RLIP76 Contributes to Pancreatic Cancer Cell Resistance to Chemotherapy and Radiation

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Newswise — LAKE TAHOE, Nev. — Researchers at the City of Hope Comprehensive Cancer Center have not only identified a protein that allows pancreatic cancer cells to resist therapy but also developed a way to block it, according to results presented at the American Association for Cancer Research’s Pancreatic Cancer: Progress and Challenges conference, held here June 18-21.

“Cancer of the pancreas is notoriously difficult to treat because it usually presents at late stages due to minimal or nonspecific symptoms in the early stages. Thus, surgical treatment is either not possible or fails to cure most patients, resulting in a dismal prognosis — 90 percent of patients are likely to die within a year,” said Sanjay Awasthi, M.D., professor of medical oncology and therapeutics research, and diabetes, endocrinology and metabolism at City of Hope in Duarte, Calif.

One of the reasons that pancreatic cancer is so difficult to treat is that the tumor cells are largely resistant to the cell death caused by radiation and chemotherapy. Awasthi and colleagues have previously shown that the protein RLIP76 plays a key role in defending cells from other types of cancers from death caused by exposure to chemotherapy or radiation.

“RLIP76 works like an exhaust system,” said Awasthi. “It pumps out the toxic chemicals that accumulate in the cancer cell as a result of chemo- or radiotherapy before they can cause cell death.”

Now, Awasthi and colleagues have found that there is more RLIP76 in human pancreatic cancer cells than there is in normal human pancreatic cells. Depleting levels of RLIP76 killed human pancreatic cancer cells in culture and shrank established human pancreatic tumors in mice. Moreover, blocking RLIP76 or depleting levels of the protein dramatically enhanced the ability of radiation and the chemotherapeutic agent doxorubicin to destroy human pancreatic cells in culture. “Pancreatic cancer patients are a special case of the particularly unlucky, and in many ways the most miserable. The drug and radiation resistance of this cancer is legendary,” Awasthi said.

“Because we have defined RLIP76 as an essential drug- and radiation-resistance mediating protein, and because blockade of RLIP76 caused regression of less therapy-resistant cancers (lung, colon, breast), as well as equally resistant cancers (kidney, prostate, resistant neuroblastoma), we undertook the present studies to determine whether this treatment could be effective in pancreatic cancer.

“Fortunately, our data show that the seemingly unconquerable pancreatic cancer has an Achilles heel; its toxin exhaust system,” he explained. “Moreover, plugging this exhaust caused pancreatic cancer cell death, leaving normal cells relatively unfazed. We hope to translate these studies into clinical trials in the near future.”

According to Awasthi, they saw an added benefit to depleting levels of RLIP76 in the mice with established tumors: a decrease in blood sugar, cholesterol and triglycerides. “These findings indicate that it might be possible to develop a single class of medications that have potent antidiabetic and anticancer effects,” he said.

Awasthi is the founder of Terapio, which makes the recombinant RLIP76 protein for treatment of radiation poisoning. However, Terapio does not work on the RLIP76-blockading technology that is the crux of the anticancer treatment strategy.

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Targeted Therapy for Pancreatic Cancer. Kathryn Leake, Sharad S. Singhal, Sanjay Awasthi. Beckman Research Institute, City of Hope, Duarte, CA. Pancreatic cancer is a highly aggressive and lethal neoplasm, generally resistant to chemotherapy and radiation. In-vitro studies have indicated a role of PI3K/AKT as mechanisms of apoptosis resistance in pancreatic cancer. RLIP76 is a multi-functional cell membrane protein that functions as a major mercapturic acid pathway transporter as well as key regulator of receptor-ligand complexes through regulation of the rate of clathrin-dependent endocytosis. Inhibition of RLIP76 has been shown to antagonize both PI3K and AKT in other cancers. Present studies were undertaken to determine whether targeted depletion of RLIP76 can be an effective antineoplastic therapy that overcomes chemo-radio-resistance in pancreatic cancer, and whether the PI3K/Akt signaling pathway is affected. Cell survival was assessed by MTT and colony forming assays. Cellular levels of proteins and phosphorylation was determined by Western blot analyses. The impact on apoptosis was determined by TUNEL assay. The anti-cancer effects of RLIP76 targeted interventions in vivo were determined using mice xenograft model of the pancreatic cancer. Doxorubicin transport was measured using 14C-DOX, and radiation sensitivity was determined by colony forming assays, respectively. RLIP76 depletion caused marked and sustained regression of established human BxPC-3 pancreatic cancer tumors in nude mouse xenograft model. The ability of RLIP76 to mediate drug and radiation resistance was confirmed. Results of signaling studies are consistent with an encompassing model for the role of RLIP76 in regulating the levels of fundamental proteins like PI3K, Akt, E-cadherin, CDK4, Bcl2 and PCNA. These studies show that RLIP76 represents a mechanistically significant target for developing effective interventions drug and radiation-resistant pancreatic cancers. (Supported by USPHS-NIH R01-CA77495 to SA)