An investigation into the role of lamin A in the progression of colorectal cancer
Nuclear lamins are type V intermediate filaments which form a proteinaceous meshwork, termed the nuclear lamina, which underlines the inner nuclear membrane and provides mechanical strength to the nucleus and maintains nuclear shape. A-type lamins in particular have been implicated in DNA replication, the regulation of gene transcription, apoptosis and nuclear migration. Expression of lamin A/C is closely associated with the differentiated phenotype and loss of lamin A/C xpression has been correlated with increased proliferation, especially in tumours. I sought to investigate the expression and regulation of A- and B-type lamins during colorectal cancer (CRC) progression. Preferential down-regulation of lamin A expression over lamin C was observed in the most dedifferentiated CRC cell lines. Semi-quantitative RT-PCR suggested that this was achieved by both transcriptional and post-transcriptional mechanisms. A connection between loss of lamin A/C and proliferation was ruled out. Instead immunohistochemical analysis of CRC tissue sections indicated loss of lamin A may correlate with the differentiation status of cells. In normal colonic crypts lamin A/C expression was greatest in the differentiated compartment, whereas lamin A was absent and lamin A/C was present at barely detectable levels in Dukes' A malignant polyps with high grade dysplasia. Stable re- expression of lamin A constructs in SW480 colon cancer cells whichexpressed almost no endogenous lamin A rescued two-dimensional growth. Subsequent RNA profiling of 325 genes with reported relevance to colorectal carcinogenesis and general tumourigenesis confirmed that proliferation indices were unaffected by changes in the level of lamin A. Synemin, a cytoskeletal linker protein, was found to be significantly down-regulated in SW480 GFP-lamin A transfected cells versus SW480 GFP transfected cells. This suggests that lamin A functions to maintain nuclear and cellular integrity by indirect modulation of components of cytoskeletal architecture.