Abstract 4768: Determination of Rab GTPase-mediated pathways critical for the antimyeloma activity of Rab GGTase inhibitors

• Published in: Cancer research (Chicago, Ill.) Ill.), 2016-07, Vol.76 (14_Supplement), p.4768-4768
• Main Authors: Dykstra, Kaitlyn M., Allen, Cheryl L., Holstein, Sarah A.
• Format: Article
• Language: English
• ISSN: 0008-5472; 1538-7445
• DOI: 10.1158/1538-7445.AM2016-4768

Multiple myeloma (MM) is a plasma cell malignancy characterized by the production of high levels of monoclonal protein (MP). This leads to an increased protein folding burden in the endoplasmic reticulum (ER) in MM cells, resulting in a constitutively upregulated unfolded protein response (UPR) and a lower threshold for ER stress-mediated apoptosis. We have previously shown that inhibitors of Rab GGTase, the enzyme responsible for Rab GTPase geranylgeranylation, disrupt MP trafficking in MM cells, leading to an accumulation of MP in the ER and induction of the UPR and apoptosis. There are over 60 different Rab GTPases in mammalian cells involved in regulating a wide range of membrane trafficking events including exocytic, endocytic, and lysosomal pathways. In this study, we aimed to further define how disruption of RabGTPase function in MM cells leads to apoptosis by knocking down individual Rab proteins. Given the accumulation of MP and induction of UPR, our initial focus was on secretory Rabs. Knockdown of Rab1A/B (>50%) using siRNA in RPMI 8226 cells lead to moderate increases in intracellular lambda light chain in Rab1A/B knockdowns after 72 hours (∼120-140% of control levels by ELISA). However, despite the increase in intracellular MP, no increases in UPR or apoptosis markers were seen by western blot analysis or annexin-V/propidium iodide flow cytometry. Extended time course experiments (up to 120 hours) also did not exhibit any increase in UPR or apoptosis. Knockdown of Rab6A (>60%), another RabGTPase associated with the secretory pathway, did not lead to increases in intracellular lambda light chain or induction of apoptosis. These results suggest that although individual knockdown of Rab1 is sufficient to partially disrupt MP trafficking, either more complete knockdown of the individual Rabs or knockdown of multiple Rabs is necessary for induction of the UPR and apoptosis. Additional optimization of the knockdowns will be performed in order to verify these results. These studies support the further development of Rab GGTase inhibitors that globally effect Rab GTPase geranylgeranylation as a novel anti-myeloma strategy. Citation Format: Kaitlyn M. Dykstra, Cheryl L. Allen, Sarah A. Holstein. Determination of Rab GTPase-mediated pathways critical for the antimyeloma activity of Rab GGTase inhibitors. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia