Nucleoplasmic LAP2α–lamin A complexes are required to maintain a proliferative state in human fibroblasts

Abstract

In human diploid fibroblasts (HDFs), expression of lamina-associated polypeptide 2 α (LAP2α) upon entry and exit from G0 is tightly correlated with phosphorylation and subnuclear localization of retinoblastoma protein (Rb). Phosphoisoforms of Rb and LAP2α are down-regulated in G0. Although RbS780 phosphoform and LAP2α are up-regulated upon reentry into G1 and colocalize in the nucleoplasm, RbS795 migrates between nucleoplasmic and speckle compartments. In HDFs, which are null for lamins A/C, LAP2α is mislocalized within nuclear aggregates, and this is correlated with cell cycle arrest and accumulation of Rb within speckles. Nuclear retention of nucleoplasmic Rb during G1 phase but not of speckle-associated Rb depends on lamin A/C. siRNA knock down of LAP2α or lamin A/C in HDFs leads to accumulation of Rb in speckles and G1 arrest, probably because of activation of a cell cycle checkpoint. Our results suggest that LAP2α and lamin A/C are involved in controlling Rb localization and phosphorylation, and a lack or mislocalization of either protein leads to cell cycle arrest in HDFs.

Conserved cysteine residues in the mammalian lamin A tail are essential for cellular responses to ROS generation

“…Specifically, lamin A has been shown to regulate a p53-dependent DNA damage response pathway Varela et al, 2005), the pRb ⁄ E2F cell proliferation pathway (Markiewicz et al, 2002;Johnson et al, 2004;van Berlo et al, 2005) and the AP1 pathway (Ivorra et al, 2006;Gonzalez et al, 2008). In particular, lamin A has been shown to be a key regulator of pRb function, and siRNA knockdown of lamin A in human cells has been shown to lead to impaired cell cycle progression and premature cell cycle arrest (Pekovic et al, 2007). Thus, we propose that lamin A is essential for an adaptive response to oxidative stress in two ways: as a ROS 'sink', and because lamin A interacts with proteins that regulate cell cycle progression.…”

“…Human dermal fibroblasts were cultured in either atmospheric (20.6%) or low (5%) O 2 incubators in Dulbecco's modified Eagle's medium (DMEM; Invitrogen, Paisley, UK) supplemented with 10% fetal bovine serum (FBS) as previously described (Pekovic et al, 2007). Human dermal fibroblasts were used between PDL 12-40 and harvested 3 days after subculture.…”

“…A-type lamins include two major isoforms, lamins A and C, produced by alternative splicing of the LMNA gene and are essential for post-natal development and differentiation . In addition to providing structural support to the nucleus, the nuclear lamins are involved in transcriptional silencing , cell cycle control (Johnson et al, 2004;van Berlo et al, 2005;Pekovic et al, 2007), DNA repair , and signal transduction (Pekovic & Hutchison, 2008).…”

Nuclear lamins: key regulators of nuclear structure and activities

“…Because lack of A-type lamins or the expression of disease-causing mutant lamin A affect the cell cycle, as well as differentiation of myocytes and adipocytes [117][118][119], it has been proposed that lamins A and C-LAP-2␣ complexes also control the balance between proliferation and differentiation of adult stem cells in tissue homeostasis and regeneration (see below) [120,121]. Interestingly, while mouse fibroblasts derived from Lmnanull or LAP-2␣-null cells showed accelerated proliferation, human dermal fibroblasts down-regulation of either lamins A and C or LAP-2␣ showed G1 cell cycle arrest [122]. Further studies are required to understand these differences.…”

Non-Thermal Effects of Near-Infrared Irradiation on Melanoma

“…Remarkably, many tissues affected by mutations in the lamin A gene are also affected in many degenerative conditions common to old age. The compromised tissue functions in laminopathy diseases are proposed to be a consequence of decreased cellular proliferation (Pekovic et al, 2007), a failure to maintain a differentiated state, and/or a loss of tissue repair during regeneration (Mounkes & Stewart, 2004;Bakay et al, 2006). Lamin A knock-out mouse models as well as mutated lamin A knock-in or transgenic mice manifesting either muscular dystrophy (Sullivan et al, 1999;Arimura et al, 2005) or premature ageing (Mounkes et al, 2003;Yang et al, 2005;Varga et al, 2006) have shortened lifespans and die prematurely.…”