The Role of UNC-45 in Heart Development and Human Breast Cancer
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The C. elegans UNC-45 has been demonstrated as a molecular chaperone of body-wall muscle myosins. In human and other vertebrates, two UNC-45 genes code two UNC-45 protein isoforms: UNC-45A/a and UNC-45B/b. UNC-45A/a is expressed in all cell types, whereas UNC-45B/b only exists in heart and skeletal muscles. This thesis investigated the distinct roles of UNC-45 isoforms in heart development and breast cancer progression. As a molecular chaperone, mouse UNC-45b directly bound cardiac muscle myosin II and transcription factor GATA4, facilitating the folding of those chaperone client proteins. The recessive loss-of-function mutation of UNC-45b produced embryonic lethality at E9.5, due to the failure of contracting heart and arrest of cardiac morphogenesis of right heart formation. The decreased accumulation of UNC-45b led to the degradation of client proteins, but not their mRNAs. The consequent reduction in sarcomeric cardiac muscle myosins, which serve as motor proteins for cardiac contraction, decreased the heart contractility. The diminished accumulation of GATA4 protein led to decreased expression of its downstream targets, such as Hand1, Hand2, and Nkx-2.5, leading to the arrest of cardiac morphogenesis of the right heart. UNC-45A molecular chaperone bound to non-muscle myosin II, which were critical for cell division and motility. In human breast cancer patient specimens and cell lines, UNC-45A mRNA and protein were up-regulated. The knockdowns of UNC-45A by shRNAs in MDA-MB-231 breast cancer cells decreased their cell proliferation and migration, consistent with a reduction in UNC-45A leading to decreased function of its client protein non-muscle myosin II. Furthermore, we found that UNC-45A was alternatively expressed at both mRNA and protein levels as two isoforms. The only difference between two isoforms was a proline-rich 15 amino acid sequence, which appears to be critical for protein degradation through ubiquitin-proteasome system.