meera Shah, Gabriel E. DiMattia. Stress management: LKB1 controls growth and survival of dormant epithelial ovarian cancer spheroid cells. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research: Exploiting Vulnerabilities; Oct 17-20, 2015; Orlando, FL. Philadelphia (PA): AACR; Clin Cancer Res 2016;22(2 Suppl):Abstract nr B51.