CYB5D2 Possesses Tumour Suppressing Activities

Abstract

<p>Loss of chromosome 17p is frequently observed in various cancers. One of the most commonly mutated targets p53 is on chromosome 17p13.1. However, studies have also reported loss of the 17p13.2 region in breast and medulloblastoma, thereby suggesting the residence of potential tumour suppressors in 17p13.2. Cytochrome b5 domain containing 2 (CYB5D2) is located on 17p13.2 implying CYB5D2 being a candidate tumour suppressor. CYB5D2 (neuferricin) belongs to the family of membrane associated progesterone receptors (MAPR). The archetypal member of the family, progesterone receptor membrane component 1 (PGRMC1), has been shown to play a role in domains independently of its function in mediating progesterone signalling. Consistent with this, CYB5D2 was reported to promote neurogenesis and inhibit the proliferation of Neuro2a cells. However, its role in tumorigenesis remains unknown.</p> <p>To investigate the role of CYB5D2 in tumorigenesis, western blot analysis was performed on 20 matched clear cell renal cell carcinomas (ccRCC) and the adjacent non-tumour kidney (ANK) tissues; significant down-regulation of CYB5D2 was demonstrated in ccRCC in comparison to ANK tissues, an observation that was confirmed by immunohistochemistry (IHC) analysis of 9 pairs of ccRCC-ANK tissues. Ectopic expression of CYB5D2 inhibited the proliferation and the invasion of A498 ccRCC along with the inhibition of AKT activation. Collectively, the above results support the possibility of CYB5D2 being a potential tumour suppressor.</p> <p>In support of the results obtained in ccRCC, we were able to show a significant reduction of CYB5D2 in cervical squamous carcinoma compared to normal cervical tissues in our analysis of CYB5D2 expression in 35 cervical squamous tumours. In vitro, overexpression and knockdown of CYB5D2 inhibited and enhanced the invasion of HeLa cells, respectively. As a member of the MAPR family, CYB5D2 contains the signature motif of the family, the cytochrome b5 (cyt-b5) like heme/steroid binding domain. This domain is known for heme binding and research in our laboratory has shown the residue D86 being critical for heme association. Ssubstitution of D86 with G (D86G) abolished not only CYB5D2's ability to bind heme but also its capacity of inhibiting HeLa cell invasion. Taken together, we provide evidence that CYB5D2 possesses activities in suppressing tumorigenesis, at least for the tumorigenesis of ccRCC and cervical squamous carcinoma.</p>