Evidence for Separable Functions of Tuberous Sclerosis Gene Products in Mammalian Cell Cycle Regulation

Abstract

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.

Tuberin Binds p27 and Negatively Regulates Its Interaction with the SCF Component Skp2

“…This idea also fits with the earlier observation that the effects of hamartin on p27 are less pronounced than the effects of tuberin (11,12,24). 2 Interestingly, earlier findings suggested that hamartin can also regulate cell proliferation in tuberin-negative cells, but without effects on p27 protein levels (24). Although the molecular mechanism for this additional capacity of hamartin remains elusive, the finding that hamartin is not in a complex with p27 in tuberin-negative cells could explain why hamartin cannot regulate p27 in these cells.…”

“…These data suggest that the effects of tuberin on p27 expression could be due to direct interaction, whereas hamartin affects p27 by its influences on tuberin stability/activity. This idea also fits with the earlier observation that the effects of hamartin on p27 are less pronounced than the effects of tuberin (11,12,24). 2 Interestingly, earlier findings suggested that hamartin can also regulate cell proliferation in tuberin-negative cells, but without effects on p27 protein levels (24).…”

“…In the past, we and others have used this approach to demonstrate the negative effects of tuberin on cell cycle progression (23,24). These effects could also be seen in cells co-overexpressing Skp2 and tuberin (Fig.…”

“…Starting with equal cell numbers (harvesting and reseeding), the cell pools were grown under G418 selection for 7 days, and cell numbers were determined on the CASY cell counter and analyzer. For these analyses, cells growing in Petri dishes were washed with buffer and harvested by trypsinization (12,23,24).…”

mTORC1 signaling and regulation of pancreatic β-cell mass

“…Chronic administration of rapamycin has been associated with higher incidence of diabetes, although the relative contribution of β-cell dysfunction and insulin resistance to this effect is difficult to dissect. 63,64 It is possible that the alterations in insulin sensitivity could result from the signals from growth factors and nutrients, 69,[100][101][102][103][104][105] induces controlled growth and is rarely associated with malignancy. 106,107 The current evidence supports the concept that mTORC1 is active in states of increased insulin demand and plays a major role in β-cell expansion and proliferation induced by AKT and insulin resistance.…”