1,25-Dihydroxyvitamin D₃ increases the growth-promoting activity of autocrine epidermal growth factor receptor ligands in keratinocytes

Topical treatment of normal skin with 1,25-dihydroxyvitamin D₃ [1,25- (OH)₂D₃] or its synthetic analogs results in enhanced keratinocyte proliferation. Autocrine growth factors belonging to the epidermal growth factor (EGF) family play a major role in controlling keratinocyte proliferation. 1,25-(OH)₂D₃ enhanced the autonomous proliferation of HaCaT human keratinocytes in the absence of exogenous growth factors. Autonomous and 1,25-(OH)₂D₃-stimulated proliferations were inhibited by a specific inhibitor of EGF receptor (EGFR) tyrosine kinase, an EGFR-neutralizing antibody, heparin, the heparin antagonist hexadimethrine, and the proteoglycan sulfation inhibitor chlorate. These results indicate the involvement of proteoglycan-dependent EGFR ligands. The initial events in EGFR (i.e. ErbB1) mitogenic signal transduction are dimer formation with another ErbB protein and tyrosine cross-phosphorylation. By immunoprecipitation followed by Western blotting we showed that ErbB1/ErbB3 heterodimers are the major mitogenic signaling entity in 1,25(OH)₂D₃- stimulated cells. 1,25-(OH)₂D₃ did not affect the levels of the proteoglycan-dependent EGFR ligands amphiregulin and heparin-binding EGF nor the synthesis of proteoglycans, as assessed by ³⁵S labeling and ion exchange chromatography. 1,25-(OH)₂D₃ caused a marked increase in the cellular contents of ErbB1, ErbB2, and ErbB3 proteins. The increase in ErbB proteins that mediates signal transduction by EGFR ligands can account for the stimulatory effect of 1,25-(OH)₂D₃ on autonomous keratinocyte proliferation.