The current work will directly address the Focus Areas, “Gaining a deeper knowledge of TSC signaling pathways and the cellular consequences of TSC deficiency” and “Facilitating therapeutics, biomarkers, and clinical trials research.” The lysosome is a key intracellular organelle that is bound by membranes and functions to degrade proteins and macromolecules within the cell. This activity is particularly important in the setting of nutrient shortages, such as those that might occur when a cancer cell outgrows its blood supply, as it enables the cell to recycle components of proteins and nutrients. Accordingly, the number and activity of lysosomes within the cell are tightly controlled at the gene expression level by a set of transcription factors belonging to the MiT/TFE family. Work over the last decade has suggested that mTORC1 activity generally leads to decreased lysosomes within the cell, by suppressing the activity of MiT/TFE transcription factors. However, this work was largely conducted with short-term pharmacologic inhibitors of mTORC1. In contrast, we have found that constitutive mTORC1 activity (as occurs with TSC1/2 loss) actually leads to dramatically increased protein levels of MiT/TFE family transcription factors, with increased lysosomal gene transcription and increased lysosomal content within the cell. Interestingly, the two types of renal tumors occurring in tuberous sclerosis (angiomyolipomas [AML] or renal cell carcinoma [RCC], which can both also occur sporadically) may be caused by either loss of the TSC1/2 tumor suppressors (as seen in tuberous sclerosis or sporadically) or increased MiT/TFE transcription factor levels due to translocations of these genes (as seen sporadically). This is consistent with the possibility that these two different gene alterations may in fact have similar cellular consequences: TSC1/2 loss could be associated with upregulation of MiT/TFE levels and consequent lysosomal biogenesis. In fact, both AML and TSC-associated RCC can be recognized pathologically by overexpression of lysosomal proteins.
If it is the case that lysosomal proteins are highly upregulated in tuberous sclerosis-associated tumors, this has important implications for potential therapies in the disease. Lysosomes are crucial to cancer cell survival and the degradative enzymes within lysosomes can facilitate tumor cell migration. Increased lysosomal activity may also be responsible for the fragility and increased bleeding observed in AML tumors of the kidney. Accordingly, there are a number of pharmacologic compounds available in current clinical trials that target and inhibit lysosomal activity. Here, we propose to study the potential mechanism for our unexpected finding in the laboratory and to use human and mouse renal tumors with TSC1/2 loss to begin to understand whether lysosomal biogenesis is a potential driver of tumorigenesis and therapeutic target in tuberous sclerosis. In the short term, we will gain a better understanding of the cellular consequences of TSC1/2 loss, and in the long term, we may develop novel diagnostic biomarkers and therapies for this devastating disease.