Motogenic growth factors: HGF/SF and MSP

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Motogenic growth factors: HGF/SF and MSP / M. SANTORO. - In: MINERVA. - ISSN 0026-4695. - 9(1997), pp. 85-92.

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Motogenic growth factors: HGF/SF and MSP

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REVIEW

MINERVA BIOTEC 1 997;9;85-92

Motogenic

growth

factors:

HGF/SF

and

MSP

MASSIMO M. SANTORO. GIOVANNI GAUDINO

Cell movement is a critical step in norrnal ernbryonic

development,

tumour

metastasis,

inflammatory

responses, and wound healing. Cell migration has been described as continuous cycles

of

specific and subsequent morphological changes of the cell body, due

to

cytoskeletal modifications and forrnation

of

adhesive contacts. There is increasing evidence that growth îactor receptors can modulate cell motility, interfering either

with

the assembVdisassembly

of

the focal adhesion sites and

with

the mechanism

of

polymerisation,/depolymerisation

of the

actin fila-ments. One particular subfamily

of

growth factors has treen characterised by its ability

to

induce

deve-lopmental, as

well

as growth, cell dissociation, and

motility

stimuli:

Hepatocyte Growth Factor/Scatter Factor (HGF/SF) _{and Macrophage Stirnulating Protein}

(MSP). HGF/SF protein has the property of dispersing

or

scattering epithelial cell colonies into single isola-ted cells, with enhanced random, non-polarised moti-lity. The biological activity of the recepto! Met tyrosi-ne kinase, depends

on the

presence

of two

pho-sphotyrosine residues

in

the carboxy-terminal tail, acting as a multifrrnctional docking site

for

SH2-con-taining

effectors. Macrophage Stimulating Protein

(MSP),

originally

discovered

by

its

effect on

rna-crophages, is also active on several epithelial cells. Its receptor, the Ron tyrosine kinase, can be constituti-vely activated

in

the absence of the ligand.

In

these conditions the signalling evoked

by

active tyrosine kinase induces a strong motile and invasive phenoty-pe. The îact that HGF/SF and MSP elicit motile-invasi-ve respoflses indicates the important role played by

This work has been supportecl by AIRC (Associazione ltaliana per ia Ricerca sr.rl Cancro) and by CNR Prolect 95.00367.1,F39.

Address reprint requests tor G. Gaudino, Department of Science

ancl Aclvanced Technologlies, University of 'forino at Aìessandria,

Corso Borsalino i4, 15100 Alcssandria, ltaly.

From the Dqpartment of Science and Aduancecl Technologies

Uníuersity cl Torino at Alessandria, Ital1

these factors and

their

receptors

in

cell physiology and pathology.

KEv wonos: Cell movement

-

Growth substances

-

Sisnal

transduction HGF/SF

-

Macrophage stimulating proiein (MSP) Hepatocyte growth facror (HGF/SF).

f

eÌ1

migration plays

a crucial role

in

a

wide

\--r spsclpm of

lriological events

not

only

for

sim-ple organisms but also

for

animals. The aptitude of several

cells

to

migrate

is an

important

process during embryogenesis, ranging from gastrulation to development

of the

nelvous

system. Moreover

it

lasts also

during

the

adult

1ife, remaining promi-nent during physiological processes (e.g. inflamma-tory response, wound healing, and tissue

regenera-tion)

as

well

as

in

disease,

during the

metastatic process.l Not every ce1l type is specialised for loco-motion,

but in

given circumstances some

of

them (e.g. neutrophils, fibroblasts, neurons) are able to

move.

In

many

tissues

cell motility is

normally

repressed

but

it

is activated only

by

certain physio-logical and pathophysio-logical conditions (e.g. oncogenic transformation, inflammatory process, tissue rege-neration).

Cell locomotion

has

been

described as

conti-nuous cycies of specific and subsequent

morpholo-gical

changes

of

the cell

body.z These

ma1or

morphological changes can

be

summarised as

SANTO]ìO

lows:

(i)

cytoskeletal-mediatecl extensions

(prc.ttru-sion), (.i1) _fbrmation

_of

_{adhesive contacts at the cell}

leading edge (.aclhesicti), (11i) _{movement along the}

substrate (trctctictn),

(iv)

breaking adhesive contacts ( _{cle acl h e s ion'),} (.v') _{q-t()skeletrl-dè penclent retraction}

of

the trailing

eclge

(tail

retrctctiori). Every step

of

these cycles neecls

a

complex array

of

molecular events,

that

requires

the

physically

co-ordinated involvement, both spatially and temporally, of cyto-skeletal network, plasma-membrane compartment, and adhesion system.

Cell migration has been described as the

fctrma-tion

of

intimate and

extensive adhesive contacts between cells and substratum. This results

from

a

co-operation between the adhesive system and the actin cytoskeleton,

followed

by

generation

of

for-ces across

the

cell.:

Therefore.

it's

of

paramount importance

to

investigate

the

mechanisms

control-ling:

(i)

the

status

of

adhesive contacts n

irh

the

matrix

or with the

sr:bstrate, deterrninecl

by

the

assembly/disassembly

of

the focal

adhesion sites;

(ii)

the

cytoskeletal reorganisation inside

the

cell. visible as stress fibers and lamellipodia formation. based

on

polymerisation/de-polymerisation

of

the actin filaments.

Focal adhesion

is the

common tvDe

of

adhesive contact made

by

cells

with

the

Extiicellular

Matrix (nClf).+ _{Focal adhesions are characterisecl}

_by

inte-grins

as

the major

adhesion receptors s e

_and

_by

associated cytoplasmic plaque proteins, including

vinculin, talin, paxillin

and

a

number

of

protein

tyrosine

kinases,

such as

FAK

(Focal

Adhesion

Kinase).4 r They are the major sites of actin fìlament attachment at the contact interface. their formation is associatecl

with

the process

of

cell spreading and are sites

for

co-ordination between

cell

adhesion and cell migration.s

The assembly

of

the fbcal adhesions is regulated by ECM ligancl binding events. The combination

of

integrin

receptor occupancy

and

clustering,

indu-ced

by

the

ECM

ligands triggers

a

synergistic response that includes re-orÉlanisation

of

the

cyto-skeleton, association

of

cytoplasmic

plaque

pro-teins, and activzttion of a local signalling pathway.6'r

On the

other hand, adhesive complexes assembly

and disassembly can also be regulated

hy

intracel-lular

signal5,10 11 _{These signals are}

Éaenerated by biochemical modifications and oroduction

of

soh,r-ble

second messengers

typical

of

the

signal tran-sduction pathways.

N{()TOGllNtC GRONTII lìAC-lOlìS: HGF/ SF AND ì\,lSP

It

has been postr-rlated that one of the mechanisms

of

adhesive complexes formation

is

tyrosine pho-sphorylation ctf integrins,

paxillin,

tensin, and FAK by specific tyrosine kinases. The phosphorylation on tyrosine residues generates ctn

the focal

aclhesion components, specific recognition sites

for

proteins containing SH2 (Src-homology

2)

clomains 12 f

i

_In adclition, FAK exhibits tyrosine kinase activity ancl

phosphorylates cytoskeletal-associated substrates such as

talin,

Src ancl paxilli1.15 16

_{This might}

_be

necessary

fbr

recruiting

aclclitìonal structural and signalling components

of

the

focal adhesion

cc-rm-plex. FurthermoÍe,

the

focal adhesion prcteins can aiso bind to each other and to actin filaments.

throu-gh

SH3 (Src Homology-3)-binding domains

or

by

LIM

domains.t; The

mctlecular interconnections

between the components of the focal adhe.sion

com-plex

and

cytoskeletal actin can

be

enhanced and modified by these multiple biochemical interactions,

in

order

to

transmit mechanical forces

for

cell loco-motlon.

Focal adhesions and actin-membrane interactions

are also regulated

by the

Rho

subfamily

of

thc

GTP-binding proteins.tE Cdc 42, Rac, and Rho sti-mulate the assembly

of

structuÍes resembling focal adhesions

in

association witl-i

filopodia,

lamellipct-dia,

and

actin

stress

fibers,

respectively.le Inte-restingly, Rho can regulate actin polymerisation 20

through a pathway involving the increase

in

phcts-phatidylinositol 4,5

bisphosphate

(PIP,)

levels.2t

PIP, promotes actin

filament

polymerisation

by

direct

interaction

with

actin-binding proteins.22 In

addition, Rho and other related GTPases may also

function

in

a local

signaliing pathway,

coupling

ligand

binding

of

integrins

to

focal

aclhesion assembly.'r Given

the

roles played

by

Cdc42, Rac

and

Rho

in

the

regulation

of the

different

actin-membrane interactions. these transclucers cor-rld be sr,rfficient

to

clrive

the

entire process co-ordinating the cycles

of

cell extension, adhesion, and detach-ment that are implicated

rn

cell n-rotility. Howcver, the mechanisms exerted

by

Cdc42, Rac ancl Rho in

the

processes relevant

to

cell

locomotion are not

well

characterised yet. Many protein tyrosine kina-ses

and tyrosine

phosphatases as

well

as

lipid

enzymes,

including the focal

adhesion

kinase

(FAK), phosphatídylinositol

(PI)

_3-kinase

(PI

3-kinase),

phosphatidylinositol

phosphate

(PIP)

5-kinase

(PIP

5-kinase),

and

the

phospholipase

C-(PLC-y) _{have been iclentified as}

_key

_mediators

_of

NIOTOCIENIC CìRO\\IH FACTORS: HCìI-'/SF AND NiSP

the

signalling pathway induced

by

these small G proteins.l FAK probably initiates formation of

acll-re-sions, PIP-5 kinase generates PIP, implicated

in

the assembly

of

actin filaments, PI 3-kinase is involved

in

chemotactic responses ancl

in

the modulation

of

integrin

affinitv,

PLC-y mobilise actin-binding pro-teins uia h).drolysis 9f PIP,.r

Tlre

multitucle

of

intracellular

comolex

interac-tions, leading

to

cell motility arc

dcpènclent upon several stimuli evoked

by

the extracelh,rlar enl iron-ment, as grou'th and clifferentiation f:rctors,

cltoki-nes,

and

chemoattractants.

In

particular,

the

rapid effects on cell adhesion ancl on celi motility, ererted

by

some

growth

factors and

their

receptors are mediated

by

some

of

the transducers and effectors mentioned above.

In

additkrn, growth factor recep-tors trigger signalling pathn'ays very similar

to

the Iocal signals of focal aclhesion regulation. For

exam-ple,

Cdc42, Rac

and

Rho

are

activated

by

rnany growth factors,23 2a _ancl

_PI

_netabolisn

_is

_{linkecl tcr}

the growth factor receptor-mecliated ceil rnotility.:;

:-In

this review, attention

will

be focusecl on the spe-cific gror,vth factors inclucing cell migration.

Motogenic

growth

factors

In

general, tissue growtl-ì factors are soluble pro-teins acting on the gror,vth and on the proliferation

of

specific target cells. Nevertheless, mzrny gror,vth factors

work

as nultifr-Lnctional agents. thzLt induce

not only cell

proliferation,

but

also

a

variety of

additional effects including cell movement.

Gror.r'th factors exefi their effect through binding

to

the ceil surface, where

a

specific transmembrane receptor rvith protein tyrosine kinase activity (PTK-R) is located. Binding

of

the gr()wth factor to the extra-celluiar domain

of

these receDtors leads

to

the tran-sient activation

of

the kinase ancl

of

rhe sienzrl tran-sduction cascade reactions.2il 31 _{Ligand bincling}

intlu-ces dimerization

of

receptor molecules,

which

in

turn

leads

to

an

increase

in

catalytic activity. This

al1ows auto-transphosphorylation

of

the intracellular domain of the receptor on tyrosine residues

embecl-ded

in

specific amino acid sequences. These pho-sphotyrosines and their flanking sequences become

clocking

sites

for

proteins containing

conserved

structural modules.

known

as SH2 domains. This module

is a

typical feature

of

sequences encocling molecules involved in signal transcluction.sz

SANTORO

As

nentioned

above, tl-rere

is

increasing

eviden-ce

that

growth

factor receptors can modulate cel1

motility.

interfering either

with

the assembly/disas,

sembly

of

the

focal

adhesion sites

and

with

the mechanism

of

polymerisation/clepolyrnerisation

of

the actin filaments.

It

is

u'orth

nothing that signals elicited

by

growth factor receptors can activate the same

specific intracellular signalling

pathweys induced

by

integrins and,/or other components

of

the focel

adhesion..r3 For example, PDGF, insulin,

and IGF-1 are able

to

índuce re-orp;anisation

of

ti-re

actin cytoskeleton, as during formation

of

lamelli-podia and menbrane

ruffles,

following

activation

of PI

3-kin25s.31-36 Many data have shed

light

on

the mechanisms

by which

PI 3-kinase and its lipicl prodllcts can initiate cell movement. Some eviden-ce suggest that the

lipid

products of the PI 3-kinase

may

be

involvecl

in

mecliating interactions

with

actin filanents

and,/or

microtubular

motor5.37 38 Recentlv,

it

h:rs been clemonstratecl that one

of

the don'nstrearn effectors

of PI

3-kinase

is the

small

GTP-bincling

protein

Rac,

that can be

directly

responsible

for

the

PDGF-incluced chemotactic

response by fibroblasts

in

uitro.3e 10

There

are

several examples

of

invoivement of

gros'tll

factors

in

the

regulation

of

fbcal adhesion and actin

filanents

assembly.

It

has been

clemon-strated

that the platelet-derived growth

factor

receptor (PDGF-R),

_upon

_{ligand activation}

_is

_able to interfere

with

adhesion and cytoskeletal systems. Phosphorylated PDGF-R

induced

cytoskeletal re-organisation

in

the

skeletal muscle cell, as

well

as

the

tyrosine phosphorylation

of

paxillin,

FAK, and talin

in

Sn-iss 3T3 fibroblasts.+l 12

ln

_{adherent Swiss}

3T3 cells

the

activated PDGF-R induced

the

asso-ciation

of

PI 3-kinase

n'ith

làK.2.) PDGF bindins to

its

receptor indnced tyrosine phosphorylarion

òf

a

190 kDa protein that co-immunoprecipiteted

specr-fically urith integrin

o..B,.13

Furthermore,

PDGF induces neurite outgrowth

of

PC12 ce11s

in

a

PI

3-kinase and PLC-y dependent manner.4l

Another

growth

factor,

the

Epidermal

Grctwth

Factor (EGF) is directly involved

in

the cyoskeletal organisation of the cell, because its receptor (EGF-R) contains an F-actin binding domain

in

its intracellu-lar region.as

Upon

activation

of

their

respective receptors, both PDGF and EGF incluce the formation

of

focal adhesion throursh

the

stimulation

of

the

GTP-bin-ding proteins

oi

the Ras family.

In

addition, PDGF

SANTOIìO

and EGF receptors phosphorylated PLC-y1,16

a

pre-requisite

for

the

dissociation

of

the

profilin:PlP, '+;

and

gelsolin:PIP, complexes,4s important steps in

the

regulation

of

actin polymerisation/depolymeri-sation. Moreover, stimulation

with

insulin has been found

to

induce the direct association between the phosphorylated specific Insulin Receptor Substrate-1

(IRS-1)

and

the

integrin

cr,.Br. Similarly,

the

o"B, ligand enhanced several

fold

DNA synthesis indu-ced

by

insulin,

in

ce1ls plated on vitronectin.is This

result

reveals

the

existence

of

a

direct

cross-talk

between these

two

classes

of

molecules: Integrins and Tyrosine Kinase Receptors.

On the

other hand, there are synergistic effects

among

Growth

Factors anC integrins

involved

in

the

regulation

of

cell

rnigration and

cell

adhesion. The misration

of

FG carcinoma cells on vitronectin

matricei

needs

the

activation

of the

signal

tran-sduction pathway

by

EGF-R,

including

PKC and

PlC-activation.25 50

In

_addition,

_Stem

_Cell

_Factor

(SCF) enhanced mast ce1ls adhesion

to

fibronectin,

uia PI

J-kinase

aclivation upon hinding

tt-r

it:

receptor Kit.;t This synergism is strictiy reqttired

ior

a complete biological effect conveyetl

hy

a growth

factor. The

addition

of

PDGF

to

non-adherent

fibroblasts,

does

not

induce 1,5

PIPz hydrolysis

(althor-rgh PLC-y becomes activated), neither Car+ mobilisation and PKC activation, three

well

known events exerted by PDGF and involved

in

its prolife-rative response.

The failure

of

PLC-y

to

hydrolyse PIP, stems

from the lack

of

substrate (.4,5

PIP),

whose synthesis depends

on

PIP 5-kinase activity,

induced

hy

integrin binding

to

ECM.

The full

responsiveness

of

fibrobiasts

to

growth

factor

sti-mulation

can

be

rescued,

only after plating

on fibronectin.iz

r;

Adherence

to

the

ECM. and subse-quent integrin activation stimulated the small GTP-binding protein Rho, resulting

in

an increase of PIP

5-kinase actlvity and synthesis

of

4,5-PIPr, substrate

of PLC-Y.54

According

to

this model,

mitogenic

stimuli

are

under

the

double

control

of

growth

factors and

extracellular matrices:

cell

oroiiferation

is

induced

by

the growth

factor,

through the

activation

of

a

specific enzyme (PLC-y),

but

is modulated

by

inte-grins, that influence

the

cellular responsiveness to growth stimuli, regulating the level

of

the substrate (PIPr) for the effector enzyme.

One

particular subfamily

of

growth

factors has been characterised

by

its ability

to

induce

a

wide

NIOTOGENIC GRO\VTII FACTORS: HGF/SF AND N{SP

spectrum

of

biological activities. Among thesc are

differentiative) as

well

as growth, cell dissociation,

and

motility

stimuli,

all

conveyed

by two

heterodi-meric proteins,

known

as Hepatocyte Growth

Factor,/Scatter Factor (HGF/SF)

and

Macrophage Stimulating Prorein (MSP).

Hepatoclte

Growth

Factor/Scatter

Factor

(HGF/SF)

This protein

has

the

property

of

dispersing or

scattering epithelial

cell

colonies

into

single

isola-ted

celis,

by

rupture

of

intercellular junctions and desmosomes.

The

scattered cells have enhanced

motilit)-,

shc,u-ing

random,

non

polarised

move-ment.:: The determination

of

its

amino

acid

seqlÌence showed

that

it

is

identicai

to

a

strong mitogen

for

hepatocytes and

for

a

witle

variety

of

other

epithelial

ce11 types.r6 Hepatocyte Growth

Factor/Scatter Factor (HGF/SF), as

it

is now named, has been generally found to be secreted

by

mesen-chymal ce1ls (fìbroblasts and smooth mr-rscle cells)

rather

than

by

epithelial and endothelial

cells,

although its effects are mainly obvious on the latter

two

cell types.

HGF/SF is a heterodimer

with

a larger cr chain

of

about 60

fOa

and

a

smaller

_$

chain

of

about 30

KDa, linked by a single interchain disulphide bond. The protein

is

glycosylated

to

a

significant extent.

It

is translated from a single nRNA, and the active

form is

produced

by

cieavage

of

the

biologically

inert

precursor chain. Anaiysis

of the

structure

of

the molecule revealed several features that are not shared

with

any other _{Élrowth factor.} The cr chain conlains

four

kringles,

structural

motifs that

also occurs

in

plasminogen, tissue-type and

urokinase-type

plasminogen activators, factor

XII

and

proth-rombin.

The

_{B chain} consists

of a

serine protease domain that

is

inactive, because

of

substitution

of

two

criticai residues in the catalytic site.5r

The high affinity

receptor

of

HGF/SF has been

identified as the MET proto-oncogene product. Met is a transmembrane receptor tyrosine kinase, made

of a

745

KDa

_B subunit and

a

50 KDa cr subunit, that is synthesised as a single chain precursor. The c, chain and the N-terminal

portion

of

the

_{B chain}

are exposed on the cell surface, whereas the C-ter-minal portion

of

the B chain is located

in

the

cyto-plasm and contains the tyrosine kinase domain and

N{OTOGE\IC GIìO'$íTH FACTORS: HGF,,SF AND NISP

phosphorylation sites involved

in

the regulation

of

enzyme activity and signal 112n5ds611en.58 5e

The biological activity

of

the

receptor, including cell motility, depends on the presence

of two

pho-sphotyrosine residues

in

the

carboxy-terminal tail, which act as a multifunctional dockins site for SH2-containing effectors

and activlte

,n

ìrrry

of

tran-sductional pathways.6tr

The molecular

mechanisms

resoonsible

for

HGF/SF

and Met receptor mediateJ

cytoskeletal rearrangement

and

consequent

cell

migration are

not

fully

understood. The role played

by

the

PI

3-kinase

in

HGF/SF-mediated

motility

and scattering seems

well

defined. The HGF/SF induced scatterins

on

MDCK cells can be blockecl using

thc

selectivè

PI

3-kinase

inhibitor, \fortmannin.6l

Similarly,

\Tortmannin treatment abrosates HGF-induced che-motaxis and tr-rbulogenesis

in

renal epithelial cells.63

These results show that

the

activation

of

the

PI

3-kinase is critical

in

HGF/Met-mediated cell dissocia-tion and motility of epithelial cells.

An

interesting

point is the

involvement

of

the small GTP-binding proteins Ras, Rac and Rho, in the regulation

of

the

migratory responses induced by HGF/SF. Different data have been reoofied

on

the

Ras

involvement

in

Met-mediatecl cèI1 migration.

Expression

of

a dominant negative mutant Ras

pro-tein

or

the

injection

of a

neutralising antibody for

Ras

in

MDCK cells blocked HGF/SF-mediatecl cell dissociation and scatter. This demonstrates that the Ras pathu,'ay

is

essential

to

mediate

the

motility

signal

of

HGF/SF-Met receptor

to

the cell-ceil adhe-sion system and

the

cyoskeleton

of

MDCK epithe-lial ce11s.6:

In

addition. microiniection of an activated

form

of

Ras promotecl constituent

cell

spreacling.6a

On the other hand. MDCK cultured cells transfected

with

a Met mutant, where the Grb2 binding site was

specifically abrogated,

but all

other effectors could bind, were still able

to

dissociate and migrate. This shows that

a

direct

link with

Grb2lRas

is

required

for

transformation,

but

it

is not

essential

to

trig€aer

the scatter response in MDCK cells.65

Moreover, microinjection

of

MDCK cells

with

a

dominant

negative

mutant

of

Rac

inhibits

cell

spreading

and actin

reorganisation

induced

by

HGF/SF, showing

that

Rac plays

a

cn:cial role

in mediating the HGF/SF induction

of

these events.61

On the

other hand,

it

has

been shown that

the HGF/SF-mediatecl

cell motility of

cultured mouse keratinocytes requires the involvement

of

Rho, but

not

of

Rac

and

Ras.66

However,

PI

3-kinase has been shown

to

be involved

in

the selective

activa-tion of

Rac and Rho,(,' therefore the possibility that

Met involves Rac and Rho

via

activation

of

the

PI 3-kinase cannot be ruled out.

Interestingly those, has been observed a

redistri-bution

of

E-cadherin

and

desmoplakins

I/II

fo1-lowing

HGF/SF stimulation

of

epithelial ce11s,6s as

well

as

the phosphorylation

of

_B-catenin

and plakoglobin,6e demonstrating

the

existence

of

a

control-effect

of

the Met

tyrosine kinase receptor

on the

adhesive elements

connecting

cells

and

regulating

cell

locomotion. Moreover,

it

has been demonstrated

that

HGF stimulates

motility

in

oral sqlramous carcinoma

cells

mediating

the

assem-bly/disassembly

of

focal adhesions

by

involvement of _P12lnak.;o

Recently,

an

increasing

number

of

signalling molecules

with

properties similar

to

HGF/SF have

been

characterised,

both

in

physiological and

in

pathoiogical cells.

A

Scatter Factor-like factor (SFL)

has been identified as

a

Daîàcrine effector molecu-le. produced by a metastatic

vrrirnt

of

a

trrcinomr

cell

line.rr Another

newly

discovered

factor with

scattering activity has been identified

in

a monocy-te-conditioned medium as stimulator

of

tumor

cell motility.T2 Many

well

characterised cytokines (e.g. aFGF, IL-6, EGF, and TNFcr) have also been found

to

induce dispersion

of

epithelial cell

colonies,

migration, and invasion

of

human

carcinoma

cells.6e 13 -1

These scattering agents

all

share the property

of

scattering epithelial cell colonies, but have different

cell type

specificity:

SFL

and

HGF/SF

do

scatter MDCK cells, whereas aFGF, TNFcx, EGF

and

the Monocyte Factor

do

not. On the

other hand, EGF induces MDCK ce1l proliferation. Thus, the specific effects elicited

by

a factor depends

on

the type

of

target cells as

n'ell

as

on

the extracellular environ-ment.

Macrophage

Stimulating

Factor (MSP) Macrophage stimulating Protein (MSP) was origi-nal1y discovered

by its ability

to

make

resiclent

peritoneal

macrophages responsive

to

the

che-moattractant C5a of the complement.T5

Macrophage stimulating Protein (MSP)

is

an

80

SA\'IOIìO

KDa clisurlphicle-linked heteroclimer

which

belongs to the HGFr'SF farnily

-t

MSP is synthesizecl

by

liver cells, circulates

in

bloocl as

a

biologically inactive precLlrsor and is clea'u.ecl

by

members

of

the

kallik-rein

family, as n'el1 as

by

nacrophage bouncl

pro-teases.'- tii

Functional stllclies har.e revealed that,

in

adclition

to

macrophages, MSP acts :rlso

on

other ce11 linea-ges. These inclucle

gron'th

stimr,rlation

of

certain epithellal

cell

lines, suppression

of

colony

forma-tion of

human bone m:rrrou,' cells indr,rcecl

by

steel

factor plr-rs GA4-CSF, and incluction

of

IL-6

produc-tion

in

primary

hurnan

rnarrow

megakaryctcvtes.

The receptol

for

NISP n-:rs icientified as

the

RON

gene prodLlct, zr transmemltrane receptor protcin tyrosine kinase, clonecl fror-n a human keratinoel-tcs cell line.r'r T1-re Ron gene encodes

a

185 KDa hete-roclimeric protein ccxnposecl ctf

a

35 KDa extracel-1ular cx-chain

and

a

150

KDa

transmembrane

B-chain

u'ith

intrinsic tyrosine kinase acti\.ity.s0 sr Ron

belongs

to

a

subfamill-

of

receptor t1'rosine kinase

tl-rat includes Sea and the proto-oncogene N{et.

As

the

HGF/SF

plototypc

r.,f

thc family,

À4SP is

eqr-raily able

to

incluce cell prolÌferation rìs q,ell :rs

celi

rnotility

on

epithelial

cells 82 _ancl

_on

_mnrine

kcr:Ltinocytes.s3

The À4SP/Ron receptor can he constitlltiveiy acti-vated

in

the absence of t1-re lisancl erther as

I

natu-rally

occnrling splicing verient

r

A-Ron)

or

_by

exprcssing molecr-rl:rr chimaeras

with

constitr-ÌtÌvely climelizecl kinase clomalns (Tpr-Ron).

In

these con-ditions the signalling evoked

by

the active tyrosine kinase incluces

a

strong

nìotile

and invasirre

phe-not,vpe.8'i

Recently

it

has been clemonstrated that

the

acti-r.ation of PI 3-kinase is an absoluLte recluirement for MSP-incluced

cell

migration

in

keratinocytes and epitl-relial cel1s.85

Motility

incluced

by

Ron

is

incle-penclent

of

the

threshold

of

N{AP-kinzrse

level of

activation. suggesring that the Ras pathwey is not a

critic:rl

step

for

MSP-induced

cell

r"nigration. The

Ras threshold required

for

the

scattering response is far lower that the necessary

for

grow'th and tran-sfÌrrmation.s(r

fte1

fulfils the

requirements

for

acti-vating

cell

dissoci:rtion

and matrix invasion

and

provides zr naturally occurring example

of

dissocra-tion

between

the

two

arms

of

the

biolosical

responses triggerecl by tyrosine kinase recept()rs.

Cell

morrement

is

a

critical

steo

in

nornal

embryonic clevelopment,

tlrmor

metaitrsis,

inflam-NIOTOGENTC Cì}ìO\ÍI-H FACTORS: HGF/SF ANI) I,{SP

matory

responses,

and

wound

healing. The

fact

that HGF/SF and MSP elicit motile-invasive respon-ses indicates

the

important

role

played

by

these factors anci

their

receptors

in

cell

physiology ancl

pathology. In particular. a possible role for Met and Ron can be envisaged

in

tumor progressictn towarcl metastasis.

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().fr3 () ll i1

tl

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