Comparison of human and mouse AKR1C enzymes: implications for modeling human cancer
Veliça, Pedro Miguel do Nascimento (2010)
Ph.D. thesis, University of Birmingham.
Human aldo-keto reductases (AKR) of the 1C subfamily have been implicated in the progression of prostate, breast and endometrial carcinomas as well as leukaemias due to their ability to modify key signaling molecules: steroid hormones and prostaglandins (PGs). In leukaemia, the AKR1C3 isoform has been identified as a novel therapeutic target since its PGD2 reductase activity prevents cell differentiation. Mice are ideal organisms for in vivo studies, using knock-out or over-expression strains. However, no mouse models for AKR1C enzymes have been generated to date since the functional conservation of these enzymes between human and mice is yet to be described. This study compared and characterised the phylogeny, substrate preference and tissue expression profile of the four human (AKR1C1,-1C2, -1C3 and -1C4) and the eight mouse (AKR1C6, -1C12, -1C13, -1C14, -1C18, -1C19, -1C20 and -1C21) isoforms. Despite being orthologues, AKR1C enzymes of mouse and humans have undergone significant divergence in number and sequence which was reflected in different substrate preference and tissue distribution. Mouse AKR1C isoforms lacked the PGD2 reductase activity but AKR1C6 was able to reduce PGE2 instead, an activity absent amongst human isoforms. Reduction of the key steroids androstenedione, 5\(\alpha\)-dihydrotestosterone, progesterone and estrone was performed by 4 of the murine isoforms. However, unlike humans, no AKR1C isoforms were detected in murine prostate, testes, uterus and haemopoietic progenitors. This study exposes significant lack of phylogenetic and functional conservation between human and murine AKR1C enzymes. Therefore, it is concluded that mice are not suitable to model the role of AKR1C in human carcinomas and leukemia. Additionally, the role of PGD2 in adult muscle differentiation was investigated using the mouse myoblast cell line C2C12. PGD2, but not PGE2 or PGF2\(\alpha\), inhibited myotube formation in a dose dependent manner. Exposure to PGD2 disrupted the expression of myogenic regulators (MyoD and myogenin) during differentiation and suppressed cell fusion, \(\alpha\)-actin expression and creatine kinase activity. Inhibition of myogenesis was independent of PGD2 surface receptors DP1 and DP2 and activation of the peroxisome proliferator-activated receptor \(\gamma\) (PPAR\(\gamma\)) suggesting a new form of signaling is involved. This discovery has implications in the interplay between inflammation, where high levels of PGD2 are secreted, and adult muscle regeneration, two processes that are intimately related.
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