question: Which are the main functions of the human HuR (ELAVL1) protein in fibroblasts?
answer: HuR is an RNA-binding protein that can stabilize labile mRNAs containing AU-rich elements in their 3' untranslated regions and has been shown to shuttle between the nucleus and cytoplasm. HuR function was previously shown to be implicated in the maintenance of a "young cell" phenotype in models of replicative cellular senescence. Loss of HuR is linked to reduced expression of proliferative genes during replicative senescence. Importantly, overexpression of HuR in senescent cells restored a "younger" phenotype, while a reduction in HuR expression accentuated the senescent phenotype. HuR associated with the 3' untranslated region of the mRNA encoding the longevity and stress-response protein SIRT1, stabilized the SIRT1 mRNA, and increased SIRT1 expression levels. In mesenchymal cells HuR plays a dominant role in lung development and as a key post-transcriptional regulator of networks guiding tissue remodeling during branching morphogenesis. In fibroblasts knockdown of HuR decreased the endogenous expression of TGFβ1 under exogenous TGFβ1 treatment, simultaneously with the decrease of Col1a, Col3a and fibronectin expression.  HuR (human antigen R), represses ARF mRNA translation, thereby maintaining the replicative life span of mouse embryonic fibroblasts (MEFs). HuR is considered a global regulator of cell-cycle progression and tumorigenesis. Through its post-transcriptional influence on specific target mRNAs, HuR can alter the cellular response to proliferative, stress, apoptotic, differentiation, senescence, inflammatory and immune stimuli.