As we have demonstrated here, modulation in the levels of SM22 in fibroblast cells alters actin morphology which in turn affects their adhesion and migration phenotype. These changes are consistent with increases or decreases in the overall actin bundling activity in the cell resulting in decreased or increased actin dynamics respectively. Studies in yeast have revealed similar consequences for the effects of Scp1p, the yeast homologue of SM22, in actin dynamics and actin dependent processes such as endocytosis [11, 12, 28]. The actin binding and bundling activity of SM22 and related proteins is conferred via sequences c-terminal of the calponin homology (CH) domain including a short linker peptide and one to three calponin-like or 'CLIK23' repeats . The presence of multiple CLIK repeats can increase the actin bundling properties of the protein thus stabilizing the actin cytoskeleton of the cell . By the use of the C. elegans UNC-87 protein, which contains 7 CLIK repeats , Gimona and colleagues elegantly demonstrated that expressing increasing numbers of UNC-87 CLIK repeats progressively stabilized the actin cytoskeleton and inhibited the cell motility and growth in soft agar of breast cancer cell lines .
Stabilization of the actin cytoskeleton by the overexpression of an actin bundling protein might be expected to have an effect on properties of the cells that rely upon a dynamic actin cytoskeleton. However, the ability of CLIK repeat-containing proteins and for SM22 in particular to achieve this effect appears to be a more specific function that could be associated with suppression of the tumour phenotype. Numerous studies have identified SM22 as being downregulated in cancer [26, 30–35] and some have even gone so far as to claim it as a tumour suppressor . However, there are contradictory reports suggesting a positive correlation between an increase in SM22 levels and colon cancer metastasis to lymph node  and increased SM22 levels in gastric cancer , though in the latter study it was acknowledged that this was likely due to SM22 associated with the increased vascularisation of the tumour rather than in the tumour cells themselves. Proteomic profiling of prostate cancer cell lines revealed considerable variation in levels of SM22 in PC3 and LNCaP cells, even between different clonal variants of the same cell line . Similar studies comparing prostate, colorectal and hepatocellular carcinoma also identified increased levels of SM22 associated with invasiveness .
Whilst the various studies investigating SM22 levels in different tumours appear to be at odds, one has to consider the cellular basis for the various cancers under scrutiny. SM22 is normally expressed in mesenchymal cells and not in epithelial cells, however in tumour cells undergoing an epithelial to mesenchymal transition (EMT), it may be that SM22 is re-expressed as a consequence of the adoption of the mesenchymal phenotype. A similar phenomenon occurs with other epithelial proteins in EMT, such as the E-cadherin to N-cadherin switch . Therefore differences observed in the levels of SM22 in different tumours may be a reflection of several factors, including the tissue/cell of origin, whether it is a primary or secondary tumour, the cellular phenotype with respect to EMT, the precision of the sampling of the tumour if it was conducted on a tissue sample, and/or the relationship between a cell line and the original tumour if it is an in vitro study. Therefore SM22 may be upregulated in an adenocarcinoma during EMT, but downregulated in a sarcoma. Additionally SM22 appears to have a role in regulating transcription, with some genes being regulated in an SM22-dependent way [31, 36] which could also modulate the tumour phenotype. In particular, relief of the repression of the matrix metalloproteinase MMP9 expression by SM22 in tumour cells where SM22 is downregulated  would lead to increased MMP9 levels. This could have a significant impact on cancer progression, especially invasion and metastasis. Whether the effect of SM22 re-expression suppressing the Matrigel invasion phenotype of PC3 cells is a consequence of reduced MMP9 expression or simply an effect on the dynamics of the actin cytoskeleton will require further analysis. This coupled with our observation that a reduction in SM22 levels also caused an increase in podosome and rosette-like structures in non-transformed REF52 cells might tend to argue in favour of SM22 acting as a tumour suppressor . But the lack of a consistent association between a reduction in SM22 levels and tumour phenotype, and the contrary evidence of SM22 overexpression reducing growth in soft agar,  would tend to argue against SM22 being a true tumour suppressor. One additional explanation for why loss of SM22 might be advantageous in a tumour environment comes from its effect on the levels of reactive oxygen species (ROS) [13, 37, 38]. As we show here, SM22 depletion results in REF52 cells being less susceptible to stress-induced ROS production, in a manner analogous to the reduction in ROS levels seen upon Scp1P depletion in S. cerevisiae. Moreover, oxidative stress in diploid fibroblasts specifically upregulates SM22 expression via a TGF-β dependent mechanism, contributing to the senescent phenotype . Reduction in SM22 levels would therefore tend to promote cell survival when cells are under stress, such as in a tumour environment, and may also contribute to increases in actin dynamics that favour metastatic potential.