Unfolded protein response (UPR) is a stress response induced in tumor cells because of higher rate of protein synthesis and subsequent misfolding of newly synthesized proteins in the endoplasmic reticulum (ER). To restore normal function of the cell, ER induces the expression of molecular chaperones, such as heat-shock proteins (Hsps) that try to correct protein misfolding. If the correction machinery fails, programmed cell death (apoptosis) could be induced in these cells. Therapeutic focused ultrasound is a promising non-invasive approach to ablate solid tumors. We have demonstrated previously that low intensity focused ultrasound (LOFU) induces mechanical stress in tumor cells and induce UPR. We hypothesized that induction of unfolded/misfolded protein burden in ER by LOFU could sensitize the tumor tissue for chemotherapeutic agents, such as, 17AAG, a HSP90 inhibitor by increasing the ER stress and switching on the apoptotic pathway. We explored whether concomitant application of 17AAG along with LOFU could increase the therapeutic ratio in murine prostate cancer model. RM-1 (murine prostate cancer cells) tumors were subjected to intratumor injection of 17AAG (10mg/kg of body weight, 2/week) and then treated with LOFU (3W at a frequency of 1Mhz, 100% duty cycle) (1/week) for three weeks. Tumor growth was measured twice a week. Three weeks after initiation of LOFU-17AAG treatment cycle, animals were sacrificed and histopathological analysis (H&E and TUNEL) and flowcytometric analysis were done on the isolated tumor tissues. The expression of the UPR target genes was determined by qRT- PCR.

Significant reduction of tumor growth was observed in 17AAG + LOFU treated cohort compared to only 17AAG treatment (p<0.001) and untreated control (p<0.004). LOFU alone does not affect tumor growth but increased the expression of UPR target genes IRE1α (2.6±0.54 fold) and CHOP (2±0.23 fold). However application of 17AAG along with LOFU activated the protein kinase RNA (PKR)-like ER kinase (PERK), a key regulator of UPR mediated apoptotic pathway and increased the CHOP mRNA level by 23±0.82 folds, the major target genes to switch on the apoptotic pathway. Histopathological analysis further confirmed this result and showed increase in the number of apoptotic cells compared to 17AAG treatment groups (p<0.04) and untreated control (p<0.01). Flowcytometric analysis of isolated tumor tissue showed significant decrease in VEGF expression in the mice received combination treatment compared to control (p<0.006) suggesting that combination treatment might have a role to control over tumor angiogenesis. These experiments indicate that a noninvasive, nonablative LOFU treatment in combination with 17AAG could be promising in the treatment of recurrent cancer. Further studies are underway to understand the molecular crosstalk between LOFU, UPR and apoptotic signaling.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1776. doi:10.1158/1538-7445.AM2011-1776