The roles of the unfolded protein response (UPR) and chaperone-mediated autophagy (CMA) in the pathogenesis and treatment of multiple myeloma

  • Nicholas Nikesitch

Western Sydney University thesis: Doctoral thesis

Abstract

Multiple myeloma (MM) remains as a predominantly incurable malignancy despite high-dose chemotherapy, autologous stem cell transplant and novel agents. MM is a genetically heterogeneous disease with increasing genetic complexity as the disease progresses to a more aggressive stage. The disease is characterised by the proliferation of plasma cell clone/s, and the continual production of immunoglobulin fragments, known as paraprotein. The malignancy most commonly affects an aged population with approximately 1200 new cases diagnosed in Australia each year. The introduction of proteasome inhibitors (PIs), such as bortezomib (Bz), has improved the life expectancy of MM patients significantly. However, not all MM patients respond to Bz. The response rate to Bz and dexamethasome in newly diagnosed MM is about 40-80%, while only 30% in relapsed/refractory MM. Therefore, relapsed refractory MM, in particular those who are refractory to proteasome inhibitors, remain the biggest hurdle in treating the disease. Myeloma cells are highly dependent upon the unfolded protein response (UPR) to modulate ER stress levels and restore cellular proteostasis. The excessive levels of paraprotein produced by myeloma cells surpass the ERs protein folding ability, leading to proteotoxicity and ER stress. The removal of protein levels by proteasomal degradation is one way in which the UPR in myeloma cells reduces ER stress levels, whilst also orchestrating the reduction of gene transcription and protein translation. The activation of the UPR is regulated through three endoplasmic reticulum transmembrane proteins: IRE- 1 (inositol-requiring enzyme-1), PERK (double-stranded RNA-activated protein kinase (PKR) - like endoplasmic reticulum kinase) and ATF6 (activating transcription factor 6). One of the most important regulators of this pathway, downstream of IRE1, is XBP-1 (X-box Binding Protein 1). XBP-1 is not only critical in the activation and regulation of the UPR, but is also important in secretory cells, as well as being necessary in plasma cell differentiation. During UPR activation, XBP-1 is activated through the splicing of a 26bp intron from XBP-1 mRNA into its active isoform. This is facilitated by IRE1, but can also be achieved by ATF6. The expression of XBP-1 is able to predict PI sensitivity in MM patients. The mRNA expression of both the total amount and spliced (active) isoform of XBP-1 are substantially reduced under increasing Bz resistance in MM, although XBP-1 is not a direct target of Bz. Its expression is possibly a surrogate marker for dependence on the UPR in MM. Therefore, an alternative stress mechanism is likely to be compensating reduced UPR activity and alleviating ER stress in Bz resistance. It is possible that chaperone-mediated autophagy (CMA) is compensating the UPR in Bz resistance. CMA is a highly specific degradation pathway responsible for the removal of damaged and unwanted soluble cytosolic proteins. The CMA pathway is already known to be important in modulating oxidative stress, nutrient starvation, and eliminating damaged proteins. The activity of CMA is directly proportional to the expression of LAMP2A, as LAMP2A is the rate-limiting factor in CMA activity. The expression of LAMP2A is known to be upregulated in a variety of tumours relative to untransformed cells, and also promotes tumour proliferation. LAMP2A protein expression is also significantly higher in a number of tumour tissues relative to their surrounding untransformed healthy tissue, thereby establishing the importance of CMA in tumour biology. The purpose of this project was to improve the existing understanding surrounding the role of the UPR in Bz sensitivity in MM. The project further aimed to determine the importance of CMA in Bz resistance, and whether CMA compensated the UPR in Bz resistant myeloma cells, conferring resistance. We hypothesised that reduced dependence on the UPR mediates Bz resistance in MM. We further hypothesised that CMA would be upregulated in Bz resistant cells, which subsequently would compensate the UPR in alleviating ER stress. Therefore, the inhibition of CMA in Bz resistant cells was hypothesised to result in cell death, providing a novel therapeutic target in treating Bz resistance in MM.
Date of Award2017
Original languageEnglish

Keywords

  • proteins
  • autophagy
  • physiology
  • multiple myeloma
  • pathology
  • treatment

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