GDF15 and idiopathic pulmonary fibrosis: Nevertheless, IPF- and COPD-derived phLF showed increased expression of the well-known senescence-related markers: CDKN1A/P21, CDKN2A/P16, tumor suppressor protein 53 (TP53), Growth Differentiation Factor 15 (GDF-15), and Matrix Metalloproteinase 3 (MMP3) (Figure 2D), although not reaching statistical significance.