MOLECULAR AND FUNCTIONAL INVESTIGATION OF CANCER-TYPE AND LIVER-TYPE VARIANTS OF ORGANIC ANION TRANSPORTING POLYPEPTIDE 1B3
Year of Publication
Doctor of Philosophy (PhD)
Dr. Wooin Lee
Dr. Patrick McNamara
OATP1B3 belongs to the OATP (organic anion transporting polypeptides) superfamily, responsible for mediating the transport of various endogenous and xenobiotic substrates. OATP1B3 was initially reported to be expressed exclusively in the hepatocytes where it mediates the uptake of numerous endogenous substrates (e.g. bile acids, steroid hormone conjugates) and several clinically relevant drugs including anticancer drugs. Later, a number of studies reported that OATP1B3 is also frequently expressed in multiple types of cancers and may be associated with differing clinical outcomes. However, a detailed investigation on the expression, localization and functions of OATP1B3 expressed in cancer has been lacking. In this thesis work, we confirmed that colon and pancreatic cancer cells express a cancer-specific OATP1B3 variant (csOATP1B3), different from OATP1B3 wild-type (WT) expressed in the normal liver. The csOATP1B3 utilizes an alternative transcription initiation site and the translated product of csOATP1B3 lacks the first 28 amino acids at the N-terminus of OATP1B3 WT. Our results show that csOATP1B3 has modest uptake transporter functions and reduced plasma membrane localization compared to OATP1B3 WT. In our efforts to investigate the regulatory mechanism underlying the expression of csOATP1B3, we found that hypoxia inducible factor-1α (HIF-1α) may play a key role in the regulation of csOATP1B3 in colon and pancreatic cancer cells. In a separate study, we tested whether the N-terminal sequence of OATP1B3 WT plays an important role in the membrane trafficking. This is based on the observation that csOATP1B3 lacking the first 28 amino acids at N-terminus of OATP1B3 WT displays a predominantly cytoplasmic localization pattern. Using the constructs with N-terminal truncations and point mutations, we verified that the N-terminus of OATP1B3 WT contains important motifs in its membrane trafficking. In particular, the amino acids within a putative β-turn-forming tetrapeptide appear to be important in regulating the membrane trafficking of OATP1B3 WT. The findings from this thesis work provide important insights into the functional and clinical significance of OATP1B3 in cancer and normal liver.
Thakkar, Nilay, "MOLECULAR AND FUNCTIONAL INVESTIGATION OF CANCER-TYPE AND LIVER-TYPE VARIANTS OF ORGANIC ANION TRANSPORTING POLYPEPTIDE 1B3" (2015). Theses and Dissertations--Pharmacy. 44.