Degradation of epidermal growth factor receptors by cathepsin L-like protease: inhibition of the degradation by c-Ha-ras gene products

Defective cathepsin Z affects EGFR expression and causes autosomal dominant palmoplantar keratoderma

BACKGROUND

The abnormal function of epidermal growth factor receptor (EGFR) has recently been shown to underlie various disorders of cornification.

OBJECTIVES

To delineate the genetic basis of a novel dominant form of palmoplantar keratoderma (PPK).

METHODS

Whole-exome (WES) and direct sequencing, quantitative real-time polymerase chain reaction, protein modelling, confocal immunofluorescence microscopy, immunoblotting, three-dimensional skin equivalents and an enzyme activity assay were used to delineate the genetic basis of a novel dominant form of PPK.

RESULTS

WES revealed heterozygous variants (c.274T > C and c.305C > T) in CTSZ (encoding cathepsin Z) in four individuals (belonging to three unrelated families) with focal PPK. Bioinformatics and protein modelling predicted the variants to be pathogenic. Previous studies have suggested that EGFR expression may be subject to cathepsin regulation. Immunofluorescence revealed reduced cathepsin Z expression in the upper epidermal layers and concomitant increased epidermal EGFR expression in patients harbouring CTSZ variants. Accordingly, human keratinocytes transfected with constructs expressing PPK-causing variants in CTSZ displayed reduced cathepsin Z enzymatic activity, as well as increased EGFR expression. In line with the role played by EGFR in the regulation of keratinocyte proliferation, human keratinocytes transfected with the PPK-causing variants showed significantly increased proliferation that was abolished upon exposure to erlotinib, an EGFR inhibitor. Similarly, downregulation of CTSZ resulted in increased EGFR expression and increased proliferation in human keratinocytes, suggestive of a loss-of-function effect of the pathogenic variants. Finally, three-dimensional organotypic skin equivalents grown from CTSZ-downregulated cells showed increased epidermal thickness and EGFR expression as seen in patient skin; here, too, erlotinib was found to rescue the abnormal phenotype.

CONCLUSIONS

Taken collectively, these observations attribute to cathepsin Z a hitherto unrecognized function in epidermal differentiation.

The lysosomal cysteine protease cathepsin L regulates keratinocyte proliferation by control of growth factor recycling

Mice deficient for cathepsin L (CTSL) show epidermal hyperplasia due to a hyperproliferation of basal keratinocytes. Here we show that the critical function of CTSL in the skin is keratinocyte specific. This is revealed by transgenic re-expression of CTSL in the keratinocytes of ctsl-/- mice, resulting in a rescue of the ctsl-/- skin phenotype. Cultivation of primary mouse keratinocytes with fibroblast- and keratinocyte-conditioned media, as well as heterologous organotypic co-cultures of mouse fibroblasts and human keratinocytes, showed that the altered keratinocyte proliferation is caused primarily by CTSL-deficiency in keratinocytes. In the absence of EGF, wild type and CTSL-knockout keratinocytes proliferate with the same rates, while in presence of EGF, ctsl-/- keratinocytes showed enhanced proliferation compared with controls. Internalization and degradation of radioactively labeled EGF was identical in both ctsl-/- and ctsl+/+ keratinocytes. However, ctsl-/- keratinocytes recycled more EGF to the cell surface, where it is bound to the EGF-receptor, which is also more abundant in ctsl-/- cells. We conclude that the hyperproliferation of keratinocytes in CTSL-knockout mice is caused by an enhanced recycling of growth factors and growth factor receptors from the endosomes to the keratinocyte plasma membrane, which result in sustained growth stimulation.

Cathepsin L-deficient mice exhibit abnormal skin and bone development and show increased resistance to osteoporosis following ovariectomy

The role of cathepsin L in normal physiological processes was assessed using cathepsin L homozygous knockout mice (B6;129-Ctsl(tm1Alpk)). These mice were generated using gene targeting in embryonic stem cells. Null mice fail to express mRNA and protein to cathepsin L. They developed normally and were fertile. The distinct phenotypic change exhibited was a progressive hair loss, culminating in extensive alopecia by 9 months of age. Histological analysis of the skin from homozygous mice revealed diffuse epithelial hyperplasia, hypotrichosis, hair shaft fragmentation and utricle formation. These findings provide evidence that cathepsin L is involved in the regulation of epithelial cell proliferation and differentiation in the skin. In addition, the role of cathepsin L in bone remodelling was evaluated. Using bone histomorphometric measurements, trabecular, but not cortical, bone volume was found to be significantly decreased in the cathepsin L heterozygote and homozygote mice compared to the wild-type mice. Following ovariectomy, it was observed that loss of trabecular bone, the most metabolically active component of bone, occurred to a lesser extent in homozygote, and heterozygote mice, than was seen in wild-type mice. These observations suggest that cathepsin L is likely to have a role in controlling bone turnover during normal development and in pathological states.

3-Hydroxy-3-methylglutaryl Coenzyme A Reductase Is Sterol-Dependently Cleaved by Cathepsin L-Type Cysteine Protease in the Isolated Endoplasmic Reticulum

We have recently shown that 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, an endoplasmic reticulum (ER) membrane protein, is degraded in ER membranes prepared from sterol pretreated cells and that such degradation is catalyzed by a cysteine protease within the reductase membrane domain. The use of various protease inhibitors suggested that degradation of HMG-CoA reductase in vitro is catalyzed by a cathepsin L-type cysteine protease. Purified ER contains E-64-sensitive cathepsin L activity whose inhibitor sensitivity was well matched to that of HMG-CoA reductase degradation in vitro. CLIK-148 (cathepsin L inhibitor) inhibited degradation of HMG-CoA reductase in vitro. Purified cathepsin L also efficiently cleaved HMG-CoA reductase in isolated ER preparations. To determine whether a cathepsin L-type cysteine protease is involved in sterol-regulated degradation of HMG-CoA reductase in vivo, we examined the effect of E-64d, a membrane-permeable cysteine protease inhibitor, in living cells. While lactacystin, a proteasome-specific inhibitor, inhibited sterol-dependent degradation of HMG-CoA reductase, E-64d failed to do so. In contrast, degradation of HMG-CoA reductase in sonicated cells was inhibited by E-64d, CLIK-148, and leupeptin but not by lactacystin. Our results indicate that HMG-CoA reductase is degraded by the proteasome under normal conditions in living cells and that it is cleaved by cathepsin L leaked from lysosomes during preparation of the ER, thus clarifying the apparently paradoxical in vivo and in vitro results. Cathepsin L-dependent proteolysis was observed to occur preferentially in sterol-pretreated cells, suggesting that sterol treatment results in conformational changes in HMG-CoA reductase that make it more susceptible to such cleavage.

Negative regulation of epidermal growth factor signaling by selective proteolytic mechanisms in the endosome mediated by cathepsin B

We have investigated the relevant protease activity in rat liver, which is responsible for most of the receptor-mediated epidermal growth factor (EGF) degradation in vivo. EGF was sequentially cleaved by endosomal proteases at a limited number of sites, which were identified by high performance liquid chromatography and mass spectrometry. EGF proteolysis is initiated by hydrolysis at the C-terminal Glu(51)-Leu(52) bond. Three additional minor cleavage sites were identified at positions Arg(48)-Trp(49), Trp(49)-Trp(50), and Trp(50)-Glu(51) after prolonged incubation. Using nondenaturating immunoprecipitation and cross-linking procedures, the major proteolytic activity was identified as that of the cysteine protease cathepsin-B. The effect of injected EGF on subsequent endosomal EGF receptor (EGFR) proteolysis was further evaluated by immunoblotting. Using endosomal fractions prepared from EGF-injected rats and incubated in vitro, the EGFR was lost with a time course superimposable with the loss of phosphotyrosine content. The cathepsin-B proinhibitor CA074-Me inhibited both in vivo and in vitro the endosomal degradation of the EGFR and increased the tyrosine phosphorylation states of the EGFR protein and the molecule SHC within endosomes. The data, therefore, describe a unique pathway for the endosomal processing of internalized EGF receptor complexes, which involves the sequential function of cathepsin-B through selective degradation of both the ligand and receptor.

Role of cysteine proteases and protease inhibitors in gastric mucosal damage induced by ethanol or ammonia in the rat

Since recent studies suggest an imbalance between cathepsin B and its tissue protease inhibitors (PI) in the pathogenesis of acute and chronic diseases, we tested the hypothesis that release of activated cysteine proteases (P) such as cathepsins B, H, and L might play a role in the pathogenesis of gastric hemorrhagic mucosal lesions (HML) induced by ethanol (E) or ammonia (A). Anesthetized rats received 1 ml of 50% E or 1% A solution intragastrically for 1 min during in situ gastric luminal perfusion. Rapid activation and release of cathepsins B, L, and H into the luminal perfusate preceded the formation of HML quantified by planimetry. Mucosal presence and activity of cysteine PI and cathepsin B have also been investigated in the pathogenesis of chemically induced HML. We extracted and partially isolated acid and thermostable inhibitors of cathepsin B in the gastric mucosa, and found rapid inactivation of PI and activation of cathepsin B in the early phase of E- or A-induced HML. Negative correlations were found between P and PI activities by E or A solutions. Both the activation of cathepsins B, L, and H and the development of E-induced HML were prevented by pretreatment with the sulfhydryl alkylator N-ethylmaleimide. These results suggest that cysteine P may be activated in the rat stomach after E or A exposure, and cysteine P may have a role in the pathogenesis of E- or A-induced gastric HML. Endogenous PI may also participate in the mechanisms of gastric mucosal lesions and gastroprotection.

Role of cysteine endopeptidases in cancerogenesis

Role of cysteine endopeptidases in cancerogenesis steps: neoplastic transformation, invasion and metastasis is reviewed and discussed. Positive correlation between tumor invasiveness, as well as its metastatic potential and secretion of cysteine endopeptidase (particularly cathepsins B and L) has been documented well in literature. Based on our recent results we postulate that serum endopeptidase-like activity could be used as a marker of cancer aggressiveness in diagnostic procedures in oncology. We also propose that the cysteine endopeptidase inhibitor levels (total, active and latent) could be useful factors for recognising the activation of the organism self-defence mechanisms against cancer. In addition, our idea of use of urinary cysteine peptidase inhibitors (UCPI) as potential anticancer agents is presented and discussed.

Effects of thiol protease inhibitors on cell cycle and proliferation of vascular smooth muscle cells in culture

Smooth muscle proliferation is a prominent feature of the vascular response to mechanical injury. Accordingly, modulation of proliferation has important therapeutic implications for angioplasty restenosis. We have identified a subclass of thiol protease inhibitors (TPIs) that reversibly inhibit bovine aortic smooth muscle cell (BASMC) proliferation in vitro. To define the nature of this inhibition, an evaluation of selected steps in the cell cycle was undertaken. Treatment of BASMCs with benzyloxycarbonyl-Leu-norleucinal (calpeptin) at 100 microM and acetyl-Leu-Leu-norleucinal (TPI-1) at 50 microM was shown to cause a block of platelet-derived growth factor-BB as well as serum-inducible cell cycle progression at a point before the G1-S boundary, reducing the percentage of bromodeoxyuridine-positive cells from 87% to 5% over a 24-hour labeling period. Addition of TPI-1 at various times after serum addition to serum-deprived BASMCs showed 80% of the maximal block of DNA synthesis even when added 6 hours after serum. The cell cycle progression block was gradually lost as the delay from serum to TPI-1 application was increased from 6 to 12 hours. By Northern analysis of mRNA after serum addition, TPI-1 caused a fourfold decrease in the transient elevation of fos and myc proto-oncogene as well as a decrease in the levels of both muscle and nonmuscle actin mRNA induced early after serum addition. Flow cytometric analysis of DNA content and synthesis in BASMCs treated with TPI-1 or calpeptin additionally revealed the presence of a distinct cell cycle block in the G2-M compartment. In the aggregate, these results suggest the existence of more than one molecular site potentially involved in inhibition by TPI of cell cycling in BASMCs.

Different effects of two proteinase inhibitors on insulin-induced cellular responses in rat adipocytes

Among various proteinase inhibitors, N-acetyl-L-tyrosine ethyl ester (ATEE), a chymotrypsin substrate analog, and N alpha-p-tosyl-L-lysine chloromethyl ketone (TLCK), a trypsin inhibitor, showed significant inhibitory effects on insulin stimulated glucose transport in rat adipocytes. ATEE did not affect insulin binding, but inhibited insulin internalization. In intact adipocytes, ATEE inhibited tyrosine phosphorylation of the beta-subunit of the insulin receptor, a 170 kDa protein and a 60 kDa protein at almost the same concentration (ID50 = 0.24 +/- 0.05 mM, n = 4, mean +/- S.E.), but in a plasma membrane fraction, ATEE did not appreciably inhibit the tyrosine phosphorylation of the beta-subunit of the insulin receptor, TLCK did not inhibit insulin binding. At 0.25 mM, TLCK did not inhibit insulin internalization, but inhibited 70% of the insulin-stimulated glucose transport (ID50 = 0.19 +/- 0.02 mM, n = 7). TLCK inhibited insulin internalization at more than 0.25 mM. TLCK did not inhibit the tyrosine phosphorylation of the beta-subunit of the insulin receptor in intact cells or in the plasma membrane fraction. In intact cells, TLCK inhibited the phosphorylation of the 60 kDa protein and simultaneously it stimulated the phosphorylation of the 170 kDa protein more than 3-fold. These results indicate that there are at least two sites in the insulin-induced signal transduction pathway where proteinase inhibitors act to suppress the insulin signal transduction. A major ATEE site is very close to phosphorylation of the beta-subunit of the insulin receptor. On the other hand, TLCK inhibits a step(s) in the signal transduction pathway after the insulin receptor but before the glucose transporter.

New biological functions of intracellular proteases and their endogenous inhibitors as bioreactants

Many unexpected biological functions as bioreactants of the intracellular proteases and their endogenous inhibitors have been found recently. Chymase and tryptase in histamine granules of mast cells and basophile cells play an important role in the process of IgE-mediated degranulation and in the formation of an allergic inflammation profile. Furthermore, the relationship between membrane proteases and their endogenous inhibitor has been taken up as a key and key-hole relation which plays an important role for special recognition apparatus of biological information like the relation of peptide hormones (growth factors) and their specific receptors. Amino acid sequences of the active site of trypstatin are homologous with the neutralizing epitope beta of gp120 of AIDS virus (HIV-1). The trypstatin and anti-tryptase M antibody inhibited syncytium formation in HIV infected Molt 4, clone 8 cells. Therefore, the relationship between tryptase M with trypstatin and the recognition site of epitope beta of HIV-1 with the receptor of helper T-cells are the common keys. The precursor of Alzheimer's deposition protein contains a Kunitz-type trypsin inhibitor domain. The A4-precursor proteins are located in axons of pyramidal neurons in brain and secretory granules of chromaffin cells in adrenal medulla. Those may be secreted into the extracellular milieu. We propose that the A4 inhibitor inhibits a special type of tryptase in the brain and disturbs the complete degradation of secreted A4-precursor protein causing amyloid deposition in alzheimer disease by abnormal proteolysis. Human c-Ha-ras p21 shows 58% homology with cystatin beta, an endogenous inhibitor of cathepsin. Actually, p21 inhibits cathepsin L specifically, but not cathepsin H, papain and cathepsin B. However, the metabolic significance of this inhibitory activity is still unknown.