A-type lamins are necessary for the stabilization of the retinoblastoma protein

Very few genes have generated as much interest as the LMNA gene, which encodes all A-type lamins, including lamin A and lamin C. Lamin A/C are intermediate filament proteins that contribute to the organization and maintenance of the nuclear structure and in comprise a protein meshwork underlying the inner nuclear membrane known as the nuclear lamina. Over 180 mutations in LMNA are associated with at least 13 different tissue-specific degenerative diseases, known as laminopathies. Little is known about the pathogenesis of laminopathies, but one theory posits that lamin A/C regulates the activity of a number of transcription factors necessary for maintaining the balance between proper cellular proliferation and terminal differentiation. However, the precise mechanism by which this intermediate filament protein regulates these transcription factors is unknown.; Previous research has indicated that A-type lamins interact with the retinoblastoma protein (pRB), a critical regulator of cell proliferation and differentiation and an important tumor suppressor. Here, I examine pRB function in cells lacking lamin A/C, and I discovered that pRB levels are dramatically reduced. I demonstrate that A-type lamins protect pRB from ubiquitination and proteasomal degradation. Since pRB is required for cell cycle arrest by p16ink4a, I probed the functional consequence of this association by testing the responsiveness of cells lines lacking A-type lamins to overexpression of this CDK inhibitor and tumor suppressor. I find that the reduction of pRB in cells lacking lamin A/C expression renders cells resistant to p16 ink4a-mediated cell cycle G1 arrest. Reintroduction of lamin A or lamin C restores pRB levels and p16ink4a responsiveness to Lmna-/- cells. An array of lamin A mutations associated with various laminopathies was introduced into the Lmna -/- cells. Of these mutations, one mutant associated with mandibuloacral dysplasia (MAD R527H), as well as two lamin A processing mutants, failed to restore p16ink4a responsiveness. My findings do not support a connection between laminopathies and altered pRB function. However, they do link lamin A/C to the functional activation of a critical tumor suppressor pathway and further support the possibility that somatic mutations in LMNA contribute to tumor progression.