Brca1 and differentiation.

Breast cancer is one of the most frequent malignancies affecting women. The human breast cancer gene 1 (BRCA1) gene is mutated in a distinct proportion of hereditary breast and ovarian cancers. Tumourigenesis in individuals with germline BRCA1 mutations requires somatic inactivation of the remaining wild-type allelle. Although, this evidence supports a role for BRCA1 as a tumour suppressor, the mechanisms through which its loss leads to tumourigenesis remain to be determined. Neither the expression pattern nor the described functions of human BRCA1 and murine breast cancer gene 1 (Brca1) can explain the specific association of mutations in this gene with the development of breast and ovarian cancer. Investigation of the role of Brca1 in normal cell differentiation processes might provide the basis to understand the tissue-restricted properties.

FIP-2, an IkappaB-kinase-gamma-related protein, is associated with the Golgi apparatus and translocates to the marginal band during chicken erythroblast differentiation.

Using an antiserum directed against marginal band associated proteins of chicken erythrocytes we isolated clones encoding the chicken homolog of 14.7K-interacting protein 2 (FIP-2), a protein potentially involved in tumor necrosis factor-alpha/nuclear factor-kappaB signaling, from a chicken erythroblast cDNA library. We found that chicken FIP-2 was expressed in a variety of tissues and cell types, but unlike its human counterpart, alternative splicing does not appear to take place. Analysis of intracellular localization revealed that FIP-2 was concentrated at the Golgi apparatus in most cells. Perturbation of the Golgi structure without loss of Golgi function (by treatment with nocodazole) resulted in a retention of FIP-2 at the dispersed Golgi fragments. In contrast, disruption of both Golgi structure and function (by brefeldin A) led to a loss of FIP-2 from Golgi membranes. Remarkably, during erythroblast differentiation FIP-2 was found to translocate from the Golgi to the marginal band. Our results support the hypothesis of a function of the Golgi apparatus in signal transduction. Moreover, our results raise the possibility that the marginal band of chicken erythrocytes, in addition to its role in morphogenesis, has a function in signal transduction and that FIP-2 is in some way involved in its formation.

Evidence for separable functions of tuberous sclerosis gene products in mammalian cell cycle regulation.

Tuberous sclerosis is an autosomal dominant disease affecting approximately 1 in 6,000 individuals. It is caused by mutations in either TSC1 on chromosome 9q34, which encodes hamartin, or TSC2 on chromosome 16p13.3, which encodes tuberin. The growths, named hamartomas, characteristically occur in different organs of patients and are speculated to result from defects in proliferation control. The observation that hamartin and tuberin can interact in vivo suggests that they might function in the same complex. Here we show that hamartin can affect proliferation control independent of the presence of functional tuberin and that binding to hamartin is not essential for tuberin to affect proliferation. Ectopic expression of hamartin negatively regulates proliferation to a similar extent in tuberin-positive and tuberin-negative cells; this is accompanied by binding to tuberin and upregulation of endogenous p27 in tuberin-positive cells and is without effects on p27 expression in the latter. Our data show for the first time that TSC proteins possess separable functions. We further demonstrate that hamartin can deregulate proliferation control by different mechanisms depending on the presence of tuberin. Besides an overlap in many features of patients with TSC1 and TSC2 mutations, data has accumulated that provides evidence for specific clinical differences. This study provides new insights into the cellular roles of TSC proteins and initiates a discussion of whether separable functions of these proteins might be associated with the clinical differences of TSC1- and TSC2-associated disease.