Retinoblastoma protein phosphorylation via PI 3-kinase and mTOR pathway regulates adipocyte differentiation

In the early phase of adipocyte differentiation, transient increase of DNA synthesis, called clonal expansion, and transient hyperphosphorylation of retinoblastoma protein (Rb) are observed. We investigated the role of these phenomena in insulin-induced adipocyte differentiation of 3T3-L1 cells. Insulin-induced clonal expansion, Rb phosphorylation and adipocyte differentiation were all inhibited by the PI 3-kinase inhibitors and rapamycin, but not the MEK inhibitor, whereas the MEK inhibitor, but not PI 3-kinase inhibitors or rapamycin, decreased c-fos induction. We conclude that insulin induces hyperphosphorylation of Rb via PI 3-kinase and mTOR dependent pathway, which promotes clonal expansion and adipocyte differentiation of 3T3-L1 cells.     

The evolution of two mutations during clonal expansion

Knudson's two-hit hypothesis proposes that two genetic changes in the RB1 gene are the rate-limiting steps of retinoblastoma. In the inherited form of this childhood eye cancer, only one mutation emerges during somatic cell divisions while in sporadic cases, both alleles of RB1 are inactivated in the growing retina. Sporadic retinoblastoma serves as an example of a situation in which two mutations are accumulated during clonal expansion of a cell population. Other examples include evolution of resistance against anticancer combination therapy and inactivation of both alleles of a metastasis-suppressor gene during tumor growth. In this article, we consider an exponentially growing population of cells that must evolve two mutations to (i) evade treatment, (ii) make a step toward (invasive) cancer, or (iii) display a disease phenotype. We calculate the probability that the population has evolved both mutations before it reaches a certain size. This probability depends on the rates at which the two mutations arise; the growth and death rates of cells carrying none, one, or both mutations; and the size the cell population reaches. Further, we develop a formula for the expected number of cells carrying both mutations when the final population size is reached. Our theory establishes an understanding of the dynamics of two mutations during clonal expansion.     

Surgical reconstruction of the contracted orbit

Anophthalmic patients and patients afflicted with retinoblastoma incur severe deformity of the orbit. Treatment of the severely contracted orbit is very difficult, and patient satisfaction is often poor. Since 1988, we have performed temporalis muscle transfer and surgical expansion of the contracted bony orbit in 26 patients. Satisfactory results were obtained. Gradual expansion of the orbit was performed in case of congenital anophthalmic patients. The treatment should be established in multiplicity, among many methods available for contracted eye sockets, according to the degree of orbital deformity and the amount of residual conjunctiva. In case of severe deformity, volume expansion surgery and temporalis muscle transfer are necessary. If augmentation is required in the periorbital region, rib bone onlay graft must be performed. We were able to shorten the operative time by modifying the three-wall orbital expansion technique of Tessier and Wolfe to a more simplified method. Our observations show that our procedures achieved symmetry in both eyes in all patients, and there have been no remarkable complications.     

Differential CRX and OTX2 expression in human retina and retinoblastoma

The histogenesis of retinoblastoma tumors remains controversial, with the cell-of-origin variably proposed to be an uncommitted retinal progenitor cell, a bipotent committed cell, or a cell committed to a specific lineage. Here, we examine the expression of two members of the orthodenticle family implicated in photoreceptor and bipolar cell differentiation, cone-rod homeobox, CRX, and orthodenticle homeobox 2, OTX2, in normal human retina, retinoblastoma cell lines and retinoblastoma tumors. We show that CRX and OTX2 have distinct expression profiles in the developing human retina, with CRX first expressed in proliferating cells and cells committed to the bipolar lineage, and OTX2 first appearing in the photoreceptor lineage. In the mature retina, CRX levels are highest in photoreceptor cells whereas OTX2 is preferentially found in bipolar cells and in the retinal pigmented epithelium. Both CRX and OTX2 are widely expressed in retinoblastoma cell lines and in retinoblastoma tumors, although CRX is more abundant than OTX2 in the differentiated elements of retinoblastoma tumors such as large rosettes, Flexner-Wintersteiner rosettes and fleurettes. Widespread expression of CRX and OTX2 in retinoblastoma tumors and cell lines suggests a close link between the cell-of-origin of retinoblastoma tumors and cells expressing CRX and OTX2.     

Loss of the retinoblastoma tumor suppressor protein in murine calvaria facilitates immortalization of osteoblast-adipocyte bipotent progenitor cells characterized by low expression of N-cadherin

The retinoblastoma gene, RB1, is frequently inactivated in a subset of tumors, including retinoblastoma and osteosarcoma (OS). One characteristic of OS, as well as other tumors in which RB1 is frequently inactivated, is the lack of N-cadherin-mediated cell-cell adhesions. The frequent inactivation of RB1 and parallel loss of N-cadherin expression in OS prompted us to ask whether these observations are directly related to each other. In this study, we observed reduced N-cadherin expression in RB1(-/-) calvarial osteoblasts. In addition, RB1(-/-) cell lines had increased migration potential compared to their RB1(+/+) counterparts. These properties of RB1(-/-) cell lines correlated with an adipogenic potential lacking in RB1(+/+) cell lines, suggesting that each property is present in an immature progenitor cell. The isolation of a cell population with low surface expression of N-cadherin and enhanced adipogenic ability supports this view. Interestingly, the acute loss of pRb does not affect N-cadherin expression or migration or confer adipogenic potential to immortalized RB1(+/+) calvarial cells, suggesting that these traits are not a direct consequence of pRb loss; rather, pRb loss leads to the expansion and immortalization of an immature progenitor pool characterized by these properties.     

Inhibitory activity of bevacizumab to differentiation of retinoblastoma cells

Vascular endothelial growth factor (VEGF) is a major regulator in retinal and choroidal angiogenesis, which are common causes of blindness in all age groups. Recently anti-VEGF treatment using anti-VEGF antibody has revolutionarily improved the visual outcome in patients with vaso-proliferative retinopathies. Herein, we demonstrated that bevacizumab as an anti-VEGF antibody could inhibit differentiation of retinoblastoma cells without affection to cellular viability, which would be mediated via blockade of extracellular signal-regulated kinase (ERK) 1/2 activation. The retinoblastoma cells expressed VEGFR-2 as well as TrkA which is a neurotrophin receptor associated with differentiation of retinoblastoma cells. TrkA in retinoblastoma cells was activated with VEGF treatment. Interestingly even in the concentration of no cellular death, bevascizumab significantly attenuated the neurite formation of differentiated retinoblastoma cells, which was accompanied by inhibition of neurofilament and shank2 expression. Furthermore, bevacizumab inhibited differentiation of retinoblastoma cells by blockade of ERK 1/2 activation. Therefore, based on that the differentiated retinoblastoma cells are mostly photoreceptors, our results suggest that anti-VEGF therapies would affect to the maintenance or function of photoreceptors in mature retina.     

Single-cell transcriptome profiling reveals intratumoural heterogeneity and malignant progression in retinoblastoma

Retinoblastoma is a childhood retinal tumour that is the most common primary malignant intraocular tumour. However, it has been challenging to identify the cell types associated with genetic complexity. Here, we performed single-cell RNA sequencing on 14,739 cells from two retinoblastoma samples to delineate the heterogeneity and the underlying mechanism of retinoblastoma progression. Using a multiresolution network-based analysis, we identified two major cell types in human retinoblastoma. Cell trajectory analysis yielded a total of 5 cell states organized into two main branches, and the cell cycle-associated cone precursors were the cells of origin of retinoblastoma that were required for initiating the differentiation and malignancy process of retinoblastoma. Tumour cells differentiation reprogramming trajectory analysis revealed that cell-type components of multiple tumour-related pathways and predominantly expressed UBE2C were associated with an activation state in the malignant progression of the tumour, providing a potential novel “switch gene” marker during early critical stages in human retinoblastoma development. Thus, our findings improve our current understanding of the mechanism of retinoblastoma progression and are potentially valuable in providing novel prognostic markers for retinoblastoma.Subject terms: Eye cancer, Differentiation, Prognostic markers     

Retinoblastoma: Host resistance and 13q- chromosomal deletion

Summary Data for 27 cases of retinoblastoma that developed in patients with 13q-were collected from the literature and analyzed. The distribution of unilateral and bilateral cases of retinoblastoma differed significantly from the expectation that the degree of expressivity does not differ between the retinoblastoma gene and deletion of 13q. The excess of unilateral cases among the patients with 13q-, which could not be accounted for by ascertainment bias, was attributed to somewhat lowered carcinogenic potential of deletion of 13q14 as compared with the retinoblastoma gene. It was argued that the retinoblastoma gene is probably not located on 13q, and perhaps 20% or more of the individuals with a deletion of 13q14 would not develop retinoblastoma. The normal allele at the retinoblastoma locus, the haplicon in the segment of 13q14, and the suppressor genes as defined by the host resistance model, may be all concerned, in their function additively and without dominance, with normal differentiation of the embryonic retinal cells.     

Separation of retinoblastoma and esterase D loci in a patient with sporadic retinoblastoma and del(13)(q14.1q22.3)

Summary A chromosome 13 deletion in a patient with sporadic retinoblastoma appears to have separated the loci for retinoblastoma and esterase D. This study indicates that: (1) the retinoblastoma locus is distinct from the esterase D locus; and (2) the linear order of these genes is centromere-esterase D-retinoblastoma.     

Linkage analysis of families with hereditary retinoblastoma: nonpenetrance of mutation, revealed by combined use of markers within and flanking the RB1 gene.

Nonpenetrance of the inherited mutation responsible for retinoblastoma has been reported. By DNA analysis in families with hereditary retinoblastoma, it is possible to identify healthy individuals in whom the mutation is nonpenetrant. This requires the use of DNA markers both within and flanking the retinoblastoma gene. We have analyzed the segregation of several markers in 19 families (69 meioses) with hereditary retinoblastoma. In two families a carrier was identified who showed nonpenetrance of the mutation predisposing to retinoblastoma. The intragenic markers were informative in 15 pedigrees. The use of flanking markers from the same chromosomal region caused an increase of the number of informative families to 18. No crossing-over within the gene was observed. In one family an inherited deletion involving one of the RB1 alleles was detected. Our findings emphasize the use of a combination of both intragenic and flanking markers to obtain both the highest reliability of carrier detection in families with hereditary retinoblastoma and an accurate estimate of the frequency of nonpenetrance.     

Nucleolar protein B23.1 binds to retinoblastoma protein and synergistically stimulates DNA polymerase alpha activity

Phosphorylated retinoblastoma protein and nucleolar protein B23 are putative stimulatory factors for DNA polymerase alpha. We showed that these two factors interacted with each other and stimulated the activity of DNA polymerase alpha synergistically. B23 exists in two isoforms designated as B23.1 and B23.2. While B23.1 bound to a retinoblastoma protein-conjugated column, B23.2 did not. These results indicate that B23.1 can directly bind to retinoblastoma protein. It was also shown that B23 was co-immunoprecipitated with both retinoblastoma protein and DNA polymerase alpha from a HeLa cell extract by monoclonal antibodies raised against these components. These results suggest that these three proteins exist as a complex in cells, at least in part. The simultaneous addition of both B23.1 and retinoblastoma protein caused stimulation of DNA polymerase alpha activity that is much higher than the sum of the stimulation by retinoblastoma protein and B23.1 alone. The maximal stimulation was attained at the molar ratio of DNA polymerase alpha/retinoblastoma protein/B23.1 = 1:1:12. Since B23 exists as a hexamer in solution, it may act as a stimulator of DNA polymerase alpha in a form of double-hexamer, in concert with the phosphorylated retinoblastoma protein.     

Retinoblastoma in a boy with a de novo mutation of a 13/18 translocation: The assumption that the retinoblastoma locus is at 13q141, particularly at the distal portion of it

Summary In serial cytogenetic examinations of peripheral lymphocytes from retinoblastoma patients, we found a patient with sporadic bilateral retinoblastoma with a de novo mutation of a 13/18 translocation, with their respective breakpoints at 13q141 and 18q122. The simultaneous de novo occurrence of retinoblastoma and the chromosomal rearrangement involving 13q14 in the proband suggests that the gene locus for retinoblastoma is at 13q141, particularly at the distal portion of it. Deletion mapping data are compatible with this suggestion.     

Detection of submicroscopic deletions and a DNA polymorphism at the retinoblastoma locus

Summary DNA samples from 60 unrelated patients with retinoblastoma were screened by Southern blot hybridization using two probes that are closely linked to the retinoblastoma locus within human chromosome band 13q14. Seven of 44 patients with bilateral or multifocal unilateral retinoblastoma and one patient with unifocal unilateral retinoblastoma were found to have a heterozygous deletion for the anonymous DNA sequence H3-8. Three of the eight deletions did not include the esterase D locus and were undetectable by conventional cytogenetic analysis. The findings are compatible with the deletions being the cause of retinoblastoma in these cases and provide a basis for DNA diagnosis in nearly 20% of patients with bilateral and multifocal unilateral retinoblastoma. The H3-8 probe also detects a restriction fragment length polymorphism that is a useful genetic marker in some families.     

Fibroblasts from Retinoblastoma patients: Enhanced growth in fetal calf serum and a normal response to ionizing radiation

Retinoblastoma is a rare malignant eye tumor found almost exclusively in young children. In 30% of cases, the tumor is bilateral and is inherited as an autosomal dominant trait. In such patients, all of the cells in the body must carry the mutation predisposing to retinoblastoma. To search for the expression of the gene in cells outside the retina, we have studied several in vitro properties of skin fibroblasts from patients with bilateral retinoblastoma. Measurement in low concentrations of fetal calf serum of the initial growth rate and the plating efficiency show that fibroblasts from retinoblastoma donors grow significantly better than those from normal donors. However, we were unable to confirm the results of other investigators that fibroblasts from donors with bilateral retinoblastoma are unusually sensitive to ionizing radiation. In family studies, skin fibroblasts from normal siblings had the same radiation sensitivity as fibroblasts from sibling with retinoblastoma.