Differential expression of cell survival and cell cycle regulatory proteins in cutaneous squamoproliferative lesions

Previous models of cutaneous carcinogenesis have primarily focused on the r egulation of keratinocyte (KC) proliferation and differentiation. However, it has become clear in many neoplastic systems that altered rates of cell d eath and;or inabilty to undergo growth arrest can also contribute to the de velopment of cancer. Apoptosis-regulatory proteins include those that block apoptosis such as Bcl-2 and Bcl-x, whilst a related protein Bar promotes a poptosis. Cell cycle regulatory proteins include those associated with grow th arrest, i.e. p21(waf1), p53, and those associated with proliferation, i. e. Ki-67. Paraffin embedded samples from ten different lesions of squamous cell carcinoma (SCC), Bowen's disease (BD), keratoacanthomas (KA), and nine normal adult skin samples were stained by immunohistochemistry to detect e xpression of Bcl-2, Bcl-x, Bar, Ki-67, p21(waf1), p53 and apoptosis (TUNEL assay). Compared to low levels of Bcl-x and Bcl-2 immunostaining in normal skin, all the squamoproliferative lesions had strong and diffuse KC express ion of Bcl-x (> 80%) but minimal to absent KC Bcl-2 expression (< 15%). Bar immunopositivity was limited to the basal layer in normal skin and ED. In contrast, by examining serial sections both Bcl-x and Bar appeared to be co expressed by the majority of malignant KCs in KA and SCC (> 70%). These imm unostaining profiles reveal that squamoproliferative lesions, including inv asive transformed KCs, preferentially express Bcl-x over Bcl-2, in addition to upregulating their Bar levels. Even though there were numerous TUNEL po sitive cells in these squamoproliferative lesions, no other evidence of apo ptosis was seen reinforcing the necessity to use caution when relying on TU NEL staining for identification of programmed cell death in skin biopsies. Normal sun-exposed skin had low but detectable p53 and rare p21(waf1) KC ex pression. Significantly higher numbers of p21(waf1) and p53 immunopositive KCs were noted throughout the lesions in ED and SCC in contrast to KA where p53 and ran p21(waf1) immunopositive KCs were primarily limited to the per iphery of the tumor cell islands. In general, p53 KC expression was higher in all squamoproliferative lesions and sun-exposed normal skin compared to p21(waf1) expression. Summary of the expression of cell cycle regulatory pr oteins for both p21(waf1) and p53 KC expression was: SCC > ED > KA, in mark ed contrast to Ki-67 KC expression which was: BD > KA > SCC. The relatively few malignant cells in SCC that were actively participating in the cell cy cle (i.e. Ki-67 positive) suggests that these neoplasms may arise primarily by increased cell survival and resistance to apoptosis rather than by hype rproliferation. These studies emphasize the importance of examining multipl e members of protein families that regulate apoptosis, proliferation, growt h arrest, and differentiation. It is the overall balance between these cell ular phenomena that determine whether a cell remains viable or undergoes pr ogrammed cell death and contributes to the appearance of a neoplasm. The ov erexpression of Bcl-x may confer a survival advantage to malignant KCs unab le to growth arrest to repair damaged DNA (mutant p53) and/or undergo termi nal differentiation (increased p21(waf1)). Thus, mutation or aberrant expre ssion of such proteins may participate in the multistep process of carcinog enesis that gives rise to these squamoproliferative lesions. (C) 1999 Elsev ier Science Ireland Ltd. All rights reserved.