The role of ligand-receptor interactions in disease

Journal of Dermatological Science, 1 (1990) 59-64 Elsevier

59

DESC 00014

Review Article

The role of ligand-receptor B.J. Vermeer’

interactions in disease

and J.A. Gevers Leuven2

‘Department of Dermatology, University Medical Center, Leiden and ‘Gaubius Institute TNO. Leiden The Netherlands (Received

3 August 1989; accepted 20 September

1989)

Abstract

Ligand-receptor interactions play a determining role in many cell biological interactions [ 11. Mutations of ligands or receptors can both cause a disturbance in these interactions. In this paper an example will be given of these types of defects. Finally it will be speculated which role receptor mechanisms may play in the field of dermatology.

Receptor Defect The metabolic disorder familial hypercholesterolaemia (FH) is characterized by elevated low density lipoprotein (LDL) serum levels, tendinous xanthomas and an increased risk for premature coronary sclerosis [ 21. Since Goldstein and Brown discovered in 1974 that defective LDL receptors, are responsible for FH, these receptors have been studied intensively. On the cellular level it could be demonstrated that LDL-particles are internalized via a coated pit-vesicle system. After internalization of the LDL-receptor complex the LDL particles are degraded in the lysosomes while the receptors are recycled back to the plasma membrane. In such a way the receptors function like a shuttle bus which allows the ligands to be internalized into the cell [3,4]. Detailed studies on cultured keratinocytes and epithelial tumor cells have shown that in these cell systems a defective binding and internalization of Correspondence to: B.J. Vermeer, M.D., Department of Dermatology, University Medical Center Leiden, P.O. Box 9600, 2300 RC Leiden, The Netherlands. 0923-181 l/90/$03.50

0 1990 Elsevier Science Publishers

LDL particles correlate with the process of terminal differentiation [ 5 -71. In vivo studies and studies on liver membranes have demonstrated that LDL receptor activity of liver cells play an essential role in the catabolic degradation of these LDL particles. In such a way the LDL receptor activity on the liver cells determines the LDL serum levels [8]. Since 1980 new therapeutic modalities have been discovered which enable us to lower the LDL cholesterol level very efficiently. The principle of this treatment is to increase the LDL receptor activity on liver cell membranes via cholesterol depletion of the liver cells. This is primarily achieved via blocking the cellular cholesterol synthesis [ 9, lo]. Molecular biological studies have shown that the LDL receptor gene is 46 kb long and is located on chromosome 19. Also a large part of the amino acid sequence of the LDL receptor is known. Detailed studies of cultured fibroblasts from homozygous FH have revealed that various defects of LDL receptors can be responsible for the development of FH. These LDL receptor defects can be divided into four different classes [3]:

B.V. (Biomedical

Division)

60

1. Defective LDL receptor synthesis; 2. Defective intracellular transport of LDL receptors ; 3. Defective ligand-binding site of LDL receptor; 4. Defective internalization of LDL receptor. Nevertheless the different LDL receptor defects cause the same type of disease. By analyzing the amino acid sequence of the defective LDL receptors in various homozygous FH cell lines it could be shown that several types of mutation (insertion, deletion or point mutation) can give rise to a particular LDL receptor defect. When certain LDL-receptor gene mutants were analyzed its structure-function relationship could be unravelled (Fig. 1). However, in practice DNA analysis can only be used for early diagnosis of FH, when a founder phenomenon is present. In that case only one type of mutation has caused FH in a restricted population. Otherwise the clini. 1

NH2

.

2

. 3

4

COOH

Fig. 1. The receptor has live domains: Domain 1. Ligandbinding domain contains 7 repeats of each 40 amino acid residues; amino acid l-292. Domain 2. This part is needed for uncoupling the receptor from the ligand within the celluar compartment; amino acid 293-659. Domain 3. Aminoacids with OH-containing sidechains which can bind sugars. May play a role in transport of receptor from golgi-to plasma membrane; 58 amino acid residues. Domain 4. Intramembranous part of receptor. Many fatbinding aminoacids; 22 amino acid residues. Domain 5. Cytoplasmic part of receptor needed for clustering receptor to coated pit; 50 amino acids.

cal description is the only basis for the diagnosis FH. In conclusion we can state that FH is a good example of a disease caused by a receptor defect. The large amount of knowledge regarding LDLreceptor interactions has enabled us to develop new therapeutic modalities for this disease. In the future gene therapy for LDL receptor-defects might become feasible. Ligand Defect In the field of lipoprotein metabolism dysbetalipoproteinaemia is a good example of a disease caused by a ligand defect. The clinical picture of dysbetalipoproteinaemia = hyperlipoprotainaemia type III is characterized by elevated intermediate density lipoproteins (IDL), xanthochromia striata palmaris, and increased risk for premature arteriosclerosis (aa coronaria and aa femorales) [ 111. Under normal conditions the very low density lipoprotein (VLDL) particles are converted into LDL particles. In this conversion an enzymatic two step process takes place. In the first step VLDL particles are degraded into IDL and IDL particles are subsequently converted into LDL. Like LDL particles also IDL particles can be degraded via the LDL receptor. The binding of the ligand LDL to the receptor occurs via the apolipoprotein B and in the case of IDL the binding takes place via the apolipoprotein E. For this reason the LDL receptor is also nominated B-E receptor [ 12,131. Besides the B-E receptor also another E-receptor has been postulated. But to date there is no direct proof for its existence in humans. In binding studies it could be shown that the binding affinity of apolipoprotein E for the B-E receptor is 4 times higher than the binding affinity of apolipoprotein B. The apolipoprotein E proved to be a protein of low MW, 34 kD [ 141. Using immunoblotting techniques three different isoforms were analysed (Apo E2, E3 and E4). These isoforms were codominantly inherited in an allelic version and as a consequence every individual has

61

a certain phenotype of Apo E, i.e. E 2/2, E 3/3, E 4/4, E 3/2, E 4/2 and E 4/3. The distribution of these Apo E typing patterns was investigated amongst various populations. In all studies the majority of the cases carried the Apo E3 isoform, Apo E2 was less abundant. The Apo E2/2 homozygous form was only found in approx. 1 y0 of the population [ 15,161. Utermann was the first to demonstrate that Apo E2/2 was nearly always present in patients suffering from dysbetalipoproteinaemia [ 171. Functional studies in vitro showed that the Apo E2 had only a low aftinity binding to the B-E receptor, compared with Apo E3 and Apo E4. The deficient binding of Apo E2 resulted in a retarded clearance of the IDL particles via the B-E receptor. However the lipoprotein disorder dysbetalipoproteinaemia was only present in 4% of the persons homozygous for Apo E2/2. Therefore, in addition to Apo E2/2 a second unknown factor had been postulated, which could induce hyperlipidaemia. Several studies have shown that in families with dysbetalipoproteinaemia other genetically determined lipid disorders (e.g. combined hyperlipidaemia) are also present [ 181. It is postulated that a deficient clearance of the IDL combined with an increased synthesis of lipo-

TABLE

proteins (Apo B) can finally result in dysbetalipoproteinaemia. Mutations of the ligand (Apo E) In the chromosome 19 a gene cluster coding for Apo E has been detected. Subsequently the amino acid sequence of the common isoforms has been determined. The three isoforms ‘differ by substitution of one or two amino acid residues (at positions 112 and 158) of the mature Apo E protein [ 16,191. The majority of mutations associated with overt clinical dysbetalipoproteinaemia are located in the docking (binding) site of the Apo Emolecule (amino acid residues 140-160) [20,21]. Thus, the substitution of the arginine residue at the 158th position by cysteine causes the loss of one positive charge necessary for a proper ligandreceptor interaction [ 221. The same holds for substitution of arginine at position 145 by cysteine and as discussed later for lysine at position 146 by glutamine [23]. Further analysis of apolipoprotein E in patients with dysbetalipoproteinaemia has disclosed some rare Apo E mutations (Table I). In our group of patients two had the phenotype Apo E3/3 and not Apo E2/2 [24].

I

Apolipoprotein

E variants in dysbetalipoproteinaemia

(DH)

Variant

Mutation

Receptor binding activity (%)

E3 E4

wild tyde CYS llZ+arg

100 100

E2” E2 E2 E2-Christchurch E3-Leiden El-Harrisburg El-Bethesda

arg arg lys arg

t2 45 40 41 30 ? ?

158 + cys 145 -+ cys 146 + gln 136 + ser

Mode of inheritance (autosomal)

recessive recessive dominant dominant dominant dominant dominant

a Most common E2 type, in 4% of homozygous form is DH present. Note that E2 arg 158 + cys has larger defect in binding than the E2 lys 146 + gln. In spite of this the latter mutant causes dysbetalipoproteinaemia in a dominant way.

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In vitro studies revealed that this particular apolipoprotein E3 had an insufficient binding to the receptor due to a, to date, unknown mutation. This Apo E has been termed Apo E3-Leiden. In three other unrelated patients with dysbetalipoproteinaemia the isoform Apo E3/2 was found. This finding is compatible with a dominant inheritance, due to a particular defect of Apo E2. Using mutation-specific oligonucleotide probes in combination with amplified DNA it could be shown that this particular Apo E2 had glutamine instead of lysine at amino acid position 146 (lys 146 -P gln) [25]. In accordance with the finding that the person heterozygous for this rare mutation of Apo E2 has dysbetalipoptroteinaemia, this particular mutation was not found in a random population. Also Apo E3-Leiden and other rare mutations induce dysbetalipoproteinaemia in an autosomal dominant way. In conclusion we can state that a change of one amino acid can have a severe effect in the binding of a ligand to a receptor. In one situation the point mutation causes dysbetalipoproteinaemia in a recessive way (E2/2) but when the mutation takes place at another position (E2/3, E3-Leiden) the same type of disease may be inherited in a dominant way. Ligand receptor interactions in dermatology In the field of dermatology the ligand receptor mechanisms may also play an important role. Recent investigations have shown the important role of cytoplasmic receptors in the field of dermatology. The findings of Sawaya et al. [26] that the activity of androgens is higher in skin specimens from the scalp of bald men than in skin specimens from controls suggest that this androgen-receptor activity determines the process of balding. It is conceivable that new therapy modalities will be developed which can influence the hairgrowth via modulation of this receptoractivity. It has already been known for a long time that the androgen receptors also determine the pattern of hairgrowth and the activity of the sebaceous glands [ 271.

A second cytosolic receptor of importance for dermatology is the retinoid receptor. There is an accumulation of evidence that this receptor plays a determining role in the differentiation of keratinocytes [28,29]. As discussed by Chambon at the Tricontinental Meeting in Washington, a specific type of retinoid receptor (type 3) is especially present in epithelial cells [30]. It is conceivable that various genetic determined skin diseases characterized by a disturbance in differentiation will be found to be caused by a (retinoid) receptor defect. Also the finding that various growth factors (ligands) are produced by the epithelial cells suggests that growth-factor receptor interactions may play a role in the homeostasis of proliferation and differentiation of keratinocytes [ 3 11. Finally, in the area of immunology ligandreceptor interactions determine the communication and regulation of the immunocompetent cells. Antigens combined with membrane constituents or interleukines and cytokines can be regarded as ligands, while T cell receptors or interleukin receptors are plasmamembrane receptors

[321. A particularly interesting phenomenon is the way the antigen-T cell receptor interaction can be restricted by plasmamembrane constituents (HLA antigens) [ 331. This restriction enables the body to regulate the immunoresponse on an individual base. The antigen presentation via the Langerhans cell restricted via the HLA class II antigens is an example of such a phenomenon [ 341. The use of crystallography has enabled us to localise the antigen binding site in the ‘groove’ of the HLA molecule [ 35,361. Looking at this model it can be understood that certain small differences in the amino acid sequence of the HLA antigen in this antigen binding groove can have a profound effect on the regulation of the immune response. It is an intriguing hypothesis that exogenous influences (e.g. UV light, drugs) via such a mechanism may modulate the immune response in the skin or induce autoimmune disease [37-401.

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