Asymmetric distribution of oncogene products at mitosis

The contribution of asymmetric cell division to phenotypic heterogeneity in cancer

Cells have evolved intricate mechanisms for dividing their contents in the most symmetric way during mitosis. However, a small proportion of cell divisions results in asymmetric segregation of cellular components, which leads to differences in the characteristics of daughter cells. Although the classical function of asymmetric cell division (ACD) in the regulation of pluripotency is the generation of one differentiated daughter cell and one self-renewing stem cell, recent evidence suggests that ACD plays a role in other physiological processes. In cancer, tumor heterogeneity can result from the asymmetric segregation of genetic material and other cellular components, resulting in cell-to-cell differences in fitness and response to therapy. Defining the contribution of ACD in generating differences in key features relevant to cancer biology is crucial to advancing our understanding of the causes of tumor heterogeneity and developing strategies to mitigate or counteract it. In this Review, we delve into the occurrence of asymmetric mitosis in cancer cells and consider how ACD contributes to the variability of several phenotypes. By synthesizing the current literature, we explore the molecular mechanisms underlying ACD, the implications of phenotypic heterogeneity in cancer, and the complex interplay between these two phenomena.

Cell cycle proliferation decisions: the impact of single cell analyses

Cell proliferation is a fundamental requirement for organismal development and homeostasis. The mammalian cell division cycle is tightly controlled to ensure complete and precise genome duplication and segregation of replicated chromosomes to daughter cells. The onset of DNA replication marks an irreversible commitment to cell division, and the accumulated efforts of many decades of molecular and cellular studies have probed this cellular decision, commonly called the restriction point. Despite a long-standing conceptual framework of the restriction point for progression through G1 phase into S phase or exit from G1 phase to quiescence (G0), recent technical advances in quantitative single cell analysis of mammalian cells have provided new insights. Significant intercellular heterogeneity revealed by single cell studies and the discovery of discrete subpopulations in proliferating cultures suggests the need for an even more nuanced understanding of cell proliferation decisions. In this review, we describe some of the recent developments in the cell cycle field made possible by quantitative single cell experimental approaches.

Epigenetic biomarkers in urothelial bladder cancer

Bladder cancers comprise heterogeneous cell populations, and numerous factors are likely to be involved in dictating recurrence, progression and patient survival. While several molecular markers that are used to evaluate the development and prognosis of bladder cancer have been studied, the limited value of these established markers has created the need for new molecular indicators of bladder cancer prognosis. Of particular interest is the silencing of tumor-suppressor genes by epigenetic alteration. Recent progress in understanding epigenetic modification and gene silencing has led to new opportunities for the understanding, detection, treatment and prevention of cancer. Moreover, epigenetic silencing of tumor-suppressor genes is interesting from a clinical standpoint, because of the possibility of reversing epigenetic changes and restoring gene function in a cell. This review focuses on the prognostic relevance of epigenetic markers in bladder cancer.

A case of HER2-positive male breast cancer with lung metastases showing a good response to trastuzumab and paclitaxel treatment

We present a case of advanced HER2-positive male breast cancer, which showed a good response to a combined treatment of trastuzumab and paclitaxel. A 78-year-old man was diagnosed with invasive ductal carcinoma (T4d N3 M1, stage IV). He had advanced breast cancer consisting of multiple tumors with skin involvement and redness over the entire left chest region. A computed tomography (CT) scan of the chest revealed a metastatic tumor in the left lung. Histologically, both the primary breast cancer and the metastatic lung tumor were identified as invasive ductal carcinoma that was estrogen receptor-negative (ER)(-) and progesterone receptor-negative (PgR)(-), with a HER2 score of 3+ (IHC). The patient received a combination chemotherapy using trastuzumab and paclitaxel. Two months later, a follow-up chest CT scan showed that the left lung tumor had disappeared, suggesting a good response to trastuzumab and paclitaxel. During trastuzumab treatment, no severe adverse events above grade 3 were observed. This is the first reported case of advanced HER2-positive male breast cancer in which a good response to trastuzumab and paclitaxel was demonstrated at both primary breast cancer and metastatic sites.

RUNX3 inactivation by point mutations and aberrant DNA methylation in bladder tumors

RUNX3 is inactivated at high frequency in many tumors. However, in most cases, inactivation is caused by silencing of the gene due to promoter hypermethylation. Because epigenetic silencing is known to affect many major tumor suppressor genes in cancer cells, it is not clear whether RUNX3 is primarily responsible for the induction of carcinogenesis in these cases, except for the gastric cancer cases that we reported previously. We investigated genetic and epigenetic alterations of RUNX3 in 124 bladder tumor cases and seven bladder tumor-derived cell lines. Here we show that RUNX3 is inactivated by aberrant DNA methylation in 73% (90 of 124) of primary bladder tumor specimens and 86% (six of seven) of bladder tumor cell lines. In contrast, the promoter regions of 20 normal bladder mucosae were unmethylated. Importantly, one patient bore missense mutations, each of which resulted in amino acid substitutions in the highly conserved Runt domain. The mutations abolished the DNA-binding ability of RUNX3. A second patient had a single nucleotide deletion within the Runt domain coding region that resulted in truncation of the protein. RUNX3 methylation was a significant risk factor for bladder tumor development, superficial bladder tumor recurrence, and subsequent tumor progression. These results strongly suggest that inactivation of RUNX3 may contribute to bladder tumor development and that promoter methylation and silencing of RUNX3 could be useful prognostic markers for both bladder tumor recurrence and progression.

Differential expression of cyclin D1 in the human hair follicle

The proliferation of keratinocytes in the hair follicle varies from slowly cycling, intermittently proliferating stem cells in the bulge to rapidly proliferating, transient cells in the bulb. To better understand the biological differences between these two compartments, we sought to identify differentially expressed genes using cDNA macroarray analysis. Cyclin D1 was one of 13 genes increased in the bulge compared to the bulb, and its differential expression was corroborated by quantitative real-time polymerase chain reaction (PCR) on the original samples. Using immunohistochemical staining, laser-capture microdissection (LCM) and quantitative real-time PCR, we localized cyclin D1 to the suprabasal cells of the telogen bulge and anagen outer root sheath (ORS). Surprisingly, cyclin D1, D2, and D3 were not detectable by immunohistochemistry in the rapidly proliferating hair-producing cells of the anagen bulb (matrix cells), while these cells were strongly positive for Ki-67 and retinoblastoma protein. In contrast, pilomatricoma, a tumor thought to be derived from matrix cells, was positive for cyclin D1, D2, and D3. Our results suggest that cyclin D1 may mediate the proliferation of stem cells in the bulge to more differentiated transient amplifying cells in the suprabasal ORS. In contrast, non-cyclin D1-proteins appear to control cell division of the highly proliferative bulb matrix cells. This non-cyclin D1-mediated proliferation may provide a protective mechanism against tumorigenesis, which is overridden in pilomatricomas. Our data also demonstrate that the combination of DNA macroarray, LCM and quantitative real-time PCR is a powerful approach for the study of gene expression in defined cell populations with limited starting material.

Genesis of clone size heterogeneity in megakaryocytic and other hemopoietic colonies: the stochastic model revisited

OBJECTIVE

We previously showed that the distributions of the numbers of doublings (NbD) undergone by individual megakaryocyte progenitors before commitment to polyploidization are markedly skewed and can consistently be fitted to straight lines when plotted on semilogarithmic coordinates. The slope of such lines, which yields the probability of polyploidization per doubling, is made less steep by stimulators of megakaryocyte colony formation and is less steep in mixed erythroid-megakaryocyte than in pure megakaryocyte colonies. Therefore, megakaryocytopoiesis provides a unique model for the study of clonal heterogeneity in a hemopoietic lineage, which is the subject of this review.

DATA SOURCES

Articles relevant to the interpretation of these data were selected from the authors' and public databases.

DATA SYNTHESIS

Exponential NbD distributions were first explained by postulating that following the assembly of thrombopoiesis-specific regulators, megakaryocyte progenitors require only a single random event to arrest proliferation and commit to polyploidization. However, this stochastic model was refuted by data indicating that intrinsic properties of individual progenitors affect the NbD they achieve. We suggest that the unequal repartition of critical compounds (including receptors, signaling molecules, and gene regulators) inherent in the stem cell-progenitor transition causes a heritable heterogeneity in megakaryocyte progenitor responsiveness to polyploidization inducers. This model would be compatible with 1) the evidence for intraclonal synchronization in megakaryocyte and other hemopoietic clones generated by committed progenitors; 2) the low probability of polyploidization of the relatively insensitive bipotent megakaryocyte progenitors; and 3) the thesis that stimulators act in part by recruiting megakaryocyte progenitor cells endowed with lesser responsiveness to polyploidization inducers and higher proliferative potential.

CONCLUSION

The responsiveness of individual megakaryocyte progenitors to polyploidization inducers may be a major determinant of the exponential shape of NbD distributions.

Divergent effects of hyperosmolality on stress-response (heat shock) protein expression in cultured human tumor cells: an immunocytochemical study

Exposing cells to adverse conditions usually elicits expression of stress-response (heat shock) proteins (srp). Here we show that hyperosmolar growth conditions do not uniformly affect srp expression in MCF-7 and HeLa S3 cells, derived from carcinoma of the breast and cervix, respectively. Thus, whereas srp 27 expression was increased in MCF-7, but not in HeLa S3, the opposite was the case with srp 72. On the other hand, hyperosmolality did not induce alpha B-crystallin or ubiquitin in either cell line. These findings show that srp expression by the human tumor cells studied is non-coordinate, suggesting that each srp is independently modulated.

Self-renewal of the long-term repopulating stem cell

Self-renewal implies maintenance of all attributes of the original in the offspring and is considered characteristic of the hemopoietic stem cells. Yet, it has been questioned whether one of the most primitive hemopoietic stem cells, the long-term repopulating cell (LTRC), has that capacity. The present experiments demonstrate that single LTRCs can repopulate the lymphohemopoietic system of a lethally irradiated mouse and that the progeny of a single LTRC in a primary recipient again contains LTRCs capable of repopulating lethally irradiated secondary recipients. The transfusion of very small numbers of marrow cells (10,000-20,000 cells containing one or no LTRCs) unexpectedly provided insight into competitive marrow repopulation. At these low levels of stem cells, irradiated host stem cells or their progeny competed successfully with unirradiated donor cells. This parallels the known reemergence and marrow repopulation by host cells when the number of nonirradiated donor stem cells is reduced by serial transplantation.