ErbB2 and EGFR are downmodulated during the differentiation of 3T3‐L1 preadipocytes

Abstract

The expression of receptors belonging to the epidermal growth factor receptor subfamily has been largely studied these last years in epithelial cells mainly as involved in cell proliferation and malignant progression. Although much work has focused on the role of these growth factor receptors in the differentiation of a variety of tissues, there is little information in regards to normal stromal cells. We investigated erbB2 expression in the murine fibroblast cell line Swiss 3T3L1, which naturally or hormonally induced undergoes adipocyte differentiation. We found that the Swiss 3T3-L1 fibroblasts express erbB2, in addition to EGFR, and in a quantity comparable to or even greater than the breast cancer cell line T47D. Proliferating cells increased erbB2 and EGFR levels when reaching confluence up to 4- and 10-fold, respectively. This expression showed a significant decrease when growth-arrested cells were stimulated to differentiate with dexamethasone and isobutyl-methylxanthine. Differentiated cells presented a decreased expression of both erbB2 and EGFR regardless of whether the cells were hormonally or spontaneously differentiated. EGF stimulation of serum-starved cells increased erbB2 tyrosine phosphorylation and retarded erbB2 migration in SDS-PAGE, suggesting receptor association and activation. Heregulin-alpha1 and -beta1, two EGF related factors, had no effect on erbB2 or EGFR phosphorylation. Although 3T3-L1 cells expressed heregulin, its specific receptors, erbB3 and erbB4, were not found. This is the first time in which erbB2 is reported to be expressed in an adipocytic cell line which does not depend on non EGF family growth factors (thyroid hormone, growth hormone, etc.) to accomplish adipose differentiation. Since erbB2 and EGFR expression were downmodulated as differentiation progressed it is conceivable that a mechanism of switching from a mitogenic to a differentiating signaling pathway may be involved, through regulation of the expression of these growth factor receptors.

Expression of erythroblastic leukemia viral oncogene homolog (erbBS) mRNAs and possible splice variants in 3T3-L1 preadipocytes

“…Expression of erbB2 at the protein level in 3T3-L1 cells (20) and its involvement in the proliferation/differentiation processes of these cells were previously observed (21). The present study completes our understanding of the expression, modulation and activation of the erbB receptors in the 3T3-L1 cell line and poses new questions regarding possible roles for erbB soluble forms in this adipogenic model.…”

“…5). We have already discussed the presence of receptor isoforms at the protein level in a previous study (20).…”

“…erbB3 also has different isoforms, of which p85-serbB3 (soluble) is capable of inhibiting heregulinmediated erbB2, erbB3 and erbB4 activation (39,40). However, in this study, only the wild-type receptor was investigated [erbB3 expression in 3T3-L1 cells at the protein level, previously published (20)]. Based on the erbB3 murine sequence, we designed a pair of primers corresponding to mRNA 5' end that amplified a fragment of the expected size (308 pb) plus two more fragments of variable size (Fig.…”

“…Previously, we studied erbB1/EGFR, erbB2, erbB3 and erbB4 as well as ligands, heregulin and EGF, in terms of expression, activation and modulation during the adipogenic process in 3T3-L1 cells (20,21). In this study, the presence of full-length erbB mRNAs and, of splice or proteolytic variants were examined using RT-PCR.…”

“…Given that 3T3-L1 cells differentiate from preadipocytes (fibroblasts) to adipocytes, a mechanism of this type is to be expected. The occurence of these receptor isoforms in 3T3-L1 cells may be of interest since they have not been previously described in adipocytes and their presence may explain the modulation that erbB2 and EGFR exhibit at the protein level during and after the differentiation induction phase (20).…”