THE REGULATION OF GSK3β NUCLEAR LOCALIZATION DURING MACROPINOCYTOSIS AND AUTOPHAGY INDUCTION

Candidate: Shafi Rahman
Supervisor: Dr. Costin Antonescu

ABSTRACT

Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer, representing approximately 15% of cases diagnosed among Canadian women each year. TNBC cells face limited access to nutrients such as amino acids in the tumour environment. To overcome such limitations, cells can replenish amino acids through protein recycling processes called macropinocytosis and autophagy. The mechanisms by which TNBC cells sense and respond to nutrient depletion, specifically by triggering autophagy and macropinocytosis, are not well understood. An important protein in this nutrient-sensing process is glycogen synthase kinase 3 beta (GSK3β), which plays a dual role in growth regulation: promoting tumour growth when in the cytoplasm and inhibiting it when located in the nucleus. The transport of GSK3β between these cellular compartments is tightly regulated by mammalian target of rapamycin complex 1 (mTORC1). However, the regulation of GSK3β localization during the activation of bulk recycling processes such as macropinocytosis and autophagy remains unclear. Here, I examine how macropinocytosis and autophagy influence the localization of GSK3β and the regulatory mechanisms that control this movement in and out of the nucleus. Using a starvation-refeeding assay, I found that a significant amount of GSK3β exits the nucleus when amino acid starved MDA-MB-231 cells, a model cell line for TNBC, are supplied with both albumin and glutamine. Furthermore, using an mTORC1 inhibition assay, I found that GSK3β is able to escape the nucleus when treated with rapamycin for a prolonged period of time. These findings reveal novel regulation of GSK3β localization during 1) refeeding of amino acid starved cells, and 2) extended inhibition of the key metabolic regulator mTORC1, suggesting that targeting GSK3β’s context-specific regulation may be a promising strategy for treating TNBC.