Peptidyl-prolyl cis-trans isomerase C (PPIC) is an enzyme that in humans is encoded by the PPICgene on chromosome 5. As a member of the peptidyl-prolyl cis-trans isomerase (PPIase) family, this protein catalyzes the cis-transisomerization of proline imidic peptide bonds, which allows it to facilitate folding or repair of proteins.[5][6][7] In addition, PPIC participates in many biological processes, including mitochondrial metabolism, apoptosis, redox, and inflammation, as well as in related diseases and conditions, such as ischemic reperfusion injury, AIDS, and cancer.[8][9][10][11]
Like other cyclophilins, PPIC forms a β-barrel structure with a hydrophobic core. This β-barrel is composed of eight anti-parallel β-strands and capped by two α-helices at the top and bottom. In addition, the β-turns and loops in the strands contribute to the flexibility of the barrel.[10] PPIC in particular is composed of 212 residues and contains a hydrophobic, ER-targeting sequence at the N-terminal. The PPIase domain is homologous to PPIA and can be bound and inhibited by CsA.[6]
The protein encoded by this gene is a member of the peptidyl-prolyl cis-trans isomerase (PPIase) family. PPIases catalyze the cis-trans isomerization of proline imidic peptide bonds in oligopeptides and accelerate the folding of proteins.[7] Generally, PPIases are found in all eubacteria and eukaryotes, as well as in a few archaebacteria, and thus are highly conserved.[8][12] The PPIase family is further divided into three structurally distinct subfamilies: cyclophilin (CyP), FK506-binding protein (FKBP), and parvulin (Pvn).[8][10] As a cyclophilin, PPI binds cyclosporin A (CsA) and can be found within the cell or secreted by the cell.[9] In eukaryotes, cyclophilins localize ubiquitously to many cell and tissue types, though PPIC especially is highly expressed in kidney.[9][10][13] In addition to PPIase and protein chaperone activities, cyclophilins function in mitochondrial metabolism, apoptosis, immunological response, inflammation, and cell growth and proliferation.[8][9][10] Along with PPIB, PPIC localizes to the endoplasmic reticulum (ER), where it maintains redox homeostasis. Depletion of these two cyclophilins lead to hyperoxidation of the ER.[11] In the brain, PPIC complexes with cyclophilin C-associated protein (CyCAP) to activate microglia and macrophage function via the calcineurin/NFAT pathway.[13]
As a cyclophilin, PPIC binds the immunosuppressive drug CsA to form a CsA-cyclophilin complex, which then targets calcineurin to inhibit the signaling pathway for T-cell activation.[9]
In cardiac myogenic cells, cyclophilins have been observed to be activated by heat shock and hypoxia-reoxygenation as well as complex with heat shock proteins. Thus, cyclophilins may function in cardioprotection during ischemia-reperfusion injury.[9] Similarly, PPIC may confer neuroprotection by forming a complex with CyCAP to activate survival mechanisms and mitigate ischemic damage in the brain.[13]
Currently, cyclophilin expression is highly correlated with cancer pathogenesis, but the specific mechanisms remain to be elucidated.[9] For instance, studies identify PPIC as a reliable indicator of circulating tumor cells in epithelial ovarian cancer (EOC) and, thus, may serve as a biomarker for detection and treatment of the cancer.[14]
^ abcdeWang T, Yun CH, Gu SY, Chang WR, Liang DC (Aug 2005). "1.88 A crystal structure of the C domain of hCyP33: a novel domain of peptidyl-prolyl cis-trans isomerase". Biochemical and Biophysical Research Communications. 333 (3): 845–9. doi:10.1016/j.bbrc.2005.06.006. PMID15963461.
^ abcdefgYamaguchi R, Hosaka M, Torii S, Hou N, Saito N, Yoshimoto Y, Imai H, Takeuchi T (Jun 2011). "Cyclophilin C-associated protein regulation of phagocytic functions via NFAT activation in macrophages". Brain Research. 1397: 55–65. doi:10.1016/j.brainres.2011.03.036. PMID21435337. S2CID28737748.
^Obermayr E, Castillo-Tong DC, Pils D, Speiser P, Braicu I, Van Gorp T, Mahner S, Sehouli J, Vergote I, Zeillinger R (Jan 2013). "Molecular characterization of circulating tumor cells in patients with ovarian cancer improves their prognostic significance -- a study of the OVCAD consortium". Gynecologic Oncology. 128 (1): 15–21. doi:10.1016/j.ygyno.2012.09.021. PMID23017820.
