Molecular Impact of Platinum Drugs on the Proteasome and SQSTM1_P62 Complexes_A Paradigm Shift in Resistance

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Lung cancer-related death is primarily due to disease recurrence, drug resistance, and metastasis. Platinum compounds such as cisplatin (CDDP) and carboplatin (CBDCA) and their derivatives are widely used in the treatment of lung cancer. Although the tumors initially respond to platinum drugs, they adeptly develop resistance thereby escaping therapy. Therefore, understanding the mechanisms by which cancer cells evade therapy and develop resistance is essential for developing new therapeutic approaches for lung cancer. This application addresses a highly innovative and high-impact area of translational research that focuses on investigating how platinum-based drugs impact the proteasome and sequestosome (SQSTM1)/P62 function in cancer cells to produce drug resistance in lung cancer. Further, a nanodelivery approach targeted towards the proteasome and SQSTM1/P62 in combination with CDDP for overcoming resistance is proposed. Our interest in testing the proteasome and SQSTM1/P62 in chemoresistance stems from a serendipitous observation made in the laboratory. We observed beta 5 (β5) expression, a subunit of the large 26S proteasome complex was markedly reduced in cisplatin-resistant (CDDPR) cancer cell lines when compared to its isogenic cisplatin-sensitive (CDDPS) cell lines. β5 is the chymotryptic component of the proteasome that is required for degrading ubiquitinated proteins and recycling of amino-acids for synthesis of new proteins in the cell. Associated with reduced β5 expression in the CDDPR cells was the intracellular accumulation of proteins. Investigation into how the cellular stress induced by intracellular accumulation of proteins is overcome by the cells revealed a role for SQSTM1/P62. The primary function of SQSTM1/P62, a scaffolding protein that is activated in response to cellular stress, is to prevent cell death by aggregating intracellular accumulated polyubiquitinated proteins into aggresomes and directing towards autophagy, thereby promoting cell survival. Further, analysis for P62 expression in a subset of human lung tumor tissues showed that chemoexperienced lung tumors had higher P62 expression compared to chemonaive tumors. Although, reduced proteasome function and increased SQSTM1/P62 expression have previously been reported in cancer cells and in stem cells, the impact of chemotherapy drugs on these cellular machineries and their role in contributing to resistance has not been previously investigated and is the basis of this innovative proposal. Based on our preliminary results, we hypothesize that alterations in the proteasome and SQSTM1/p62 function in cancer cells contributes to platinum resistance. To test our hypothesis we have identified three specific aims. Aim 1. Investigate how modulating the proteasome and SQSTM1/P62 in CDDPR and CDDPS cancer cells and in normal cells alters the therapeutic response to platinum drugs in vitro. In this aim, the requirement of beta-5 subunit of the proteasome and SQSTM1/P62 to platinum sensitivity and cross-resistance to other anticancer drugs will be investigated using isogenic human cancer cell lines and compared to normal cells. Aim 2. Demonstrate restoring beta-5 subunit of the proteasome with simultaneous knock-down of SQSTM1/p62 using multifunctional nanoparticle reverts CDDPR lung tumor sensitivity to cisplatin in vivo. In this aim, we will use lung tumor xenograft and patient-derived xenograft models (PDX) to test a) whether restoration of beta-5 with simultaneous silencing of P62 in CDDPR tumors restores platinum sensitivity and b) knock-down of beta-5 with concomitant overexpression of P62 in CDDPS tumors results in platinum resistance. Aim 3. Determine the biological significance of beta-5 and SQSTM1/P62 protein expression in chemonaïve and chemotreated human lung tumor tissue specimens with clinical benefit. In this aim, archival lung tumor specimens representing primary and metastatic tumor will be examined for beta-5 and SQSTM1/P62 expression and correlate with clinical benefit.