Splice variants of human cathepsin L mRNA show different expression rates.

Human cathepsin L (hCATL) mRNA occurs in vivo in at least three splice variants. They differ in the length of exon 1, which comprises 278 nucleotides (hCATL-A), 188 nucleotides (hCATL-A2) and 132 nucleotides (hCATL-A3), respectively. We describe here the shortest variant for the first time. This form is predominant in all tissues and cells examined so far, including malignant tumors. We studied the expression rate of the three mRNA variants in order to explain why malignant kidney tumors show low cathepsin L activity despite of high mRNA levels. The variant hCATL-A3 showed the highest expression rate in vitro and in vivo. Based on these results, we suggest a cis-acting element on human cathepsin L mRNA which can be bound by a negative trans-acting regulator, thus leading to reduced expression rates.     

Post-transcriptional regulation of human cathepsin L expression

The expression of cathepsin L, a lysosomal protease, is known to be elevated in cancer and other pathologies. Multiple splice variants of human cathepsin L with variable 5'UTRs exist, which encode for the same protein. Previously we have observed that variant hCATL A (bearing the longest 5'UTR) was translated in vitro with significantly lower efficiency than variant hCATL AIII (bearing the shortest 5'UTR). Contrary to these findings, results of the present study reveal that in cancer cells, hCATL A mRNA exhibits higher translatability in spite of having lower stability than AIII. This is the first report demonstrating a highly contrasting trend in translation efficiencies of hCATL variants in rabbit reticulocytes and live cells. Expression from chimeric mRNAs containing 5'UTRs of A or AIII upstream to luciferase reporter cDNA established the A UTR to be the sole determinant for this effect. Transient transfections of bicistronic plasmids and mRNAs confirmed the presence of a functional Internal Ribosome Entry Site in this UTR. Our data suggest that differential stability and translation initiation modes mediated by the 5'UTRs of human cathepsin L variants are involved in regulating its expression.     

Cathepsin B-like proteinase as a marker for metastatic tumor cell variants

Serial transplantation of a spontaneous BDX rat tumor, classified as an anaplastic sarcoma, gives rise to two variants; a rapidly growing nonmetastatic line (AS) and a slowly growing, invasive, and highly metastatic variant (ASML). The availability of two cell lines of the same origin but with markedly differing metastatic potential offers an ideal model for the identification of the cellular properties involved in invasive and/or metastatic behavior. The present work focuses on the pattern of various proteinases in the two tumor cell variants. The findings disclosed one major consistent difference which relates to a cathepsin B-like cysteine proteinase. The metastatic ASML variant manifests exceedingly high intracellular cathepsin B-like activity; in the nonmetastatic AS variant, the activity of this proteinase is significantly lower. Other proteinases, in particular elastase-like, chymotrypsin-like, collagenase-like enzymes and plasminogen activator, showed low, essentially comparable activity patterns. Thus, cathepsin B-like proteinase is a marker enzyme of the metastatic ASML tumor cell variant.     

Two new double-stranded RNA molecules showing non-mendelian inheritance and heat inducibility in Saccharomyces cerevisiae.

Certain strains of Saccharomyces cerevisiae were found to have a complex nuclear defect (designated clo-) that makes cells unable to maintain some L-B and some L-C double-stranded RNAs at 25 degrees C. The clo- strains were not defective in maintenance of L-A, M1, or M2 double-stranded RNAs. Most clo-strains lacking L and M carry small amounts of two double-stranded RNA species intermediate in size between L and M and denoted T (2.7 kilobase pairs) and W (2.25 kilobase pairs). Some strains carry both T and W, some carry neither, and some carry only W; no strains carrying only T have been found. Both T and W show 4+:0 segregation in meiosis and efficient transmission by cytoplasmic mixing (cytoduction), indicating that they are non-Mendelian genetic elements. T and W do not cross-hybridize with each other or with L-A, L-B, L-C, M1, M2, or chromosomal DNA. T and W are apparently distinct from other known non-Mendelian genetic elements (2mu DNA, [rho], [psi], 20S RNA, [URE3]). In most strains the copy number of both T and W is increased about 10-fold by the growth of cells at 37 degrees C. This heat inducibility of T and W is under control of a cytoplasmic gene. T and W double-stranded RNAs are not found in a purified L-containing virus-like particle preparation from a strain containing L-B, T, and W double-stranded RNAs. The role, if any, of T or W in the killer systems is not known.     

Killer systems in Saccharomyces cerevisiae: three distinct modes of exclusion of M2 double-stranded RNA by three species of double-stranded RNA, M1, L-A-E, and L-A-HN.

M1 and M2 double-stranded RNAs (dsRNAs) code for the K1R1 and K2R2 killer toxin and resistance functions, respectively. Natural variants of a larger dsRNA (L-A) carry various combinations of the [EXL], [HOK], and [NEX] genes, which affect the K1 and K2 killer systems. Other dsRNAs, the same size as L-A, called L-B and L-C, are often present with L-A. We show that K1 killer strains have [HOK] and [NEX] but not [EXL] on their L-A (in disagreement with Field et al., Cell 31:193-200, 1982). These strains also carry other L-size molecules detectable after heat-curing has eliminated L-A. The exclusion of M2 dsRNA observed on mating K2 strains with K1 strains is due to the M1 dsRNA (not the L-A dsRNA as claimed by Field et al.) in the K1 strains. Four independent mutants of a [KIL-k2] [NEX-o] [HOK-o] strain were selected for resistance to [EXL] exclusion of M2 ([EXLR] phenotype). The [EXLR] phenotype showed non-Mendelian inheritance in each case, and these mutants had simultaneously each acquired [HOK]. The mutations were located on L-A and not on M2, and did not confer resistance to M1 exclusion of M2.     

Analysis of Fasciola cathepsin L5 by S2 subsite substitutions and determination of the P1-P4 specificity reveals an unusual preference

Fasciola parasites (liver flukes) express numerous cathepsin L proteases that are believed to be involved in important functions related to host invasion and parasite survival. These proteases are evolutionarily divided into clades that are proposed to reflect their substrate specificity, most noticeably through the S(2) subsite. Single amino acid substitutions to residues lining this site, including amino acid residue 69 (aa69; mature cathepsin L5 numbering) can have profound influences on subsite architecture and influence enzyme specificity. Variations at aa69 among known Fasciola cathepsin L proteases include leucine, tyrosine, tryptophan, phenylalanine and glycine. Other amino acids (cysteine, serine) might have been expected at this site due to codon usage as cathepsin L isoenzymes evolved, but C69 and S69 have not been observed. The introduction of L69C and L69S substitutions into FhCatL5 resulted in low overall activity indicating their expression provides no functional advantage, thus explaining the absence of such variants in Fasciola. An FhCatL5 L69F variant showed an increase in the ability to cleave substrates with P(2) proline, indicating F69 variants expressed by the fluke would likely have this ability. An FhCatL2 Y69L variant showed a decreased acceptance of P(2) proline, further highlighting the importance of Y69 for FhCatL2 P(2) proline acceptance. Finally, the P(1)-P(4) specificity of Fasciola cathepsin L5 was determined and, unexpectedly, aspartic acid was shown to be well accepted at P(2,) which is unique amongst Fasciola cathepsins examined to date.     

Differential effect toward inhibition of papain and cathepsin C by recombinant human salivary cystatin SN and its variants produced by a baculovirus system

Human salivary cystatin SN (CsnSN) is a member of the cystatin superfamily of cysteine proteinase inhibitors. In this study we used a baculovirus expression system to produce a full-length unaltered CsnSN and its variants. The variants were constructed with the changes in the three predicted proteinase-binding regions: the N-terminus (variant N(12-13), G12A-G13A), beta-hairpin loop I (variant L(56-58), Q56G-T57G-V58G) and beta-hairpin loop II (variant L(106-107), P106G-W107G). The secreted CsnSNs were purified using sequential spiral cartridge ultrafiltration and DE-52 radial flow chromatography. The purified proteins were examined for papain- and cathepsin C-inhibition. The wild-type CsnSN, and variants N(12-13) and L(106-107) bound tightly to papain (K(i) < 10 pM), whereas mutation in the loop I reduced binding affinity 5700-fold (K(i) = 57 nM). On the other hand, the wild-type CsnSN bound to cathepsin C less tightly (K(i) = 100 nM). The mutation in the N-terminus or loop I reduced binding affinity by 16 (K(i) = 1.6 microM)- and 19-fold (K(i) = 1.9 microM), respectively, while mutation in loop II resulted in an ineffective cathepsin C inhibitor (K(i) = 14 microM). Collectively, these results suggest that the N-terminal G12-G13 residues of CsnSN are not essential for papain inhibition but play a role in cathepsin C inhibition; residues Q56-T57-V58 in the loop I are essential for both papain and cathepsin C inhibitions, and residues P106-W107 in the loop II are not important for papain inhibition but essential for cathepsin C inhibition. These results demonstrated that CsnSN variants have different effects toward different cysteine proteinases.     

Breast cancer cells express cathepsins B and L but not cathepsins K or H

Lysosomal cysteine proteinases (cathepsins) are considered to play a role in bone degradation mediated by metastatic breast cancers. To evaluate which cathepsin contributes to the osteolysis, we quantitatively determined the expression levels of four cathepsins in two breast cancer cell lines, MCF-7 and MDA-MB-231, by competitive RT-PCR. Cathepsin K, which is the most abundant cathepsin in osteoclasts, was not detected in either cell lines. We also failed to detect cathepsin H mRNA. By contrast, we found significant expression of cathepsins B and L in both cell lines. By Northern blot analysis cathepsin B mRNA was detected in a single form in these cells, whereas osteoclasts contained multiple forms of the mRNA. Cathepsin B protein was also detected by Western blotting as a single immunoreactive band corresponding to its mature enzyme. These findings suggest that osteolysis associated with metastatic breast cancers takes place in a different way from osteoclast-mediated bone resorption.     

Stress-resistant Translation of Cathepsin L mRNA in Breast Cancer Progression

The cysteine protease cathepsin L (CTSL) is often thought to act as a tumor promoter by enhancing tumor progression and metastasis. This goes along with increased CTSL activity in various tumor entities; however, the mechanisms leading to high CTSL levels are incompletely understood. With the help of the polyoma middle T oncogene driven breast cancer mouse model expressing a human CTSL genomic transgene, we show that CTSL indeed promotes breast cancer metastasis to the lung. During tumor formation and progression high expression levels of CTSL are maintained by enduring translation of CTSL mRNA. Interestingly, human breast cancer specimens expressed the same pattern of 5′ untranslated region (UTR) splice variants as the transgenic mice and the human cancer cell line MDA-MB 321. By polyribosome profiling of tumor tissues and human breast cancer cells, we observe an intrinsic resistance of CTSL to stress-induced shutdown of translation. This ability can be attributed to all 5′ UTR variants of CTSL and is not dependent on a previously described internal ribosomal entry site motif. In conclusion, we provide in vivo functional evidence for overexpressed CTSL as a promoter of lung metastasis, whereas high CTSL levels are maintained during tumor progression due to stress-resistant mRNA translation.     

Podocyte migration during nephrotic syndrome requires a coordinated interplay between cathepsin L and alpha3 integrin

Podocyte foot process effacement and disruption of the slit diaphragm are typically associated with glomerular proteinuria and can be induced in rats by the injection of puromycin aminonucleoside. Here, we show that the induction of puromycin aminonucleoside nephrosis involves podocyte migration conducted by a coordinated interplay between the cysteine protease cathepsin L and alpha(3) integrin. Puromycin aminonucleoside treatment up-regulates cathepsin L expression in podocytes in vivo as well as expression and enzymatic activity of cathepsin L in podocytes in vitro. Isolated podocytes from mice lacking cathepsin L are protected from cell puromycin aminonucleoside-induced cell detachment. The functional significance of cathepsin L expression was underscored by the observation that puromycin aminonucleoside-induced cell migration was slowed down in cathepsin L-deficient podocytes and by the preservation of cell-cell contacts and expression of vital slit diaphragm protein CD2AP. Cathepsin L expression and activity were induced in podocytes lacking alpha(3) integrin. Similarly, acute functional inhibition of alpha(3) integrin in wild type podocytes with a blocking antibody increased the expression of cathepsin L activity. Down-regulation of alpha(3) integrin protected against puromycin aminonucleoside-induced podocyte detachment. In summary, these data establish that podocyte foot process effacement is a migratory event involving a novel interplay between cathepsin L and alpha(3) integrin.     

Estrogen receptor variants and mutations

There is a large and increasing body of experimental and clinical data supporting the existence of variant estrogen receptor (ER) proteins in both normal and neoplastic estrogen target tissues, including human breast. Therefore, future examination of ER signal transduction and/or measurement of ER protein must take into account variant ER expression. The functions of variant ER proteins, either physiological or pathological, remain unclear, although a role(s) for some ER variants in breast tumorigenesis and breast cancer progression would be consistent with the accumulated data. Possible tissue specific expression leads to the speculation that ER variants may have a role in tissue specific estrogen action. The following review focuses on the current knowledge available in the scientific literature with respect to the type and characteristics of estrogen receptor variants and mutations that have been identified to occur naturally in tissues and cell lines.     

Transforming potential of alternatively spliced variants of fibroblast growth factor receptor 2 in human mammary epithelial cells

A breast cancer cell line developed in our laboratory (SUM-52PE) has a 12-fold amplification and high-level overexpression of the oncogene fibroblast growth factor receptor 2 (FGFR2). Previously, nine different alternatively spliced FGFR2 variants were isolated from this cell line. Overexpression of two variants that differ only in their carboxyl termini (C1 and C3) has been successfully accomplished in the immortalized human mammary epithelial cell line H16N2. FGFR2 expression led to the activation of the mitogen-activated protein kinase and phosphatidylinositol 3-kinase signaling cascades. Phosphorylation of the adapter protein FGF receptor substrate 2 is much more robust in the cells expressing the C3 variant of FGFR2 compared with the C1 variant. H16N2 cells expressing the full-length FGFR2 with the C1 or C3 carboxyl terminus were tested for their ability to grow under epidermal growth factor (EGF)-independent conditions, in soft agar, and for their ability to invade naturally occurring basement membranes and compared with the parental SUM-52PE cell line. All three cell lines grew under EGF-independent conditions and all were inhibited by the FGFR family specific inhibitor PD173074. The full-length FGFR2-C1 and FGFR2-C3 variants grew robustly in soft agar similar to the parental cell line SUM-52PE. However, cells expressing the C3 variant formed large colonies in agar in both insulin-free and EGF-free medium, whereas the cells expressing the C1 variant required insulin for growth. Soft agar growth was also inhibited by PD173074. Because SUM-52PE was developed from a metastatic breast carcinoma, the FGFR2-overexpressing cell lines were assessed for their ability to invade sea urchin embryo cell membranes. H16N2 cells expressing the C1 carboxyl terminus failed to invade sea urchin embryo cell membranes. By contrast, FGFR2-C3-expressing cells were as invasive as the SUM-52 breast cancer cells and erbB-2-overexpressing H16N2 cells. These results indicate that FGFR2 is a transforming oncogene in human mammary epithelial cells when expressed to levels similar to that found in breast cancer cells with FGFR2 gene amplification. Furthermore, the results suggest that different splice variants have differing transforming activities and that signaling from variants expressing the C3 carboxyl terminus results in more autonomous signaling, cell growth, and invasion.     

Modification of alternative splicing of Bcl-x pre-mRNA in prostate and breast cancer cells. analysis of apoptosis and cell death

There is ample evidence that deregulation of apoptosis results in the development, progression, and/or maintenance of cancer. Since many apoptotic regulatory genes (e.g. bcl-x) code for alternatively spliced protein variants with opposing functions, the manipulation of alternative splicing presents a unique way of regulating the apoptotic response. Here we have targeted oligonucleotides antisense to the 5'-splice site of bcl-x(L), an anti-apoptotic gene that is overexpressed in various cancers, and shifted the splicing pattern of Bcl-x pre-mRNA from Bcl-x(L) to Bcl-x(S), a pro-apoptotic splice variant. This approach induced significant apoptosis in PC-3 prostate cancer cells. In contrast, the same oligonucleotide treatment elicited a much weaker apoptotic response in MCF-7 breast cancer cells. Moreover, although the shift in Bcl-x pre-mRNA splicing inhibited colony formation in both cell lines, this effect was much less pronounced in MCF-7 cells. These differences in responses to oligonucleotide treatment were analyzed in the context of expression of Bcl-x(L), Bcl-x(S), and Bcl-2 proteins. The results indicate that despite the presence of Bcl-x pre-mRNA in a number of cell types, the effects of modification of its splicing by antisense oligonucleotides vary depending on the expression profile of the treated cells.     

Mak10, a Glucose-Repressible Gene Necessary for Replication of a Dsrna Virus of Saccharomyces Cerevisiae, Has T Cell Receptor α-Subunit Motifs

The MAK10 gene is necessary for the propagation of the L-A dsRNA virus of the yeast Saccharomyces cerevisiae. We have isolated MAK10 from selected phage lambda genomic DNA clones that map near MAK10. This gene encodes a 733-amino acid protein with several regions of similarity to T cell receptor alpha-subunit V (variable) regions. We show that MAK10 is essential for optimal growth on nonfermentable carbon sources independent of its effect on L-A. Although loss of L-A by mak10-1 mutants is partially suppressed by loss of the mitochondrial genome, no such suppression of a mak10::URA3 mutation was observed. Using MAK10-lacZ fusions we show that MAK10 is expressed at a very low level and that it is glucose repressed. The highest levels of expression were seen in tup1 and cyc8 mutants, known to be defective in glucose repression. These results suggest that the mitochondrial genome and L-A dsRNA compete for the MAK10 protein.     

Motility of fibronectin receptor-deficient cells on fibronectin and vitronectin: collaborative interactions among integrins.

Cells are capable of adhering to and migrating on protein components of the extracellular matrix. These cell-matrix interactions are thought to be mediated largely through a family of cell surface receptors termed integrins. However, the manner in which individual integrins are involved in cell adhesion and motility has not been fully determined. To explore this issue, we previously selected a series of CHO variants that are deficient in expression of the integrin alpha 5 beta 1, the "classical" fibronectin receptor. Two sets of subclones of these variants were defined which respectively express approximately 20% or 2% of fibronectin receptor on the cell surface when compared to wild-type cells (Schreiner, C. L., J. S. Bauer, Y. N. Danilov, S. Hussein, M. M. Sczekan, and R. L. Juliano. 1989. J. Cell Biol. 109:3157-3167). In the current study, the variant clones were tested for haptotactic motility on substrata coated with fibronectin or vitronectin. Data from assays using fibronectin show that cellular motility of the 20% variants was substantially decreased (30-75% of wild type), while the motility of the 2% variants was nearly abolished (2-20% of wild type). Surprisingly, a similar pattern was seen for haptotactic motility of both 2% and 20% variants when vitronectin was used (approximately 20-30% of wild type). The reduced haptotactic motility of the fibronectin receptor-deficient variant clones on vitronectin was shown not to be due to reduced vitronectin receptor (alpha v beta 3) expression nor to a failure of these variants to adhere to vitronectin substrata. Transfection of the deficient variants with a cDNA for the human alpha 5 subunit resulted in normal levels of fibronectin receptor expression (as a human alpha 5/hamster beta 1 chimera) and restored the motility of the CHO variants on fibronectin and vitronectin. This indicates that expression of the alpha 5 subunit is required for normal haptotactic motility on vitronectin substrata and suggests that the fibronectin receptor (alpha 5 beta 1) plays a cooperative role with vitronectin receptors in cell motility.     

Gene Expression Profiling Reveals Epithelial Mesenchymal Transition (EMT) Genes Can Selectively Differentiate Eribulin Sensitive Breast Cancer Cells

Objectives

Eribulin mesylate is a synthetic macrocyclic ketone analog of the marine sponge natural product halichondrin B. Eribulin is a mechanistically unique inhibitor of microtubule dynamics. In this study, we investigated whether selective signal pathways were associated with eribulin activity compared to paclitaxel, which stabilizes microtubules, based on gene expression profiling of cell line panels of breast, endometrial, and ovarian cancer in vitro.

Results

We determined the sets of genes that were differentially altered between eribulin and paclitaxel treatment in breast, endometrial, and ovarian cancer cell line panels. Our unsupervised clustering analyses revealed that expression profiles of gene sets altered with treatments were correlated with the in vitro antiproliferative activities of the drugs. Several tubulin isotypes had significantly lower expression in cell lines treated with eribulin compared to paclitaxel. Pathway enrichment analyses of gene sets revealed that the common pathways altered between treatments in the 3 cancer panels were related to cytoskeleton remodeling and cell cycle regulation. The epithelial-mesenchymal transition (EMT) pathway was enriched in genes with significantly altered expression between the two drugs for breast and endometrial cancers, but not for ovarian cancer. Expression of genes from the EMT pathway correlated with eribulin sensitivity in breast cancer and with paclitaxel sensitivity in endometrial cancer. Alteration of expression profiles of EMT genes between sensitive and resistant cell lines allowed us to predict drug sensitivity for breast and endometrial cancers.

Conclusion

Gene expression analysis showed that gene sets that were altered between eribulin and paclitaxel correlated with drug in vitro antiproliferative activities in breast and endometrial cancer cell line panels. Among the panels, breast cancer provided the strongest differentiation between eribulin and paclitaxel sensitivities based on gene expression. In addition, EMT genes were predictive of eribulin sensitivity in the breast and endometrial cancer panels.