Expression of the human Cathepsin L inhibitor hurpin in mice: skin alterations and increased carcinogenesis

Mechanisms Applied by Protein Inhibitors to Inhibit Cysteine Proteases

Protein inhibitors of proteases are an important tool of nature to regulate and control proteolysis in living organisms under physiological and pathological conditions. In this review, we analyzed the mechanisms of inhibition of cysteine proteases on the basis of structural information and compiled kinetic data. The gathered structural data indicate that the protein fold is not a major obstacle for the evolution of a protease inhibitor. It appears that nature can convert almost any starting fold into an inhibitor of a protease. In addition, there appears to be no general rule governing the inhibitory mechanism. The structural data make it clear that the "lock and key" mechanism is a historical concept with limited validity. However, the analysis suggests that the shape of the active site cleft of proteases imposes some restraints. When the S1 binding site is shaped as a pocket buried in the structure of protease, inhibitors can apply substrate-like binding mechanisms. In contrast, when the S1 binding site is in part exposed to solvent, the substrate-like inhibition cannot be employed. It appears that all proteases, with the exception of papain-like proteases, belong to the first group of proteases. Finally, we show a number of examples and provide hints on how to engineer protein inhibitors.

Cathepsin L Regulates Metabolic Networks Controlling Rapid Cell Growth and Proliferation

Rapidly proliferating cells reshape their metabolism to satisfy their ever-lasting need for cellular building blocks. This phenomenon is exemplified in certain malignant conditions such as cancer but also during embryonic development when cells rely heavily on glycolytic metabolism to exploit its metabolic intermediates for biosynthetic processes. How cells reshape their metabolism is not fully understood. Here we report that loss of cathepsin L (Cts L) is associated with a fast proliferation rate and enhanced glycolytic metabolism that depend on lactate dehydrogenase A (LDHA) activity. Using mass spectrometry analysis of cells treated with a pan cathepsin inhibitor, we observed an increased abundance of proteins involved in central carbon metabolism. Further inspection of putative Cts L targets revealed an enrichment for glycolytic metabolism that was independently confirmed by metabolomic and biochemical analyses. Moreover, proteomic analysis of Cts L-knockout cells identified LDHA overexpression that was demonstrated to be a key metabolic junction in these cells. Lastly, we show that Cts L inhibition led to increased LDHA protein expression, suggesting a causal relationship between LDHA expression and function. In conclusion, we propose that Cts L regulates this metabolic circuit to keep cell division under control, suggesting the therapeutic potential of targeting this protein and its networks in cancer.

Ultraviolet A Enhances Cathepsin L Expression and Activity via JNK Pathway in Human Dermal Fibroblasts

BACKGROUND

Cathepsin L (CatL) is a cysteine protease with strong matrix degradation activity that contributes to photoaging. Mannose phosphate-independent sorting pathways mediate ultraviolet A (UVA)-induced alternate trafficking of CatL. Little is known about signaling pathways involved in the regulation of UVA-induced CatL expression and activity. This study aims to investigate whether a single UVA irradiation affects CatL expression and activity and whether mitogen-activated protein kinase (MAPK)/activator protein-1 (AP-1) pathway is involved in the regulation of UVA-induced CatL expression and activity in human dermal fibroblasts (HDFs).

METHODS

Primary HDFs were exposed to UVA. Cell proliferation was determined by a cell counting kit. UVA-induced CatL production and activity were studied with quantitative real-time reverse transcription polymerase chain reaction (RT-PCR), Western blotting, and fluorimetric assay in cell lysates collected on three consecutive days after irradiation. Time courses of UVA-activated JNK and p38MAPK signaling were examined by Western blotting. Effects of MAPK inhibitors and knockdown of Jun and Fos on UVA-induced CatL expression and activity were investigated by RT-PCR, Western blotting, and fluorimetric assay. Data were analyzed by one-way analysis of variance.

RESULTS

UVA significantly increased CatL gene expression, protein abundance, and enzymatic activity for three consecutive days after irradiation (F = 83.11, 56.14, and 71.19, respectively; all P < 0.05). Further investigation demonstrated phosphorylation of JNK and p38MAPK activated by UVA. Importantly, inactivation of JNK pathway significantly decreased UVA-induced CatL expression and activity, which were not affected by p38MAPK inhibition. Moreover, knockdown of Jun and Fos significantly attenuated basal and UVA-induced CatL expression and activity.

CONCLUSIONS

UVA enhances CatL production and activity in HDFs, probably by activating JNK and downstreaming AP-1. These findings provide a new possible molecular approach for antiphotoaging therapy.

Knockdown of cathepsin L sensitizes ovarian cancer cells to chemotherapy

Ovarian cancer is a leading gynecological malignancy associated with high mortality. The development of acquired drug resistance is the primary cause of chemotherapy failure in the treatment of ovarian cancer. To examine the mechanism underlying paclitaxel resistance in ovarian cancer and attempt to reverse it, the present study induced a TAX-resistant ovarian cancer cell line, SKOV3/TAX. Cathepsin L (CTSL) has been found to be overexpressed in ovarian cancer. The aim of the present study was to investigate the possible involvement of CTSL in the development of TAX resistance in ovarian cancer. CTSL expression was knocked down in SKOV3 ovarian cancer cells and their phenotypic changes were analyzed. The effects of silenced CTSL on the resistant cell line were investigated by proliferation and apoptosis analysis compared with control SKOV3 cells. CTSL was more highly expressed in SKOV3/TAX cells compared with SKOV3 cells. Paclitaxel treatment downregulated the expression of CTSL in SKOV-3 but not in the paclitaxel-resistant SKOV3/TAX cells. CTSL small hairpin RNA (shRNA) knockdown significantly potentiated apoptosis induced by paclitaxel compared with SKOV3/TAX cells transfected with control shRNA, suggesting that CTSL contributes to paclitaxel resistance in ovarian cancer cells and that CTSL silencing can enhance paclitaxel-mediated cell apoptosis. Thus, CTSL should be explored as a candidate of therapeutic target for modulating paclitaxel sensitivity in ovarian cancer.

Cancer-associated lysosomal changes: friends or foes?

Rapidly dividing and invasive cancer cells are strongly dependent on effective lysosomal function. Accordingly, transformation and cancer progression are characterized by dramatic changes in lysosomal volume, composition and cellular distribution. Depending on one's point of view, the cancer-associated changes in the lysosomal compartment can be regarded as friends or foes. Most of them are clearly transforming as they promote invasive growth, angiogenesis and drug resistance. The same changes can, however, strongly sensitize cells to lysosomal membrane permeabilization and thereby to lysosome-targeting anti-cancer drugs. In this review we compile our current knowledge on cancer-associated changes in lysosomal composition and discuss the consequences of these alterations to cancer progression and the possibilities they can bring to cancer therapy.

Nephrogenic systemic fibrosis: evidence for oxidative stress and bone marrow-derived fibrocytes in skin, liver, and heart lesions using a 5/6 nephrectomy rodent model

Nephrogenic systemic fibrosis (NSF) is associated with gadolinium-based magnetic resonance imaging (MRI) contrast exposure in the setting of acute or chronic renal compromise. It has been proposed that circulating fibrocytes mediate the disease. A study was conducted to determine whether bone marrow-derived fibroblast precursors are involved in contributing to organ fibrosis in MRI contrast-treated rodents with renal insufficiency. Rats status post 5/6 nephrectomy underwent bone marrow transplant from human placental alkaline phosphatase (hPAP)-expressing donors. After engraftment, animals were treated with gadolinium-based MRI contrast (2.5 mmol/kg IP), during weekdays for 4 weeks, or an equivalent volume of normal saline. Dermal cellularity in the contrast-treated group was fourfold that of control. Skin cells from the contrast-treated group demonstrated greater hPAP expression with co-expression of pro-collagen I and α-smooth muscle actin-positive stress fibers. Donor and host cells expressed CD34. Dihydroethidium staining of skin was greater in the contrast-treated animals, indicating oxidative stress. This was abrogated when the animals were co-administered the superoxide dismutase mimetic tempol. In conclusion, a bone marrow-derived cell population is increased in the dermis of MRI contrast-treated rodents. The cell markers are consistent with fibrocytes mediating the disease. These changes correlate with oxidative stress and expression of Nox4, suggestive of a novel therapeutic target. Elucidation of the mechanisms of MRI contrast-induced fibrosis may aid in discovering therapies to this devastating disease.

Protective role of cathepsin L in mouse skin carcinogenesis

Lysosomal cysteine protease cathepsin L (CTSL) is believed to play a role in tumor progression and is considered a marker for clinically invasive tumors. Studies from our laboratory using the classical mouse skin carcinogenesis model, with 7,12-dimethyl-benz[a]anthracene (DMBA) for initiation and 12-O-tetradecanoylphorbol-13-acetate (TPA) for promotion, showed that expression of CTSL is increased in papillomas and squamous cell carcinomas (SCC). We also carried out carcinogenesis studies using Ctsl-deficient nackt (nkt) mutant mice on three different inbred backgrounds. Unexpectedly, the multiplicity of papillomas was significantly higher in Ctsl-deficient than in wild-type mice on two unrelated backgrounds. Topical applications of TPA or DMBA alone to the skin of nkt/nkt mice did not induce papillomas, and there was no increase in spontaneous tumors in nkt/nkt mice on any of the three inbred backgrounds. Reduced epidermal cell proliferation in Ctsl-deficient nkt/nkt mice after TPA treatment suggested that they are not more sensitive than wild-type mice to TPA promotion. We also showed that deficiency of CTSL delays terminal differentiation of keratinocytes, and we propose that decreased elimination of initiated cells is at least partially responsible for the increased papilloma formation in the nackt model.

Deficiency for the cysteine protease cathepsin L promotes tumor progression in mouse epidermis

To define a functional role for the endosomal/lysosomal cysteine protease cathepsin L (Ctsl) during squamous carcinogenesis, we generated mice harboring a constitutive Ctsl deficiency in addition to epithelial expression of the human papillomavirus type 16 oncogenes (human cytokeratin 14 (K14)-HPV16). We found enhanced tumor progression and metastasis in the absence of Ctsl. As tumor progression in K14-HPV16 mice is dependent on inflammation and angiogenesis, we examined immune cell infiltration and vascularization without finding any effect of the Ctsl genotype. In contrast, keratinocyte-specific transgenic expression of cathepsin V, the human orthologue of mouse Ctsl, in otherwise Ctsl-deficient K14-HPV16 mice restored the phenotype observed in the control HPV16 skin. To better understand this phenotype at the molecular level, we measured several oncogenic signal transduction pathways in primary keratinocytes on stimulation with keratinocyte-conditioned cell culture medium. We found increased activation of protein kinase B/Akt and mitogen-activated protein kinase pathways in protease-deficient cells, especially if treated with media conditioned by Ctsl-deficient keratinocytes. Similarly, the level of active GTP-Ras was increased in Ctsl-deficient epidermis. We conclude that Ctsl is critical for the termination of growth factor signaling in the endosomal/lysosomal compartment of keratinocytes and, therefore, functions as an anti-tumor protease.

Cathepsin L, target in cancer treatment?

Cathepsin L, a cysteine protease, is considered to be a potential therapeutic target in cancer treatment. Proteases are involved in the development and progression of cancer. Inhibition of activity of specific proteases may slow down cancer progression. In this review, we evaluate recent studies on the inhibition of cathepsin L in cancer. The effects of cathepsin L inhibition as a monotherapy on apoptosis and angiogenesis in cancer are ambiguous. Cathepsin L inhibition seems to reduce invasion and metastasis, but there is concern that selective cathepsin L inhibition induces compensatory activity by other cathepsins. The combination of cathepsin L inhibition with conventional chemotherapy seems to be more promising and has yielded more consistent results. Future research should be focused on the mechanisms and effects of this combination therapy.