Opposite regulation of thrombospondin-1 and corticotropin-induced secreted protein/thrombospondin-2 expression by adrenocorticotropic hormone in adrenocortical cells

Thrombospondin-2 acts as a critical regulator of cartilage regeneration: A review

The degeneration of articular cartilage tissue is the most common cause of articular cartilage diseases such as osteoarthritis. There are limitations in chondrocyte self-renewal and conventional treatments. During cartilage regeneration and repair, growth factors are typically used to induce cartilage differentiation in stem cells. The role of thrombospondin-2 in cartilage formation has received much attention in recent years. This paper reviews the role of thrombospondin-2 in cartilage regeneration and the important role it plays in protecting cartilage from damage caused by inflammation or trauma and in the regenerative repair of cartilage by binding to different receptors and activating different intracellular signaling pathways. These studies provide new ideas for cartilage repair in clinical settings.

A comparison between the equine and bovine hypothalamus-pituitary-adrenocortical axis

In this review, we address the function of the hypothalamus-pituitary-adrenocortical (HPA) axis with special emphasis on the comparison between the bovine and equine species. The pars intermedia of the pituitary gland is particularly well developed in horses and cattle. However, its function is not well appreciated in cattle yet. The Wulzen's cone of the adenohypophysis is a special feature of ruminants. Total basal cortisol concentration is much higher in horses than that in cows with similar free cortisol fractions. Corticotropin-releasing factor (CRF) concentrations in equine pituitary venous blood are lower compared with other species, whereas plasma ACTH concentrations in cows are higher than those in horses. A CRF challenge test induced a more pronounced cortisol response in horses compared with cattle, whereas regarding ACTH challenge testing, the opposite seems true. Based on data from literature, the bovine species is characterized by relatively high basal blood CRF and ACTH and low cortisol and glucose concentrations. Obviously, further lowering of blood cortisol in cattle is easily prevented by the high sensitivity to ACTH, and as a consequence, subsequent increased gluconeogenesis prevents imminent hypoglycemia. Hypoglycemia is less likely in horses given their high muscle glycogen content and their relatively high cortisol concentration. When assessing HPA axis reactivity, response patterns to exogenous ACTH or CRH might be used as a reliable indicator of animal welfare status in cows and horses, respectively, although it is emphasized that considerable caution should be exercised in using measures of HPA activity solely to assess animal welfare.

Steroidogenesis-adrenal cell signal transduction

The purpose of this article is to review fundamentals in adrenal gland histophysiology. Key findings regarding the important signaling pathways involved in the regulation of steroidogenesis and adrenal growth are summarized. We illustrate how adrenal gland morphology and function are deeply interconnected in which novel signaling pathways (Wnt, Sonic hedgehog, Notch, β-catenin) or ionic channels are required for their integrity. Emphasis is given to exploring the mechanisms and challenges underlying the regulation of proliferation, growth, and functionality. Also addressed is the fact that while it is now well-accepted that steroidogenesis results from an enzymatic shuttle between mitochondria and endoplasmic reticulum, key questions still remain on the various aspects related to cellular uptake and delivery of free cholesterol. The significant progress achieved over the past decade regarding the precise molecular mechanisms by which the two main regulators of adrenal cortex, adrenocorticotropin hormone (ACTH) and angiotensin II act on their receptors is reviewed, including structure-activity relationships and their potential applications. Particular attention has been given to crucial second messengers and how various kinases, phosphatases, and cytoskeleton-associated proteins interact to ensure homeostasis and/or meet physiological demands. References to animal studies are also made in an attempt to unravel associated clinical conditions. Many of the aspects addressed in this article still represent a challenge for future studies, their outcome aimed at providing evidence that the adrenal gland, through its steroid hormones, occupies a central position in many situations where homeostasis is disrupted, thus highlighting the relevance of exploring and understanding how this key organ is regulated. © 2014 American Physiological Society. Compr Physiol 4:889-964, 2014.

Regulating the angiogenic balance in tissues

A balance between angiogenesis inducers and inhibitors in the microenvironment controls the rate of new blood vessel formation. We hypothesized that fibroblasts, an important cellular constituent of the tissue stroma, secrete molecules that contribute to this balance. We further hypothesized that fibroblasts secrete molecules that promote angiogenesis when they are in a proliferative state and molecules that inhibit angiogenesis when they are not actively cycling (quiescent). Microarray analysis revealed that angiogenesis inducers and inhibitors are regulated as fibroblasts transition into a quiescent state and reenter the cell cycle in response to changes in serum. To assess whether changes in transcript levels result in changes in the levels of secreted proteins, we collected conditioned medium from proliferating and quiescent fibroblasts and performed immunoblotting for selected proteins. Secreted protein levels of the angiogenesis inhibitor pigment epithelium derived factor (PEDF) were higher in quiescent than proliferating fibroblasts. Conversely, proliferating fibroblasts secreted increased levels of the angiogenesis inducer vascular endothelial growth factor-C (VEGF-C). For the angiogenesis inhibitor thrombospondin-2, quiescent cells secreted a prominent 160 kDa form in addition to the 200 kDa form secreted by proliferating and restimulated fibroblasts. Using immunohistochemistry we discovered that fibroblasts surround blood vessels and that the angiogenesis inhibitor PEDF is expressed by quiescent fibroblasts in uterine tissue, supporting a role for PEDF in maintaining quiescence of the vasculature. This work takes a new approach to the study of angiogenesis by examining the expression of multiple angiogenesis regulators secreted from a key stromal cell, the fibroblast.

Thrombospondin expression in aldosterone-producing adenomas

Thrombospondin (TSP) 1 and 2 are extracellular matrix proteins that appear to play a role in cell adhesion and cell migration. It has been demonstrated that the pattern of TSP expression is shifted from TSP1 to TSP2 under adrenocorticotrophic hormone treatment in bovine adrenocortical cells. We investigated the expression in human adrenal tissues by Northern blot analysis and correlated these data with the expression of the adrenocorticotrophic hormone-receptor (ACTH-R). All adrenal tissues (control adrenals, nonfunctional adenomas and ACTH-dependent aldosterone-producing adenomas (APA)) expressed both TSP1 and TSP2 mRNAs. Compared to control adrenals (TSP1 and TSP2 expression = 100 +/- 12%, respectively), TSP1 expression was negatively (51 +/- 10%, p < 0.01) and TSP2 expression was positively (289 +/- 36%, p < 0.01) regulated in APA. No significant differences in TSP1 and TSP2 expressions were found between control adrenals and nonfunctional adenomas. In APA, TSP1 (r = -0.86, p<0.01) and TSP2 (r = 0.88, p < 0.01) expressions correlated closely with the expression of ACTH-R. These results suggest that ACTH activity plays an important role in regulating the expression of TSPs in human adrenal tissues. We speculate that the shift of expression observed in APA may be associated with the phenotype of the tumors.

Identification of two novel ACTH-responsive genes encoding manganese-dependent superoxide dismutase (SOD2) and the zinc finger protein TIS11b [tetradecanoyl phorbol acetate (TPA)-inducible sequence 11b]

ACTH is the major trophic factor regulating and maintaining adrenocortical function, affecting such diverse processes as steroidogenesis, cell proliferation, cell migration, and cell survival. We used differential display RT-PCR to identify genes that are rapidly induced by ACTH in the bovine adrenal cortex. Of 42 PCR products differentially amplified from primary cultures of bovine adrenocortical cells treated with 10 nM ACTH, six identified mRNAs that were confirmed by Northern blot analysis to be induced by ACTH. Four of these amplicons encoded noninformative repetitive sequences. Of the other two sequenced amplicons, one encoded a partial sequence for mitochondrial manganese-dependent superoxide dismutase (SOD2), an enzyme that is likely to protect adrenocortical cells from the cytotoxic effects of radical oxygen species generated during steroid biosynthesis. The second was identified as TIS11b (phorbol-12-myristate-13-acetate-inducible sequence 11b)/ERF-1/cMG, a member of the CCCH double-zinc finger protein family. SOD2 induction by ACTH was independent of extracellular steroid concentration or oxidative stress. SOD2 and TIS11b mRNA expressions were rapidly induced by ACTH, reaching a maximal level after 8 h and 3 h of treatment, respectively. These ACTH effects were mimicked by forskolin but appeared independent of cortisol secretion. Upon ACTH treatment, induction of TIS11b expression closely followed the previously characterized peak of vascular endothelial growth factor (VEGF) expression. Transfection of a TIS11b expression plasmid into 3T3 fibroblasts induced a decrease in the expression of a reporter gene placed upstream of the VEGF 3'-untranslated region, indicating that TIS11b may be an important regulator of VEGF expression through interaction with its 3'-untranslated region.

Tumor angiogenesis

In order to grow beyond minimal size and to metastasize, tumors need to induce the growth of new blood vessels (angiogenesis). Whereas in normal tissues, vascular quiescence is maintained by the dominant influence of endogenous angiogenesis inhibitors over angiogenic stimuli, tumor angiogenesis is induced by increased secretion of angiogenic factors and/or by downregulation of angiogenesis inhibitors. Recent evidence suggests vascular endothelial growth factor (VEGF) as the major tumor angiogenesis factor, promoting tumor growth, invasion, and metastasis. Conversely, blocking of VEGF function inhibits angiogenesis and suppresses tumor growth in vivo. Newly identified members of the VEGF family of angiogenesis factors include placental growth factor, VEGF-B, VEGF-C, and VEGF-D, and show overlapping binding patterns to specific endothelial cell receptors. VEGF-C appears to play a major role as a lymphangiogenesis factor and as a growth factor for Kaposi's sarcoma. In contrast, endogenous inhibitors prevent blood vessel growth in normal tissues. In particular, thrombospondin-1 (TSP-1) and TSP-2 are expressed in normal skin and, when introduced into squamous cell carcinomas, potently inhibit malignant tumor growth via inhibition of tumor angiogenesis.

Thrombospondin 2, a matricellular protein with diverse functions

Thrombospondin (TSP) 2 is a close relative of TSP1 but differs in its temporal and spatial distribution in the mouse. This difference in expression undoubtedly reflects the marked disparity in the DNA sequences of the promoters in the genes encoding the two proteins. The synthesis of TSP2 occurs primarily in connective tissues of the developing and growing mouse. In the adult animal the protein is again produced in response to tissue injury and in association with the growth of tumors. Despite the abnormalities in collagen fibrillogenesis, fragility of skin, and laxity of tendons and ligaments observed in the TSP2-null mouse, TSP2 does not appear to contribute directly to the structural integrity of connective tissue elements. Instead, emerging evidence supports a mode of action of TSP2 'at a distance', i.e. by modulating the activity and bioavailability of proteases and growth factors in the pericellular environment and, very likely, by interaction with cell-surface receptors. Thus, TSP2 qualifies as a matricellular protein, as defined in the introduction to this minireview series. The phenotype of TSP2-null mice has been very helpful in providing clues to the functions of TSP2. In addition to histological and functional abnormalities in connective tissues, these mice display an increased vascularity of the dermis and subdermal tissues, increased endosteal bone growth, a bleeding defect, and a marked adhesive defect of dermal fibroblasts. Our laboratory has established that TSP2 binds matrix metalloproteinase 2 (MMP2) and that the adhesive defect in TSP2-null fibroblasts results from increased MMP2 activity. The investigation of the basis for the other defects in the TSP2-null mouse is likely to yield equally interesting results.

ACTH-regulated expression of vascular endothelial growth factor in the adult bovine adrenal cortex: a possible role in the maintenance of the microvasculature

Endothelial cells lining vessels of endocrine tissues are fenestrated. Interactions with the local environment via either soluble factors or cell-cell interactions appear to govern this terminal endothelial differentiation. Adrenocorticotropin (ACTH) has previously been reported to modulate endothelial fenestration in the rat adrenal cortex. Since vascular endothelial growth factor (VEGF) has been characterized as a potent inducer of endothelial fenestration, we aimed to characterize the status of VEGF expression in the bovine adult adrenal cortex and asked whether ACTH may regulate VEGF expression. By immunohistochemical analysis, we observed VEGF expression in steroidogenic cells from both zona glomerulosa and zona fasciculata of the bovine adrenal cortex. Double-labeling experiments performed on isolated cells in primary culture revealed VEGF immunoreactivity, essentially colocalized with the Golgi apparatus. The expression of two predominant VEGF isoforms, VEGF(121) and VEGF(165), was observed by RT-PCR analysis. ACTH (10 nM) was found to rapidly (within 2-4 h) increase the abundance of these VEGF transcripts, as assessed by both RT-PCR and Northern blot analysis. In parallel, ACTH significantly induced VEGF secretion into the medium of fasciculata cells in primary culture. Thus, our data are consistent with the involvement of ACTH, through its regulation of VEGF expression, in the maintenance of the adult adrenal cortex endothelium.

Thrombospondin-2: a potent endogenous inhibitor of tumor growth and angiogenesis

Recent evidence suggests a potential role for thrombospondin-2 (TSP-2), a matricellular glycoprotein, in the regulation of primary angiogenesis. To directly examine the biological effect of TSP-2 expression on tumor growth and angiogenesis, human A431 squamous cell carcinoma cells, which do not express TSP-2, were stably transfected with a murine TSP-2 expression vector or with vector alone. A431 cells expressing TSP-2 did not show an altered growth rate, colony-forming ability, or susceptibility to induction of apoptosis in vitro. However, injection of TSP-2-transfected clones into the dermis of nude mice resulted in pronounced inhibition of tumor growth that was significantly stronger than the inhibition observed in A431 clones stably transfected with a thrombospondin-1 (TSP-1) expression vector, and combined overexpression of TSP-1 and TSP-2 completely prevented tumor formation. Extensive areas of necrosis were observed in TSP-2-expressing tumors, and both the density and the size of tumor vessels were significantly reduced, although tumor cell expression of the major tumor angiogenesis factor, vascular endothelial growth factor, was maintained at high levels. These findings establish TSP-2 as a potent endogenous inhibitor of tumor growth and angiogenesis.

Tissue inhibitor of metalloproteinase-2 (TIMP-2) expression is strongly induced by ACTH in adrenocortical cells

Besides its acute and chronic effects on corticosteroid synthesis, the pituitary adrenocorticotropic hormone (ACTH) regulates diverse adrenocortical biological functions including the synthesis of a number of mitochondrial, cytoplasmic, and secreted proteins. ACTH-induced secreted proteins are candidates to act as local extracellular relays of the hormone in either an autocrine or a paracrine manner. In the present study, we report that stimulation of primary cultures of bovine adrenocortical (BAC) fasciculata cells with 10 nM ACTH for 24 h results in a mean 8 +/- 4-fold induction of the synthesis of a secreted protein presenting an apparent Mr of 21 kDa. Peptide microsequencing and Western blotting allowed us to identify this 21-kDa ACTH-induced protein as the tissue inhibitor of metalloproteinase-2 (TIMP-2). The induction of TIMP-2 by ACTH required transcription, was mimicked by 8-bromo cyclic 3'-5' adenosine monophosphate, but was not observed in response to angiotensin II, IGF-1, fibroblast growth factor-2, transforming growth factor-beta1, or cortisol treatments. ACTH stimulated TIMP-2 mRNA levels by a factor 4, whereas TIMP-1 mRNA levels were not affected and TIMP-3 mRNA remained undetectable. The biological activity of TIMP-2 varied accordingly, as we observed that the conditioned medium of ACTH-treated BAC cells was four times more potent at inhibiting gelatinolytic activity than was the conditioned medium of control cells. Because the proteolytic activity of both progelatinase-B and progelatinase-A secreted by BAC cells remained latent, whether in the presence or in the absence of ACTH, a paracrine rather than autocrine role is proposed for TIMP-2 in the adrenal cortex.

Detection of exon skipping and retained introns in transcription of the human thrombospondin 2 gene

Alternative transcripts of the gene for human thrombospondin 2 (TSP2), an extracellular glycoprotein, have been identified using a PCR procedure. Species that show exon skipping and/or retained introns were revealed. In particular, a species was characterized in which exon 3 has been spliced from the sequence. This transcript is otherwise intact from exon 1A to exon 22. Transcripts which retain intron 1A, intron 1B, or intron 2 were also identified. Tissue-specific differences were evident in the transcripts with retained introns. In addition, a species which both retained intron 1B and skipped exon 3 was revealed. The existence of these alternative transcripts of the human TSP2 gene could be important for understanding the diverse functions and tissue-specific expression implicated for TSP2.

Bovine thrombospondin-2: complete complementary deoxyribonucleic acid sequence and immunolocalization in the external zones of the adrenal cortex

Given the variety of biological functions in the adrenal cortex that are controlled by ACTH, we hypothesized that some extracellular proteins act as biological relays for this systemic hormone. One candidate protein [corticotropin-induced secreted protein (CISP)] was purified from the conditioned medium of bovine adrenocortical cells on the basis of a 5- to 14-fold increase in its synthesis after the addition of ACTH. We report here the cloning of overlapping complementary DNAs that span the sequence encoding the full-length protein (1170 amino acids). The deduced CISP protein sequence is 89% identical to that of human thrombospondin-2 (TSP2), but only 61% identical to that of bovine TSP1, confirming that CISP is the bovine ortholog of TSP2. The bovine TSP2 sequence aligned perfectly with human, mouse, and chicken TSP2 sequences, except for a gap of 2 amino acids located in a linker region. All 58 cysteine residues that are conserved in other species were present in the bovine sequence as well as most of the functional domains. Most endocrine tissues (adrenal cortex, testis, ovary, and placenta) appeared to express TSP2, as determined by Western blot analysis. The highest levels of TSP2 protein were found in the adrenal cortex, followed by the heart, spleen, brain, and kidney. A differential extent of N-glycosylation or tissular proteolytic maturation may be responsible for the mol wt differences observed between bovine TSP2 detected in the medium from primary cultures and that in fresh tissue extracts. The immunohistochemical analysis of the distribution of TSP2 in the bovine adrenal gland revealed that the protein is much more abundant in the external zones (zona glomerulosa and zona fasciculata) than in the internal reticularis zone, a pattern similar to that reported for ACTH receptors. This distribution clearly suggests that TSP2 is a candidate relay protein for a subset of ACTH actions in the adrenal cortex.

Regulation of the three-dimensional organization of thyroid epithelial cells into follicle structures by the matricellular protein, thrombospondin-1

Thyroid epithelial cells in primary culture have the capacity to organize into thyroid-specific three-dimensional structures, the follicles, in response to TSH. We studied whether thrombospondin 1 (TSP1), which represents, besides thyroglobulin, the main protein secreted by thyroid cells, could play a role in the process of folliculogenesis. TSH promoted follicle formation and inhibited TSP1 production. On the contrary, the phorbol ester, 12-O-tetradecanoyl-phorbol 13-acetate (1-100 nM) prevented TSH-induced follicle formation and strongly increased the synthesis of TSP1. Activation of TSP1 synthesis was dependent upon messenger RNA synthesis. Transforming growth factor-beta, like 12-O-tetradecanoyl-phorbol 13-acetate, increased TSP1 synthesis and prevented TSH-induced follicle formation. Thus, signaling molecules that depressed or conversely activated TSP1 production, respectively promoted or prevented thyroid folliculogenesis. TSP1, purified from platelets, was devoid of effect on cell substratum attachment, but exerted a concentration-dependent inhibition of the TSH-activated reconstitution of thyroid follicles (half-inhibition at 40 microg/ml). TSP1 exhibited the same effect when added to thyroid cell aggregates representing primitive follicle structures. Our data suggest that the control of thyroid follicle formation may operate at least in part through regulation of the production of the matricellular protein TSP1, which acts as a negative modulator of the cell-cell adhesion process involved in thyroid follicle morphogenesis.

The distribution of the matricellular protein thrombospondin 2 in tissues of embryonic and adult mice

Mice that lack the matricellular protein thrombospondin 2 (TSP2) develop a pleiotropic phenotype characterized by morphological changes in connective tissues, an increase in vascular density, and a propensity for bleeding. Furthermore, dermal cells derived from TSP2-null mice display adhesion defects, a finding that implicates TSP2 in cell-matrix interactions. To gain a better understanding of the participation of TSP2 in the development and maturation of the mouse, we examined its distribution in embryonic and adult tissues. Special attention was paid to the presence of TSP2 in collagen fibers, because collagen fibrils in the TSP2-null mouse appear to be irregular in size and contour by electron microscopy. Immunohistochemical analysis of Day 15 and Day 18 embryos revealed TSP2 in areas of chondrogenesis, osteogenesis, and vasculogenesis, and in dermal and other connective tissue-forming cells. Distinctly different patterns of deposition of TSP2 were observed in areas of developing cartilage and bone at Days 15 and 18 of embryonic development. A survey of adult tissues revealed TSP2 in dermal fibroblasts, articular chondrocytes, Purkinje cells in the cerebellum, Leidig cells in the testis, and in the adrenal cortex. Dermal fibroblasts were also shown to synthesize TSP2 in vitro. The distribution of TSP2 during development is in keeping with its participation in the formation of a variety of connective tissues. In adult tissues, TSP2 is located in the pericellular environment, where it can potentially influence the cell-matrix interactions associated with cell movement and tissue repair.

A far upstream, cell type-specific enhancer of the mouse thrombospondin 3 gene is located within intron 6 of the adjacent metaxin gene

Thrombospondin 3 (TSP3) is a secreted, pentameric glycoprotein whose regulation of expression and function are not well understood. Mouse Thbs3 is located just downstream from the divergently transcribed metaxin gene (Mtx), which encodes an outer mitochondrial membrane import protein. Although Thbs3 and Mtx share a common promoter region, previous studies showed that Mtx is regulated by proximal elements that had little effect on Thbs3 expression. In this study, transient transfection of rat chondrosarcoma cells and NIH-3T3 fibroblasts demonstrated that Thbs3 is regulated in a cell type-specific manner by a position- and orientation-independent far upstream enhancer located within intron 6 of Mtx. Despite its greater proximity to the transcription start site of Mtx, the Thbs3 enhancer did not have a significant effect on Mtx expression. Two DNA-protein complexes, which were both required for activity, were identified when nuclear extracts were assayed with a probe containing the enhancer sequence. The protein in one of these complexes was identified as Sp1, while the other DNA-protein complex remains uncharacterized. A 6-kilobase pair promoter containing the enhancer was able to direct specific expression of the E. coli lacZ gene in transgenic mice, whereas a 2-kilobase pair promoter that lacked the enhancer was inactive. Thus, despite their close proximity, the genes of the Mtx/Thbs3 gene cluster are regulated independently.

Mice that lack thrombospondin 2 display connective tissue abnormalities that are associated with disordered collagen fibrillogenesis, an increased vascular density, and a bleeding diathesis

Thrombospondin (TSP) 2, and its close relative TSP1, are extracellular proteins whose functions are complex, poorly understood, and controversial. In an attempt to determine the function of TSP2, we disrupted the Thbs2 gene by homologous recombination in embryonic stem cells, and generated TSP2-null mice by blastocyst injection and appropriate breeding of mutant animals. Thbs2-/- mice were produced with the expected Mendelian frequency, appeared overtly normal, and were fertile. However, on closer examination, these mice displayed a wide variety of abnormalities. Collagen fiber patterns in skin were disordered, and abnormally large fibrils with irregular contours were observed by electron microscopy in both skin and tendon. As a functional correlate of these findings, the skin was fragile and had reduced tensile strength, and the tail was unusually flexible. Mutant skin fibroblasts were defective in attachment to a substratum. An increase in total density and in cortical thickness of long bones was documented by histology and quantitative computer tomography. Mutant mice also manifested an abnormal bleeding time, and histologic surveys of mouse tissues, stained with an antibody to von Willebrand factor, showed a significant increase in blood vessels. The basis for the unusual phenotype of the TSP2-null mouse could derive from the structural role that TSP2 might play in collagen fibrillogenesis in skin and tendon. However, it seems likely that some of the diverse manifestations of this genetic disorder result from the ability of TSP2 to modulate the cell surface properties of mesenchymal cells, and thus, to affect cell functions such as adhesion and migration.

Cloning and expression of murine SC1, a gene product homologous to SPARC

A number of cDNAs (SC1, QR1, and hevin) have been shown to be similar to SPARC (secreted protein acidic and rich in cysteine), a matricellular protein that regulates cell adhesion, cell cycle, and matrix assembly and remodeling. These proteins are 61-65% identical in the final 200 residues of their C-termini; their N-terminal sequences are related but more divergent. All have an overall acidic pl, with a follistatin-like region that is rich in cysteine, and a Ca+2 binding consensus sequence at the C-terminus. Using degenerate primers representing the most highly conserved region in SPARC, SC1, and QR1, we identified a 300-BP SC1 clone in a primary polymerase chain reaction (PCR) screen of a mouse brain cDNA library. This cDNA was used to obtain a full-length clone, which hybridized to a 2.8-KB RNA abundant in brain. Mouse SC1 displays a similarity of 70% to mouse SPARC at the amino acid level. Northern blot and RNAse protection assays revealed a 2.8-KB mRNA expressed at moderate levels (relative to brain) in mouse heart, adrenal gland, epididymis, and lung, and at low levels in kidney, eye, liver, spleen, submandibular gland, and testis. In contrast to SPARC, in situ hybridization showed expression of SC1 mRNA in the tunica media and/or adventitia of medium and large vessels; transcripts were not detected in capillaries, venules, or large lymphatics. The distribution of transcripts for SC1 was also different from that of SPARC in several organs, including adrenal gland, lung, heart, liver, and spleen. Moreover, SC1 mRNA was not evident in endothelium cultured from rat heart, bovine fetal and adult aorta, mouse aorta, human omentum, and bovine retina. Cultured smooth muscle cells and fibroblasts also failed to express SC1 mRNA. The absence of SC1 transcript in cultured cells indicates that the SC1 gene is potentially sensitive to regulatory factors in serum or to a three-dimensional architecture conferred by the extracellular matrix that is lacking in vitro. In conclusion, the expression of SPARC and SC1 appears to be coincident in specific tissues (e.g., adrenal gland and brain), but these proteins exhibit distinct expression patterns in most organs of the mouse. Because SC1 and SPARC are structurally similar and exhibit counteradhesive effects on cultured cells, their overlapping and/or adjacent expression in most tissues predicts that one protein might compensate functionally, at least in part, for the other.

Expression of laminin and its possible role in adrenal cortex homeostasis

The adult mammalian adrenal cortex undergoes permanent regeneration. This process implies a cellular proliferation step restricted to the external zone of the tissue, and a subsequent centripetal cell migration during which phenotypic transition from glomerulosa into fasciculata and reticularis cells and elimination of senescent cells through apoptosis occur. As the molecular mechanisms implied in adrenocortical cell migration are still generally unknown, we addressed that question in the present study. Of several extracellular matrix proteins tested, laminin was the most potent chemotactic and haptotactic factor for bovine fasciculata adrenocortical cells. The maximal chemotactic effect (3-fold stimulation) was observed with 50-75 micrograms/ml laminin, whereas the haptotactic effect (3.5-fold stimulation) plateaued for laminin concentrations in the coating solution over 25 micrograms/ml. Using an anti-Engelbreth-Holm-Swarm laminin antibody, we could demonstrate that adrenocortical cells actively synthesize and secrete Engelbreth-Holm-Swarm-laminin, with the A chain produced in limiting quantities. ACTH treatment of adrenocortical cells specifically induced a 2.7- to 4.5-fold increase in A chain synthesis, resulting in a corresponding increase in the amount of secreted laminin. The distribution of laminin in the adrenal cortex tissue was then evaluated by standard immunohistochemistry. The protein appeared to be uniformly expressed in the three zones of the cortex. This observation does not favor the hypothesis that laminin acts as an attractant driving centripetal cell migration. Laminin, which is synthesized under the control of the systemic hormone ACTH, appears as a permissive factor that facilitates proper homeostasis of the adrenocortical tissue.