Constitutive activation of nuclear facto -kB: preferntial homodimerization of p50 subunits in cervical carcinoma

Abstract

Infection of specific types of 'high risk' HPVs such as HPV 16 and HPV 18 has been associated with the development of cervical cancer. Deregulation of specific NF-kB members has also been implicated with the development of many cancers including cervical cancer. We have studied the expression and DNA-binding activity of NF-kB during the development of cervical cancer involving cervical precancer, cancer and control tissues with or without HPV infection. We observed constitutive activation of NF-kB to a significant level in squamous-cell carcinomas while no or negligible NF-kB binding activity was observed in normal controls or precancerous lesions. Interestingly, there is a gradual increase in binding activity of NF-kB and differential expression pattern of NF-kB family of proteins from low-grade squamous intraepithelial lesions (LSIL) to high-grade squamous intraepithelial lesion (HSIL) and to invasive squamous cell carcinoma (SCC). On further dissection of NF-kB complex, the p⁵⁰ subunit which generally heterodimerizes with p65 to form an active form NF-kB appears to form a p50/p50 homodimer instead of conventional p50/p65 heterodimer. In situ analysis of expression of p50 and p65 subunits by immunohistochemistry in tissue sections from different grades of cervical lesions including invasive cancer also demonstrate a gradual increase in the expression and nuclear localization of p50 subunit only as the severity of lesions increases. We have observed a very high expression of nuclear p50 in HSIL and invasive cancers while the level of nuclear p65 is significantly lower or nil. We further observed that this activation is not dependent on HPV infection since both HPV positive and HPV negative tumors showed the same pattern of high binding activity and increased expression of NF-kB p50/p50 homodimer. Although nuclear translocation and localization of p65 was observed to a lesser extent in invasive tumor, p65 was not found to be involved in dimmer formation. Thus the gradual activation and expression of NF-kB as a function of severity of cervical lesions and the change in dimerization pattern in favor of p50/p50 homodimers appear to play an important role during the development of cervical carcinoma.