Protein kinase C family: on the crossroads of cell signaling in skin and tumor epithelium

The protein kinase C (PKC) family represents a large group of phospholipid dependent enzymes catalyzing the covalent transfer of phosphate from ATP to serine and threonine residues of proteins. Phosphorylation of the substrate proteins induces a conformational change resulting in modification of their functional properties. The PKC family consists of at least ten members, divided into three subgroups: classical PKCs (alpha, betaI, betaII, gamma), novel PKCs (delta, epsilon, eta, theta), and atypical PKCs (zeta, iota/lambda). The specific cofactor requirements, tissue distribution, and cellular compartmentalization suggest differential functions and fine tuning of specific signaling cascades for each isoform. Thus, specific stimuli can lead to differential responses via isoform specific PKC signaling regulated by their expression, localization, and phosphorylation status in particular biological settings. PKC isoforms are activated by a variety of extracellular signals and, in turn, modify the activities of cellular proteins including receptors, enzymes, cytoskeletal proteins, and transcription factors. Accordingly, the PKC family plays a central role in cellular signal processing. Accumulating data suggest that various PKC isoforms participate in the regulation of cell proliferation, differentiation, survival and death. These findings have enabled identification of abnormalities in PKC isoform function, as they occur in several cancers. Specifically, the initiation of squamous cell carcinoma formation and progression to the malignant phenotype was found to be associated with distinct changes in PKC expression, activation, distribution, and phosphorylation. These studies were recently further extended to transgenic and knockout animals, which allowed a more direct analysis of individual PKC functions. Accordingly, this review is focused on the involvement of PKC in physiology and pathology of the skin.

Activation of protein kinase C zeta induces serine phosphorylation of VAMP2 in the GLUT4 compartment and increases glucose transport in skeletal muscle

Insulin stimulates glucose uptake into skeletal muscle tissue mainly through the translocation of glucose transporter 4 (GLUT4) to the plasma membrane. The precise mechanism involved in this process is presently unknown. In the cascade of events leading to insulin-induced glucose transport, insulin activates specific protein kinase C (PKC) isoforms. In this study we investigated the roles of PKC zeta in insulin-stimulated glucose uptake and GLUT4 translocation in primary cultures of rat skeletal muscle. We found that insulin initially caused PKC zeta to associate specifically with the GLUT4 compartments and that PKC zeta together with the GLUT4 compartments were then translocated to the plasma membrane as a complex. PKC zeta and GLUT4 recycled independently of one another. To further establish the importance of PKC zeta in glucose transport, we used adenovirus constructs containing wild-type or kinase-inactive, dominant-negative PKC zeta (DNPKC zeta) cDNA to overexpress this isoform in skeletal muscle myotube cultures. We found that overexpression of PKC zeta was associated with a marked increase in the activity of this isoform. The overexpressed, active PKC zeta coprecipitated with the GLUT4 compartments. Moreover, overexpression of PKC zeta caused GLUT4 translocation to the plasma membrane and increased glucose uptake in the absence of insulin. Finally, either insulin or overexpression of PKC zeta induced serine phosphorylation of the GLUT4-compartment-associated vesicle-associated membrane protein 2. Furthermore, DNPKC zeta disrupted the GLUT4 compartment integrity and abrogated insulin-induced GLUT4 translocation and glucose uptake. These results demonstrate that PKC zeta regulates insulin-stimulated GLUT4 translocation and glucose transport through the unique colocalization of this isoform with the GLUT4 compartments.

Glucose effects on skin keratinocytes: implications for diabetes skin complications

Altered skin wound healing is a common cause of morbidity and mortality among diabetic patients. However, the molecular mechanisms whereby diabetes alters skin physiology have not been elucidated. In this study, we investigated the relative roles of hyperglycemia, insulin, and IGF-I, all of which are abnormal in diabetes, in primary murine skin keratinocytes. These cells proliferate and differentiate in vitro in a manner similar to skin in vivo. It was found that in the presence of high glucose (20 mmol/l), the glucose transport rate of primary proliferating or differentiating keratinocytes was downregulated, whereas at 2 mmol/l glucose, the transport rate was increased. These changes were associated with changes in the GLUT1 expression and with changes in the affinity constant (K(m)) of the transport. Exposure to high glucose was associated with changes in cellular morphology, as well as with decreased proliferation and enhancement of Ca(2+)-induced differentiation of keratinocytes. Furthermore, in the presence of high glucose, ligand-induced IGF-I receptor but not insulin receptor (IR) autophosphorylation was decreased. Consequently, in high glucose, the effects of IGF-I on glucose uptake and keratinocyte proliferation were inhibited. Interestingly, lack of IR expression in IR-null keratinocytes abolished insulin-induced glucose uptake and partially decreased insulin- and IGF-I-induced proliferation, demonstrating the direct involvement of the IR in these processes. Our results demonstrate that hyperglycemia and impaired insulin signaling might be directly involved in the development of chronic complications of diabetes by impairing glucose utilization of skin keratinocytes as well as skin proliferation and differentiation.

Protein kinase Cdelta-mediated phosphorylation of alpha6beta4 is associated with reduced integrin localization to the hemidesmosome and decreased keratinocyte attachment

In mammalian epidermis, expression of the alpha6beta4 integrin is restricted to the hemidesmosome complexes, which connect the proliferative basal cell layer with the underlying basement membrane. Keratinocyte differentiation is associated with down-regulation of alpha6beta4 expression and detachment of keratinocytes from the basement membrane. Neoplastic keratinocytes delay maturation, proliferate suprabasally, and retain the expression of the alpha6beta4 integrin in suprabasal cells disassociated from the hemidesmosomes. We now show that the alpha6beta4 integrin is a substrate for serine phosphorylation by protein kinase C in keratinocytes. Furthermore, protein kinase C-mediated phosphorylation of alpha6beta4 is associated with redistribution of this integrin from the hemidesmosome to the cytosol. Specifically, in vitro kinase assays identified the protein kinase Cdelta as the primary isoform phosphorylating alpha6 and beta4 integrin subunits. Using recombinant protein kinase C adenoviruses, overexpression of protein kinase Cdelta but not protein kinase Calpha in primary keratinocytes increased beta4 serine phosphorylation, decreased alpha6beta4 localization to the hemidesmosome complexes, and reduced keratinocyte attachment. Taken together, these results establish a link between protein kinase Cdelta-mediated serine phosphorylation of alpha6beta4 integrin and its effects on alpha6beta4 subcellular localization and keratinocyte attachment to the laminin underlying matrix.

Insulin stimulates PKCzeta -mediated phosphorylation of insulin receptor substrate-1 (IRS-1). A self-attenuated mechanism to negatively regulate the function of IRS proteins

Incubation of rat hepatoma Fao cells with insulin leads to a transient rise in Tyr phosphorylation of insulin receptor substrate (IRS) proteins. This is followed by elevation in their P-Ser/Thr content, and their dissociation from the insulin receptor (IR). Wortmannin, a phosphatidylinositol 3-kinase (PI3K) inhibitor, abolished the increase in the P-Ser/Thr content of IRS-1, its dissociation from the IR, and the decrease in its P-Tyr content following 60 min of insulin treatment, indicating that the Ser kinases that negatively regulate IRS-1 function are downstream effectors of PI3K. PKCzeta fulfills this criterion, being an insulin-activated downstream effector of PI3K. Overexpression of PKCzeta in Fao cells, by infection of the cells with adenovirus-based PKCzeta construct, had no effect on its own, but it accelerated the rate of insulin-stimulated dissociation of IR.IRS-1 complexes and the rate of Tyr dephosphorylation of IRS-1. The insulin-stimulated negative regulatory role of PKCzeta was specific and could not be mimic by infecting Fao cells with adenoviral constructs encoding for PKC alpha, delta, or eta. Because the reduction in P-Tyr content of IRS-1 was accompanied by a reduced association of IRS-1 with p85, the regulatory subunit of PI3K, it suggests that this negative regulatory process induced by PKCzeta, has a built-in attenuation signal. Hence, insulin triggers a sequential cascade in which PI3K-mediated activation of PKCzeta inhibits IRS-1 functions, reduces complex formation between IRS-1 and PI3K, and inhibits further activation of PKCzeta itself. These findings implicate PKCzeta as a key element in a multistep negative feedback control mechanism of IRS-1 functions.

Insulin induces specific interaction between insulin receptor and protein kinase C delta in primary cultured skeletal muscle

Certain protein kinase C (PKC) isoforms, in particular PKCs beta II, delta, and zeta, are activated by insulin stimulation. In primary cultures of skeletal muscle, PKCs beta II and zeta, but not PKC delta, are activated via a phosphatidylinositol 3-kinase (PI3K)-dependent pathway. The purpose of this study was to investigate the possibility that PKC delta may be activated upstream of PI3K by direct interaction with insulin receptor (IR). Experiments were done on primary cultures of newborn rat skeletal muscle, age 5--6 days in vitro. The time course of insulin-induced activation of PKC delta closely paralleled that of IR. Insulin stimulation caused a selective coprecipitation of PKC delta with IR, and these IR immunoprecipitates from insulin-stimulated cells displayed a striking induction of PKC activity due specifically to PKC delta. To examine the involvement of PKC delta in the IR signaling cascade, we used recombinant adenovirus constructs of wild-type (W.T.) or dominant negative (D.N.) PKC delta. Overexpression of W.T.PKC delta induced PKC delta activity and coassociation of PKC delta and IR without addition of insulin. Overexpression of D.N.PKC delta abrogated insulin- induced coassociation of PKC delta and IR. Insulin-induced tyrosine phosphorylation of IR was greatly attenuated in cells overexpressing W.T.PKC delta, whereas in myotubes overexpressing D.N.PKC delta, tyrosine phosphorylation occurred without addition of insulin and was sustained longer than that in control myotubes. In control myotubes IR displayed a low level of serine phosphorylation, which was increased by insulin stimulation. In cells overexpressing W.T.PKC delta, serine phosphorylation was strikingly high under basal conditions and did not increase after insulin stimulation. In contrast, in cells overexpressing D.N.PKC delta, the level of serine phosphorylation was lower than that in nonoverexpressing cells and did not change notably after addition of insulin. Overexpression of W.T.PKC delta caused IR to localize mainly in the internal membrane fractions, and blockade of PKC delta abrogated insulin-induced IR internalization. We conclude that PKC delta is involved in regulation of IR activity and routing, and this regulation may be important in subsequent steps in the IR signaling cascade.

The regulation of skin proliferation and differentiation in the IR null mouse: implications for skin complications of diabetes

Impaired wound healing of skin is one of the most serious complications of diabetes. However, the pathogenesis of this process is not known, and it is unclear whether impaired insulin signaling could directly affect skin physiology. To elucidate the role of insulin in skin, we studied skin insulin receptor (IR) null mice. The morphology of the skin of newborn IR null mice was normal; however, these mice exhibited decreased proliferation of skin keratinocytes and changes in expression of skin differentiation markers. Due to the short life span of the IR null mice, further characterization was performed in cultured skin keratinocytes that can be induced to differentiate in vitro, closely following the maturation pattern of epidermis in vivo. It was found that despite a compensatory increase in the insulin-like growth factor I receptor autophosphorylation, differentiation of cultured IR null keratinocytes was markedly impaired. In vitro proliferation was not affected as much. Furthermore, although the basal glucose transport system of the null mice was not defective, the insulin-induced increase in glucose transport was abrogated. These results suggest that insulin regulates, via the IR, the differentiation and glucose transport of skin keratinocytes, whereas proliferation is affected by the diabetes milieu of IR knockout mice.

PKCdelta activation: a divergence point in the signaling of insulin and IGF-1-induced proliferation of skin keratinocytes

Insulin and insulin-like growth factor-1 (IGF-1) are members of the family of the insulin family of growth factors, which activate similar cellular downstream pathways. In this study, we analyzed the effects of insulin and IGF-1 on the proliferation of murine skin keratinocytes in an attempt to determine whether these hormones trigger the same signaling pathways. Increasing doses of insulin and IGF-1 promote keratinocyte proliferation in an additive manner. We identified downstream pathways specifically involved in insulin signaling that are known to play a role in skin physiology; these include activation of the Na+/K+ pump and protein kinase C (PKC). Insulin, but not IGF-1, stimulated Na+/K+ pump activity. Furthermore, ouabain, a specific Na+/K+ pump inhibitor, abolished the proliferative effect of insulin but not that of IGF-1. Insulin and IGF-1 also differentially regulated PKC activation. Insulin, but not IGF-1, specifically activated and translocated the PKCB isoform to the membrane fraction. There was no effect on PKC isoforms alpha, eta, epsilon, and zeta, which are expressed in skin. PKC8 overexpression increased keratinocyte proliferation and Na+/K+ pump activity to a degree similar to that induced by insulin but had no affect on IGF-1-induced proliferation. Furthermore, a dominant negative form of PKCdelta abolished the effects of insulin on both proliferation and Na+/K+ pump activity but did not abrogate induction of keratinocyte proliferation induced by other growth factors. These data indicate that though insulin or IGF-1 stimulation induce keratinocyte proliferation, only insulin action is specifically mediated via PKC8 and involves activation of the Na+/K+ pump.

Characterization of glucose transport system in keratinocytes: insulin and IGF-1 differentially affect specific transporters

Skin is one of the major tissues displaying chronic diabetic complications. We have studied glucose transport following stimulation with insulin and IGF-1 in cultured mouse keratinocytes. In proliferating cells, acute stimulation with insulin and IGF-1 increased glucose uptake. Insulin translocated glucose transporters 1 and 5, whereas IGF-1 translocated glucose transporters 2 and 3. With differentiation, glucose transporter 3 expression increased and the expression of glucose transporters 1, 2, and 5 decreased. No increase in glucose uptake was observed, however, following stimulation with either hormone. These results indicate that insulin and IGF-1 differentially regulate glucose uptake as well as expression and translocation of specific transporters in skin keratinocytes.

FER kinase activation of Stat3 is determined by the N-terminal sequence

p94(fer) and p51(ferT) are two tyrosine kinases that share identical SH2 and kinase domains but differ in their N-terminal regions. To further explore the cellular functions of these two highly related tyrosine kinases, their subcellular distribution profiles and in vivo phosphorylation activity were followed using double immunofluorescence assay. When combined with immunoprecipitation analysis, this assay showed that p94(fer) can lead to the tyrosine phosphorylation and activation of Stat3 but not of Stat1 or Stat2. Native p94(fer) exerted this activity when residing in the cytoplasm. However, modified forms of p94(fer), which are constitutively nuclear, could also lead to the phosphorylation of Stat3. Endogenous Stat3 and p94(fer) co-immunoprecipitated with each other, thus proving the interaction of these two proteins in vivo. Unlike p94(fer), p51(ferT) did not induce the phosphorylation of Stat3 but led to the phosphorylation of other nuclear proteins. Replacing the unique 43-amino acid-long N-terminal tail of p51(ferT) with a parallel segment from the N-terminal tail of p94(fer) did not change the subcellular localization of p51(ferT) but enabled it to activate Stat3. Thus the different N-terminal sequences of p94(fer) and p51(ferT) can affect their ability to induce phosphorylation of Stat3 and most probably direct their different cellular functions.

Differential roles of insulin receptor and insulin-like growth factor-1 receptor in differentiation of murine skin keratinocytes

The insulin receptor and the insulin-like growth factor-1 receptor are widely expressed tyrosine kinases that mediate insulin and insulin-like growth factor-1 signaling. Both receptors are expressed in many cells in which insulin stimulation does not result in an increase in glucose transport, and the distinct role of the insulin receptor in these tissues, is not known. We have studied the regulation of insulin receptor and insulin-like growth factor-1 receptor in the differentiation of cultured murine keratinocytes. Both receptors are expressed in skin keratinocytes and their expression was unchanged in all stages of calcium-induced differentiation. Insulin binding to skin keratinocytes, however, increased during calcium-induced differentiation, whereas insulin-like growth factor-1 binding decreased. Ligand-induced autophosphorylation was also changed during differentiation. In proliferating keratinocytes both receptors became phosphorylated upon ligand binding, insulin-like growth factor-1 receptor to a greater extent. Terminal differentiation resulted in a decrease in insulin receptor autophosphorylation, whereas insulin-like growth factor-1 receptor autophosphorylation was abolished. There was no change in the cellular localization of the proteins, their intrinsic activity, or their internal structure. Finally, due to the change in the receptor's activity during keratinocyte differentiation, the role of insulin and insulin-like growth factor-1 in the differentiation process was examined. The expected increase in the expression of keratins 1 and 10 during calcium-induced differentiation was facilitated in the presence of insulin, whereas this induction was inhibited in the presence of insulin-like growth factor-1. In conclusion, these results demonstrate that insulin and insulin-like growth factor-1 signaling pathways are differentially involved in skin differentiation, suggesting that abnormal insulin signaling, as occurs in diabetes, may lead to skin pathology.

Differential localization of conventional protein kinase C isoforms during mouse oocyte development

Protein kinase C (PKC), the major cell target for tumor-promoting phorbol esters, plays a central role in signal transduction pathways. In many biological systems where Ca(2+) serves as a second messenger, regulatory control is mediated by PKC. The activation of PKC depends on its binding to RACK1 receptor, which is an intracellular protein anchor for activated PKC. We demonstrate that the conventional PKC (cPKC) isoforms, PKC-alpha, PKC-betaI, and PKC-betaII, as well as RACK1, are expressed in mouse oocytes (germinal vesicle [GV]) and mature eggs (metaphase II [MII]). In GV oocytes, PKC-alpha, PKC-betaII, and RACK1 were uniformly distributed in the cytoplasm, while PKC-betaI was localized in the cytoplasm and in the plasma membrane as well. Treatment of GV oocytes with the biologically active phorbol ester, 12-o-tetradecanoyl phorbol-13-acetate (TPA), resulted in a rapid translocation of the cytosolic PKC-alpha, but not PKC-betaI, PKC-betaII, or RACK1, to the plasma membrane. This was associated with inhibition of GV breakdown. In MII eggs (17 h post-hCG), PKC-alpha was uniformly distributed in the cytoplasm while PKC-betaI and -betaII were distributed in the cytoplasm and in the plasma membrane as well. Treatment with TPA resulted in a rapid translocation of PKC-alpha from the cytoplasm to the plasma membrane and a significant decrease of PKC-betaI throughout the cytoplasm, while it also remained in the cell periphery. No change in the distribution of PKC-betaII or RACK1 was observed. TPA also induced pronucleus formation. Physiological activation of MII eggs by sperm induced cortical granule exocytosis associated with significant translocation of PKC-alpha and -betaI, but not -betaII, to the plasma membrane. Overall, these results suggest a possible involvement of cPKC isoforms in the mechanism of mouse oocyte maturation and egg activation.

Protein kinase Cdelta mediates insulin-induced glucose transport in primary cultures of rat skeletal muscle

Insulin activates certain protein kinase C (PKC) isoforms that are involved in insulin-induced glucose transport. In this study, we investigated the possibility that activation of PKCdelta by insulin participates in the mediation of insulin effects on glucose transport in skeletal muscle. Studies were performed on primary cultures of rat skeletal myotubes. The role of PKCdelta in insulin-induced glucose uptake was evaluated both by selective pharmacological blockade and by over-expression of wild-type and point-mutated inactive PKCdelta isoforms in skeletal myotubes. We found that insulin induces tyrosine phosphorylation and translocation of PKCdelta to the plasma membrane and increases the activity of this isoform. Insulin-induced effects on translocation and phosphorylation of PKCdelta were blocked by a low concentration of rottlerin, whereas the effects of insulin on other PKC isoforms were not. This selective blockade of PKCdelta by rottlerin also inhibited insulin-induced translocation of glucose transporter 4 (GLUT4), but not glucose transporter 3 (GLUT3), and significantly reduced the stimulation of glucose uptake by insulin. When overexpressed in skeletal muscle, PKCdelta and PKCdelta were both active. Overexpression of PKCdelta induced the translocation of GLUT4 to the plasma membrane and increased basal glucose uptake to levels attained by insulin. Moreover, insulin did not increase glucose uptake further in cells overexpressing PKCdelta. Overexpression of PKCdelta did not affect basal glucose uptake or GLUT4 location. Stimulation of glucose uptake by insulin in cells overexpressing PKCdelta was similar to that in untransfected cells. Transfection of skeletal myotubes with dominant negative mutant PKCdelta did not alter basal glucose uptake but blocked insulin-induced GLUT4 translocation and glucose transport. These results demonstrate that insulin activates PKCdelta and that activated PKCdelta is a major signaling molecule in insulin-induced glucose transport.

Tyrosine phosphorylation of specific protein kinase C isoenzymes participates in insulin stimulation of glucose transport in primary cultures of rat skeletal muscle

Several reports indicate that protein kinase C (PKC) plays a role in insulin-induced glucose transport in certain cells. The precise effects of insulin on specific PKC isoforms are as yet unknown. Utilizing primary cultures of rat skeletal muscle, we investigated the possibility that insulin may influence the activation state of PKC isoenzymes by inducing their translocation and tyrosine phosphorylation. This, in turn, may mediate insulin effects on glucose transport. We identified and determined the glucose transporters and PKC isoforms affected by insulin and 12-O-tetradecanoylphorbol-13-acetate (TPA). Insulin and TPA each caused an increase in glucose uptake. Insulin translocated GLUT3 and GLUT4 without affecting GLUT1. In contrast, TPA translocated GLUT1 and GLUT3 without affecting GLUT4. Insulin translocated and tyrosine phosphorylated and activated PKC-beta2 and -zeta; these effects were blocked by phosphatidylinositol 3-kinase (PI3K) inhibitors. TPA translocated and activated PKC-alpha, -beta2, and -delta; these effects were not noticeably affected by PI3K inhibitors. Furthermore, wortmannin significantly inhibited both insulin and TPA effects on GLUT translocation and glucose uptake. Finally, insulin-induced glucose transport was blocked by the specific PKC-beta2 inhibitor LY379196. These results indicate that specific PKC isoenzymes, when tyrosine-phosphorylated, are implicated in insulin-induced glucose transport in primary cultures of skeletal muscle.

Cell cycle-dependent nuclear accumulation of the p94fer tyrosine kinase is regulated by its NH2 terminus and is affected by kinase domain integrity and ATP binding

p94fer and p51ferT are two tyrosine kinases that are encoded by differentially spliced transcripts of the FER locus in the mouse. The two tyrosine kinases share identical SH2 and kinase domains but differ in their NH2-terminal amino acid sequence. Unlike p94fer, the presence of which has been demonstrated in most mammalian cell lines analyzed, the expression of p51ferT is restricted to meiotic cells. Here, we show that the two related tyrosine kinases also differ in their subcellular localization profiles. Although p51ferT accumulates constitutively in the cell nucleus, p94fer is cytoplasmic in quiescent cells and enters the nucleus concomitantly with the onset of S phase. The nuclear translocation of the FER proteins is driven by a nuclear localization signal (NLS), which is located within the kinase domain of these enzymes. The functioning of that NLS depends on the integrity of the kinase domain but was not affected by inactivation of the kinase activity. The NH2 terminus of p94fer dictated the cell cycle-dependent functioning of the NLS of FER kinase. This process was governed by coiled-coil forming sequences that are present in the NH2 terminus of the kinase. The regulatory effect of the p94fer NH2-terminal sequences was not affected by kinase activity but was perturbed by mutations in the kinase domain ATP binding site. Ectopic expression of the constitutively nuclear p51ferT in CHO cells interfered with S-phase progression in these cells. This was not seen in p94fer-overexpressing cells. The FER tyrosine kinases seem, thus, to be regulated by novel mechanisms that direct their different subcellular distribution profiles and may, consequently, control their cellular functioning.

Insulin receptor regulation of cell surface integrins: a possible mechanism contributing to the development of diabetic complications

Insulin plays a central role in regulating cellular growth in addition to its classic effects to regulate fuel metabolism. In a previous study, we have identified a patient who was homozygous for a deletion of the insulin receptor gene. In our current investigation, we used cultured skin fibroblasts from this patient as a model system in which to investigate the mechanisms whereby insulin regulates cellular growth in vitro. After cell division, skin fibroblasts from normal individuals migrate on the tissue culture plate and appear to be distributed randomly over the surface of the plate. In contrast, the patient's cells grew in clumps. Furthermore, the patient's fibroblasts exhibited a marked increase in the expression of several integrin subunits, especially the alpha5- and beta1-subunits that comprise the fibronectin receptor. Because the cellular growth pattern was restored to normal when cells were cultivated in the presence of blocking antibodies directed against either alpha5- or beta1-integrin subunits, we infer that increased expression of alpha5beta1-integrin may be the cause of the observed abnormality in the growth of the patient's cells in vitro. Furthermore, insulin stimulation led to downregulation of the levels of the alpha5- and beta1-integrin subunits in normal human fibroblasts but not in the patient's cells that lacked insulin receptors. Taken together, these data suggest that insulin's ability to regulate the expression of cell surface integrins may contribute to the mechanisms whereby insulin regulates cell growth. In light of the important role of integrins in mediating interactions between cells and the basement membrane, we suggest that dysregulation of integrin expression might contribute to the abnormalities in the structure of the basement membranes associated with the chronic microvascular complications of diabetes.

Topical retinoic acid reduces skin papilloma formation but resistant papillomas are at high risk for malignant conversion

Retinoic acid (RA) was topically applied to the skin of Sencar mice during the promotion phase of specific tumor induction protocols that produce papillomas at low (12-O-tetradecanoylphorbol-13-acetate promoted, TPA) or high (mezerein-promoted) risk for premalignant progression and malignant conversion. RA consistently reduced the yield of papillomas and carcinomas in both protocols, but the frequency of malignant conversion in papillomas that emerged during RA treatment was not reduced. When TPA was reapplied after cessation of RA treatment, the number of papillomas increased 2-fold, suggesting that RA had not eliminated initiated cells. In vitro, RA prevented the emergence of transformed keratinocytes in an assay that mimics malignant conversion, suggesting that RA can suppress conversion if applied during the stage of premalignant progression. Examination of tumor markers at weeks 14 and 22 of the tumor-induction experiments in vivo indicated that papillomas evolving during RA treatment exhibited a phenotype of high progression risk, even in the TPA-promoted groups. In the majority of these tumors, the alpha6beta4 integrin and retinoid X receptor alpha transcripts were detected suprabasally, indicating an advanced state of premalignant progression. RA-treated tumors also expressed higher levels of transcripts for transforming growth factor (TGF)-beta1 and localized TGF-beta1 peptide in the basal portions of the tumor fronds. Because up-regulated expression of TGF-beta1 suppresses papilloma formation, these studies suggest a mechanism whereby RA can prevent papilloma eruption via a TGF-beta intermediate, but papillomas resistant to RA may have altered TGF-beta signaling and progress to carcinomas at an increased frequency.

Loss of retinoic acid receptors in mouse skin and skin tumors is associated with activation of the ras(Ha) oncogene and high risk for premalignant progression

Retinoic acid receptor transcripts (RARalpha and RARgamma) are decreased in benign mouse epidermal tumors relative to normal skin and are almost absent in carcinomas. In this report, the expression of RARalpha and RARgamma proteins was analyzed by immunoblotting in benign skin tumors induced by two different promotion protocols designed to yield tumors at low or high risk for malignant conversion. RARalpha was slightly reduced in papillomas promoted with 12-O-tetradecanoylphorbol-13-acetate (low risk) and markedly decreased or absent in papillomas promoted by mezerein (high risk). However, mezerein also caused substantial reduction of RARalpha in nontumorous skin. RARgamma was not detected in tumors from either protocol and was greatly reduced in skin treated by either promoter. Both RARalpha and RARgamma proteins were decreased in keratinocytes overexpressing an oncogenic v-ras(Ha) gene, and RARalpha was underexpressed in a benign keratinocyte cell line carrying a mutated c-ras(Ha) gene. Introduction of a recombinant RARalpha expression vector into benign keratinocyte tumor cells reduced the S-phase population and inhibited [3H]thymidine incorporation in response to retinoic acid. Furthermore, transactivation of B-RARE-tk-LUC by retinoic acid was markedly decreased in keratinocytes transduced with the v-ras(Ha) oncogene (v-ras(Ha)-keratinocytes). Blocking protein kinase C function in v-ras(Ha)-keratinocytes with bryostatin restored RARalpha protein to near normal levels, reflecting the involvement of protein kinase C in RARalpha regulation. Both RARalpha and RARgamma are down-regulated in cultured keratinocytes by 12-O-tetradecanoylphorbol-13-acetate, further implicating PKC in the regulation of retinoid receptors. Our data suggest that modulation of RARs could contribute to the neoplastic phenotype in mouse skin carcinogenesis and may be involved in the differential promoting activity of mezerein and 12-O-tetradecanoylphorbol-13-acetate, particularly for selecting tumors at high risk for malignant conversion.

Selective changes in laminin adhesion and alpha 6 beta 4 integrin regulation are associated with the initial steps in keratinocyte maturation

In skin, the distribution of integrins is compartmentalized. Whereas the alpha 6 beta 4 integrin complex is polarized to the basal portion of proliferating cells in the basal layer juxtaposed to the basement membrane, alpha 3 beta 1 integrin receptors are localized on the cell surface surrounding basal and suprabasal cells, suggesting beta 1 integrins mediate both cell-matrix and cell-cell interactions. As initiation of maturation in skin is associated with the detachment of cells from the basement membrane, the early loss of alpha 6 beta 4, but not alpha 3 beta 1, integrin expression could be a determining factor in the transition from the proliferating to a differentiating keratinocyte. We have studied the regulation of adhesion potential and integrin expression during differentiation of mouse basal keratinocytes culture in 0.05 mM Ca2+ medium and induced to differentiate in 0.12 mM Ca2+ medium. Within 12-24 h after elevation of Ca2+, a selective loss of the alpha 6 beta 4 integrin complex is associated with the induction of the spinous cell marker keratin 1. This early differentiation phenotype coincides with loss of cell attachment mediated by alpha 6 beta 4 to laminins 1 and 5 but not a fibronectin or collagen IV. Selective loss of attachment to laminin is also detected in spinous cells isolated from newborn epidermis in vivo. The loss of alpha 6 and beta 4 protein expression is a consequence of transcriptional and posttranscriptional events, including reduction in mRNA transcripts, reduced synthesis of the alpha 6 protein, and enhanced processing of the alpha 6 and beta 4 chains as determined by Western blots and pulse-chase experiments in metabolically labeled keratinocytes. Selective processing of the beta 4 intracellular domain is detected before loss of beta 4 from the cell surface in basal keratinocytes, and this process is accelerated during differentiation. Whereas early keratinocyte maturation is linked to the selective loss of the alpha 6 beta 4 complex, loss of both beta 1 and beta 4 integrin mRNA and protein occurs as cells proceed to later stages in the differentiation program as induced by 0.5 mM Ca2+ or suspension culture. These conditions are characterized by accelerated expression of transglutaminase; reduced keratin 1 protein; loss of adhesion to fibronectin, laminin 1, laminin 5, and collagen IV; and rapid cell death. Contributing to the down-regulation of beta 1 integrins during terminal differentiation is a selective sensitivity of alpha 3 beta 1 but not alpha 6 beta 4 to down-regulation by transforming growth factors beta 1 and beta 2, factors that are also expressed differentially in normal skin. This study indicates that down-regulation of the alpha 6 beta 4 but not beta 1 integrins occurs during the initial steps of keratinocyte differentiation and is associated with detachment from the laminin matrix. Such changes could contribute an important signal to initiate the process of terminal keratinocyte differentiation.

Differential regulation of integrins and extracellular matrix binding in epidermal differentiation and squamous tumor progression

Cell surface receptors of the integrin family regulate physiological and pathological processes in skin, including proliferation, differentiation, and malignant transformation. In skin, integrins are compartmentalized. While alpha 6 beta 4 is restricted to the basal surface of basal cells, beta 1 integrins are expressed in basal and suprabasal layers. In vivo and in Ca(2+)-induced differentiation of mouse keratinocytes in vitro, the loss of attachment to laminin via alpha 6 beta 4 integrin is an early event associated with initiation of spinous differentiation. The restricted expression of alpha 6 beta 4 to the basal cells in normal skin is disrupted early in the development of squamous cancer, where benign papillomas at high risk for malignant progression express alpha 6 beta 4 suprabasally in an expanded proliferative compartment. The aberrant suprabasal alpha 6 beta 4 is associated with reduced keratin 1 expression and upregulation of keratin 13, keratin 8, and gamma-glutamyltranspeptidase. During malignant conversion, the increase in alpha 6 beta 4 protein and mRNA is associated with novel expression of an alternatively spliced form of the alpha 6 subunit, alpha 6B. The induction of alpha 6B both in vivo and in vitro is particularly high in malignant cells produced by transduction of both v-fos and v-rasHa oncogenes into normal keratinocytes where it was associated with increased attachment to laminin. Furthermore, binding to laminin is increased by introduction of alpha 6B into a papilloma cell line. These results establish a link between squamous tumor progression and the upregulation of the alpha 6 beta 4 integrin and suggest that expression of alpha 6B could be functionally relevant to interaction of tumor cells with the laminin matrix during malignant conversion.

Suspension-induced murine keratinocyte differentiation is mediated by calcium

Modulating extracellular Ca2+ (Cao) and suspension culture are two frequently used methods to induce maturation of cultured human and mouse keratinocytes. To determine if the two methods share a common mechanism, changes in Ca2+ metabolism were studied in suspension cultures of mouse keratinocytes. Spontaneously detached and suspension- cultured keratinocytes in 0.05 microM Ca2+ medium express markers of suprabasal differentiation, while 0.05 microM Ca2+ is not permissive for marker expression by attached keratinocytes. Intracellular free Ca2+ (Cai) increased rapidly after placing keratinocytes in suspension in 0.05 microM Ca2+, reaching levels up to 3- to 4-fold higher than Cai in attached cells after 4-5 h. In suspended cells, the increase in Cai was associated with a 2- to 6- fold increase in Ca2+ transport across plasma membrane as well as depletion of intracellular Ca2+ -stores. Differentiation marker expression and terminal differentiation were inhibited in suspension-cultured keratinocytes by preventing the rise of Cai using either 1,2-bis (o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid to chelate intracellular Ca2+ or ethyleneglycol-bis (beta-aminoethyl ether)- N,N,N',N' -tetraacetic acid to reduce Cao. Together these results indicate that a rise in CAi is a common mechanism controlling differentiation in cultured mouse keratinocytes, and suspension of keratinocytes enhances Ca2+ transport and alters intracellular Ca2+ sequestration producing a rise in Cai.

A splice variant of alpha 6 integrin is associated with malignant conversion in mouse skin tumorigenesis

The epithelial-specific integrin alpha 6 beta 4 is suprabasally expressed in benign skin tumors (papillomas) and is diffusely expressed in carcinomas associated with an increase in the proliferating compartment. Analysis of RNA samples by reverse transcriptase-PCR and DNA sequencing revealed that chemically or oncogenically induced papillomas (n = 8) expressed a single transcript of the alpha 6 subunit, identified as the alpha 6 A splice variant. In contrast, carcinomas (n = 13) expressed both alpha 6A and an alternatively spliced form, alpha 6B. Primary keratinocytes and a number of keratinocyte cell lines that vary in biological potential from normal skin, to benign papillomas, to well-differentiated slowly growing carcinomas exclusively expressed alpha 6A. However, I7, an oncogene-induced cell line that produces highly invasive carcinomas, expressed both alpha 6A and alpha 6B transcript and protein. The expression of alpha 6B in I7 cells was associated with increased attachment to a laminin matrix compared to cell lines exclusively expressing alpha 6A. Furthermore, introduction of an alpha 6B expression vector into a papilloma cell line expressing alpha 6A increased laminin attachment. When a papilloma cell line was converted to an invasive carcinoma by introduction of the v-fos oncogene, the malignant cells expressed both alpha 6A and alpha 6B, while the parent cell line and cells transduced with v-jun or c-myc, which retained the papilloma phenotype, expressed only alpha 6A. Comparative analysis of alpha 6B expression in cell lines and their derived tumors indicate that alpha 6B transcripts are more abundant in tumors than cell lines, and alpha 6B is expressed to a greater extent in poorly differentiated tumors. These results establish a link between malignant conversion and invasion of squamous tumor cells and the regulation of transcript processing of the alpha 6 beta 4 integrin.

Targeted disruption of mouse EGF receptor: effect of genetic background on mutant phenotype

Gene targeting was used to create a null allele at the epidermal growth factor receptor locus (Egfr). The phenotype was dependent on genetic background. EGFR deficiency on a CF-1 background resulted in peri-implantation death due to degeneration of the inner cell mass. On a 129/Sv background, homozygous mutants died at mid-gestation due to placental defects; on a CD-1 background, the mutants lived for up to 3 weeks and showed abnormalities in skin, kidney, brain, liver, and gastrointestinal tract. The multiple abnormalities associated with EGFR deficiency indicate that the receptor is involved in a wide range of cellular activities.

Mouse skin tumor progression results in differential expression of retinoic acid and retinoid X receptors

Retinoids are powerful regulators of epidermal cell growth and differentiation and are widely used in the prevention and treatment of skin disorders and cancers in humans. Since many of the effects of retinoids on cell growth and differentiation are mediated by nuclear retinoid receptors (RARs and RXRs), we were interested in determining RAR and RXR gene expression during mouse skin tumor progression. The two-stage system of mouse skin carcinogenesis was used to generate papillomas and carcinomas, and the different stages of malignant progression (papillomas, differentiated squamous cell carcinomas, undifferentiated squamous cell carcinomas, and spindle cell carcinomas) were characterized in each tumor by specific keratin expression prior to receptor characterization. Using in situ hybridization analysis, we show that the two major RAR isoforms (alpha 1 and gamma 1), which account for most of RARs in the skin, were expressed in both the basal and suprabasal layers in mouse epidermis. In contrast, RXR alpha transcripts were compartmentalized to the basal cell layers and concentrated in hair follicles. During skin tumor progression, RAR (alpha 1 and gamma 1) transcripts were down-modulated in malignant tumor cells, whereas RXR (alpha and beta) transcript expression was expanded in papillomas and carcinomas as the number of undifferentiated cells also increased. RXR gamma was not detected in the skin or at any stage during skin tumor progression. Spindle cell tumors lacked markers of the keratinocyte phenotype and lost RAR expression, yet retained expression of RXR alpha and beta. The increased abundance of transcripts for RXRs and decreased presence of RARs in skin tumor progression may favor other nuclear signal transduction pathways requiring RXR for heterodimer formation and contribute to phenotypic progression of cancer cells.

Role of oncogenes and tumor suppressor genes in multistage carcinogenesis

The introduction of the techniques of molecular biology as tools to study skin carcinogenesis has provided more precise localization of biochemical pathways that regulate the tumor phenotype. This approach has identified genetic changes that are characteristic of each of the specific stages of squamous cancer pathogenesis: initiation, exogenous promotion, premalignant progression, and malignant conversion. Initiation can result from mutations in a single gene, and the Harvey allele of the ras gene family has been identified as a frequent site for initiating mutations. Heterozygous activating mutations in c-rasHa are dominant, and affected keratinocytes hyperproliferate and are resistant to signals for terminal differentiation. An important pathway impacted by c-rasHa activation is the protein kinase C (PKC) pathway, a major regulator of keratinocyte differentiation. Increased activity of PKC alpha and suppression of PKC delta by tyrosine phosphorylation contribute to the phenotypic consequences of rasHa gene activation in keratinocytes. Tumor promoters disturb epidermal homeostasis and cause selective clonal expansion of initiated cells to produce multiple benign squamous papillomas. Resistance to differentiation and enhanced growth rate of initiated cells impart a growth advantage when the epidermis is exposed to promoters. The frequency of premalignant progression varies among papillomas, and subpopulations at high risk for progression have been identified. These high-risk papillomas overexpress the alpha 6 beta 4 integrin and are deficient in transforming growth factor beta 1 and beta 2 peptides, two changes associated with a very high proliferation rate in this subset of tumors. The introduction of an oncogenic rasHa gene into epidermal cells derived from transgenic mice with a null mutation in the TGF beta 1 gene have an accelerated rate of malignant progression when examined in vivo. Thus members of the TGF beta gene family contribute a tumor-suppressor function in carcinogenesis. Accelerated malignant progression is also found with v-rasHa transduced keratinocytes from skin of mice with a null mutation in the p53 gene. The similarities in risk for malignant conversion by initiated keratinocytes from TG beta 1 and p53 null geneotypes suggest that a common, growth-related pathway may underly the tumor-suppressive functions of these proteins in the skin carcinogenesis model.

The suprabasal expression of alpha 6 beta 4 integrin is associated with a high risk for malignant progression in mouse skin carcinogenesis

Enhanced expression of the alpha 6 beta 4 integrin complex has been linked to malignant progression in mouse skin carcinogenesis. To determine if alpha 6 beta 4 expression can predict risk for malignant conversion among populations of benign skin tumors, we analyzed the distribution of alpha 6 beta 4 and other markers of progression in papillomas at high and low risk for malignant progression. After initiation with 7,12-dimethylbenz[a]anthracene, mice were promoted with 12-O-tetradecanoylphorbol-13-acetate to induce predominantly low risk tumors or promoted with mezerein to produce predominantly high risk tumors. When tumors first appeared at 8 weeks after promotion, high risk papillomas demonstrated basal and suprabasal alpha 6 beta 4 expression, loss of keratin 1, and aberrant expression of keratin 13. In these tumors alpha 6 beta 4 expression coincided with an expansion of the proliferating compartment as indicated by suprabasal bromodeoxyuridine labeling. In contrast, alpha 6 beta 4 immunostaining was confined to basal cells in low risk tumors, keratin 1 was abundant, and keratin 13 was absent in the majority of this group, while proliferating cells were largely in the basal compartment. By 33 weeks, alpha 6 beta 4 suprabasal expression continued to distinguish groups at higher risk for malignant conversion, but keratin 13 was expressed in all groups. At this time, high risk tumors displayed focal expression of keratin 8 and gamma-glutamyltranspeptidase, markers also found in chemically induced carcinomas. Keratin 8 and gamma-glutamyltranspeptidase were expressed only in alpha 6 beta 4 positive cells. These results indicate that expression of alpha 6 beta 4 integrin in suprabasal strata serves as an early predictive marker to identify benign squamous tumors at high risk for malignant progression.

Loss of expression of transforming growth factor beta in skin and skin tumors is associated with hyperproliferation and a high risk for malignant conversion

Mouse skin carcinomas arise from a small subpopulation of benign papillomas with an increased risk of malignant conversion. These papillomas arise with limited stimulation by tumor promoters, appear rapidly, and do not regress, suggesting that they differ in growth properties from the majority of benign tumors. The transforming growth factor beta (TGF-beta) proteins are expressed in the epidermis and are growth inhibitors for mouse keratinocytes in vitro; altered TGF-beta expression could influence the growth properties of high-risk papillomas. Normal epidermis, tumor promoter-treated epidermis, and skin papillomas at low risk for malignant conversion express TGF-beta 1 in the basal cell compartment and TGF-beta 2 in the suprabasal strata. In low-risk tumors, 90% of the proliferating cells are confined to the basal compartment. In contrast, the majority of high-risk papillomas are devoid of both TGF-beta 1 and TGF-beta 2 as soon as they arise; these tumors have up to 40% of the proliferating cells in the suprabasal layers. Squamous cell carcinomas are also devoid of TGF-beta, suggesting that they arise from the TGF-beta-deficient high-risk papillomas. In some high-risk papillomas, TGF-beta 1 loss can occur first and correlates with basal cell hyperproliferation, while TGF-beta 2 loss correlates with suprabasal hyperproliferation. Similarly, TGF-beta 1-null transgenic mice, which express wild-type levels of TGF-beta 2 in epidermis but no TGF-beta 1 in the basal layer, have a hyperproliferative basal cell layer without suprabasal proliferation. In tumors, loss of TGF-beta is controlled at the posttranscriptional level and is associated with expression of keratin 13, a documented marker of malignant progression. These results show that TGF-beta expression and function are compartmentalized in epidermis and epidermal tumors and that loss of TGF-beta is an early, biologically relevant risk factor for malignant progression.

Epidermal growth factor receptor ligands regulate keratin 8 expression in keratinocytes, and transforming growth factor alpha mediates the induction of keratin 8 by the v-rasHa oncogene

Cytokeratins 8 and 18 (Endo A and B) are among the earliest expressed embryonic genes and the major components of the cytoskeleton in simple epithelia of the adult. Recent data indicate that these cytokeratins are aberrantly expressed in several epithelial tumor types and that expression in cultured mouse keratinocytes is linked to activation of the rasHa oncogene. Furthermore, up-regulation of K8/K18 in keratinocytes is associated with reciprocal suppression of K1. We now show that the aberrant expression of K8 and K18 and suppression of K1 in cultured keratinocytes transduced with the v-rasHa gene are mediated by a factor secreted into the culture medium. Furthermore, transforming growth factor alpha (TGF-alpha) and epidermal growth factor elicit an identical pattern of K8/K18 expression and K1 suppression in normal keratinocytes. The factor in medium from v-rasHa keratinocytes is TGF-alpha, as a specific blocking antibody for rat and mouse TGF-alpha prevents the expression of K8 and restores expression of K1. The tyrosine kinase inhibitor genistein also prevents K8 induction in v-rasHa keratinocytes and in normal keratinocytes treated with TGF-alpha- or v-rasHa-conditioned medium. However, simply stimulating proliferation of keratinocytes by cholera toxin does not result in expression of K8 or suppression of K1. Finally, tumor grafts from neoplastic epidermal cells overexpressing TGF-alpha via retroviral transduction of human TGF-alpha complementary DNA in vitro show coordinate expression of K8 and human TGF-alpha. These studies indicate that K8 expression in keratinocytes, and derivative neoplastic cells, in vivo and in vitro is regulated by epidermal growth factor receptor ligands. Since the expression of cytokines and K8/K18 in early embryogenesis is often coincident, cytokines may be the physiological mediators of K8/K18 expression in embryonic cells.

Critical aspects of initiation, promotion, and progression in multistage epidermal carcinogenesis

Carcinogenesis in mouse skin can be divided into three distinct stages: initiation, promotion, and progression (malignant conversion). Initiation, induced by a single exposure to a genotoxic carcinogen, can result from a mutation in a single critical gene (e.g., rasHa), apparently in only a few epidermal cells. The change is irreversible. Promotion, resulting in the development of numerous benign tumors (papillomas), is accomplished by the repeated application of a nonmutagenic tumor promoter. The effects of single applications of tumor promoters are reversible since papillomas do not develop after insufficient exposure of initiated skin to promoters or when the interval between individual promoter applications is increased sufficiently. The reversibility of promotion suggests an epigenetic mechanism. Promoter treatment provides an environment that allows the selective clonal expansion of foci of initiated cells. The conversion of squamous papillomas to carcinomas (termed progression or malignant conversion) occurs spontaneously at a low frequency. The rate of progression to malignancy can be significantly increased by treatment of papilloma-bearing mice with certain genotoxic agents. These progressor agents or converting agents are likely to act via a second genetic change in papillomas already bearing the initiating mutation. Progression in the skin is characterized by genetic changes that result in several distinct changes in the levels or activity of structural proteins, growth factors, and proteases. The mechanisms involved in progression are being studied in epidermal cell culture. In order to determine the in vivo phenotype of cultured cells, a grafting system was developed in which the cells were transferred from culture to a prepared skin bed in athymic mice. Introduction of an activated v-fos oncogene into initiated cells bearing an activated rasHa gene produced cells with a carcinoma phenotype, i.e., carcinomas formed when the cells were grafted as part of reconstituted skin. Grafted keratinocytes containing the rasHa gene alone produced papillomas; with v-fos alone, normal skin formed when grafted. The rasHa/fos carcinomas showed changes in differentiation markers characteristic of chemically induced carcinomas. A cell culture assay utilizing cells initiated by the introduction of an activated rasHa oncogene was developed to study progression. After exposure of initiated cells to progressor agents under conditions in which the proliferation of the rasHa-initiated cells was suppressed, proliferating foci developed, with a good correlation of activity in the assay with activity in the progression stage in vivo. The cell culture assay provides a quantitative model to study chemically induced neoplastic progression and may be useful to identify potential progressor agents.

Extracellular matrix receptors and mouse skin carcinogenesis: altered expression linked to appearance of early markers of tumor progression

Interaction of cells with the basement membrane is important for cell proliferation and differentiation. Disruption of the basement membrane is an early event during progression of benign tumors to cancer. Using the techniques of immunohistochemistry and immunofluorescence, we show that cell-matrix interactions via the cell surface integrin receptors alpha 3 beta 1, alpha 5 beta 1, alpha 6 beta 4, the Mr 67,000 laminin receptor (67LR) laminin-binding protein, and the secreted matrix protein laminin are strictly regulated during differentiation of mouse epidermis. While alpha 6 beta 4 and alpha 5 beta 1 are polarized to the basal surface of basal cells in contact with the basement membrane, alpha 3 beta 1 and the non-integrin 67LR are primarily detected in the cell periphery of suprabasal cells, where cell to cell contacts are found. Sequential changes in expression of matrix receptors occur following multistage carcinogenesis of mouse skin. In an analysis of benign and malignant skin tumors induced by chemical carcinogens or oncogene transduction, we found that alpha 3 beta 1 and alpha 5 beta 1 as well as the non-integrin 67LR are sequentially down-regulated in the progression from benign to malignant, while alpha 6 beta 4 is the predominant receptor expressed in the carcinomas. Tumor expression of alpha 6 beta 4 is not polarized and is dissociated from its colocalized normal partner bullous pemphigoid antigen, which remains restricted to the basement membrane. The changes in matrix receptors are linked to appearance of keratin 13 in suprabasal regions, but always in alpha 6 beta 4 negative cells. The predominance of alpha 6 beta 4 in the proliferating cells during progression is associated with decreased expression of keratin 13 in carcinomas. These results suggest that matrix interactions with its receptors are important determinants of ordered differentiation in normal skin and show characteristic alterations during carcinogenesis that parallel changes in differentiation of the tumors.

Magnesium and phosphate enrichment of culture medium stimulates the proliferation of epidermal cells from newborn and adult mice

The proliferation and differentiation of mouse epidermal cells can be sequentially analyzed by modification of extracellular calcium. Newborn cells cultured in low calcium medium (less than 0.1 mM) proliferate as a monolayer and maintain a typical basal cell phenotype in culture but have a limited proliferative capacity and short lifespan. Elevation of the magnesium content of the culture medium from 1 to 5 mM stimulated the proliferation of newborn mouse (1-3 days old) keratinocytes. Maximal DNA synthesis rates, as determined on day 5 of culture, were up to 2-3-fold higher in the magnesium-enriched cultures. Exposure to high magnesium caused 3-4-fold increases in the DNA content of newborn keratinocyte cultures, and extended the confluent phase of epidermal cell growth to over 10 days. Other divalent cations (strontium, copper, zinc, nickel, beryllium, and barium) did not improve keratinocyte growth in culture. Keratinocytes from the tail skin of adult (3 months old) mice displayed an absolute requirement for high phosphate in the culture medium. The medium containing an optimal (10 mM) phosphate concentration prevented the cell detachment caused by the standard low (1 mM) phosphate medium, and in combination with an elevated magnesium content (10-15 mM) it markedly increased both DNA synthesis rates and DNA content of the adult cell cultures. Optimally growing, newborn or adult cultures contained less cells in the G1 phase of the cell cycle and more cells in S and G2 +M. The addition of phosphate and magnesium per se did not induce keratinocyte differentiation and did not interfere with the high calcium (1 mM)-induced differentiation.

A rapid procedure for flow cytometric DNA analysis in cultures of normal and transformed epidermal cells

A simple, rapid, and highly reproducible procedure for flow cytometric DNA analysis has been adapted for studying cell cycle kinetics in epidermal cell cultures. The preparation of cell nuclei and their staining with the fluorescent dye propidium iodide were performed directly on the culture dish, without prior suspension and fixation of the cells. Singly dispersed nuclei were produced by mild trypsinization of cells in the presence of the nonionic detergent Nonidet P-40 and spermine. The culture dishes could be kept frozen for prolonged periods of time before trypsinization and staining, without affecting either the recovery of nuclei or the cell cycle distribution profiles. This remarkable stability of cell nuclei greatly simplified the analysis of multiple samples in cell cycle kinetic studies. This method was used to analyze the cell cycle distribution in cultures of normal and transformed mouse epidermal cells, human colon carcinoma cells, primary bovine aortic endothelial cells, and fibroblastic and myogenic cell lines. This procedure should be very useful in studying growth kinetics, differentiation, and transformation of epidermal as well as other adherent cell types.