Oncomodulin in normal and transformed cells

Oncomodulin: The Enigmatic Parvalbumin Protein

EF-hand Ca²⁺-binding protein family members, α- and β-parvalbumins have been studied for decades. Yet, considerable information is lacking distinguishing functional differences between mammalian α-parvalbumin (PVALB) and oncomodulin (OCM), a branded β-parvalbumin. Herein, we provide an overview detailing the current body of work centered around OCM as an EF-Hand Ca²⁺-binding protein and describe potential mechanisms of OCM function within the inner ear and immune cells. Additionally, we posit that OCM is evolutionarily distinct from PVALB and most other β-parvalbumins. This review summarizes recent studies pertaining to the function of OCM and emphasizes OCM as a parvalbumin possessing a unique cell and tissue distribution, Ca²⁺ buffering capacity and phylogenetic origin.

The LTR promoter of the rat oncomodulin gene is regulated by cell-line specific accessibility in the LTR U3 region

By germline insertion, a long terminal repeat (LTR) of an intracisternal A-particle type IAP retrovirus has overtaken the transcriptional control of the rat oncomodulin (OM) gene, which codes for a high affinity Ca2+-binding protein with modulatory capacity. In order to get insights into regulatory mechanisms of LTR directed OM gene expression we tested promoter activity of this LTR by transient transfection of transformed rat fibroblasts with this sequence placed 5' of the human growth hormone hGH reporter gene. The OM LTR is a strong promoter but does not follow an expression pattern similar to the one of the OM gene. Genomic sequencing showed a good correlation between CpG hypomethylation in the OM LTR and OM transcription among various cell lines and tissues. DNase I mapping of a 18 kb fragment containing the OM gene and 5' flanking sequences revealed cell-line specific hypersensitivity sites located within the U3 region of the LTR element. Several cis-elements in the OM LTR promoter exhibiting cell-line specific occupancy were identified by in vivo DMS-footprinting. Detailed analysis of protein interactions with two such sequence elements in vitro revealed binding of ubiquitously expressed nuclear factors within an AP-1 (activator protein 1) and a intracisternal A-particle upstream enhancer recognition sequence. Protein occupancy to the latter sequence is significantly reduced by CpG methylation. These results indicate that cell-line specificity of OM expression is dictated by factor accessibility to the LTR promoter.

Calcium Homeostasis in Human Placenta: Role of Calcium‐Handling Proteins

The human placenta is a transitory organ, representing during pregnancy the unique connection between the mother and her fetus. The syncytiotrophoblast represents the specialized unit in the placenta that is directly involved in fetal nutrition, mainly involving essential nutrients, such as lipids, amino acids, and calcium. This ion is of particular interest since it is actively transported by the placenta throughout pregnancy and is associated with many roles during intrauterine life. At term, the human fetus has accumulated about 25-30 g of calcium. This transfer allows adequate fetal growth and development, since calcium is vital for fetal skeleton mineralization and many cellular functions, such as signal transduction, neurotransmitter release, and cellular growth. Thus, there are many proteins involved in calcium homeostasis in the human placenta.

Calbindin-d(28k): a marker of recurrence for medulloblastomas

BACKGROUND

The expression of the Ca(2+)-binding protein calbindin-D(28k) was analyzed in medulloblastomas in relation to clinical features and other biologic markers related to cell proliferation, differentiation, p53, and cerebellar developmental regulated gene expression.

METHODS

Immunohistochemistry was carried out on histologic slides from a first retrospective series of 29 nonmetastatic and 10 metastatic medulloblastoma formalin-fixed, paraffin-embedded tissues, using specific antibodies against calbindin-D(28k), calretinin, alpha-parvalbumin and beta-parvalbumin, and S100 proteins. Informed consent was obtained from the subjects and/or guardians. Other biologic markers for differentiation, cell proliferation, the expression of the p53 tumor suppressor gene protein, and cerebellar developmental regulated genes were similarly investigated. A second series of 16 medulloblastomas from young patients (younger than 15 years) was added in order to validate the results obtained in the first series.

RESULTS

Of all the markers investigated, only calbindin-D(28k) was significantly associated with prognosis. Survival and remission (i.e. recurrence free) time analysis performed on all the cases (n = 55) confirmed a high risk of death (P = 0.004) and recurrence (P = 0.003) associated with calbindin-positivity. As calbindin-positivity was predominantly observed in tumors from young patients, the authors confirmed its prognostic value in the subgroup of patients younger than 15 years (n = 37). Cox regression analysis showed a significant and independent prognostic value for calbindin expression and, to a lesser extent, the type of surgery (total or subtotal). Three risk groups were thus identified, distinguishing among the cases characterized by a total resection and calbindin-negativity (good prognosis), by a subtotal resection and calbindin-negativity (intermediary), and by calbindin-positivity (bad prognosis).

CONCLUSIONS

The current study suggests that calbindin-positive medulloblastomas represent a subclass of aggressive tumors more frequently seen in younger patients.

Conformational stabilities of the rat alpha- and beta-parvalbumins

It is widely believed that beta-parvalbumin (PV) isoforms are intrinsically less stable than alpha-parvalbumins, due to greater electrostatic repulsion and an abbreviated C-terminal helix. However, when examined by differential scanning calorimetry, the apo-form of the rat beta-PV (i.e. oncomodulin) actually displays greater thermal stability than the alpha-PV. Whereas the melting temperature of the a isoform is 45.8 degrees C at physiological pH and ionic strength, the Tm for the beta isoform is more than 7 degrees higher (53.6 degrees C). This result suggests that factors besides net charge and C-terminal helix length strongly influence parvalbumin conformational stability. Extension of the F helix in the beta-PV, by insertion of Ser-109, has a modest stabilizing effect, raising the Tm, by 1.1 degrees. Truncation of the alpha-PV F helix, by removal of Glu-108, has a more profound impact, lowering the Tm by 4.0 degrees.

Structure and chromosomal localization of the mouse oncomodulin gene

The rat gene encoding oncomodulin (OM), a small calcium-binding protein, is under the control of a solo LTR derived from an endogenous intracisternal A-particle. The latter sequence is the only OM promoter analyzed so far. In order to study cell type-specific OM expression in a species lacking LTR sequences in the OM locus, we initially synthesized an OM cDNA from mouse placenta. By sequencing, we found a 137-bp-long 5'leader region that differed markedly from its rat counterpart but had high similarity to several mouse genomic sequences. Primers specific to this sequence in addition with primers specific for an exon 2/intron 2 sequence were used to screen a mouse ES cell line genomic P1 library. One positive clone contained the whole OM gene, including intron 1 of 25kb and a 5' flanking region of 27 kb lacking an LTR. The region upstream of exon 1 contains no TATA or CCAAT boxes but has a homopurine/homopyrimidine stretch of 102 bp as well as a (CA)22 repeat. The latter sequence is polymorphic and was therefore, used to map the OM gene to the distal end of the long arm of mouse Chromosome (Chr) 5 by interspecific backcross analysis. Additionally we localized the OM gene by in situ hybridization to the region G1-3 on Chr 5, confirming the genetic linkage results. Finally, the OM gene was found to be structurally conserved and to exist in a single copy in mammals.

The intracisternal A particle derived solo LTR promoter of the rat oncomodulin gene is not present in the mouse gene

The rat gene encoding oncomodulin, a small calcium-binding protein related to parvalbumin, is under the control of a solo long terminal repeat (LTR) derived from an endogenous intracisternal A-particle (IAP). This gene was the first example of a mammalian gene regulated in normal cells by a promoter of retroviral origin (see also article by D. Robins and L. Samuelson in this volume). We show here that the oncomodulin LTR is a member of a small subset of sequence related solo LTR elements present in the rat genome and that a full length IAP genome containing LTRs of this type is no longer present in the rat genome. We have assayed the transcriptional activity of the oncomodulin LTR coupled to the human growth hormone gene as a reporter. Transfections in both Hela cells and 293 cells indicate the oncomodulin LTR promoter is sufficient to efficiently initiate transcription. In 293 cells (human embryo kidney cells transformed with human adenovirus type 5 DNA), the oncomodulin LTR is a strong promoter, capable of bidirectional transcription. Finally, we have determined the structure and the sequence of the mouse oncomodulin gene. Our results suggest that the integration of the IAP particle genome within the rat oncomodulin gene occurred after the rat and the mouse became distinct species.