PPIC

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]

PPIC has been shown to interact with:

• CsA,^[13][15]
• CyCAP,^[13][15]
• calcineurin,^[13] and
• NFATc1.^[13]