Camptothecin resistance is determined by the regulation of topoisomerase I degradation mediated by ubiquitin proteasome pathway.

Proteasomal degradation of topoisomerase I (topoI) is one of the most remarkable cellular phenomena observed in response to camptothecin (CPT). Importantly, the rate of topoI degradation is linked to CPT resistance. Formation of the topoI-DNA-CPT cleavable complex inhibits DNA re-ligation resulting in DNA-double strand break (DSB). The degradation of topoI marks the first step in the ubiquitin proteasome pathway (UPP) dependent DNA damage response (DDR). Here, we show that the Ku70/Ku80 heterodimer binds with topoI, and that the DNA-dependent protein kinase (DNA-PKcs) phosphorylates topoI on serine 10 (topoI-pS10), which is subsequently ubiquitinated by BRCA1. A higher basal level of topoI-pS10 ensures rapid topoI degradation leading to CPT resistance. Importantly, PTEN regulates DNA-PKcs kinase activity in this pathway and PTEN deletion ensures DNA-PKcs dependent higher topoI-pS10, rapid topoI degradation and CPT resistance.

DIFFERENTIAL SERUM LEVEL OF SPECIFIC HAPTOGLOBIN ISOFORMS IN SMALL CELL LUNG CANCER.

Lung cancer is the leading cause of cancer death for both men and women in the United States, and similar trends are seen world wide. The lack of early diagnosis is one of the primary reasons for the high mortality rate. A number of biomarkers have been evaluated in lung cancer patients, however, their specificity and early stage diagnostic values are limited. Using traditional protein chemistry and proteomics tool we have demonstrated higher serum haptoglobin levels in small cell lung cancer (SCLC). Similar findings have been reported for other cancers including ovarian cancer and glioblastoma. Haptoglobin is an acute phase protein with at least six possible phenotypes. The six phenotypes, in combination with two post translational modifications, glycosylation and deamidation, lead to large numbers of possible haptoglobin isoforms. Recent studies indicate a possible correlation between specific haptoglobin glycosylation and particular disease conditions. In our current study, we have fractionated control and SCLC patient serum by 2-D gel electrophoresis to identify differentially expressed haptoglobin isoforms in SCLC serum samples.