Regulation and drug insensitivity of F-actin association with adhesion areas of transformed cells

Emerging role of caldesmon in cancer: A potential biomarker for colorectal cancer and other cancers

Colorectal cancer (CRC) is a devastating disease, mainly because of metastasis. As a result, there is a need to better understand the molecular basis of invasion and metastasis and to identify new biomarkers and therapeutic targets to aid in managing these tumors. The actin cytoskeleton and actin-binding proteins are known to play an important role in the process of cancer metastasis because they control and execute essential steps in cell motility and contractility as well as cell division. Caldesmon (CaD) is an actin-binding protein encoded by the CALD1 gene as multiple transcripts that mainly encode two protein isoforms: High-molecular-weight CaD, expressed in smooth muscle, and low-molecular weight CaD (l-CaD), expressed in nonsmooth muscle cells. According to our comprehensive review of the literature, CaD, particularly l-CaD, plays a key role in the development, metastasis, and resistance to chemoradiotherapy in colorectal, breast, and urinary bladder cancers and gliomas, among other malignancies. CaD is involved in many aspects of the carcinogenic hallmarks, including epithelial mesenchymal transition via transforming growth factor-beta signaling, angiogenesis, resistance to hormonal therapy, and immune evasion. Recent data show that CaD is expressed in tumor cells as well as in stromal cells, such as cancer-associated fibroblasts, where it modulates the tumor microenvironment to favor the tumor. Interestingly, CaD undergoes selective tumor-specific splicing, and the resulting isoforms are generally not expressed in normal tissues, making these transcripts ideal targets for drug design. In this review, we will analyze these features of CaD with a focus on CRC and show how the currently available data qualify CaD as a potential candidate for targeted therapy in addition to its role in the diagnosis and prognosis of cancer.

Subversion of the actin cytoskeleton during viral infection

Viral infection converts the normal functions of a cell to optimize viral replication and virion production. One striking observation of this conversion is the reconfiguration and reorganization of cellular actin, affecting every stage of the viral life cycle, from entry through assembly to egress. The extent and degree of cytoskeletal reorganization varies among different viral infections, suggesting the evolution of myriad viral strategies. In this Review, we describe how the interaction of viral proteins with the cell modulates the structure and function of the actin cytoskeleton to initiate, sustain and spread infections. The molecular biology of such interactions continues to engage virologists in their quest to understand viral replication and informs cell biologists about the role of the cytoskeleton in the uninfected cell.

Altered actin organization in an N-methyl-N'-nitro-N-nitrosoguanidine-transformed pulmonary epithelial cell line

After in vitro-induced neoplastic transformation by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), altered actin organization in pulmonary epithelial cells was examined. A certain correlation was found between anchorage independency (AI), tumorigenicity in nude mice and altered actin organization. DNase I inhibition assay demonstrated significant loss of detergent-insoluble F-actin (stress fibers, p less than 0.02) and soluble F-actin (single filaments, p less than 0.0001) after AI transformation in comparison with untransformed cells, while the level of G-actin increased significantly (p less than 0.0001). At the same time, fluorescence and electron microscopy also revealed that after AI transformation there was a striking loss of stress fibers usually accompanied by reorganization of at least some of the lost stress fibers into F-actin aggregations. After s.c. implantation of AI-transformed cells into nude mice followed by recultivation of the developed tumors, DNase I-inhibition assay showed a significant increase (p less than 0.001) in the level of detergent-insoluble F-actin as compared with untransformed cells, but no significant difference in the amount of G-actin. However, most of this increased detergent-insoluble F-actin was in the form of aggregations as revealed by immunofluorescence and electron microscopy. This growth behaviour-dependent alteration in actin organization occurring after exposure to MNNG may be causally related to the progressive development of neoplastic phenotypes, although the biological significance of actin aggregation formation remains unclear. The results have also pointed out the importance of parallel investigations into both the biochemical and morphological statuses of actin, particularly when it may be regarded as an indicator of neoplastic transformation and malignancy.

Gene regulation in reverse transformation: cyclic AMP-induced actin homolog in CHO cells

Reverse transformation (RT) presents a challenge in understanding of the role of protein-genome interaction in regulating gene expression in normal and transformed cells. Early during RT of CHO-K1 cells by cyclic AMP a new protein, mol wt = 43,000 and pI = 5.3 +/- 0.2, was rapidly and specifically induced. This cAMP-induced protein (CIP) is a phosphorylated actin homolog. Induction required new protein synthesis. Actinomycin D treatment failed to inhibit CIP induction, suggesting the existence of an untranslated or sequestered mRNA in untreated cells. Expression of CIP was not dependent upon cell shape or cytoskeletal integrity as are other steps in RT. CIP was detectable only in cAMP-treated cells, whether transformed or nontransformed, and cAMP treatment inhibited growth of both cell types. CIP was associated with soluble cell fractions and not with F-actin. We propose that CIP plays an early role in RT, that is necessary but not sufficient for the complete RT process, and that it participates in the cAMP signaling pathway of cells through changes in the cytoskeleton. This pathway inhibits cell growth as required in the differentiated phenotype. A molecular model is presented for the RT reaction in CHO-K1, which also explains cAMP effects on transformed cells such as the S49 lymphoma and other malignancies.

Clonal selection by fluorescence redistribution after photobleaching (FRAP)--a "fast" lateral mobility fibroblast mutant (E7G1)

Fluorescence redistribution after photobleaching (FRAP) was utilized to select a "fast" lateral mobility clone from Kirsten murine sarcoma virus-transformed 3T3 (KMSV-3T3) fibroblasts. The clone, E7G1, demonstrated a lateral mobility for membrane wheat germ agglutinin (WGA) and succinylated concanavalin A (sCon A) receptors of (2.1 +/- 1.6) X 10(-9) cm2/s and (2.7 +/- 2.3) X 10(-9) cm2/s, respectively. These mobilities were approximately equivalent to phospholipid mobility (2.8 +/- 1.9 X 10(-9) cm2/s). The fast mobility phenotype is observed when the cells are unattached and spherical. Upon attachment, the mobility decreases to (0.19 +/- 0.19) X 10(-9) cm2/s. In addition, the ability of Con A to initiate global modulation was completely lost in spread as well as spherical cells in the E7G1 fast mobility clone. A comparison of F-actin patterns between untransformed Balb/c fibroblasts and the E7G1-transformed line suggests a correlation between well-developed stress fiber assemblies and the ability to induce global modulation. The fast mobility clone was stable for at least 23 passages.

Enucleation of normal and transformed cells

A quantitative analysis based on centrifugal force requirements for enucleation was developed to examine the response of a number of untransformed and transformed cell lines to cytochalasin mediated enucleation. Examination of the extent of cell enucleation as a function of centrifugal force resulted in a series of response curves demonstrating that enucleation g force requirements varied between Balb/c 3T3, Swiss 3T3, and Kirsten sarcoma virus transformed Balb/c 3T3 (3T3-K). A four times greater centrifugal force was required to reach 50% enucleation for transformed Balb/c 3T3-K when compared to Swiss 3T3. A qualitative correlation could be observed between ease of enucleation and the existence of a well-formed stress fiber network. A comparison of cytochalasin B and D suggested that cytochalasin D was far more effective in the enucleation of transformed cells. Experiments with 2-deoxyglucose and monensin provided evidence that decreasing cellular ATP levels, either directly or potentially by uncoupling ion transport from ATP generation, can decrease the efficiency of enucleation. It is suggested that the organization of the cytoskeleton is affected by the altered cellular ATP levels which can affect the centrifugal requirements of enucleation.

Effect of adenosine 3',5'-cyclic monophosphate on volume and cytoskeleton of MDCK cells

We examined the effect of N6,2'-O-dibutyryladenosine 3',5'-cyclic monophosphate (DBcAMP) on the volume and cytoskeleton of confluent cultures of Madin-Darby canine kidney (MDCK) cells. A 90-min exposure to 1 mM DBcAMP resulted in a 20% reduction in volume as measured by [14C]-urea water space. The volume in cells exposed to isobutylmethylxanthine (IBMX, 0.1 mM) was reduced by 24%. In control cultures F-actin, revealed by staining with nitrobenzoxadiazole-phallacidin, was found at the base of the cell as fibers, at the junctional region as a circumferential band, and on the apical cell surface as a mottled fluorescence. A dense pattern of microtubules, revealed by indirect immunofluorescence, was seen throughout the cell. Exposure to DBcAMP for 90 min resulted in a change of F-actin fibers into dense bundles near the periphery of the cell. This effect was even more striking when cells were exposed to IBMX. Cytochalasin B disrupted F-actin and resulted in a volume reduction similar to that in DBcAMP. Neither DBcAMP nor IBMX affected the distribution of microtubules. Moreover, colchicine, which completely disrupted the microtubules, did not change MDCK cell volume. The results suggest that DBcAMP and F-actin play a role in volume control in MDCK cells.